A111D1 HO1^

NBS

PUBLICATIONS

NBSIR 82-2487

Hail Impact Testing Procedure for

Solar Collector Covers

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Materials Division

Washington, DC 20234

April 1982

-QO

10U

.1156

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

" Active Heating and Cooling Division

Washington, DC 20585

62-2487

19d2

NBSIR 82-2487

HAIL IMPACT TESTING PROCEDURE FOR

SOLAR COLLECTOR COVERS

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MAY 1 4 1982

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David R. Jenkins

Robert G. Mathey

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U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Materials Division

Washington, DC 20234

April 1982

Prepared for U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

HAIL IMPACT TESTING PROCEDURE FOR SOLAR COLLECTOR COVERS

by

David R. Jenkins* and Robert G. Mathey**

ABSTRACT

This report presents laboratory test results which simulate hail impact on solar collector covers. The general objective of the work is to contribute to

the development of a test method for evaluating the resistance of solar collector covers to this type of loading. A procedure for such testing is described

as well as results obtained with ice balls impacting four typical collector

cover materials. Aspects which are discussed include the preparation of ice

balls, the design and operation of a launcher for ice ball propulsion, the

method of mounting cover panel specimens, the selection of ice ball velocity

and impact location, and techniques for failure or damage assessment.

The research results show that ice balls of consistent diameter and mass can

be prepared in the laboratory. Further, both analysis and results tend to

show that acceptable simulation for evaluation or testing can be achieved with

normal impacts of ice balls traveling at a resultant velocity which is the

vector sura of the terminal velocity and a horizontal wind component. Results

for a variety of impact locations are presented and for comparison purposes,

arbitrarily selected points near the collector cover boundaries appear to be a reasonable choice. Finally, it is shown that for some collector cover materials,

more than one kind of failure must be considered when evaluating test results.

Test data for two types of tempered glass, semirigid fiber reinforced plastic,

and flexible thin plastic film covers are presented.

Key Words: hail damage, hail impact testing, hail launcher, simulated hail

testing, solar collector covers, test method development.

* Professor of Engineering, University of Central Florida, Orlando, Florida,

on assignment to the National Bureau of Standards from September 1978 to

September 1980 under provisions of the Intergovernmental Personnel Act.

** Center for Building Technology, National Bureau of Standards, Washington, DC.

iii
ACKNOWLEDGMENT

This report was prepared under the sponsorship of the DoE Active Heating and

Cooling Division, Office of Solar Heat Technologies.

The authors wish to acknowledge with thanks the contribution of Mr. Jessie C.

Hairston of the National Bureau of Standards (NBS) for conducting the laboratory

tests and for the fabrication of some of the test apparatus. The authors also

recognize the contribution of David Waksman, Elmer R. Streed, Thomas W. Reichard,

and Louis E. Cattaneo of NBS for the development of the first draft of a standard for use in the evaluation of the hail impact resistance of solar collector

covers. This draft standard was published in National Bureau of Standards

Report NBSIR 78-1305A, "Provisional Flat Plate Solar Collector Testing

Procedures: First Revision," June 1978.

In addition, the authors are grateful for valuable comments provided by

Ms. Elizabeth J. Clark of NBS regarding the selection of cover materials for

test and for information pertaining to a proposed standard for evaluation of

hail impact resistance of solar collector covers which was under consideration

by ASTM E44.04.02 Cover Plate Task Group. This proposed standard was approved

by ASTM in June 1981 and is designated as ASTM E822-81, "Practice for Determining Resistance of Solar Collector Covers to Hail by Impact with Propelled

Ice Balls."

iv
TABLE OF CONTENTS

Page

ABSTRACT ill

ACKNOWLEDGMENT iv

LIST OF TABLES vii

LIST OF FIGURES x CONVERSION FACTORS FOR METRIC (SI) UNITS xii

1. INTRODUCTION AND PURPOSE 1 1.1 COVER MATERIALS AND FUNCTION 1

1.2 BACKGROUND 1 1.3 PURPOSE 2 1.4 SCOPE 2

2. HAIL TESTING CONCEPTS AND VARIABLES 4

2 . 1 MAJOR PARAMETERS 4

2.2 IMPACT LOADINGS AND STRESSES 5 2.3 SIMULATED HAIL TESTING BY OTHER INVESTIGATORS 6

2.4 LABORATORY TESTING 8

3. PREPARATION OF ICE BALLS 9

3.1 SIMULATION 9

3.2 THE SPLIT MOLD APPROACH 9

3.3 CASTING PROCEDURE 11

4. LAUNCHER 14

4.1 COMPRESSED AIR GUN 14

4.2 INTERCHANGEABLE BARRELS 14

4.3 TIMING RANGE 20

4.4 OPTICAL POINTER 20

5. MOUNTING OF COVER SPECIMENS 23

5.1 COVER SUPPORT FRAMES 23

5.2 PLACING SPECIMENS IN THE FRAMES 23

5.3 SUPPORT FRAME POSITIONER 25

6. ICE BALL VELOCITY AND IMPACT POINTS 32

6.1 ALTERNATIVE LABORATORY VELOCITIES 32

6.2 VALUES OF TERMINAL VELOCITY AND WIND VELOCITY 32

6.3 MAGNITUDE OF THE NORMAL VELOCITY 34

6.4 IMPACT LOCATIONS 38

7 . FAILURE OR DAMAGE ASSESSMENT 40

8. TEST PROCEDURE 48

v
TABLE OF CONTENTS (Continued)

Page

9. TEST RESULTS 49

9.1 GENERAL 49

9.2 TEMPERED GLASS, TEXTURED SIDE EXPOSED 51

9.3 TEMPERED GLASS, BOTH SIDES SMOOTH 52

9.4 SEMIRIGID FIBER REINFORCED PLASTIC SHEET 52

9.5 FLEXIBLE THIN PLASTIC FILM 53

9.6 TEMPERED GLASS MOUNTED IN SOLAR COLLECTORS 54

10. CONCLUSIONS 57

11. REFERENCES 58

APPENDIX A IMPACT TEST DATA A1

vi
LIST OF TABLES

Page

Table 1. Interchangeable Barrel Materials and Dimensions .... 17

Table 2. Ice Ball Parameters for Laboratory Testing 33

Table 3. Summary of Ice Ball Impact Test Results 50

Table A-l Tempered Glass Exposed Side Textured 1/8 in.

Thick A2

Table A-2 Tempered Glass Exposed Side Textured 1/8 in.

Thick A2

Table A-3 Tempered Glass Exposed Side Textured 1/8 in.

Thick A2

Table A-4 Tempered Glass Exposed Side Textured 1/8 in.

Thick A3

Table A-5 Tempered Glass Exposed Side Textured 1/8 in.

Thick A3

Table A-6 Tempered Glass Exposed Side Textured 1/8 in.

Thick A3

Table A-7 Tempered Glass Exposed Side Textured 1/8 in.

Thick A3

Table A-8 Tempered Glass Exposed Side Textured 1/8 in.

Thick A4

Table A-9 Tempered Glass Exposed Side Textured 1/8 in.

Thick A4

Table A-10 Tempered Glass Exposed Side Textured 1/8 in.

Thick A4

Table A-ll Tempered Glass Exposed Side Textured 1/8 in.

Thick A4

Table A-12 Tempered Gla ;s Both Sides Smooth 1/8 in. Thick A5

Table A-13 Tempered Glass Both Sides Smooth 1/8 in. Thick A5

Table A-14 Tempered Glass Both Sides Smooth 1/8 in. Thick A5

Table A-15 Tempered Glass Both Sides Smooth 1/8 in. Thick A6

vii
LIST OF TABLES (continued)

Page

Table A-16 Tempered Glass Both Sides Smooth 1/8 in. Thick A6

Table A-17 Tempered Glass Both Sides Smooth 1/8 in. Thick A6

Table A-18 Tempered Glass Both Sides Smooth 1/8 in. Thick A7

Table A-19 Tempered Glass Both Sides Smooth 1/8 in. Thick A7

Table A-20 Tempered Glass Both Sides Smooth 1/8 in. Thick ..... A7

Table A-21 Tempered Glass Both Sides Smooth 1/8 in. Thick A7

Table A-22 Tempered Glass Both Sides Smooth 1/8 in. Thick A8

Table A-23 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A8

Table A-24 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A8

Table A-25 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A9

Table A-26 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A9

Table A-27 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A9

Table A-28 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A10

Table A-29 Reinforced Plastic Polyester Resin With Glass

Fiber Mat 0.040 in. Thick A10

Table A-30 Poly(Vinyl Fluoride) Film 0.004 in. Thick A10

Table A-31 Poly(Vinyl Fluoride) Film 0.004 in. Thick All

Table A-32 Poly(Vinyl Fluoride) Film 0.004 in. Thick All

Table A-33 Poly(Vinyl Fluoride) Film 0.004 in. Thick All

Table A-34 Poly(Vinyl Fluoride) Film 0.004 in. Thick A12

Table A-35 Poly(Vinyl Fluoride) Film 0.004 in. Thick A12

viii
LIST OF TABLES (continued)

Page

Table A-36 Poly(Vinyl Fluoride) Film 0.004 in. Thick A12

Table A-37 Poly(Vinyl Fluoride) Film 0.004 in. Thick A13

Table A-38 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A13

Table A-39 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A13

Table A-40 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A14

Table A-41 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A14

Table A-42 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A14

Table A-43 Tempered Glass Both Sides Smooth 1/8 in. Thick Mounted

in Solar Collector A14

ix
LIST OF FIGURES

Page

Figure 1. Cross Section of Metal Mold Pattern for Making Rubber

Molds Used to Cast Ice Balls . 10

Figure 2. Silicone Rubber Ice Ball Molds 12

Figure 3. Overall View of Launcher System 15

Figure 4. Compressed Air Gun 16

Figure 5. Interchangeable Barrels for the Compressed Air Gun 18

Figure 6. Installation of Barrel in the Compressed Air Gun 19

Figure 7. Optical Pointer for the Hail Gun 21

Figure 8. Schematic of the Optical Pointer 22

Figure 9. Sketch of Assembled Support Frame 22

Figure 10. Typical Support Frame Cross Sections 24

Figure 11. Corner Strap for Support Frames 24

Figure 12. Tensioning of Film Cover Material 26

Figure 13. C-Clamp Weights Attached to Film Cover Material ........ 27

Figure 14. Hole in Film Cover Material to Permit Bolt Passage 28

Figure 15. Transverse Loading of Film Cover to Check Consistency

of Tensioning 29

Figure 16. Lift Truck Modified as a Support Frame Positioner ...... 31

Figure 17. Three-Dimensional Relationship of Resultant Velocity and

Normal Component of Resultant Velocity 35

Figure 18. Two-Dimensional Relationship of the Hailstone Velocity

Components and the Normal Velocity Component for Wind

Direction Perpendicular to the Horizontal Edge of An

Inclined Collector 35

Figure 19. Assumed and Actual Normal Velocity Values for

Various Terminal Velocities 37

Figure 20. Layout of Impact Points 39

x
LIST OF FIGURES (continued)

Page

Figure 21. Overall View of a Textured Surface Tempered Glass Cover

Specimen After Fracture 41

Figure 22. Hole in Tempered Glass Cover Specimen Caused by Impact

of an Ice Ball 42

Figure 23. Localized Cracking Failure in Reinforced Plastic Cover

Caused by Impact of an Ice Ball 43

Figure 24. Cylinder Sealed to Reinforced Plastic Material for Leak

Testing 44

Figure 25. Tearing and Penetration Through Reinforced Plastic Cover

Material Caused by Impacting Ice Ball 45

Figure 26. Indentations in Plastic Film Cover 47

Figure 27. Puncture Failures of Plastic Film Cover 47

Figure 28. Detail of Solar Collector Edge Support 56
CONVERSION FACTORS FOR METRIC (SI) UNITS

Physical

Quantity

To Convert

From To Multiply By

Length ft m 3.048 x 10_1

Length in m 2.54 x 10~2

Force lbf N 4.448

Mass lbm kg 4.536 x lO" 1

Force/length lbf /ft N/m 1.459 x 10

Velocity ft/s m/s 3.048 x 10" 1

Temperature °F °C t° c = (t° F-32)/l

Energy ft-lbf J 1.356

Momentum lbf-s N-s 4.448

xii
1. INTRODUCTION AND PURPOSE

1.1 COVER MATERIALS AND FUNCTION

This report presents testing procedures and test results for flat plate solar

collector cover materials subjected to simulated hail impact. The cover materials examined are representative of the three classes normally encountered,

namely, rigid, semirigid, and flexible. Tempered glass is a typical rigid

cover, fiber reinforced plastic is a typical semirigid cover, and polymeric

thin film is a typical flexible cover.

In a solar collector, the cover plate (or cover plates since double covers are often used) in addition to protecting the interior of the collector from the

weather has at least three functions which affect collector efficiency. Most

importantly, the cover permits transmission of solar energy to the absorber

plate and consequently its transmittance should be as high as possible.

Secondly, the cover may suppress infrared reradiation from the absorber plate.

Thirdly, by maintaining a confined air space between the cover and the absorber

plate, the cover plate tends to reduce convective heat loss from the absorber

plate. Thus, cover failure can substantially degrade the operation of the

entire system. Failure of a collector cover by hail impact can occur by several

means. Unquestionably, complete rupture of the cover material where the cover

plate is destroyed is one type of failure. Also, extensive crazing, fine crack

development, or surface distortion which substantially reduce transmission

could be looked upon as failure. Further, local rupture of the cover material

which allows air exchange from inside to outside may be considered to be a failure. Such a local rupture may also permit moisture penetration into the

collector which could cause degradation of the absorber plate and insulation

and result in a decrease in thermal performance of the collector. It is noted

that cover plate failures affect the function or serviceability of collectors

and are not in general a safety problem. A possible exception to this could

be with annealed glass or other materials which break in a similar manner and

could give off large flying shards.

1.2 BACKGROUND

Laboratory evaluation of hail impact resistance may be performed for several

reasons: (1) to evaluate impact resistance of a single material or collector;

(2) to compare the impact resistance of several materials or collectors; (3) to

provide a common basis for selection of cover materials or collectors for use

in various geographic areas; or (4) to evaluate changes in impact resistance

due to environmental factors such as weather. The purpose of the testing may

influence the choice of testing parameters. If the purpose of the testing is

to determine the minimum size hail ball causing damage, testing will proceed

with progressively larger ice balls until the cover material sustains damage.

However, if the purpose of the testing is to determine whether a collector

could be used in a specific geographic region, it would be necessary to test

the cover using ice balls of the selected size and corresponding velocity

1
JL for satisfactory service in that region. Simiu and Cattaneo [1] proposed

maximum hailstone sizes corresponding to a 20-year mean recurrence interval

for various regions of the United States. An appropriate hailstone size and

corresponding velocity can be selected from reference [1] or from a similar

governing or advisory document [2].

Prior to the approval of the American Society for Testing and Materials

Standard designated as ASTM E 822-81 in June 1981, "Standard Practice for

Determining Resistance of Solar Collector Covers to Hail by Impact with Propelled Ice Balls," there was no standard method available to evaluate the

impact resistance of covers. The early Department of Housing and Urban

Development (HUD) Minimum Property Standards [3] and the Interim Performance

Criteria prepared by National Bureau of Standards (NBS) for Department of

Energy (DoE) and HUD, respectively [4, 5], had requirements for testing such

as hail ball size for geographic regions, but did not have a test method.

1.3 PURPOSE

The primary purpose of the research reported here is to contribute to the

development of a test method for simulating hail impact on solar collector

covers. For some time now, the general outlines of a test method have been in

existence [6, 7], namely to propel ice balls or other hail simulators with a known velocity at a target consisting of the material to be evaluated. The

broad requirements for the test method were that the laboratory procedure provide good simulation of hail storm events and that the method be a practical

one that can be reproduced by a number of laboratories at reasonable cost.

Consequently, the work reported herein has emphasized the refinement of testing

procedures and data evaluation procedures rather than the accumulation of extensive additional data on hail impact resistance of specific solar collector cover materials

.

1.4 SCOPE

The hail impact test procedure for covers used in flat plate solar collectors

was based on many factors. Most importantly were those dealing with the

velocities of ice balls used to simulate hail, impact loads caused by falling

hail and propelled ice balls, impact locations on the collector cover, support

conditions for the covers, and failure or damage assessment of collector

covers caused by impacting ice balls.

Other factors considered were the procedures, techniques, equipment, and methods

of test and fabrication for the different phases of the hail impact testing

procedure. A procedure is given for the preparation of ice balls which

includes a method for making ice ball molds. A launcher for propelling ice

balls is described along with a timing device for measuring their velocity.

An optical pointer technique for alignment of the barrel of the launcher on a

Numbers in brackets indicate references listed in section 11.

2
selected target area on the solar collector cover Is described. Also, a method

for mounting the cover material in a cover support frame is given together with

a method to position the cover support frame during testing.

Test results are presented for the impact resistance of rigid, semirigid, and

flexible cover materials mounted in cover support frames and for tempered glass

mounted in flat plate solar collectors. This testing was performed to evaluate

the test method. It was not intended to develop a comprehensive data base that

could be used to determined the suitability of specific products. Since the

properties of a specific product are highly dependent on the formulation, method

of manufacture, and other variables it is not possible to draw conclusions for

generic classes of materials.

Based upon the laboratory impact testing experience at the National Bureau of

Standards (NBS) a standard for use in evaluation of hail impact resistance of

solar collector covers was drafted and submitted to ASTM Committee E44 on Solar

Energy Conversion, Subcommittee E44.04 on Materials Performance. The ASTM

Cover Plate Task Group, E44.04.02, prepared the standard practice designated

as ASTM E822-81, "Practice for Determining Resistance of Solar Collector Covers

to Hail by Impact with Propelled Ice Balls." During the conduct of this study

to develop a hail impact testing procedure for solar collector covers, information was provided by NBS to the Task Group for consideration for their draft

standard. The laboratory work described in this report supports and explains

the major elements of the ASTM standard.

3
2. HAIL TESTING CONCEPTS AND VARIABLES

2.1 MAJOR PARAMETERS

When evaluating the damage risk which hail presents to solar collector

installations, a number of parameters must be considered. Only a brief overview of the significant parameters will be presented. The parameters pertaining to hail which will be discussed are frequency of occurrence, size, specific gravity, speed, angle of velocity vector with collector surface, and kinetic energy. Other parameters to be considered are the mounting method of the cover and the location of hail impact. Simiu and Cattaneo [1] have presented a summary of published data applicable to hail load determination. As is true for

loadings associated with most natural phenomena, randomness suggests a statistical rather than deterministic approach. The frequencies of occurrence of

storms containing hail and the hailstone diameter distribution vary with the

region of the United States. Hail frequency maps are available which show the

average number of days with hail in various areas. In recent years, the availability of increased hail data and the use of more sophisticated statistical

analyses has made it possible to estimate the maximum size of hail to be

expected under various conditions [1].

Specific gravity of hail is an important consideration since it appears to be

less than that of ice due to air entrapped in the hailstone as it freezes [1].

However reported values often do not show a great variation from solid ice

having a specific gravity of 0.92 and thus the use of ice balls in simulating

hail impact does not seem to be excessively conservative [8, 9].

The speed at which a hailstone strikes an object is also a critically important

parameter. Much attention has been devoted to this subject by other investigators and their work is discussed in section 6.2. In general it has been

observed that hailstones fall at a terminal velocity which occurs when the

weight of the hailstone equals the drag force. Since most hailstones are accompanied by wind it seems reasonable to add vectorially a horizontal wind

component to the vertical terminal velocity component. For this research program, a resultant velocity was used which was based on a 66 ft/s horizontal wind

velocity and the appropriate terminal velocity for a given hailstone diameter

(section 6.2). The angle which the velocity vector makes with the collector

surface is another important consideration and the conservative assumption of

normal impact at terminal or at a resultant velocity is usually made in testing

(section 6.3).

Finally, the kinetic energy of a hailstone,

Kinetic energy = 1/2 mV^ (1)

where: m = mass of hailstone

V = velocity of hailstone,

while it is a combination of other parameters, is one measure of the intensity

of impact on the cover surface. Since kinetic energy is not conserved when

the ice ball or hailstone hits the cover, part of the kinetic energy is consumed

4
in fracturing of the ice ball, part goes into work done to deform the cover,

and part is used by the ice ball or its fragments which rebound from the cover

surface

.

An alternate measure of the intensity of impact on the cover surface is the

momentum of the hailstone, that is,

Momentum = mV (2)

where m and V are as defined above. While momentum is conserved in the impact,

this fact is not easy to exploit since the masses and flight paths of all hail

ball fragments after impact are difficult to determine.

It has been shown experimentally that the method of mounting a cover (i.e„,

its support condition) and the location at which ice balls impact the cover

affect its resistance to impact. As an example, the corners are in general

the most critical impact points (see section 6.4).

2.2 IMPACT LOADINGS AND STRESSES

The exact nature of the impact loading, or impulse loading, which occurs when

the cover is struck by an ice ball is complex and may be difficult to fully

define. Impulse is by definition, the load versus time history at the point

of impact. This aspect of hail impact has not yet been well studied and might

be an appropriate subject for future investigation. The exact nature of the

impulse depends on the deformation of the ice ball and of the cover material

while they are in contact, on whether fragmentation of the ice ball occurs during the contact period, and on whether fracture of the cover material occurs during this period. Consequently, it is extremely difficult to experimentally

define the load-time curve for the full duration of the impulse. Alternatively,

it is possible to determine the duration of the impulse or the length of time

the ice ball and cover are in contact and it may be possible to determine the

total impulse or area under the load-time curve since total impulse is equal

to the change in momentum of the impacting objects, that is

2 / F dt = £ Am^«Av^2 - m + Cover momentum (3)

*1

where:

F = normal force

t = t ime m = mass of ice ball

Am^ = mass of pieces of ice ball

V]_ = velocity of ice ball, initial

Avi 2 = velocity of pieces of ice ball, final.

The duration of the impulse is t 2 - t^ . When the duration of the impulse is

compared to the natural period of vibration of the plate-like collector cover,

characterizing the impact response can sometimes be simplified. For example,

3
an approximate analysis of dynamic load response for a single degree of freedom

system indicates that , if the duration of the impulse is short relative to the

natural period of the structure, then the maximum displacement amplitude and

consequently the maximum stress depend only on the impulse height or magnitude

[10]. Impulse durations were not determined in this research program.

If the form of the impulse can be approximated, the relationship between the

time varying forces and the stresses in the cover material may be clarified by

the use of structural mechanics. Since the collector cover can in most cases be considered as a thin plate by structural mechanics definition and since the

rapid loading probably assures elastic material behavior up to failure, dynamic

elastic plate analysis may give reasonable predictions of cover deflections

and stresses. The largest stresses in a plate would be normal to its cross section at the plate surfaces. The technique of modal analysis would seem to

be appropriate and would be applicable to point impact loadings [11]. However,

the edge support conditions of a cover in a solar collector or in a laboratory

support frame are not usually the same as the simply supported edges assumed

in most published analysis. By definition a simple support is one in which

there is no vertical deflection of the plate and there is unrestrained rotation

at the supports about lines parallel to the edges of the plate. This uncertainty in the support conditions of covers in collectors could bring into

question the validity of quantitative predictions made by dynamic plate theory.

Qualitatively this analytical technique may be used to compare the stress

levels produced by impacts at various points on the plate. Later it is noted

that failure appears to initiate on the "back" side of the cover, that is the

surface opposite to the contact surface. This suggests that plate action

may predominate.

Mechanics analysis may also provide some insight into the localized stresses

caused by contact between the spherical ice ball and the flat cover plate.

These so-called contact stresses were originally calculated by Hertz [12] and

tend to cause failure below the contact surface due to large shearing stresses

in that region. Again, elastic material behavior of the cover plate and ice

balls is assumed. The radius of curvature of the spherical ice ball would

affect the stress level produced as would the instantaneous value of the load

acting between the two bodies.

To summarize this discussion of impact stress prediction, several possibily

significant parameters can be added to those already mentioned in section 2.1.

These include the duration of the impulse, the total impulse, the support con- ditions of the cover in its supporting frame, and the radius of curvature of

the ice ball.

2.3 SIMULATED HAIL TESTING BY OTHER INVESTIGATORS

The potential damage which can be caused by hail to exposed portions of

buildings and to equipment, such as solar devices, mounted on buildings has

made simulated hail testing a subject of interest to a number of investigators.

Greenfeld [6], while a Research Associate at the National Bureau of Standards,

used an early configuration of the equipment used in the research of this

report, and is described later, to study hail resistance of roofing products.

6
Mathey [7] used basically the same equipment with a modified velocity timer to

investigate hail resistance of aluminum skin honeycomb panels. Although details

of the current configuration will be presented later , it is noted here for

information purposes that this test apparatus uses a compressed air gun for

firing an ice ball horizontally at controlled velocity toward the "exposed”

surface of a test object. A timer for velocity measurement is included. Ice

balls cast in silicone rubber molds are used.

Smith [13], at Texas Tech University, has developed equipment for hail impact

testing of solar reflector panel mirror materials and other materials. He also

uses a compressed air gun or air cannon with interchangeable barrels to accommodate ice balls of 1.0 in., 1.5 in., 2.0 in., and 2.5 in. diameter. His simulated hailstones or ice balls are cast in a spherical shape by a unique process.

In this process, enough water is poured into a rubber balloon for the desired ice

ball size. The portion of the balloon containing the water is placed in a spherical mold and then the mold is placed in a freezer. When an ice ball is

fired, photoelectric timing gates are actuated by ice ball passage and are used

to start and stop an electronic timer. In Smith's test procedure ice balls of

a selected size are fired at the target at increasing velocities until failure

occurs or until terminal velocity is exceeded by some margin.

Moore, Wilson, and Ross [14, 15], of the Jet Propulsion Laboratory have reported

a simulated hail impact apparatus for testing photovoltaic solar panels. Their

compressed air gun fires ice balls vertically upward. Interchangeable barrels

for various ice ball sizes are employed. A photoelectric velocity measuring

system is used. Instead of casting ice balls, Moore, Wilson, and Ross mold

pieces of ice into a spherical shape in a hand operated press after which the

ice balls are frozen. These authors in addition to observing the type of

mechanical failure produced, made high speed movies of ice ball impacts. From

an analysis of the individual frames of the high speed movies, they concluded

that ice ball deceleration is complete in about 0.001 s after initial contact,

that failure initiates in less than 0.002 s after initial contact in glass

panels, and that failure is initiated on the back side of a glass panel in a plate bending mode. In most of their tests on glass panels, ice balls shattered

on impact. With a view to developing a simpler alternative test method, steel

balls of various diameters were dropped from various heights on the cover plate

materials and the results of damage were compared with the damage caused by

ice balls. It was difficult to find a correlation between the steel and ice

ball data for similar kinds of failure but momentum similitude did give

agreement for some cover materials.

A somewhat different approach has been taken by F. Rupp [16]. He used a polyamide sphere rather than an ice ball to simulate hail in a test apparatus

that was otherwise quite similar to those already described. A compressed air

gun with interchangeable barrels for various sizes of spheres was oriented to

fire vertically downward. Velocity was measured by a sequence of three electrooptical sensors. The use of the polyamide ball as a simulator for hail was justified by Rupp on the basis that the failure produced in the test apparatus

was similar to failure produced by hail in the field. However, polyamide's

specific gravity of 1.2 to 1.4 would make it impossible to have both kinetic

energy and momentum similitude. Further, the difference in stiffness and

7
strength between polyamide and ice would make it difficult to simulate all

aspects of the impact of failure process.

2.4 LABORATORY TESTING

When it is necessary to evaluate cover performance, decisions regarding the

values of the parameters (section 2.1) must be made. Simiu and Cattaneo [1]

proposed maximum hailstone sizes corresponding to a 20-year mean recurrence

interval for various regions of the United States. An appropriate hailstone

size and corresponding velocity can be selected from Reference [1] or a similar

governing or advisory document [2]. The collector cover would be expected to

sustain an impact or impacts of hail of the selected size for satisfactory

service in a given region of the country. Ice balls are good similators of

hailstones and are generally used in laboratory hail impact testing.

The testing apparatus for impact tests should be capable of propelling ice ball

of various diameters at various controllable velocities. The system must be

capable of being aimed at selected points on a properly mounted solar collector

cover. For a given cover material, the initial ice ball size and corresponding

velocity should be selected which would not be expected to cause failure.

Selected points on the properly supported cover surface should then be impacted

Damage, if any, should be evaluated after selected points have been impacted

with the initial size ice balls. To avoid delays, ice balls ranging from 1 in.

diameter, in 1/4 in. increments, up to the expected maximum size for the test,

possibly up to 2-3/4 in. or 3 in. diameter, should be available. If there has

been no damage to the collector cover specimen, it can be impacted with the

next larger size ice ball. However, a damaged specimen should be replaced

before the next round of impacts. This procedure is repeated with ice balls

of increasing size until failure occurs or until a "required" ice ball diameter

is exceeded.

8
3.1 PREPARATION OF ICE BALLS

SIMULATION

Ice balls have been used here, and have been used by a number of other

Investigators, to simulate hailstones in laboratory testing. Hailstones

generally consist of layers of ice which may vary in density. It is thought

that ice balls are good simulators for hailstones even though the specific

gravity of ice is slightly higher than that of actual hail [2, 3]. For example

the specific gravity of clear ice is 0.92 as compared to the specific gravities

of large hailstones (1 in. diameter or larger) ranging from 0.92 down to 0.82.

Mean values of measured specific gravities in relatively large hailstone

samples typically are about 0.88.

The selection of ice as a simulator for hail and the attendant use of a higher

specific gravity is not a trivial consideration since both terminal velocity

and kinetic energy may be influenced. For terminal velocity, T, it is found

that

T = C

1 (p H

D) 1/2

(4)

where: = Constant

= Hailstone specfic gravity

D = Hailstone diameter.

The above equation can be derived from Eq. (6) which is discussed in section

6.2 of this report. With regard to kinetic energy, defined by Eq. (1) of

section 2.1, it is found that

K.E . = C

2 pH

2 D4

(5)

where: C 2 = Constant.

Since the kinetic energy is a function of the square of pjj, an 11 percent

change in pjj from 0.92 to 0.82 could result in a 24 percent change in kinetic

energy. The change in kinetic energy would be 9 percent when comparing the p^

of and ice ball of 0.92 to 0.88.

However, clear ice balls can be made reproducibly in the laboratory by

techniques described in section 3.3. If a lower specific gravity ball is

required, the fabrication process would become more complex and probably less

reproducible. In addition, the use of the greater density clear ice tends to

be conservative in that for a given diameter of sphere both terminal velocity

and kinetic energy are higher for the higher density ice. This means that for

criteria based on hailstone size, as proposed by Simiu and Cattaneo [1], the use of solid ice balls in laboratory testing gives a somewhat more severe test

than would be true if "actual” air-containing ice balls were used.

3.2 THE SPLIT MOLD APPROACH

The ice balls were cast in silicone rubber split molds. Figure 1 is a cross section sketch of the metal mold pattern which was used for making the silicone

9
BALL BEARING (FORMS CAVITY

IN ICE BALL MOLDI

ALUMINUM BASE — %\n DIAMETER POST (FORMS

RISER OR OPENING IN ICE

BALL MOLD)

Figure 1. Cross section of metal mold pattern for making rubber molds

used to cast ice balls

10
rubber molds. The metal mold consisted of an external aluminum tube which had

an inner diameter at least 3/4 in. greater than the ice ball cavity.

The ice ball cavity was formed by a steel ball bearing supported on a 1/4-in.

diameter post which in turn fitted into an aluminum base. The post formed the

riser or opening in the silicone rubber mold. A room temperature vulcanizing

silicone rubber compound was thoroughly mixed with catalyst, degassed in a vacuum desicator, and poured into the metal mold. The usual curing procedure

was to cure over night at room temperature. The external aluminum tube and

the aluminum base plate were removed from the cured silicone rubber casting.

The rubber mold was then placed in a lathe and a cut carefully made at the

equator of the embedded ball bearing. Index marks were made on the exterior

of the halves before the casting was separated into two parts and the ball

bearing removed. Figure 2 is a photograph of some of these molds. This

procedure produced a mold which provided ice balls with virtually no trace of

the parting line in the mold. The molds were stable dimensionally and reusable

over an extended period of time.

It is noted that this procedure for mold-making is largely a refinement of the

procedure described by Greenfeld [6] and improved by Mathey [7].

3.3 CASTING PROCEDURE

Although the procedure described below may seem tedious, it was found to be

necessary if uncracked ice ball castings were to be produced. While it has been

the practice at NBS to use individual ice ball molds, gang molds can also be

fabricated and used.

The first step was to nearly fill the lower half of the mold with water,

assemble the two halves of the mold, and place the assembly in the freezer compartment of a refrigerator at about 0°F. Note that a thin layer of silicone

stop cock grease was placed on the mating surfaces of the mold before assembly

to prevent water from leaking and to hold the two halves of the molds together.

The index marks on the exterior of the molds were lined up to assure that the

mold halves were assembled in the same relative orientation in which they were originally formed. This practically assured that there was no offset in the

halves of the mold at the parting line.

When the water in the lower half of this mold was frozen, the remaining cavity

was filled about one half full. The second step was completed by freezing the

added water.

Finally, the third step was to fill the mold with water into the riser or opening. During this step it was very important not to entrap air in the mold

while making the final fill. One way to do this was to use a wash bottle with

a dispensing tube which was inserted into the mold through the riser so that

water entered the mold at the surface of the ice. In this way, air was pushed

out of the riser while the mold filled.

This entire procedure required about 24 hours from start to finished ice ball.

The ice balls produced using this procedure were of consistent size and mass.

11
12

Figure

2.

Silicone

rubber

ice

ball

molds
Ice balls with diameters of 3/4 in., 1 in., 1-1/4 in., 1-1/2 in., 1-3/4 in., 2 in., 2-1/4 in., and 2-1/2 in. were cast and used in obtaining the test results

reported in section 9. Ice balls were stored in plastic bags in a freezer at

-20°F. A given bag contained ice balls of the same diameter and was marked

with the ball diameter for easy identification.

13
4 . LAUNCHER

The launcher system for ice balls consists of a compressed air gun, a set of

interchangeable barrels, a timing range for measuring ice ball velocity, and

an optical pointer for alignment on a selected target area. Figure 3 is an overall view of the launcher system, which is very much the same as that used

in earlier work [6, 7]. It should be noted that, in the impact tests reported

here, the compressed air gun, barrel, and timing range assembly were mounted

on a stationary platform.

4.1 COMPRESSED AIR GUN

The compressed air gun, shown in figure 4, is simply a large chamber which can

be pressurized and which is connected, through a 2-in. diameter quick release

valve to a barrel adapter whose inside diameter is 3-1/4 in. During the work

reported here, a 15 psi pressure gage was used although a gage with a lower

pressure range could have been used in many cases. Chamber pressures as low

as 2 psi were all that was required to achieve resultant velocity (terminal

velocity and 66 ft/s horizontal wind component) for 3/4-in. diameter ice balls,

for example. The particular gun used at NBS was a commercially available model

4.2 INTERCHANGEABLE BARRELS

Interchangeable barrels were made of either poly(vinyl chloride), PVC, or acrylic tubing. In the earlier work at NBS, a brass barrel, 3-1/4 in. in

diameter, and polyethylene carriers for each ice ball size were used [7].

For the present work, tubing was selected so that the inside diameter was slightly larger than the ice ball diameter. When the inside diameter of the

barrel was 0.05 to 0.14 in. larger than the ice ball, the greatest velocity

for a particular barrel size and chamber pressure was obtained. If a "tighter"

fit was chosen, the ice ball decelerated by contact friction as it passed

through the barrel. On the other hand, if a "looser" fit was chosen, pressure

did not build up behind the ice ball when the quick release valve was opened

and a slower than expected velocity was obtained. Table 1 lists the barrel

materials and dimensions for each size of ice ball. Figure 5 shows the assortment of barrels used and figure 6 shows one of the barrels being installed in

the compressed air gun. Wooden adapter sleeves were attached to the breech

end of each barrel so that a snug fit in the barrel adapter on the compressed

air gun was obtained. These adapter sleeves are visible in figure 5.

In our preliminary work, brass tubing was tried for the interchangeable barrels

A note about our negative experience may be in order since other investigators

have used metallic barrels with apparent success. Brass barrels with internal

diameters 0.05 to 0.10 in. larger than the ice ball diameters resulted in ball

velocities much lower (about half) than for plastic barrels for a given air

chamber pressure. Also, for a given chamber pressure, the velocities obtained

were much more erratic with the brass barrels. Although the reasons for this

behavior are not well understood, it is possible that an increased amount of

ice ball melting in the brass barrels may be responsible. One mechanism that

could be assumed is that there is sufficient melting of the ice ball while

pressure is being adjusted and the aiming being checked, to result in a very

14
cymm

Mm- tm 03

fy&r//yr. fg %Msm CJ m

m

gUffur. tm

wm*. m

15

Figure

3.

Overall

view

of

launcher

system
Pressure

gage

OO TO

16

Figure

4.

Compressed

air

gun
Table 1 Interchangeable Barrel Materials and Dimensions

Ice Ball

Nominal

Internal

Actual

Internal

Difference Between

Actual Barrel Diameter and Ice Ball

Diameter Barrel Diameter Diameter Diameter

(in.) Material (in.) (in.) (in.)

3/4 3/4-in. PVC Pipe

Schedule 40

0.810 0.797 0.047

1 1-in. PVC Pipe

Schedule 40

1.033 1.047 0.047

1-1/4 1-1/4-in. PVC Pipe

Schedule 80

1.256 1.281 0.031

1-1/2 1-1/2-in. PVC Pipe

Schedule 40

1.592 1.594 0.094

1-3/4 2-in. PVC Pipe

Schedule 80

1.913 1.891 0.141

2 2-in. PVC Pipe

Schedule 40

2.049 2.070 0.070

2-1/4 2-1/2-in. PVC Pipe

Schedule 80

2.289 2.328 0.078

2-1/2 Cast Acrylic Tubing 2.625 2.58* 0.080

Duct tape (one layer) attached to internal surface to reduce diameter.

17
Figure 5. Interchangeable barrels for the compressed air gun

18
nil

19

Figure

6.

Installation

of

barrel

in

the

compressed

air
"loose" fit. Consequently, the released chamber pressure dissipates around the

ice ball rather than exerting a force on it. An alternate mechanism would

postulate less melting but enough to yield an ice ball that adopts the barrel's

cylindrical shape over part of its surface and, thus, does not roll freely as it

passes down the barrel. Both of these mechanisms may have some plausibility for

the situation where the barrel is horizontal.

4.3 TIMING RANGE

The timing range is the metal framework in the left foreground of figure 3.

Each end has an illuminator which projects a 5-in. -wide beam of light in a plane

perpendicular to the ice ball path. Each of these beams is focused on a photocell. The beams of light are spaced 2 ft apart and are positioned so that

the ice ball interrupts the beams sequentially. When the first beam is interrupted, the photocell output is changed abruptly, this change in output actuates

an electronic counter. When the second beam is interrupted, the counter is

stopped. The smallest count corresponds to 0.01 millisecond and it is estimated

that time measurements are accurate to better than 0.5 percent. This timing

range is very dependable and light sources and photocells are protected in

such a way that the system is very damage resistant. It was used previously

by Mathey [ 7

]

.

4.4 OPTICAL POINTER

An optical pointer was developed to allow rapid aiming of the system at a selected point on the specimen. Figure 7 shows the pointer mounted in the

launcher barrel. During testing the solar collectors and cover support frames,

containing the collector covers, were moved to bring different predetermined

impact points into alignment with the barrel of the compressed air gun. The

method of positioning is described in section 5.3. In operation, the pointer

focuses the image of the flashlight bulb filament on the collector cover surface. A sketch showing the various parts of the device is presented in

figure 8. During aiming, the unit is mounted in the muzzle end of the barrel.

Conventional flashlight batteries in a separate holder are used to illuminate

the bulb. As designed, a spot of light lying on the launcher axis was in

focus on a plane about 3 ft from the muzzle and facilitated accurate aiming.

20
Figure 7. Optical pointer for the hail gun

21
Figure 8. Schematic of optical pointer

22
5. MOUNTING OF COVER SPECIMENS

5.1 COVER SUPPORT FRAMES

There are two approaches to mounting solar collector cover materials for hail

resistance testing. One of these, the one followed here, is to mount the cover material in a support frame in a consistent manner for all materials of

that class. The other approach Is to test the cover material as mounted in an assembled collector. The first approach would tend to offer a more objective

comparison of cover materials since the support conditions would not vary

greatly from specimen to specimen. This approach is preferable even though it

would not take into account variations in impact resistance that might occur

as a result of collector size or cover mounting configuration. Since full

size collector covers are to be investigated, the cover support frames were

designed to accommodate 34 in. by 76 in. plates. This is a representative size

for flat plate collectors.

For this research, one type of cover support frame was designed to accomodate

rigid and semirigid materials while a slightly modified design was used for

flexible thin film materials. Basically the cover support frames consist of

two halves which are bolted together with the cover specimen between them. In

general, the frame simulated to some degree the support conditions found in an assembled solar collector. It is very likely that the resistance to edge

rotation is greater for the support frames used here than in many assembled

collectors

.

Figure 9 gives a simplified sketch of a support frame. The frames used here

were rectangular in cross section and were built from solid maple. The inner

surfaces of the frames contained strips of neoprene rubber to bear against the

edges of the cover specimens. Figure 10 shows typical support frame cross sections with the locations of the neoprene inserts. The neoprene strips,

3/8 in. by 3/4 in., were cemented into slots milled in the frame so that the

neoprene compressed about 20 percent when the assembly was complete. To

control the amount of compression when rigid or semirigid materials were

clamped, a spacer the same thickness as the cover specimen was inserted. The

neoprene used here had a Shore A Durometer hardness of 30 to 45. Steel straps

such as shown in figure 11, were attached to the exterior of each corner to

both stiffen and strengthen the corners of the support frames.

5.2 PLACING SPECIMENS IN THE FRAMES

For rigid and semirigid cover materials, the specimen dimensions were 34 in.

by 76 in. so that 1/2 in. of the cover was clamped between the neoprene strips.

For thin film covers, the sheet material extended through the support frame so that weights for stretching could be attached outside of the frame. No spacer

is used with the thin film cover materials.

In both cases, frames were fastened together with 3/8-in. diameter bolts at

locations indicated in figure 9. The bolts were 14 in. on centers and were

symmetrically located. Using bolts for fastening was found to be much more convenient than using C-clamps since the bolts interfere a minimum amount with

23
Figure 10. Typical support frame cross sections

24
the mounting of the assembly in the support frame positioner. Further, C-clamps

are very difficult to install in the case where thin film cover materials are used since during mounting the material extended outside the frame and had

weights attached to it.

It was found that semirigid materials, in particular fiber reinforced plastics,

required support during the assembly of the cover support frame. Since assembly

was done with the cover in a horizontal plane, the material would sag under its own weight if supported only along the edges. A block of the correct thickness

placed under the cover at its center tended to suppress this sagging. However,

semirigid materials were not pulled taut during assembly and some deviation

from flatness did occur when the cover support frame was raised to a vertical

position for testing.

As noted above, thin film cover materials, such as the poly(vinyl fluoride),

were tensioned before the halves of the support frame were bolted together.

An overall view of this tensioning operation is shown in figure 12. Figure 12,

also shows that the specimen was oversize, actually about 50 in. by 92 in., so that the material hung over the frame by about 6 in., on all sides. C-clamps

were used as weights for tensioning the flexible material as shown in both

figure 12 and in figure 13. In addition, figures 12 and 13 show bolts extending through the cover material. This was facilitated by punching slightly

oversize holes in the cover material before the bolts were inserted. This

detail is shown in figure 14. It was found that an edge loading of 5.2 Ibm/ft

to 5.5 lbm/ft used in this study was necessary to achieve moderate tautness

of the thin film. The question arises here as to how taut the material needs

to be to simulate the "heat stretching" frequently employed. Some film cover materials in an actual collector assembly would probably be "heat stretched"

over a frame and then mounted in the collector. A representative edge loading

needs to be known. The edge loading values of 5.2 lbm/ft and 5.5 lbm/ft used

here produced a taughtness in the cover material greater than has been used in some instances to simulate heat "stretched material." The consistency of

tensioning of the material by the edge loading was checked by a center deflection measuring technique. This technique was a very simple one in which the

horizontally tensioned film was loaded transversely by a 0.1 lbm weight at the

center of the specimen and the deflection measured. Fairly consistent results

from specimen to specimen were obtained. Figure 15 shows a tensioned film

cover loaded by a 0.1 lbm weight.

It should be noted that the thin film material did not always maintain its

tautness when the cover-support frame was in the vertical testing configuration.

5.3 SUPPORT FRAME POSITIONER

As noted earlier, the launcher assembly is fixed in position so that the flight

path of the ice ball is fixed. The cover frame assembly must be moved to align

the selected impact points on the cover specimen with the aimed trajectory of

the ice balls. During testing, the cover specimen in its support frame must

be held in a vertical plane by a relatively rigid supporting system and that

system must be capable of rapid horizontal and vertical positioning of the cover frame assembly.

25
26

Figure

12.

Tensioning

of

film

cover

material
RiM***,

27

Figure

13.

C-clamp

weights

attached

to

film

cover

material
Figure 14. Hole in film cover material to permit bolt passage

23
Figure 15. Transverse loading of film cover to check consistency

of tensioning

29
To do this, a battery-operated heavy-duty lift truck was used. Small steel

channels were bolted to the uprights of the lift platform and the cover frame

assembly was attached to the channels with C-clamps. This is shown in figure

16. The lift mechanism was employed for vertical positioning and horizontal

positioning was accomplished by moving the lift truck. Using this technique

in combination with the optical pointer, it was possible to reposition the

specimen from an impact point at the left end of the exposed area to one at

the right end of the exposed area in a few minutes. Clamping and unclamping

of the cover support assembly was not necessary as it would had been if a fixed supporting structure had been used.

30
Figure 16. Lift truck modified as a support frame positioner

31
6. ICE BALL VELOCITY AND IMPACT POINTS

6.1 ALTERNATIVE LABORATORY VELOCITIES

The maximum hailstone sizes which occur in nature have been determined using a statistical approach. The maximum sizes vary with the geographical area and recurrence interval selected. Simiu and Cattaneo [1] have estimated the maximum size hail to be expected under various conditions as already noted in

section 2.1. The selection of the velocities of hailstones or ice balls for

laboratory testing, if the tests are to simulate extreme but realistic field

conditions, is a question that has received considerable attention.

With regard to velocity of the impacting ice ball, there are several questions

to consider. First, should the magnitude of the velocity be equal to the

terminal velocity or to a resultant of the terminal velocity acting vertically

and a wind component acting horizontally? Second, if the laboratory test is

to require only normal impact, which seems reasonable to minimize testing

costs and testing time, what component of the resultant velocity should one assume acts normal to the laboratory specimen?

6.2 VALUES OF TERMINAL VELOCITY AND WIND VELOCITY

The normal velocity values (ice ball traveling in a direction perpendicular to

the plane of impact) used for various ice ball sizes are shown in table 2 along

with the corresponding values of kinetic energy and momentum. These data are essentially the same as those reported in 1970 by Mathey [7] and are based on a comprehensive analysis of the relevant technical literature.

As stated earlier in this report, the terminal velocity of a hailstone is the

velocity at which the hailstone falls when its weight is equal to the drag

force acting upon it. Setting weight equal to drag force in equation form

yields

P H *S* cD *my ( 6 )

where Pjj = density of hail stone

g = acceleration of gravity

D = hailstone diameter

PA = density of air at appropriate altitude and temperature

T = terminal velocity

Cq = coefficient of drag.

The coefficient of drag, Cq, depends on the Reynolds number in general, but for

the flow regime usually experienced by hailstones (i.e., for the range of sizes

given in table 2), Cp appears to be constant and to have a value of 0.5 or slightly less.

The data of table 2 were calculated from an equation like Eq . (6) proposed by

Bilhara and Relf [17]. Clearly there are a number of parameters in Eq. (6) for

32
Table 2. Ice Ball Parameters for Laboratory Testing

Diameter

(in.)

Mass

(lbm)

Velocity-*-/ Kinetic Energy^/ Momenturn Terminal

(ft/s)

Resultant^/

(ft/s)

Terminal

(ft-lbf)

Resultant

(ft-lbf)

Terminal

(lbf-s)

Resultant

(lbf-s)

3/4 0.0073 62 91 0.44 0.94 0.014 0.021

1 0.0173 73 98 1.43 2.58 0.039 0.053

1-1/4 0.0337 82 105 3.52 5.77 0.086 0.110

1-1/2 0.0584 90 112 7.35 11.38 0.163 0.203

1-3/4 0.0928 97 117 13.56 19.73 0.280 0.337

2 0.1384 105 124 23.69 33.04 0.451 0.533

2-1/4 0.1971 111 129 37.71 50.93 0.679 0.790

2-1/2 0.2705 117 134 57.50 75.42 0.983 1.126

For laboratory testing, ice balls were propelled in a direction perpendicular

to the plane of impact.

2/ Resultant velocity is the vector sum of the terminal velocity and a 66 ft/s

horizontal wind component.

3/ Corresponding to the terminal and resultant velocity.

33
which values must be selected and various investigators have proposed various

values. The value of Cp found by Bilham and Relf for ice spheres in their

experiments was about 0.5 and other investigators appear to agree with this if

the Reynolds number is in the range of 2 x 10^ to 2 x 10^. List [18] ran relatively low speed wind tunnel tests on ice spheres and found a Cp of about

0.45. He also points out that the terminal velocities of actual, non-spherical

,

hailstones will be less than that of a sphere of the same volume and density.

Macklin and Ludlam [19] measured Cp values for ice balls in a wind tunnel and

measured fall speeds of hailstones dropped from a meteorological balloon.

Radar was used for the latter fall speed measurements. Their Cp values ranged

from 0.41 to 0.51 at the higher Reynolds numbers. They further showed that

the fall speeds of the dropped hailstones could be correlated with the velocity

of an equivalent sphere of the same mass and specifc gravity of 0.9. Data for

the 2-in. diameter equivalent sphere are virtually the same as that in table 2

based on Bilham and Relf's work.

A wind component must be considered when establishing the normal velocity for

test purposes since thunderstorms which produce hail are generally accompanied

by strong winds. For example, in "Climates of the United States," Baldwin [20]

indicates that thunderstorms are occasionally accompanied by hail but most are accompanied by strong wind which can have gusts reaching speeds in excess of

65 mi/h. In connection with a 1961 study in South Africa [21], it was reported

that winds having velocities of 30 to 60 mi/h generally are associated with

hailstorms and that gust velocities are often 30 to 50 percent higher than the

average wind velocity. Finally, Schleusener [22] notes that strong winds aloft

have been found to be closely correlated with the occurrence of severe hail.

Based upon the evidence of wind accompanying hail, a 45 mi/h horizontal wind

component was used in calculating the resultant velocities in table 2. It is

noted that in earlier tests on the hail resistance of building materials and

products, such as those conducted by Greenfeld [6] and Laurie [23], ice balls

were propelled at their terminal velocity. Laurie [23] stated that hailstones

can fall at an angle of as much as 45 degrees to the vertical because of high

wind.

6.3 MAGNITUDE OF THE NORMAL VELOCITY

A brief discussion of the components of the hailstone velocity vector may

clarify the interaction. This discussion will deal with a two-dimensional

special case of the actual three-dimensional situation. Considering the latter,

for the collector cover shown in figure 17, the normal component N of the

resultant velocity, which would be the only component producing a normal impact,

makes an angle 9 with the vertical or z-axis. This follows since the collector

cover is inclined at an angle 0 to the x-y or horizontal plane. The resultant

velocity of a hailstone does not, in general, coincide with N and in fact does

not generally lie in the y-z plane as does N. The resultant vector shown in

figure 17 has components in both the x and y horizontal directions as well as the vertical z direction. However, for this orientation of the xyz axes, the

x component of the resultant has no effect on the magnitude of N so that the

two dimensional analysis of components in the y-z plane is sufficient.

34
T = Terminal velocity

W = Horizontal wind velocity

W x = W component in the x direction

W component in the

y direction R = Resultant of T and W

Ry Z = Component of R in the

yz plane N = Component of RyZ and Wy perpendicular

to collector

Plane of cover

material

Figure 17

.

Three-dimensional relationship of resultant velocity

and normal component of resultant velocity

w

T - Terminal velocity

W = Horizontal wind velocity

component that contributes

to normal impact of hailstone

R = Resultant of T and W

N = Component of R and W

perpendicular to collector

Figure 18. Two-dimensional relationship

T

of the hailstone velocity

components and the normal velocity component for wind

direction perpendicular to the horizontal edge of an inclined collector

35
Figure 18 shows a two-dimensional view of the velocity components which

contribute to the normal velocity component N. In this figure it is assumed

that the wind direction is perpendicular to the lower (horizontal) edge of the

inclined collector. This special case will result in a maximum value of N for

a given wind velocity. It is noted that the probability of the wind direction

and speed must be introduced into occurence criteria (section 2.1). T represents the terminal velocity of the hailstone, W is the horizontal wind component

contributing to normal impact, and R is the resultant of T and W, that is,

T

2 + W2 = R2

. (7)

If an angle a is used to designate the direction of the resultant velocity,

that is

,

tan a =

T

then, since N can also be considered as a component of R

N = R cos (0-a). (9)

It can also be shown that

N = T cos6 + W sine. (10)

Using Eq. (7) and Eq. (10), values of N and R were compared for various values

of 6 and T. The reason for this comparison was to determine if the normal

component, N, could be taken to be equal to the magnitude of R, for the purpose

of selecting ice ball velocities for testing of solar collector plates. Clearly,

the normal component is equal to the magnitude of R only when a = 9 , i.e.,

when the values of W and T have a particular relationship. In the calculations

for N and R, W was taken as equal to 45 mi/h or 66 ft/s, i.e., the full effect

of a horizontal wind blowing in a direction perpendicular to a horizontal edge

of an inclined collector.

The results of the analyses are presented in figure 19. The uppermost curve in

figure 19 is a plot of Eq. (7) or the magnitude of the resultant velocity as a function of terminal velocity, T, and a horizontal wind velocity of 66 ft/s.

The straight line plots lying immediately below the R versus T plot represent

N versus T for a horizontal wind velocity of 66 ft/s, or Eq. (10), for cover inclination angles, 9, of 30 deg, 40 deg, 50 deg and 60 deg. If attention is

confined to terminal velocities, T, ranging from 62 ft/s to 105 ft/s which

correspond to ice ball diameters of 3/4 in. to 2 in. respectively, these plots

show that the magnitude of R exceeds N at most by 15 ft/s or 13 percent at T = 105 ft/s. Further, this latter N situation occurs for 9 = 60 deg which is

greater than the angle of inclination for most solar heating systems in the

continental United States. Consequently, for testing purposes the magnitude of

the normal velocity was taken to be equal to the magnitude of the resultant

velocity.

To provide an estimate of the effect of neglecting horizontal wind velocity, W,

the dashed lines are presented in the lower part of figure 19. The uppermost

36
160

</> 140 1—

>-o

W=66FT/S (SOLID LINES)

>-

ho

GO

80

R = ^/T 2 +W 2

N = Tcos0 + Wsin0

V-

/

/ / _

/ ' /W=0

/ x ''(DASHED

„ / / LINES)

*' / s' s' - V /

yy 4 x ys

</ A>/

%/Si* X

/Vs'*'

'

20

0

ICE BALL DIAMETERS (in.)

% 1 1 Vi 2

20 40 60 80 100 120 140

TERMINAL VELOCITY, T, (FT/S)

Figure 19. Assumed and actual normal velocity values for various

terminal velocities

37
dashed line represents the resultant velocity R which equals T when W = 0. The

remaining dashed lines represent the values of N as a function of T, from

Eq. (10) with W = 0, for various values of 0. It is noted that, for a given

value of 0, the solid straight line and the dashed straight line are parallel

but that the solid straight line is offset vertically by a distance corresponding to W sin 0. There is a distinct difference in resultant velocity magnitudes

when a wind component is or is not considered. Again looking only at the terminal velocity range from 62 ft/s to 105 ft/s, the greatest difference between

R for W = 66 ft/s and R for W = 0 occurs at 62 ft/s or at the small ice ball

diameter end of the range. Here a difference of 29 ft/s or 32 percent occurs. Even the smallest difference between values of R over this terminal velocity

range for W = 66 ft/s and W = 0 which occurs at 105 ft/s or for the 2-in. diameter ice ball is 19 ft/s or 15 percent. These results were taken, in part,

to be a justification, for including a wind component in selecting the normal

velocity for laboratory testing.

6.4 IMPACT LOCATIONS

A layout of impact points on the mounted collector cover is shown in figure 20.

These points were chosen at the outset of the experimental program to determine,

in general, the areas of the cover which are more or less susceptible to failure

by impact. Locations 6, 7, 8, and 9 which are positioned 6 in. away from the

neighboring sides of the support frame have been proposed as preferred test

locations in some test methods [2].

It was expected that the "criticality", of an impact location, i.e., the

tendency for failure with a given ice ball diameter and velocity, would increase

roughly with the location number. Tendency for failure is equated to the strain

level at the plate surface on the reverse side at the impact point. Specifically, ascending order of "criticality" were selected as follows:

No. 1 Least Critical

No. 2,3

No. 4,5

No. 6, 7, 8,

9

No. 10,11

No. 12,13

No. 14,15,16,17 Most Critical.

This ordering of the criticality of the points was supported in general by the

test results. In addition, calculations of the maximum tensile strain in the

plane of a simply support plate at the impact location tended to demonstrate

that the strain increases in accordance with the order of "criticality"

described above. An important parameter is the maximum tensile strain in the

plate which from calculation occurs on the back surface of the cover plate at

the impact point. The calculation assumed that the nature of the impulse is

the same at all locations. However, this may not be entirely correct since

the duration of the impulse or the variation of force with time may be different at various locations, due to differences in the effective "stiffness" of

the cover at the locations where it is impacted.

38
+ CO

CO

39

Figure

20.

Layout

of

impact

points
7. FAILURE OR DAMAGE ASSESSMENT

Since there are three distinct classes of materials, i.e., rigid, semirigid,

and flexible film, used for collector covers, it might be expected that three

distinct types of failure behavior may be observed. This will be discussed in

terms of the specific cover materials tested in this research program.

Two rigid cover materials were included, namely tempered glass with both

surfaces smooth and tempered glass with the exposed surface textured and the

other smooth. Failure and/or damage by impact of these materials is quite

straightforward to describe because they shatter into small pieces. Tempered

glass behaves elastically until the tensile fracture stress is reached. At

this point, the tempered glass cover specimen develops a complex crack pattern

which, because of the residual stresses due to the tempering, progresses

throughout the material in a few minutes. An overall view of a textured surface tempered glass cover specimen after fracture is shown in figure 21. A

hole in the cover resulting from the impacting ice ball is shown in figure 22.

For impacts which do not produce fracture, there may be some microscopic surface damage but this was not apparent from visual inspection.

One semirigid cover material was tested and this was a polyester resin sheet

reinforced with chopped strand glass fiber mat. Sheet thickness was 0.040 in.

This class of cover materials is much less brittle than the glasses discussed

above. Even at the high strain rates associated with ice ball impact, these

reinforced plastic materials exhibited some inelastic deformation before fracture. However when fracture occurred, the crack or cracks were in general

confined to the immediate neighborhood of the impact point. This is referred

to by some investigators as "starring". A failure of this type is shown in

the center of figure 23. Localized failures of this type may or may not be

detrimental to solar collector operations. While a relatively small portion of

the total surface area is affected and, thus, transmission may not be reduced to

any great extent, it is possible that cracking may extend through the specimen

thickness. This would allow water penetration which could eventually deteriorate solar collector components.

To check the possibility of water penetration through the area damaged by

impact of an ice ball, a leak test procedure was developed. After impact, the

reinforced plastic specimen was removed from the support frame and was cut into

smaller pieces which contained impacted areas to accomplish the leak testing.

The next step was to attach a clear plastic cylinder to the reinforced plastic's

surface so that the cracked region was inside the boundary of the cylinder.

Melted wax was used to seal the cylinder to the cover surface. Figure 24 shows

a cylinder attached in this way. Water was poured into the cylinder to a depth of 1-1/2 in. to give an identical static head for each test. After an arbitrarily selected time period, overnight in our case, the depth of water

was recorded and used to indicate water penetration into the specimen at each

impact location.

When the reinforced plastic cover material is impacted by large ice balls

traveling at relatively high velocities, damage such as tearing and breaks

through the cover specimen may occur. Figure 25 shows a failure of this type.

40
Figure

21.

Overall

fiew

of

a

textured

surface

tempered

glass

cover

specimen

after

fracture
Figure 22. Hole in tempered glass cover specimen caused by

impact of ice ball

42
Figure 23. Localized cracking failure in reinforced plastic cover caused by impacting ice ball

43
Figure 24. Cylinder sealed to reinforced plastic material for

leak testing

44
Figure 25. Tearing and penetration through reinforced plastic

cover material caused by impacting ice ball

45
The material is not only cracked but exhibits gross tears through which water

or air could flow easily. The area covered by these failures and the ease of

water penetration through the damaged area are both much greater than for

the localized "starring" type of failure described above. When a penetration

or break in the cover material occurred, testing was stopped.

Because localized failure was observed even with relatively small ice balls,

points on a given specimen were impacted only once.

The one film cover material tested was 0.004 in. thick poly(vinyl fluoride).

This class of cover materials undergoes large deformations before breaking or tearing occurs. It could be characterized as the most ductile of the three

classes, although some of this ductility is used up by stretching the film

when it is installed in the collector or in the specimen support frame. Local

failure in poly(vinyl fluoride) film takes the form of small to large indentations (figure 26) having a depth as much as 70 percent of the impacting ice

ball radius. Once the indentations are formed, they persist for long periods

of time. Total penetration of the cover material or puncture failure may

occur for the larger ice ball sizes, as shown in figure 27.

Punctured film covers would allow much higher convection losses in the space

above the absorber than would be the case for undamaged film. Water could

also enter the collector through the punctured film. While the large indentations in the cover do not permit water access into the collector interior at the

outset, they do contain highly deformed material which may degrade rapidly

under typical environmental exposure and thus permit leakage. Another point

to consider is that it may be possible where large or deep deformations occur

in the film due to impact, that the film may come in contact with the absorber.

If this is the case, melting of the film could occur at these locations and

also part of the force at impact may be transferred to the absorber.

46
Figure 26. Indentations in plastic film cover

Figure 27 . Puncture failures of plastic film cover

47
8 . TEST PROCEDURE

To clarify the use of the various parts of the launcher and to demonstrate

how ice ball melting is minimized, the sequence of operation followed during

a test run is described.

First, the collector cover material was mounted in the cover support frame

following a procedure appropriate to the type of material being tested. This

is described in sections 5.1 and 5.2. After mounting, impact points were marked on the specimen surface with a black, felt-tipped pen (see figure 20).

The cover support frame assembly was then mounted in the support frame positioner as described in section 5.3. Then the modified lift truck or support

frame positioner was moved into position at the end of the launcher system.

This location was at the left of the timing range shown in figure 3. Figure 21

shows a mounted specimen in this testing position.

By moving the lift truck laterally and using the forks of the lift truck to

raise and lower the cover support frame assembly, a selected impact point was aligned with the compressed air gun with the aid of the optical pointer.

An ice ball size (diameter) and corresponding velocity were selected for the

particular test series. The correct barrel was installed on the compressed

air gun to match the ice ball diameter. See table 1. In addition, the

chamber pressure which would produce the desired velocity was determined from

the calibration data for the compressed air gun. The velocity of the ice ball

as measured by the timing range was in general not considered valid if it

varied more than + 10 percent from the desired or selected velocity. With

regard to ice ball size, the usual approach was to start with a small diameter

and increase the selected diameter sequentially until failure occurred.

When the launcher and specimen were ready for firing, an ice ball of the

selected size was removed from the freezer. The ice ball was immediately

weighed and muzzle-loaded into the barrel of the compressed air gun. Then

the timing raiige was actuated, pressure was introduced into the chamber and

the quick-release valve opened to allow the pressure to act on the ice ball.

The latter was, in effect, the firing sequence. The ice ball then impacted

the cover material at the desired impact point. The velocity of the ice

ball was measured using the timing range (section 4.3). Less than one minute

elapsed between the time that the ice ball was removed from the freezer and

its impact on the specimen.

After firing, the damage caused was evaluated visually and the specimen was indexed to the next impact location. Whenever failure occurred, the testing

sequence for that specimen was terminated. Failure or damage assessment is

discussed in section 7.

48
9 . TEST RESULTS

9 . 1 GENERAL

The test results presented here are intended to give an idea of the range of

ice ball sizes and corresponding velocities which result in failure for the

three class of cover materials. New or unexposed materials were used in the

tests and the effect of aging on the impact resistance of the cover materials

was not determined. Materials may degrade as a result of environmental

exposure and have considerably less impact resistance as compared to new materials. The results are not intended to be a definitive data set which can be

used to define the capabilities of a given cover material. Along with this,

the data demonstrate the types of failure or damage which are encountered and,

consequently, must be evaluated. This has already been discussed to some extent in section 7. Finally, some notion of the degree of variability of the

results can be found to the extent this is possible with a limited number of

tests. For the tempered glass covers, the effect of damage from previous

impacts that did not cause fracture cannot be estimated.

A summary of results from tests on four solar collector cover materials (two

types of tempered glass, reinforced plastic, and plastic film) is presented

in table 3. In all of these tests, cover materials were mounted in specimen

support frames and were impacted by ice balls at either resultant or terminal

velocity. Table 3 also contains data from an earlier series of tests, not

previously reported, which were conducted on tempered glass (both sides smooth)

mounted in solar collector frames. Table 3 reports the diameter, velocity,

and kinetic energy of the ice ball causing failure of the cover material;

the location of impact and type of failure; prior impact tests on the cover

specimen; and the edge loading of plastic film during mounting in the cover

support frame.

The information presented in table 3 represents only the final run of impact

tests which resulted in fracture or in damage. Prior impact tests which did

not result in fracture or other type of failure are listed for the tempered

glass cover materials. Consider, for example, the 1/8 in. thick tempered

glass with textured side exposed. Specimen No. 1 of that group had been previously subjected to impacts by 1-in. diameter ice balls at resultant velocity

and 1-1/2-in. ice balls at resultant velocity. Fracture occurred when a 1-3/4-

in. ice ball at resultant velocity impacted the specimen at Point No. 1 or the

center of the specimen as indicated in figure 20. For the above specimen, 1 R

denotes a 1-in. ice ball fired at resultant velocity and 1-1/2 R denotes a 1-1/2-in. ice ball at resultant velocity. The letter T after the diameter

denotes a run at terminal velocity.

Unless noted otherwise, a test run included impacts by ice balls of a given size

at either resultant or terminal velocity at all 17 points indicated in figure

20. Fracture in any run precluded further testing.

Data for all impacts during each test run on each specimen are found in

appendix A. For example, table A-l presents the data for tempered glass,

textured side exposed. Specimen No. 1 impacted by 1-in. diameter ice balls at

49
Table 3. Summary of Ice Ball Impact Test Results

Ice Ball Causing Failure Kinetic Impact Location^/ Prior Edge

Specimen Appendix Diameter Velocity^-' Energy and Type of Impact Loading^/ Material Number Table (in.) (ft/s) (ft-lbf

)

Failure Tests (lbm/ft)

Tempered glass. #1 A3

1-3/4 119 R 20 0 1 Fracture 1 R, 1-1/2 R

_ Exposed side 02 A5

1-3/4 115 R 19 02 Fracture 1-3/4 T textured, other 03 A6

1-3/4 122 R 20 02 Fracture None - side smooth. #4 A7

1-3/4 101 R 14 02 Fracture None - (1/8 in. thick) #5 A8

1-1/2 95 T 7.5 02 Fracture None -

#6 A- 11 1-3/4 98 T 13 02 Fracture 1-1/2 T, 1-1/2 R

Tempered glass. #1 A- 13 2 129 R 36 014 Fracture 2 T

- Both sides smooth. 02 A-17 2-1/4 131 R 49 05 Fracture 2 R, 2-1/4 T

- (1/8 in. thick) 03 A-18 2-1/4 127 R 47 #3 Fracture None -

#4 A-20 2-1/4 114 T 38 09 Fracture 2 T

- #5 A-21 2 130 R 34 04 Fracture None -

#6 A-22 2 128 R 33 01 Fracture None

Tempered glass. A A-38 2-1/2 135 R 78 01 Fracture 1-3/4 R, 2 R - Both sides smooth. F A- 39 1-3/4 119 R 20 01 Fracture None

(1/8 in. thick) B A-40 2-1/2 136 R 80 010 Fracture 2 R Mounted in Solar E A-41 2-1/2 136 R 79 010 Fracture 2 R - Collector C A-42 2-1/2 137 R 80 #14 Fracture 1-3/4 R, 2 R

-

D A-43 1-3/4 115 R 19 014 Fracture None “

Reinforced plastic.. #1 A-23 1-1/2 90 T 5. 2-9.

2

#1 to 011, Leak None - Polyester resin //I A-23 1-1/4 82 T 3. 1-5.

3

012 to 017, Leak None - with glass fiber #2 A-24 3/4 62 T 0.38-0.53 01 to 017, Cracks None

mat. (0.040 in. 03 A-25 1-1/4 82 T 3. 0-3.

9

#1, 03 to 6, 08 None

thick) to 11, 013 to 16, Leak

it 4 A-26 1-1/4 105 R 4. 4-5.

9

#2 to 012, 014, 015, None - Leak, 016, 017, Break

05 A-27 1-1/2 98 R 8.3-11.7 #1 to 017, Break None

#6 A-28 1-1/4 82 T 2. 9-3.

7

03 to 09, Leak None -

#7 A-29 1-3/4 97 T 12-18 01 to 09, Break None “

Poly (vinyl #1 A- 30 3/4 62 T 0.36-0.49 01 to 017, Slight None 3.3 side fluoride) film. damage, small 4.2 end (0.004 in. indentations

thick) #1 A-31 3/4 91 R 0.76-1.01 01 to 017, Slight 3/4 T damage, small indentations

#2 A-32 1 73 T 1.1-1.

6

01 to 017, Slight None 3.6 side

damage, indentations

5.3 end

#2 A-33 1 98 R 2. 2-2.

7

01 to 017, Slight 1 T damage, indentatations

#3 A- 34 1-1/4 82 T 2. 9-4.0 #1 to 04, #7 to 09, None 5.2 side Indentations 5.5 end

1-1/4 105 R 5. 0-5.

7

05, 06, 010 to 012,

#16, 017, Indentations

03 A- 34 1-1/2 90 T 5. 8-6.

6

013, 014, Indenta- None

tions

1-1/2 112 R 9.3 015 Indentation

#4 A-35 2 105 T 19-24 02, 06 to 09, None 5.2 side Break 5.5 end 18-20 01, 03 to 05, Indentations

#5 A-36 1-3/4 117 R 16 01, #9, Indentations None 5.2 side

17-18 #2, 03, 06 to 08, 5.5 end Break

06 A-37 1-3/4 97 T 11-15 #1 to 09, 011 to 013, . 5.2 side Indentations 5.5 end

12-15 #10, 014 to 017, None Break

R " resultant velocity, T « terminal velocity. Locations are denoted by a number as shown in figure 20.

3/ Edge loading of plastic film during mounting in the cover support frame.

50
resultant velocity. Some variation In velocity from ice ball to ice ball is

apparent in table A-l and in other tables in appendix A. As indicated in

table 2, the nominal resultant velocity for a 1-in. ice ball is 98 ft/s while

actual velocities varied from 91 ft/s to 101 ft/s (table A-l). Table A-2

also gives data for Specimen No. 1, but in this case 1-1/2-in. diameter ice

balls at resultant velocity were used. The data for the final run on Specimen

No. 1 are given in table A-3 which shows that fracture occurred when a 1-3/4-

in. ice ball impacted Point No. 1 at resultant velocity.

9.2 TEMPERED GLASS, TEXTURED SIDE EXPOSED

For the two tempered glass cover materials tested, failure implies fracture

which eventually results in total destruction of the specimen. As discussed in

section 7, the cracking associated with the ice ball impact extends throughout

the specimen because of the residual stresses due to tempering. Although the

nominal thickness of the textured glass was 1/8 in., areas of this material

were thinner due to the texturing. This may account for its impact resistance

being lower than the other tempered glass having both sides smooth.

Relatively little specimen-to-specimen variability was displayed by the results

for tempered glass with exposed side textured. It is noted, however, that a larger number of tests, which would permit a good statistical treatment of

the data, would be preferable even though variability appears to be small in

this data set. Some glass cover plate manufacturers often recommend at least

30 individual specimens be tested.

As shown in table 3, all fractures were initiated either at Point No. 1, the

center of the specimen, or at Point No. 2, 6 in. from the edge at the center of

the long side. Figure 20 gives impact point positions on the specimen. As

pointed out in section 6.4, Points 6, 7, 8, and 9 have been recommended as impact points in some test methods [2]. Since Points 6, 7, 8, and 9 have a higher degree of criticality (as described in section 6.4) than Points 1 and 2 it would be expected that smaller ice balls would cause failure at Points 6,

7, 8, and 9 than at Points 1 and 2.

Only one fracture was initiated by a 1-1/2-in. ice ball at terminal velocity.

All other fractures required a 1-3/4-in. ice ball at either terminal or resultant velocity. Values of the kinetic energy causing failure ranged from 7.5

ft-lbf to 20 ft-lbf. This is a larger difference in kinetic energy than the

limited range of ice ball sizes causing failure that might lead one to expect.

It is noted that kinetic energy level is considered by some investigators to

be a measure of damage potential.

Damage from prior impact tests did not appear to affect the fracture behavior

of Specimen No. 1 and 2 since these specimens and Specimen No. 3, which had no previous impacts, were all fractured by a 1-3/4-in. ice ball at nearly the same (resultant) velocity. Specimen No. 6 which had previous impacts did not

perform quite as well as Specimen No. 3, but it did perform better than Specimen No. 5 and about the same as Specimen No. 4. Specimen Nos. 4 and 5 had no previous impacts. Thus, there is no clear evidence of an effect of possible

damage from previous impacts.

51
The data from individual impact tests conducted on tempered glass having one

side textured are presented in tables A-l to A-ll in appendix A.

9.3 TEMPERED GLASS, BOTH SIDES SMOOTH

The results for tempered glass with both sides smooth mounted in a cover support

frame showed somewhat greater variability than did the results for tempered

glass with exposed side textured. The variety of impact points at which fracture initiated was evidence of this. There was, in fact, no consistent effect

of impact location.

All impacts that initiated fracture were caused by at least a 2-in. ice ball

at resultant velocity. It should be noted that the reported velocities for

the 2-in. ice balls for Specimens Nos. 1, 5, and 6 are somewhat higher than the

nominal resultant velocity of 124 ft/s for this size. This accounts for the

fact that a 2-1/4-in. ice ball at nominal terminal velocity, (Specimen No. 4)

had only slightly higher kinetic energy than the 2-in. ice balls at nominal

resultant velocity.

For 2-in. ice balls at resultant velocity, locations of fracture initiation

varied from Point No. 1 at the specimen center, to Point No. 4 which is 6 in.

from the edge at the center of the short side, to Point No. 14 in the corner

3 in. from each of the adjacent sides. The same situation holds for 2-1/4-in.

ice balls. When the 2-1/4-in. ice balls were fired at resultant velocity,

fracture was initiated in one case at Point No. 3 which is 6 in. from the edge at

the center of the long side or at Point No. 5 which is positioned similarly to

Point No. 4. Fracture with a 2-l/4-in. ice ball at terminal velocity occurred at

Point No. 9 in a corner at a distance of 6 in. from each of the adjacent

sides as might be expected. Here a more critical point should require a lesser

velocity for a given size ice ball.

The kinetic energies of ice balls at fracture ranged from 33 ft-lbf to 49 ft-lbf.

There seems to be little or no effect of damage from previous impacts for this

material since Specimen No. 2 had the highest number of runs prior to fracture

but exhibited fracture under the most severe conditions. Similarly, Specimen

No. 1 with one previous run performed as well as or better than other specimens

impacted by 2-in. ice balls.

Generally, this material displayed better performance than did tempered glass

with the exposed side textured. Better performance here implies that impacts

by larger sizes of ice balls could be sustained without fracture.

Tables A-12 through A-22 contain the data from the individual runs for this

material

.

9.4 SEMIRIGID FIBER REINFORCED PLASTIC SHEET

The semirigid material investigated was a polyester resin reinforced with glass

fiber mat and was 0.040 in. thick. Covers of this material are much thinner

52
than tempered glass and the effective Young’s Modulus is less than that of tempered glass. Both of these factors would lead to a notably lower plate stiffness in this case. Differences in type and extent of failure have already been

discussed in section 7.

Because of possible cumulative damage, unlike glass, multiple runs were not

carried out on this material since impacts with small ice balls at terminal

velocity produced small regions of localized cracking. Because of this, each

specimen was impacted by ice balls of only one size at one velocity.

The regions of localized cracking were larger for the larger ice balls and the

cracks in most cases extended through the specimen thickness. A leak test

procedure to determine whether cracks extended through the specimen is described

in section 7. The onset of this degree of damage occurred with a 1-1/4-in. ice

ball impacting at terminal velocity. Both Specimen No. 3 and Specimen No. 6

were subjected to these conditions and, as can be seen from table 3, leakage

occurred at 13 of 17 impact points in Specimen No. 3 and at 7 of 9 impact

points in Specimen No. 6.

Using a 1-1/4-in. ice ball but increasing the velocity to resultant level as in Specimen No. 4 maintained roughly the portion of impact points that showed

a leak or break, i.e., 15 of 17. Going a step further and impacting with a 1-1/2-in. ice ball at terminal velocity at every point of impact up to Point

No. 11 produced leaking cracks at all of them as shown in the results for

Specimen No. 1 in table A-23. Points 12-17 were then impacted with 1-1/4-in.

ice balls at terminal velocity and leakage occurred at all of these points.

The 1-1/2-in. ice balls at resultant velocity were used on Specimen No. 5 and

more severe damage began to appear. For this specimen leaks occurred at all

impact points and a large number of breaks beginning with Point No. 1 were

seen. These breaks are preceptible tears in the cover material through which

air or water could easily penetrate.

The progression of increasing impact severity and increasing damage described

above suggests relatively consistent behavior by this material. It should be

noted that Specimens No. 1 and 2 were "old" material, that is, material that

had been on hand for possibly two years while the remainder of the specimens

were material which had been purchased recently.

The kinetic energy range of 2.9 ft-lbf to 9.2 ft-lbf was associated with local

cracking and leakage. At the upper end of this range more severe damage began

to be evident. Large scale tearing or penetration occurred in the kinetic

energy range of 13.3 ft-lbf to 14.0 ft-lbf.

Data from individual reinforced plastic specimen runs are presented in tables

A-23 to A-29 of appendix A.

9.5 FLEXIBLE THIN PLASTIC FILM

The poly(vinyl fluoride) film cover material which was investigated was only

0.004 in. thick and behaved as a membrane rather than a plate in bending.

53
When new, the material is very ductile so that impacting ice balls with progressively larger diameters locally deform the film with indentations of

increasing size until breaks or penetration through the material occurred.

All poly(vinyl fluoride) specimens were tensioned when mounted in the specimen

supported frames and, since the tensioning load was not the same for all specimens, the values used are indicated in table 3. The tensioning procedure is

described in section 5.2.

For both Specimen No. 1 and No. 2, two test runs were made with a given ice

ball size. In Specimen No. 1, all impact points were subjected to 3/4-in. ice

balls at terminal velocity and then the procedure was repeated for points about

1 in. away from the originals with 3/4-in. ice balls at resultant velocity.

Specimen No. 2 was tested the same way but with 1-in. ice balls. In both cases,

small indentations which only slightly damaged the cover sheet were produced.

In Specimen No. 3, Point Nos. 1-4 and 7-9 were impacted with 1-1/4-in. ice

balls at terminal velocity while Point Nos. 5, 6, 10, 11, and 12 were impacted

at resultant velocity. See table A-34. A maximum indentation depth of 0.22

in. occurred at Point No. 10. Points 13 and 14 were then impacted with 1-1/2-

in. ice balls traveling at terminal velocity. A maximum indentation depth of

0.34 in. was found at Point No. 13. The indentation depth was 0.19 in. at

Point No. 12, a point of identical criticality. Thus, a step change in material

response occurs between 1-1/4 in. ice balls at resultant velocity and 1-1/2-in.

ice balls at terminal velocity.

When larger size ice balls were used, breaks or penetration through the material

occurred. This began with 1-3/4-in. ice balls at terminal velocity. Specimen

No. 6, in which breaks appeared at 5 of 17 points impacted. When Specimen No. 5, was impacted with 1-3/4-in. ice balls at resultant velocity, 5 of 7 impacted

points displayed breaks and the other two, large indentations. Increasing the

ice ball size to 2 in. and firing at terminal velocity did not greatly increase

the proportion of breaks at impact points. However, the depth of indentation

at points that did not break increased as shown by Specimen 4 (table A-35).

Again, the increasing damage associated with increasing impact severity tended

to indicate consistent specimen-to-specimen behavior of this material.

For this thin film material, only slight damage occurred for kinetic energy

levels up to 2.7 ft-lbf. When the kinetic energy was in the range 5.0 to 6.6

ft-lbf, rather large, permanent, indentations occurred which may be detrimental

to collector performance depending on the number of indentations and other

environmental factors. However, for kinetic energy levels greater than about

12 ft-lbf, ice balls may penetrate through the cover material. The latter

phenomenon occurred for kinetic energies ranging from 12 to 24 ft-lbf, while in some cases the material did not break at an energy level of as much as

20 ft-lbf.

9.6 TEMPERED GLASS MOUNTED IN SOLAR COLLECTORS

These data were obtained by Mathey in 1976 and have not been reported previously

in the open literature. Testing conditions were very similar to those which

54
have been described in this report and previously by Mathey [7]

.

The tempered

glass cover material, both sides smooth, was also very similar to the material

described in section 9.3.

In table 3, impact locations for this data set are reported in terms of the

points shown in figure 20. Data from the individual runs for the six specimens

are presented in tables A-38 to A-43 in appendix A.

The major difference between these tests and the ones reported in section 9.3

lies in the edge conditions or support conditions. A detail of the solar collector construction as it relates to support of the cover plate is shown in

figure 28.

When fracture results are compared for specimens mounted in the specimen support

frames and specimens in solar collectors, very little difference is seen. Two

of the six specimens in the solar collectors were fractured by 1-3/4-in. ice

balls at the resultant velocity (Point Nos. 1 and 14, specimens F and D,

respectively). The other four specimens were fractured by 2-1/2-in ice balls

at the resultant velocity.

Specimens mounted in the support frames were fractured by a 2-in. ice ball at

resultant velocity. For impacts in the corner. Points No. 14-17, the same situation applied. The cover material mounted in the solar collector appeared

to have greater resistance to ice ball impacts but the variability was greater

than was found for cover specimens mounted in the support frames.

This suggests that the specimen support frames described in section 5.1 provide

adequate simulation of the edge conditions in solar collectors. Conservative

test results appear to be produced.

55
Two

layers

of

aluminum

foil

with

bituminous

material

56

Figure

28.

Detail

of

solar

collector

edge

support
10. CONCLUSIONS

This report has presented a discussion of important parameters and testing

considerations which relate to laboratory testing of solar collector covers for

determining their resistance to hail impact. In addition to this, a fully

detailed experimental procedure for performing such testing is described. This

includes the preparation of ice balls, the launching or propulsion of ice balls,

a method for appropriately mounting cover specimens, the selection of ice ball

velocity and impact location, and various aspects of failure or damage assessment. Finally, results of simulated hail impact tests on four typical solar

collector cover materials are presented and evaluated.

It was intended that the results of this research would contribute to the

development of a general test method for simulating hail impact on solar

collector covers.

The data and analysis presented in this report appear to support the following

conclusions

:

1. Ice spheres or ice balls provide good simulation of hailstones. Clear ice

balls, without cracks and with consistent diameter and mass, can be readily

prepared in laboratory. Modifying the clear ice density is judged to be

unnecessary.

2. Ice balls should impact a specimen along a path perpendicular to the

specimen surface at a resultant velocity which is the vector sum of the

hailstone terminal velocity and a horizontal wind velocity component

corresponding to 45 mi/h or 66 ft/s.

3. A compressed air launcher with interchangeable plastic barrels of different

diameters for propelling the ice balls offers a safe and readily controllable propulsion system. This is based on the authors’ experience and

that of other investigators.

4. A system for accurate velocity measurement (within about 2 percent) of ice

balls in flight is essential. The velocity data were in general not

considered valid if they varied more than + 10 percent from the desired

or selected values.

5. A support frame for collector cover materials was found to offer good

simulation of support conditions as compared to actual collectors.

6. The preference for various impact locations and their influence on test

results is difficult to define. Arbitrarily selected points near the cover

plate boundaries appear to be a satisfactory compromise. Plate bending

response is not the only structural behavior mode present, particularly

near the corners.

7. Several kinds of failure must be considered when evaluating test results,

one of which is fracture or rupture of the cover plate. Others include

localized cracking which may permit water penetration and localized regions

of severe deformation which may alter optical properties.

57
11 REFERENCES

1. Cattaneo, L. E., Harris, J. R. , Reinhold, T. A., Simiu, E., and Yancey,

C . W . C . , Wind, Earthquake, Snow, and Hail Loads on Solar Collectors , NBSIR 81-2199, National Bureau of Standards, Washington, DC, January

1981. (See, in particular, Part IV).

2. Waksman, D., Streed, E., R., Reichard, T. W., and Cattaneo, L. E.,

Provisional Flat Plate Solar Collector Testing Procedures: First Revision , NBSIR 78-1305 A, National Bureau of Standards, Washington, DC,

June 1978.

3. Intermediate Minimum Property Standards Supplement, Solar Heating and

Domestic Hot Water Systems, 1977 Edition, HUD 4930.2, prepared for HUD

by NBS.

4. Interim Performance Criteria for Solar Heating and Cooling Systems in

Commercial Buildings, NSIR 76-1187, National Bureau of Standards,

Washington, DC, November 1976.

5. Interim Performance Criteria for Solar Heating and Combined Heating/

Cooling Systems and Dwellings, January 1975, prepared for HUD by NBS.

6. Greenfeld, S. N., Hail Resistance of Roofing Products , Building Science

Series 23, National Bureau of Standards, Washington, DC, August 1969.

7. Mathey R. G., Hail Resistance Tests of Aluminum Skin Honeycomb Panels

for the Relocatable Lewis Building, Phase II , NBS Report 10193, National

Bureau of Standards, Washington, DC, April 1970.

8. List, R., Cantin, J. G., and Ferland, M. G., "Structural Properties of

Two Hailstone Samples," Journal of the Atmospheric Sciences , Vol. 27,

pp. 1080-1090, October 1970.

9. Macklin, W. C., Strauch, E., and Ludlam, F. N., "The Density of

Hailstones Collected from a Summer Storm", Nubila, Vol. 3, pp. 12-17,

1960.

10. Clough, R. W. , and Penzien, J., Dynamics of Structures, McGraw-Hill,

1975.

11. Meirovich, L., Analytical Methods in Vibrations , Macmillan, 1967.

12. Hertz, H. (See Timoshenko, S. and Goodier, J. N., Theory of Elasticity , McGraw-Hill, 1951, Sec 125)

13. Smith, M. L. , Hail Testing of Solar Reflector Panels , Preprint: Solar

Reflector Materials Technology Workshop, Texas Tech University, Lubbock,

TX, 1978.

58
14. Moore, D., Wilson, A., and Ross, R., Simulated Hail Impact Testing of

Photovoltaic Solar Panels", Proceedings - Institute of Environmental

Sciences , pp. 419-430, 1978.

15. Moore, D., and Wilson, A., Photovoltaic Solar Panel Resistance to

Simulated Hail , D0E/JPL-1012-78/6 (Low-Cost Solar Array Project 5101-62),

Jet Propulsion Laboratory, Pasadena, CA, October 15, 1978.

16. Rupp, R. , "Structure and Mode of Operation of a Tester for Investigation

of the Resistance of Roofing Materials Against Simulated Hail-Impact",

Material and Technik , No. 4, pp. 198-201, 1976 (In German).

17. Bilham, E. G., and Relf, E. F. , "The Dynamics of Large Hailstones"

Quarterly Journal of the Royal Meterological Society, Vol. 63, pp. 149-162,

1937 .

18. List, R., "The Aerodynamics of Hailstones" Zeitschrift fiir angewandte

Mathematik and Physik , Vol. 10, No. 2, pp. 143-159, 1959, Research Translation T-G-244, Air Force Cambridge Research Laboratories , Bedford, MA,

September 1965.

19. Macklin, W. C. and Ludlam, F. N., "The Fallspeeds of Hailstones," Quarterly

Journal of the Royal Meteorological Society , Vol. 87, No. 371, 1961.

20. Baldwin, J. L., Climates of the United States , National Oceanic and

Atmospheric Administration, Environmental Data Service, Washington, DC,

1973.

21. Technical Support Package for Tech. Brief 68-10552, Simulated Hailstone

Fabrication, PB 183 582, Office of Technology Utilization, National

Aeronautics and Space Administration, Washington, DC.

22. Schleusener, R. A., "Hailstorm Characterization and the Crystal Structure

of Hail," Meterological Monographs , Vol. 5, No. 27, pp. 173-176,

September 1963.

23. Laurie, J. A. P., "Hail and Its Effects on Buildings," Bulletin 21,

National Building Research Institute, Pretoria, South Africa, August 1960.

59

APPENDIX A. IMPACT TEST DATA

The tables (A-l to A-43) presented in this appendix contain data from each

ice ball impact on each specimen of the collector cover material. In a given,

table, data are in general presented for one ice ball size corresponding to

either terminal or resultant velocity. Exceptions to this are tables A-23,

A- 34, A-38, A-40, A-41, and A-42 where more than one ice ball size and

velocity were tried on a given specimen. Measured ice ball mass, velocities,

and observed damage states are listed. The kinetic energy calculated for

each ice ball is also listed.

The tables are arranged with data from tempered glass, exposed side textured,

mounted in a cover support frame, listed first. This is followed by data for

tempered glass, both sides smooth, then fiber reinforced plastic sheet and

finally poly( vinyl fluoride) film materials mounted in a cover support frame.

The last six tables contain data from tempered glass mounted in solar

collectors

.

The numerical designation of the impact locations are shown in figure 20.

A1
Table A-l Tempered Glass Esposed Side Textured

1/8 in. Thick

Specimen No. 1

Impact Location*

Desired

Resultant Velocity

(ft/s)

Ice Ball Nominal Diameter

(in.)

Weight

Hass

(lbm) Velocity

(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 98 1 0.018 96 2.5 No visible damage

2 98 1 0.016 92 2.2 No visible damage

3 98 1 0.016 99 2.5 No visible damage

4 98 1 0.018 98 2.6 No visible damage

5 98 1 0.016 96 2.4 No visible damage

6 98 1 0.018 93 2.4 No visible damage

7 98 1 0.016 92 2.2 No visible damage

8 98 1 0.018 99 2.7 No visible damage

9 98 1 0.016 94 2.3 No visible damage

10 98 1 0.016 95 2.3 No visible damage

11 98 1 0.018 91 2.3 No visible damage

12 98 1 0.018 100 2.7 No visible damage

13 98 1 0.016 101 2.6 No visible damage

14 98 1 0.015 95 2.2 No visible damage

15 98 1 0.018 100 2.7 No visible damage

16 98 1 0.018 98 2.6 No visible damage

17 98 1 0.016 95 2.3 No visible damage

* Impact

Impact Location

locations are

Desired Resultant Velocity

(ft/s)

shown in figure 20.

Table A-2 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 1

Ice Ball Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic

Energy

(ft-lbf) Results of Impact

i 112 1-1/2 0.060 120 13 No visible damage

2 112 1-1/2 0.060 118 13 No visible damage

3 112 1-1/2 0.060 114 12 No visible damage

4 112 1-1/2 0.060 104 10 No visible damage

5 112 1-1/2 0.060 116 12 No visible damage

6 112 1-1/2 0.060 112 12 No visible damage

7 112 1-1/2 0.060 110 11 No visible damage

8 112 1-1/2 0.060 111 11 No visible damage

9 112 1-1/2 0.060 106 10 No visible damage

10 112 1-1/2 0.060 115 12 No visible damage

11 112 1-1/2 0.060 110 11 No visible damage

12 112 1-1/2 0.060 111 11 No visible damage

13 112 1-1/2 0.060 111 11 No visible damage

14 112 1-1/2 0.060 113 12 No visible damage

15 112 1-1/2 0.058 110 11 No visible damage

16 112 1-1/2 0.060 112 12 No visible damage

17 112 1-1/2 0.060 114 12 No visible damage

Table A-3 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 1

Desired Ice Ball

Impact Location

Resultant Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass Velocity

(lbm) (ft/s)

Kinetic

Energy

(ft-lbf) Results of Impact

1 117 1-3/4 0.093 119 20 Fracture

A2
Table A-4 Tempered Glass Exposed Side Textured

1/8 In. Thick

Specimen No. 2

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

i 97 1-3/4 0.093 114 19 No visible damage

2 97 1-3/4 0.090 97 13 No visible damage

3 97 1-3/4 0.090 107 16 No visible damage

4 97 1-3/4 0.089 99 14 No visible damage

5 97 1-3/4 0.093 96 13 No visible damage

6 97 1-3/4 0.088 100 14 No visible damage

7 97 1-3/4 0.089 108 16 No visible damage

8 97 1-3/4 0.090 102 15 No visible damage

9 97 1-3/4 0.090 106 16 No visible damage

10 97 1-3/4 0.088 116 18 No visible damage

11 97 1-3/4 0.088 101 14 No visible damage

12 97 1-3/4 0.090 100 14 No visible damage

13 97 1-3/4 0.099 96 13 No visible damage

14 97 1-3/4 0.089 98 13 No visible damage

15 97 1-3/4 0.088 107 16 No visible damage

16 97 1-3/4 0.088 99 13 No visible damage

17 97 1-3/4 0.088 97 13 No visible damage

Table A-5 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 2

Desired Ice Ball

Impact Location

Resultant Velocity

(ft/s)

Nominal Diameter

(in.

)

Weight

Mass Velocity

(lbm) (ft/s)

Kinetic

Energy

(ft-lbf) Results of Impact

12

117

117 1-3/4 1-3/4 0.090 129 0.090 115

24

19 No visible damageFracture

Table A-6 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No . 3

Desired Ice Ball

Impact Location

Resultant Velocity

(ft/s)

Nominal Diameter

(in.

)

Weight

Mass Velocity

(lbm) (ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

12 117

117 1-3/4

1-3/4 0.090 119 0.090 122 20

20 No visible damage

Fracture

Table A-7 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 4

Desired Ice Ball Terminal Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.

)

(lbm) (ft/s) (ft-lbf) Impact

i 97 1-3/4 0.086 100 14 No visible damage

2 97 1-3/4 0.086 101 14 Fracture

A3
Table A-8 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 5

Desired Ice Ball Terminal Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.

)

(lbm) (ft/s) (ft-lbf) Impact

1 90 1-1/2 0.051 80 5.1 No visible damage

2 90 1-1/2 0.053 95 7.5 Fracture

Table A-9 Tempered Glass Exposed Side Textured

1/8 in. 'Thick

Specimen No. 6

Desired Ice Ball Terminal Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.

)

(lbm) (ft/s) (ft-lbf) Impact

1 90 1-1/2 0.051 89 6.3 No visible damage

2 90 1-1/2 0.051 87 5.9 No visible damage

3 90 1-1/2 0.051 83 5.4 No visible damage

4 90 1-1/2 0.051 91 6.5 No visible damage

5 90 1-1/2 0.053 88 6.3 No visible damage

6 90 1-1/2 0.053 90 6.7 No visible damage

7 90 1-1/2 0.051 90 6.4 No visible damage

8 90 1-1/2 0.053 96 7.6 No visible damage

9 90 1-1/2 0.053 87 6.2 No visible damage

Table A- 10 Tempered Glass Exposed Side Textured

1/8 in . Thick

Specimen No. 6

Desired Ice Ball Resultant Nominal Weight Kinetic

impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.

)

(lbm) (ft/s) (ft-lbf) Impact

1 112 1-1/2 0.053 104 8.9 No visible damage

2 112 1-1/2 0.053 111 10.1 No visible damage

3 112 1-1/2 0.051 98 7.5 No visible damage

4 112 1-1/2 0.055 115 11.4 No visible damage

5 112 1-1/2 0.053 105 9.1 No visible damage

6 112 1-1/2 0.055 122 12.8 No visible damage

7 112 1-1/2 0.055 116 11.6 No visible damage

8 112 1-1/2 0.057 115 11.8 No visible damage

9 112 1-1/2 0.055 113 11.0 No visible damage

Table A-l 1 Tempered Glass Exposed Side Textured

1/8 in. Thick

Specimen No. 6

Desired Ice Ball Terminal Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf) Impact

1 97 1-3/4 0.086 101 14 No visible damage

2 97 1-3/4 0.086 98 13 Fracture
Table A- 12 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Specimen No. 1

Impact Location

Desired Terminal

Velocity

(ft/s)

Ice Ball Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic

Ene rgy (ft-lbf) Results of Impact

1 105 2 0.141 104 24 No visible damage

2 105 2 0.139 106 24 No visible damage

3 105 2 0.139 120 31 No visible damage

4 105 2 0.138 107 24 No visible damage

5 105 2 0.138 108 25 No visible damage

6 105 2 0.139 109 26 No visible damage

7 105 2 0.138 103 23 No visible damage

8 105 2 0.138 107 24 No visible damage

9 105 2 0.140 109 25 No visible damage

10 105 2 0.138 113 27 No visible damage

11 105 2 0.138 101 21 No visible damage

12 105 2 0.139 111 27 No visible damage

13 105 2 0.141 113 28 No visible damage

14 105 2 0.138 118 30 No visible damage

15 105 2 0.138 113 27 No visible damage

16 105 2 0.138 94 19 No visible damage

17 105 2 0.138 110 26 No visible damage

Impact Location

Desired Resultant Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A- 13 Glass Smooth Thick

No. 1

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf)

1 124 2 0.139 121 32 No visible damage

2 124 2 0.133 126 33 No visible damage

3 124 2 0.137 135 39 No visible damage

4 124 2 0.138 125 33 No visible damage

5 124 2 0.134 130 35 No visible damage

6 124 2 0.139 128 35 No visible damage

7 124 2 0.136 126 34 No visible damage

8 124 2 0.137 128 35 No visible damage

9 124 2 0.138 120 31 No visible damage

10 124 2 0.136 122 31 No visible damage

11 124 2 0.139 129 36 No visible damage

12 124 2 0.138 130 36 No visible damage

13 124 2 0.137 130 36 No visible damage

14 124 2 0.137 129 36 Fracture

Impact Location

Desired Resultant Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A- 14 Glass Smooth Thick

No. 2

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic

Energy

(ft-lbf)

10 124 2 0.137 126 34 No visible damage

11 124 2 0.134 121 30 No visible damage

12 124 2 0.136 128 34 No visible damage

13 124 2 0.139 123 33 No visible damage

14 124 2 0.137 131 36 No visible damage

15 124 2 0.139 117 30 No visible damage

16 124 2 0.139 113 38 No visible damage

17 124 2 0.137 131 37 No visible damage

A5
Table A- 15 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Specimen No. 2

Impact Location

Desired Resultant Velocity

(ft/s)

Ice Ball Nominal Diameter

(in.

)

Weight

Mass

( lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

10 124 2 0.139 128 35 No visible damage

11 124 2 0.138 122 32 No visible damage

12 124 2 0.136 123 32 No visible damage

13 124 2 0.137 126 33 No visible damage

14 124 2 0.138 125 33 No visible damage

15 124 2 0.139 133 38 No visible damage

16 124 2 0.140 132 38 No visible damage

17 124 2 0.138 120 31 No visible damage

Impact Location

Desired Terminal Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A-16

Glass Smooth Thick

No. 2

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf)

i 111 2-1/4 0.187 114 38 No visible damage

2 111 2-1/4 0.187 119 41 No visible damage

3 111 2-1/4 0.187 117 40 No visible damage

4 111 2-1/4 0.190 120 43 No visible damage

5 111 2-1/4 0.190 114 39 No visible damage

6 111 2-1/4 0.187 114 38 No visible damage

7 111 2-1/4 0.190 114 39 No visible damage

8 111 2-1/4 0.190 111 36 No visible damage

9 111 2-1/4 0.190 113 37 No visible damage

10 111 2-1/4 0.190 112 37 No visible damage

11 111 2-1/4 0.190 111 36 No visible damage

12 111 2-1/4 0.190 113 37 No visible damage

13 111 2-1/4 0.190 112 37 No visible damage

14 111 2-1/4 0.190 116 40 No visible damage

15 111 2-1/4 0.190 113 38 No visible damage

16 111 2-1/4 0.190 112 37 No visible damage

17 111 2-1/4 0.186 111 36 No visible damage

Impact Location

Desired Resultant Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A- 17 Glass Smooth

Thick

No. 2

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic

Energy

(ft-lbf)

1 129 2-1/4 0.186 127 46 No visible damage

2 129 2-1/4 0.187 127 47 No visible damage

3 129 2-1/4 0.190 134 53 No visible damage

4 129 2-1/4 0.187 129 49 No visible damage

5 129 2-1/4 0.185 131 49 Fracture
Table A- 18 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Specimen No. 3

Impact Location

Desired Resultant Velocity

(ft/s)

Ice Ball Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf

)

Results of Impact

1 129 2-1/A 0.187 131 50 No visible damage

2 129 2-1/A 0.190 127 A7 No visible damage

3 129 2-1/A 0.187 127 A7 Fracture

Impact Location

Desired

Terminal Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A-19

Glass Smooth Thick

No. A

Ice Ball

Results of Impact

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic Energy

(ft-lbf)

1 105 2 0.126 112 25 No visible damage

2 105 2 0.128 112 25 No visible damage

3 105 2 0.126 102 20 No visible damage

A 105 2 0.126 116 26 No visible damage

5 105 2 0.126 117 27 No visible damage

6 105 2 0.126 98 19 No visible damage

7 105 2 0.128 102 21 No visible damage

8 105 2 0.126 92 18 No visible damage

9 105 2 0.126 103 21 No visible damage

Impact Location

Desired

Terminal Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A-20 Glass Smooth Thick

No. A

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic Energy

(ft-lbf)

1 111 2-1/A 0.185 107 33 No visible damage

2 111 2-1/A 0.192 117 A1 No visible damage

3 111 2-1/A 0.190 116 A0 No visible damage

A 111 2-1/A 0.192 11A 38 No visible damage

8 111 2-1/A 0.190 113 37 No visible damage

9 111 2-1/A 0.190 11A 38 Fracture

Impact Location

Desired

Resultant Velocity

(ft/s)

Table Tempered

Both Sides

1/8 in.

Specimen

A-21 Glass Smooth Thick

No. 5

Ice Ball Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 12A 2 0.129 123 30 No visible damage

2 12A 2 0.123 121 28 No visible damage

3 12A 2 0.126 127 32 No visible damage

A 12A 2 0.128 130 3A Fracture

A7
Table A-22 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Specimen No. 6

Desired Ice Ball Resultant Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf

)

Impact

1 124 2 0.129 128 33 Fracture

Table A-23 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 1 ("Old" Material)

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 90 1-1/2 0.058, 101 9.2 Leaked

2 90 1-1/2 0.060 101 9.5 Leaked

3 90 1-1/2 0.057 76 5.2 Leaked

4 90 1-1/2 0.058 100 9.0 Leaked

5 90 1-1/2 0.057 97 8.4 Leaked

6 90 1-1/2 0.057 101 9.1 Leaked

7 90 1-1/2 0.057 97 8.3 Leaked

8 90 1-1/2 0.057 93 7.7 Leaked

9 90 1-1/2 0.057 77 5.3 Leaked

10 90 1-1/2 0.057 91 7.4 Leaked

11 90 1-1/2 0.058 99 8.9 Leaked

12 82 1-1/4 0.033 101 5.3 Leaked

13 82 1-1/4 0.031 81 3.1 Leaked

14 82 1-1/4 0.031 85 3.4 Leaked

15 82 1-1/4 0.032 86 3.7 Leaked

16 82 1-1/4 0.032 82 3.3 Leaked

17 82 1-1/4 0.032 88 3.8 Leaked

Table A-24 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 2 ("Old" Material)

Impact Location

Desired Terminal Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

(lbm)

Ice Ball

Velocity

(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 62 3/4 0.007 69 0.50 Fine cracking

2 62 3/4 0.007 70 0.51 Fine cracking

3 62 3/4 0.007 67 0.46 Fine cracking

4 62 3/4 0.007 60 0.37 Fine cracking

5 62 3/4 0.007 72 0.53 Fine cracking

6 62 3/4 0.007 70 0.50 Fine cracking

7 62 3/4 0.007 66 0.45 Fine cracking

8 62 3/4 0.007 61 0.38 Fine cracking

9 62 3/4 0.007 62 0.39 Fine cracking

10 62 3/4 0.007 61 0.39 Smaller cracked area

11 62 3/4 0.007 64 0.42 Smaller cracked area

12 62 3/4 0.007 63 0.40 Fine cracking

13 62 3/4 0.007 61 0.39 Fine cracking

14 62 3/4 0.007 61 0.39 Fine cracking

15 62 3/4 0.007 64 0.42 Fine cracking

16 62 3/4 0.007 61 0.38 Fine cracking

17 62 3/4 0.007 64 0.42 Fine cracking

A8
Table A-25 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 In. Thick

Specimen No. 3

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic

Energy

(ft-lbf) Results of Impact

1 82 1-1/4 0.029 82 3.0 Leaked

2 82 1-1/4 0.029 85 3.2 Fine cracking

3 82 1-1/4 0.029 82 3.0 Leaked

4 82 1-1/4 0.031 88 3.7 Leaked

5 82 1-1/4 0.031 82 3.1 Leaked

6 82 1-1/4 0.031 81 3.1 Leaked

7 82 1-1/4 0.031 83 3.3 Fine cracking

8 82 1-1/4 0.031 86 3.5 Leaked

9 82 1-1/4 0.031 84 3.4 Leaked

10 82 1-1/4 0.031 81 3.1 Leaked

11 82 1-1/4 0.031 90 3.9 Leaked

12 82 1-1/4 0.031 86 3.6 Fine cracking

13 82 1-1/4 0.031 83 3.4 Leaked

14 82 1-1/4 0.031 81 3.1 Leaked

15 82 1-1/4 0.031 81 3.2 Leaked

16 82 1-1/4 0.031 82 3.2 Leaked

17 82 1-1/4 0.031 82 3.2 Fine cracking

Impact Location

Desired Resultant Velocity

(ft/s)

Table A-26 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 4

Ice Ball Nominal Weight Kinetic

Diameter Mass Velocity Energy

(in.) (lbm) (ft/s) (ft-lbf) Results of Impact

1 105 1-1/4 0.031 111 5.9 Fine cracking

2 105 1-1/4 0.031 108 5.5 Leaked

3 105 1-1/4 0.031 105 5.3 Leaked

4 105 1-1/4 0.031 101 4.9 Leaked

5 105 1-1/4 0.031 103 5.1 Leaked

6 105 1-1/4 0.031 108 5.6 Leaked

7 105 1-1/4 0.031 104 5.1 Leaked

8 105 1-1/4 0.031 104 5.2 Leaked

9 105 1-1/4 0.031 107 5.5 Leaked

10 105 1-1/4 0.031 101 4.9 Leaked

11 105 1-1/4 0.031 105 5.3 Leaked

12 105 1-1/4 0.031 97 4.4 Leaked

13 105 1-1/4 0.031 106 5.4 Cracking

14 105 1-1/4 0.031 107 5.5 Leaked

15 105 1-1/4 0.031 99 4.7 Leaked

16 105 1-1/4 0.031 110 5.8 Break through

17 105 1-1/4 0.031 108 5.6 Break through

Impact Location

Desired Resultant Velocity

(ft/s)

Table A-27 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 5

Ice Ball

Results of Impact

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf)

1 112 1-1/2 0.055 98 8.3 Break through

2 112 1-1/2 0.055 107 9.7 Break through

3 112 1-1/2 0.055 117 11.7 Break through

4 112 1-1/2 0.055 106 9.7 Break through

5 112 1-1/2 0.055 110 10.3 Break through

6 112 1-1/2 0.055 no 10.4 Break through

7 112 1-1/2 0.054 112 10.6 Break through

8 112 1-1/2 0.055 111 10.6 Break through

9 112 1-1/2 0.055 111 10.6 Break through

10 112 1-1/2 0.055 109 10.2 Break through

11 112 1-1/2 0.055 116 11.5 Break through

12 112 1-1/2 0.054 111 10.3 Break through

13 112 1-1/2 0.055 116 11.6 Break through

14 112 1-1/2 0.055 113 11.0 Break through

15 112 1-1/2 0.055 113 11.0 Break through

16 112 1-1/2 0.055 111 10.6 Break through

17 112 1-1/2 0.055 114 11.2 Break through

A9
Table A-28 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 6

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.

)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf

)

Results of Impact

1 82 1-1/4 0.029 79 2.8 Fine cracking

2 82 1-1/4 0.029 82 3.0 Fine cracking

3 82 1-1/4 0.031 84 3.4 Leaked

4 82 1-1/4 0.029 91 3.7 Leaked

5 82 1-1/4 0.029 82 3.0 Leaked

6 82 1-1/4 0.031 83 3.3 Leaked

7 82 1-1/4 0.031 89 3.8 Leaked

8 82 1-1/4 0.029 81 2.9 Leaked

9 82 1-1/4 0.031 81 3.1 Leaked

Table A-29 Reinforced Plastic Polyester Resin With Glass Fiber Mat 0.040 in. Thick

Specimen No. 7

Desired Ice Ball Terminal Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf) Impact

1 97 1-3/4 0.086 115 18 Break through

2 97 1-3/4 0.088 102 14 Break through

3 97 1-3/4 0.088 99 14 Break. Large deformation

4 97 1-3/4 0.086 97 12 Break through

5 97 1-3/4 0.088 100 14 Break through

6 97 1-3/4 0.088 101 14 Full Penetration

7 97 1-3/4 0.088 98 13 Break through

8 97 1-3/4 0.086 97 12 Break. Large deformation

9 97 1-3/4 0.088 99 13 Break through

Table A-30

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 1

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

( lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 62 3/4 0.007 61 0.38 Slight damage

2 62 3/4 0.007 63 0.41 Slight damage

3 62 3/4 0.007 67 0.46 Slight damage

4 62 3/4 0.007 69 0.49 Slight damage

5 62 3/4 0.007 64 0.42 Slight damage

6 62 3/4 0.007 61 0.38 Slight damage

7 62 3/4 0.007 62 0.40 Slight damage

8 62 3/4 0.007 59 0.36 Slight damage

9 62 3/4 0.007 67 0.46 Slight damage

10 62 3/4 0.007 66 0.45 Slight damage

1

1

62 3/4 0.007 60 0.37 Slight damage

12 62 3/4 0.007 59 0.36 Slight damage

13 62 3/4 0.007 66 0.45 Slight damage

14 62 3/4 0.007 68 0.47 Slight damage

15 62 3/4 0.007 63 0.41 Slight damage

16 62 3/4 0.007 62 0.39 Slight damage

17 62 3/4 0.007 60 0.37 Slight damage

A10
Table A-31

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 1

Impact Location

Desired

Resultant Velocity

(ft/s)

Nominal Diameter

(in.)

Ice Ball Weight

Mass Velocity

(lbm) (ft/s)

Kinetic

Energy

(ft-lbf

)

Results of Impact

1 91 3/4 0.007 98 0.90 Slight damage

2 91 3/4 0.007 86 0.76 Slight damage

3 91 3/4 0.007 97 0.97 Slight damage

4 91 3/4 0.007 97 0.97 Slight damage

5 91 3/4 0.007 91 0.84 Slight damage

6 91 3/4 0.007 98 0.99 Slight damage

7 91 3/4 0.007 98 0.98 Slight damage

8 91 3/4 0.007 97 0.97 Slight damage

9 91 3/4 0.007 96 0.95 Slight damage

10 91 3/4 0.007 96 0.95 Slight damage

11 91 3/4 0.007 H^lError 1-32 Slight damage

12 91 3/4 0.007 112' 1.29 Slight damage

13 91 3/4 0.007 94 0.91 Slight damage

14 91 3/4 0.007 99 1.01 Slight damage

15 91 3/4 0.007 93 0.90 Slight damage

16 91 3/4 0.007 92 0.87 Slight damage

17 91 3/4 0.007 91 0.85 Slight damage

Impact Location

Desired

Resultant Velocity

(ft/s)

Table A-32

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 2

Ice Ball

Results of Impact

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic Energy

(ft-lbf)

1 73 1 0.015 81 1.6 Slight damage

2 73 1 0.015 69 1.1 Slight damage

3 73 1 0.015 72 1.2 Slight damage

4 73 1 0.015 76 1.4 Slight damage

5 73 1 0.015 71 1.2 Slight damage

6 73 1 0.015 72 1.3 Slight damage

7 73 1 0.015 75 1.3 Slight damage

8 73 1 0.015 75 1.4 Slight damage

9 73 1 0.015 74 1.3 Slight damage

10 73 1 0.015 73 1.3 Slight damage

11 73 1 0.015 74 1.3 Slight damage

12 73 1 0.015 74 1.3 Slight damage

13 73 1 0.015 77 1.4 Slight damage

14 73 1 0.015 80 1.5 Slight damage

15 73 1 0.015 79 1.5 Slight damage

16 73 1 0.015 72 1.2 Slight damage

17 73 1 0.015 74 1.3 Slight damage

Impact Location

Desired Resultant Velocity

(ft/s)

Table A-33 Poly(Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 2

Ice Ball

Results of Impact

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic Energy

(ft-lbf)

1 98 1 0.015 99 2.3 Slight damage

2 98 1 0.015 103 2.6 Slight damage

3 98 1 0.015 95 2.2 Slight damage

4 98 1 0.015 101 2.5 Slight damage

5 98 1 0.015 103 2.6 Slight damage

6 98 1 0.015 107 2.7 Slight damage

7 98 1 0.015 99 2.4 Slight damage

8 98 1 0.015 101 2.4 Slight damage

9 98 1 0.015 97 2.2 Slight damage

10 98 1 0.015 99 2.4 Slight damage

11 98 1 0.015 101 2.4 Slight damage

12 98 1 0.015 95 2.2 Slight damage

13 98 1 0.015 100 2.4 Slight damage

14 98 1 0.015 105 2.6 Slight damage

15 98 1 0.015 98 2.3 Slight damage

16 98 1 0.015 95 2.2 Slight damage

17 98 1 0.015 98 2.3 Slight damage

All
Table A-34

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 3

Ice Ball

Impact Location

Desired Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity

(ft/s)

Kinetic

Energy

(ft-lbf

)

Results of Impact

1 82 1-1/4 0.031 91 4.0 0.13 in. indent

.

2 82 1-1/4 0.031 80 3.1

3 82 1-1/4 0.031 81 3.2

4 82 1-1/4 0.031 78 2.9

5 105 1-1/4 0.031 104 5.1

6 105 1-1/4 0.031 109 5.7

7 82 1-1/4 0.031 82 3.3

8 82 1-1/4 0.031 82 3.2

9 82 1-1/4 0.031 77 2.9

10 105 1-1/4 0.031 107 5.5 0.22 in. indent

.

11 105 1-1/4 0.031 108 5.6

12 105 1-1/4 0.031 102 5.0 0.19 in

.

indent

.

13 90 1-1/2 0.055 ' 88 6.6 0.34 in. indent

.

14 90 1-1/2 0.055 82 5.8 0.34 in. indent

.

15 112 1-1/2 0.055 104 9.3

16 105 1-1/4 0.031 107 5.5

17 105 1-1/4 0.031 108 5.6

Impact Location

Desired

Terminal Velocity

(ft/s)

Table A-35

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 4

Ice Ball Nominal Weight Diameter Mass Velocity

(in.) (lbm) (ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 105 2 0.130 95 18 0.50 in. indent.

2 105 2 0.130 91 19 Break

3 105 2 0.130 100 20 0.70 in. Indent.

4 105 2 0.130 95 18 0.70 in. Indent.

5 105 2 0.130 99 20 0.72 in. indent.

6 105 2 0.132 108 24 Break

7 105 2 0.132 107 23 Break

8 105 2 0.132 105 23 Break

9 105 2 0.132 106 23 Break

Impact Location

Desired Resultant Velocity

(ft/s)

Table A-36

Poly( Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 5

Ice Ball Nominal Weight Diameter Mass Velocity

(in.) (lbm) (ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 117 1-3/4 0.086 109 16 0.60 in. indent.

2 117 1-3/4 0.086 113 17 Break

3 117 1-3/4 0.086 117 18 Break

6 117 1-3/4 0.086 115 18 Break

7 117 1-3/4 0.086 116 18 Break

8 117 1-3/4 0.086 117 18 Break

9 117 1-3/4 0.086 109 16 0.60 in. indent.

A12
Table A-37

Poly (Vinyl Fluoride) Film

0.004 in. Thick

Specimen No. 6

Desired Ice Ball

Impact Location

Terminal Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf

)

Results of Impact

1 97 1-3/4 0.086 92 11 0.40 in. Indent.

2 97 1-3/4 0.088 105 15 0.55 in. indent.

3 97 1-3/4 0.086 97 12 0.45 in. indent.

4 97 1-3/4 0.086 100 13 0.48 in. indent.

5 97 1-3/4 0.086 103 14 0.55 in. indent.

6 97 1-3/4 0.086 101 14 0.52 in. indent.

7 97 1-3/4 0.088 105 15 0.55 in. indent.

8 97 1-3/4 0.086 100 14 0.50 in. indent.

9 97 1-3/4 0.086 100 13 0.55 in. indent.

10 97 1-3/4 0.086 96 12 Break

11 97 1-3/4 0.086 98 13 0.52 in. indent.

12 97 1-3/4 0.086 98 13 0.65 in. indent.

13 97 1-3/4 0.086 97 13 0.65 in. indent.

14 97 1-3/4 0.086 104 15 Break

15 97 1-3/4 0.086 100 13 Break

16 97 1-3/4 0.086 107 15 Break

17 97 1-3/4 0.086 101 14 Break

Table A-38 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Mounted in Solar Collector

Specimen No. A

Desired Ice Ball

Impact Location

Resultant Velocity

(ft/s)

Nominal Diameter

(in.)

Weight

Mass

(lbm) Velocity(ft/s)

Kinetic Energy

(ft-lbf) Results of Impact

1 (4 in. right) 117 1-3/4 0.095 130 25 No visible damage

1 (4 in. right) 117 1-3/4 0.095 128 24 No visible damage

1 (4 in. right) 117 1-3/4 0.095 129 25 No visible damage

1 (6 in

.

left) 124 2 0.141 130 37 No visible damage

1 (6 in. left) 124 2 0.141 132 38 No visible damage

1 (6 in

.

left) 124 2 0.143 128 36 No visible damage

1 134 2 0.141 132 38 No visible damage

1 (4 in

.

right) 134 2-1/2 0.276 135 78 Fracture

Table A-39 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Mounted in Solar Collector

Specimen No. F

Desired Ice Ball Resultant Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf) Impact

1 (4 in. right) 117 1-3/4 0.093 129 24 No visible damage

1 (4 in. right) 117 1-3/4 0.093 119 20 Fracture

A13
Table A-40 Tempered Glass

Both Sides Smooth

1/8 in. Thick Mounted in Solar Collector

Specimen No. B

Impact Location

Desired Resultant Velocity

(ft/s)

Nominal Diameter

(in.

)

Ice Ball Weight

Mass Velocity

(lbm) (ft/s)

Kinetic

Energy

(ft-lbf

)

Results of Impact

11 124 2 0.134 133 37 No visible damage

11 124 2 0.143 127 36 No visible damage

11 124 2 0.141 122 33 No visible damage

11 124 2 0.139 127 35 No visible damage

11 124 2 0.141 125 34 No visible damage

10 134 2-1/2 0.276 136 79 No visible damage

10 134 2-1/2 0.276 136 79 No visible damage

10 134 2-1/2 0.276 136 79 No visible damage

10 134 2-1/2 0.273 137 80 No visible damage

10 134 2-1/2 0.278 136 80 Fracture

Table A-41 Tempered Glass

Both Sides Smooth

1/8 in. Thick Mounted in Solar Collector

Specimen No. E

Desired Ice Ball Resultant Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf) Impact

11 124 2 0.137 128 35 No visible damage

11 124 2 0.137 125 34 No visible damage

11 124 2 0.134 129 35 No visible damage

11 124 2 0.134 125 33 No visible damage

11 124 2 0.134 132 36 No visible damage

10 124 2-1/2 0.273 136 79 Fracture

Table A-42 Tempered Glass

Both Sides Smooth

1/8 in. Thick

Mounted in Solar Collector

Specimen No. C

Impact Location

Desired

Resultant Velocity

(ft/s)

Nominal Diameter

(in.

)

Ice Ball Weight

Mass Velocity

(lbm) (ft/s)

Kinetic

Energy

(ft-lbf) Results of Impact

11 117 1-3/4 0.093 119 20 No visible damage

14 117 1-3/4 0.093 118 20 No visible damage

15 124 2 0.139 130 36 No visible damage

15 124 2 0.141 132 38 No visible damage

15 124 2 0.141 129 36 No visible damage

15 124 2 0.139 126 34 No visible damage

15 124 2 0.139 130 36 No visible damage

14 124 2-1/2 0.280 138 83 No visible damage

14 124 2-1/2 0.273 137 80 Fracture

Table A-43 Tempered Glass

Both Sides Smooth

1/8 in. Thick Mounted in Solar Collector

Specimen No. D

Desired Ice Ball Resultant Nominal Weight Kinetic

Impact Velocity Diameter Mass Velocity Energy Results of Location (ft/s) (in.) (lbm) (ft/s) (ft-lbf) Impact

14 117 1-3/4 0.093 121 21 No visible damage

14 117 1-3/4 0.093 115 19 Fracture

A14
NBS-114A (rev. 2-ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

NBSIR 82-2487

2. Performing Organ. Report NoJ 3. Publication Date

March 1982

Hail Impact Testing Procedure for Solar Collector Covers

5. AUTHOR(S)

David R. Jenkins and Robert G. Mathey

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Department of Energy

Office of Solar Heat Technologies

Active Heating and Cooling Division

Washington, DC 20585

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) This report presents laboratory test results which simulate hail impact on solar collector covers. The general objective of the work is to contribute to the development of a test method for evaluating the resistance of solar collector covers to this type of

loading. A procedure for such testing is described as well as results obtained with

ice balls impacting four typical collector cover materials. Aspects which are discussed

include the preparation of ice balls, the design and operation of a launcher for ice

ball propulsion, the method of mounting cover panel specimens, the selection of ice ball

velocity and impact location, and techniques for failure or damage assessment.

The research results show that ice balls of consistent diameter and mass can be prepared

in the laboratory. Further, both analysis and results tend to show that acceptable

simulation for evaluation or testing can be achieved with normal impacts of ice balls

traveling at a resultant velocity which is the vector sum of the terminal velocity and

a horizontal wind component. Results for a variety of impact locations are presented

and for comparison purposes, arbitrarily selected points near the collector cover boundaries appear to be a reasonable choice. Finally, it is shown that for some collector cover materials, more than one kind of failure must be considered when

evaluating test results. Test data for two types of tempered glass, semirigid fiber

reinforced plastic, and flexible thin plastic film covers are presented.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) hail damage; hail impact testing; hail launcher; simulated hail testing,’ solar collector

covers; test method development

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

t)OCl Unl imited

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1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

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