A DYNAMIC STUDY OF FRAGMENTATION AND ENERGY LOSS

DURING HIGH VELOCITY IMPACT

Ralph Zee, Principal Investigator

Table of Content

ABSTRACT ................................................................ 1

INTRODUCTION ............................................................ 3

MOMENTUM AND ENERGY DISTRIBUTION IN THE DEBRIS CLOUD .................... 5

Initial Idea ..................................................... 5

Momentum versus Energy ........................................... 7

Development of Momentum Monitoring ............................... 7

Development of Energy Monitoring ................................. 46

CHARACTERIZATION OF ENERGY LOSS IN ADVANCED MATERIALS

FOR SECONDARY PROTECTION ......................................... 62

A DYNAMIC STUDY OF FRAGMENTATION AND ENERGY LOSS

DURING HIGH VELOCITY IMPACT

Ralph Zee, Principal Investigator

ABSTRACT

Research conducted under this contract can be divided into two main

areas: hypervelocity (in the range up to 7 km/s) and high velocity (less than

i km/s). Work in the former was performed at NASA-Marshall Space Flight

Center using the Light Gas Gun Facility. The lower velocity studies were

conducted at Auburn University using the ballistic gun.

The emphasis of the project was on the hypervelocity phenomenon

especially in the characterization of the debris cloud formed by the primary

impact events. Special devices were made to determine the angular

distributions of momentum and energy of the debris cloud as a function of

impact conditions. After several iteration processes, it was decided to

concentrate on the momentum effort. Prototype devices were designed,

fabricated and tested. These devices were based on the conservation of

momentum. Distributions of the debris cloud formed were measured by

determining the amount of momentum transferred from the debris cloud to

strategically placed pendulum measurement devices. The motion of the pendula

was monitored using integrated opto-interrupters. Six pendula were placed at

scattering angles ranging from I0 ° to 40 °. The device was found to be very

durable even in the hostile environment that existed in the target chamber.

An automated data acquisition system was used for data collection.

Experimental findings were in accord with calculations. The distribution of

the momentum in the debris cloud was found to be a strong function of the
impact condition. Small projectiles at high velocities were observed to

produce finely dispersed debris whereas large projectiles generated discrete

particles in the debris. Results also show that the momentum in the forward

direction was enhanced due to the impact. This phenomenon of momentum

multiplication was also observed in other studies and in computer simulations.

It was initially planned to determine the energy distribution using

deformation energy in a rod with strain gauges. Results from preliminary

studies show that this technique is acceptable but too tedious. A new

technique was explored based on measuring the heating effect of the debris

cloud using an IR camera. The feasibility and sensitivity was established at

Auburn University. This type of energy distribution measurement method can

easily be adapted to the gas gun facility at MSFC.

The objective of the lower velocity studies at Auburn was to simulate

the damage produced in advanced materials by the lower energy debris cloud.

Graphite, Kevlar and PE reinforced composites. Results show that PE based

materials possess the best impact resistance.
INTRODUCTION

The Space Station Freedom will be exposed to a hostile environment over

long periods of time. Such long term exposure of systems in space may lead to

degradation of materials as a direct result of their interactions with

environmental factors. These factors include ultraviolet radiation, atomic

oxygen, and hypervelocity particles. With the ever increasing amount of manmade debris, hyper-velocity impacts have become a critical concern of

designers. Due to the high energies involved in these particle collisions, it

is impossible to protect the space station using a single containment of

weight limitations. One solution proposed by Whipple comprises of a system of

plates which serve to intercept any incoming particle and render it harmless

to the pressure wall. This is believed to occur by disintegrating the

particle and redistributing its momentum over a large angle thus reducing its

penetrating power. It is therefore necessary to design the bumper in such a

manner as to maximize its effectiveness.

Earlier attempts designed to determine the effectiveness of proposed

bumper materials relied solely upon examinations of witness plates which

received the debris cloud after the initial impact occurred. Measurements of

crater size and areal density were taken and recorded for future comparison

with other testing conditions. This method, however, does not provide

empirical data by which the designer can evaluate the effectiveness of the

bumper for redistributing momentum. Up to the present time, several attempts

have been made to measure the momentum of the ejecta produced from the

collision between the projectile and the bumper. This was accomplished

through the use of a ballistic pendulum system. Such systems are designed to

measure the total momentum produced by such an impact by catching all of the
ejecta simultaneously. Unlike these previous attempts, the system devised in

this experiment is capable of measuring the momentumof the debris at direct

locations within the ejecta cloud. This results in the ability to

characterize momentum more thoroughly than previously possible. Momentum

profiles were obtained for a series of test conditions. In addition to

providing a method for obtaining momentum distributions, the data obtained

from this experiment will help to confirm whether or not an amplification of

momentum occurs. This is necessary in determining the role played by the

bumper in providing protection for the space vehicle. Total momentum values

in the debris cloud were calculated from the debris momentum profiles. These

results indicated that a momentum amplification exists with a multiplication

factor of between 2 and 3. Thus the role of the bumper to serve as a means

for momentum redistribution and not reduction was verified.

The main objective of this research project is to develop devices and

techniques which are capable of determining the momentum and energy

distributions in the debris cloud generated by the initial impact between the

hypervelocity projectile and the bumper plate. Since the bumper does not

completely absorb the energy of the primary impact, but rather distribute it,

damage to the pressure wall from the debris cloud also needs to be examined.

The second goal of the research is to determine the effects of the debris

cloud on potential pressure wall materials.

4
MOMENTUM AND ENERGY DISTRIBUTION IN THE DEBRIS CLOUD

Initial Idea

In the initial proposal, a momentum sensing device was proposed to

uniquely identify the mass and velocity distributions of the debris as a

function of impact parameters and scattering angle. To determine the momentum

distribution, a recoil device were to be used. The concept proposed was based

on the conservation of linear momentum during impact. The experiment setup

proposed was similar to that given in figure I. The initial hypervelocity

projectile (A) with mass M I and velocity V I will impact on the bumper material

(B). Small fragments (masses m i and speeds vl) produced will exit the back

face in the direction 8i. A stopper recoil plate (C) made of aluminum of mass

M2 (must be a soft material so that energy and momentum can be easily

absorbed) will be placed on a low friction cart. Due to the impact of the

small fragments, the stopper sheet will recoil with a velocity V 2. The total

linear momentum of all the small fragments in the undeflected direction can

then be obtained. Mathematically, this can be represented by

MaYa -Zi (mlvl cosei) [I]

This overall information is insufficient to uniquely clarify the velocity

distribution of the debris. To accomplish that, a movable aperture (D) were

to be inserted in between the bumper material (B) and the recoil stopper plate

(C). This is shown schematically in figure 2. The aperture and the movable

recoil plate assembly were to be mounted on a track pivoted about the point of

impact. The aperture (D) will then isolate the debris scattered into O i for

collision with the recoil plate. By systematically moving the aperture and

the recoil plate assembly to different positions, the overall velocity of the

debris in different Oi angles will be determined. Mathematically, this
TOP VIEW

>@

MlVl

m

vi

&

Bumper

Plate

> M2 V2

Movable recoil plate, sit

on a set of low friction

bearings

Figure I. Basic experimental assembly for measuring momentum transfer from

the debris cloud.
TOP VIEW

Bumper plate with

two rotation angles

/

Aperture

1

I _ "_ Movable recoil

plate

Figure 2. Experimental assembly with defining aperture. The schematic shown

is for the more general non-normal incident configuration. In

this research, effort was concentrated on normal incident.
condition can be represented by

M2V 2 - nlmivi [2]

where n i is the number of fragments of mass m i that pass through the aperture

at angle 8 i. Equation [2] does not contain the term cos 8 i since all the

selected debris passing through the aperture will impact the recoil plate at

normal incidence. We assume that all the debris scattered into this aperture

are uniform in size, which is acceptable if the aperture is small.

Preliminary calculations were performed to predict the parameters needed for

this recoil system. As an estimation, we can assume that the hyperveloclty

projectile weighs I00 mg and has an initial velocity of 5 km/s. Furthermore,

it is reasonable to assume that during the initial impact of the projectile

with the bumper, only half of the initial momentum is available to set the

recoil plate in motion. Of this total available momentum, only 1% is allowed

to pass through the aperture. If the recoil plate has a mass of i00 gm, the

resulting speed of this plate will be 2.5 cm/s which can be easily monitored

using a series of relays at regular intervals.

To isolate the mass and velocity contributions to the momentum, the mass

f

distribution of the debris as a function of scattered angle was to be

determined by placing a heavy shield behind the bumper plate to stop the

debris. These fragments will make indentions on the plate and the size of

these indentions will provide the necessary mass distribution profile. This

distribution is of course expected to be spherically symmetric and depends

only on the scattered angle and the initial impact conditions.
Momentum versus Energy

However earlier in the project, it was determined that such a linear

motion device is unacceptable due to the confined nature of the target chamber

in the hypervelocity gun at MSFC. At that time a decision was made to

determine both the energy and the momentum distributions in the debris cloud

as a function of scattering angle 8. This is because such dual measurements

would facilitate a direct and precise determination of both the mass and

velocity without having to identify the mass directly since

v - J(2E/p) [3]

and

m - p/v [4]

A careful review of collision physics showed that in order to determine

momentum transfer, the device must be able to response by a physical recoil

motion whereas a energy measurement device would not have such a restriction.

Development for Momentum Monltorln_

The objectives of this experiment are three-fold. The first objective

is to design, construct, and test at device by which the momentum distribution

could be monitored during a hypervelocity impact event. This requires that

any equipment which resides in the test chamber be sufficiently durable to

allow multiple uses without a substantial degradation in its capabilities.

The second objective is to obtain momentum profiles for a set of test

conditions. The final objective is to calculate the total momentum produced

and verify the estimated amplification value.

The linear motion device proposed in the initial proposal was quickly

replaced by a pendulum concept. Momentum transferred to the pendulum results
in the motion of the pendulum. Unfortunately, the period of the pendulum is

independent of the impacted momentum. This means that it is necessary to

monitor the real time motion of the pendulum instead of simply measuring its

period. One approach will be to measure the velocity of the pendulum using

two sensors at a fixed interval. After a careful study, we have adopted the

basic design which requires only one optical sensor with one mechanical

microswitch. The mechanical microswitch is for triggering. A series of

periodic slots will be machined at the bottom of the pendulum which during the

movement of the pendulum would introduce the required information in the

optical sensor. A fast-response optical sensor such as Motorola H21 series

slotted couplers/interrupter modules (response time in the microsecond range)

will be ideal for this application. It is important to emphasize that both

the duration of each cycle as well as time between cycles can be used to

determine the momentum imparted on the pendulum. An aperture/shield assembly

will be used in front of the pendulum to defined the scattering angle desired

as well as to protect the pendulum system from debris damage. This basic

design was improved during the course of the study and evolved into the final

device. Figure 3 shows the actual signal stored on a digital oscilloscope

from the motion of the pendulum.

The momentum measurement device was tested in the light gas gun facility

(LGG) located at the Marshall Space Flight Center. The layout of this

facility is shown in figure 4. The gun was used to propel spheres of various

materials and sizes at velocities ranging from 5.0 to 7.5 km/sec at the

targets. In this study aluminum (ii00 AI) spheres were used as projectile and

the bumper plates were comprised of 6061-T6 A1 sheet. The momentum monitoring

device was placed in the secondary test chamber. The bumper plate was places

i0
Figure 3. Discrete signal from the momentum device from which motion of the

pendulum induced by the debris can be quantified for momentum

determination.

ii
Barrel

Pun_ tube

Breech

Small test chamber

Extendon tube

Hallstation

(velocltycletecllon)

Intermedlate test chamber

(locationof momentum device)

Preuure tank

Ho!dlng fixture,

seal,and testarticle

Figure 4. Layout of the light gas gun facility at NASA-MSFC.

12
at a distance of 15.24 cm (6") from the front of the blast shield protecting

the device. During the tests, the chamber was evacuated to a vacuum of less

than 300 millitorr. The primary impact between the aluminum projectile and

the bumper produced a high energy debris cloud, the distribution of which is

the center of investigation for this study.

The concept employed for this device involves the transfer of momentum

from the incident ejects produced by the particle and bumper collisions onto

pendula placed at periodic locations along a horizontal plane bisecting the

debris cloud. A total of six pendula were employed at intervals of i0 °

beginning with i0 ° from the normal incident toward one side ending at 30 ° and

beginning with 15 ° from normal incident ending at 35 ° on the other side.

Figure 5 shows the typical configuration of the pendulum system while figure 6

shows the layout of all six pendulum system with respect to the bumper and

incident particle. The mild steel blast protector serves to shield the

pendula and other fragile components from the debris and gas surge while

allowing a predetermined area of the pendulum impact block to be struck by

incoming ejects. The pendula assemblies consist of an aluminum pendulum bar,

impact block, and timing blade. The velocity of each pendulum is determined

by the movement of the timing blade through an infrared photo-interrupter

diode. Holes placed at regular intervals along the bottom edge of the timing

blade serve to intermittently block and expose the beam produced by the

infrared emitter located on one side of the diode allowing it to reach the

sensor located on the opposite side of the blade. This results in a square

wave potential drop alternating between 0 and 5 volts. Figure 7 illustrates a

set of signals produced by all six pendulums simultaneously during an impact

event.

13
/_Bearing axis

BTa-_-JSr-O_'I I...... _I ............... Pendulum --

lJ -_ Configuration.

_ ,I/ Impact block

/

//-1_1[ _'_'- ///Timing blade

Aperture / _-

(2) (2) _ sensorPh°t°-interupt°r

1

|

/

/

Base support _"

Figure 5. Basic pendulum configuration for momentum measurement.

14
Bumper Btast proteotor . _

• -

MOMENTUM DEVICE

[top vlew) #, j _ Pendulum _

Figure 6. Arrangement of the pendula on the momentum measurement device.

15
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.l_ll,_m K

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6X,$I9.PPI9 .8

RY,

8.lgOOO aY

OY'

+.8OelO eV

'6Y'

e.oeeee av

;_l' 0

600' 4006

Figure 7. Signals produced by the opto-interrupters during impact. These

data are stored on the Norland system.

16
Data collection and storage is accomplished through the use of a digital

storage oscilloscope. Initial development of the device required four

channels for data acquisition with triggering performed manually. This was

accomplished through the use of _two Tektronix storage oscilloscopes. The

device used in this study requires six channels, one for the each pendulum,

and one channel for remote triggering. The data is recorded on a ten channel

Norland A200i digital oscilloscope and saved on floppy diskettes for further

analysis. Triggering is accomplished by imputing a signal produced by an xray flash detector used in determining projectile velocity into an unused

channel. Total time duration for data acquisition is about 2 seconds which

sufficiently allows for all the pendulums to pass through their upward and

downward arcs at least one.

The device worked well for more than 40 shots with no significant

reduction in sensitivity. Only a small amount of adjustment every 4 to 5

shots was required to tighten any loose bolts and replace damaged blast

shields. After approximately i0 shots the device was removed and examined for

any damage to electronic or structural components and cleaned. The diodes

were replaced after approximately 25 shots and the pendulum impact blocks were

replaced after every 15 shots.

The momentum monitoring device basically monitors the angular movement

of each pendulum induced by the ejects from the impact. This motion is

directly related to the momentum transfer from the specific portion of the

cloud to the respective pendulum. It is therefore necessary to determine the

conversion factor from the angular velocity to momentum density P' The

momentum form the debris cloud (my) must equal the angular momentum induced in

the pendulum as stated below in equation [5]. It is important to note here

17
the distinction of this momentumvalue with the particle momentum (moVo),

my - l_/r [5]

where _ - v/R [6]

Substitution of equation [6] into equation [5] yields the following which

correlates the momentum (P) with the incident debris velocity (v)

P - Iv/rR [7]

where P - mv (debris momentum)

R- radius of arc traced by the timing blade

v - velocity of the timing blade

r - distance from center of axle to center of gravity

I - moment of inertia for the pendulum

In this study, R-5.13", r-3.5" and 1-4285 gm-cm 2. Substitution of these

values into equation [7] gives

P (kg-cm/s)- A*v

where A - 0.038

[8]

[9]

Therefore, multiplication of the measured velocity, v, by the computed

constant, A, gives the desired momentum values. This parameter A is a

function of the inertias of the pendulum which was experimentally determined.

To obtain the momentum per unit area P', the values determined in the previous

step are divided by the area of the apertures in the blast protector of the

area of the impact block exposed to the debris.

Using the previously described method and equations [8] and [9], it is

then possible to calculate the momentum values from the velocities obtained

for each pendulum. More than forty shots were made using the design. Twenty

two of these shots yielded useful information on the momentum distribution in

the debris cloud. The conditions of the shots can be divided into four

18
categories: Group A: small projectile (0.25" diameter) and thin bumper (0.04"

thick), Group B: small projectile and thick bumper (0.08" thick), Group C:

large projectile (0.313" diameter) and thin bumper, Group D: large projectile

and thick bumper. Table i summarizesthe conditions for the tests conducted

in this study. These distributions are shownin figures 8 to 30. It is

evident from the profiles shownin these figures that the debris clouds formed

by the impact of small projectiles (0.25" diameter, GroupsA and B) have

momentumconcentrated in the middle and decay with scattering angle. However

the momentumdistributions in the debris generated by the larger projectiles

(Groups C and D) are more discrete and spread out to large scattering angles.

In somecases (such as Z-54) the pendula at large scattering angles actually

received more momentathan the undeflected direction. This indicate that

discrete fragments were formed under this impact condition.

The profiles from the impact of small projectiles can be fitted with

either a Gaussian or triangular shape within the limit of experimental

accuracy. However, the expected symmetry in these profiles is not always

evident. This is due in part to variations present in the LGGsystem with

respect to its ability to direct the projectile at the center of thebumper

plate. Thus, the peak values can and are often observed toward one side of

the center position or the other.

It appears that within the experimental scatters of the data, it is

appropriate to fit the momentum distribution to either a Gaussian or

triangular shape. The latter was selected to fit the data due to simplicity.

Such a distribution can be characterized by two parameters: peak value of the

momentum areal density P' and half width (radius ro) at he bass. Total

momentum in the debris cloud (Ptot) was then calculated from these values by

19
Z-26

0.25" Projectile

5 km/s

0.04" Bumper

4

5

2

kgcm/s

P( , )

cm

L • x 0 J • f

-40 -20 0 20 40

ANGLE

Figure 8. Momentum distribution obtained using the momentum device.

2O
0.25" Projectile

5 km/s

0.04" Bumper

kgcm/s

P( ) E

cm

i I 0 i

-40 -20 0 20 40

ANGLE

I

Figure 9. Momentum distribution obtained using the momentum device.

21
Z-50

0.25" Projectile

6 km/s

0.04" Bumper

@

kgcm/s

24- p( )

2O

16

12

8

_

£

cm

i A A

-- . 0 I

-40 -20 0 20 40

ANGLE

Figure i0. Momentum distribution obtained using the momentum device.

22
Z-24

0.25" Projectile

6 km/s

0.05" Bumper

4- p(

kgcm/s

E

cm

5

2

i i 0 I i

-40 -20 0 20 40

ANGLE

Figure II. Momentum distribution obtained using the momentum device.

23
Z-47

0.25" Projectile

6 km/s

0.05" Bumper

kgcm/s

lO- p( )

8

6

4

_

£

cm

1 i

-40 -20 0 20 40

ANGLE

Figure 12. Momentum distribution obtained using the momentum device.

24
Z-31

0.25" Projectile

7.2 km/s

0.04" Bumper

kgcm/s

5- p( )

2

cm

L J 0 I I

--40 --20 0 20 40

ANGLE

Figure 13. Momentum distribution obtained using the momentum device.

25
Z-38

0.25" Projectile

7.2 km/s

0.04" Bumper

kgcm/s

10

8

6

4

2

P( . )

(2m

i I 0 Av vA 1

-40 -20 0 20 40

ANGLE

Figure 14. Momentum distribution obtained using the momentum device.

26
0.25" ProjecLile

5 km/s

0.08" Bumper

kgcm/s

5 p( ) £

cm

4

3

2

I m

I i 0 l i

-40 -20 0 20 40

ANGLE

Figure 15. Momentum distribution obtained using the momentum device.

27
Z-39

0.25" Projectile

6 km/s

0.08" Bumper

kgcm/s

P( )

£

cm

-40 -20 0 20 40

ANGLE

Figure 16. Momentum distribution obtained using the momentum device.

28
Z-48

0.25" Projectile

6 km/s

0.08" Bumper

I

4

2 -

1

kgcm/s

P( . )

cm

I 0 i A i

V

-40 -20 0 20 40

ANGLE

Figure 17. Momentum distribution obtained using the momentum device.

29
Z-S1

0.25" Projectile

6 km/s

0.08" Bumper

4- P(

b -

2

1 -

kgcm/s

, )

cm

t I 0 i j

-40 -20 0 20 40

ANGLE

Figure 18. Momentum distribution obtained using the momentum device.

30
0.25" Projectile

6.5 km/s

0.08" Bumper

kgcm/s

4- p( )

I

cm

3

2

J i oi I ,

-40 -20 0 20 40

ANGLE

Figure 19. Momentum distribution obtained using the momentum device.

31
0.25" Projectile

7 km/s

0.08" Bumper

4-- p(

3

2

• • 1

kgcm/s

. )

em

l l -0- _ j

-40 -20 0 20 40

ANGLE

Figure 20. Momentum distribution obtained using the momentum device.

32
Z-33

0.25" Projectile

7.2 km/s

0.08" Bumper

kgcm/s

4- p( )

5

£

cm

L i 0 i i

-40 -20 0 20 40

ANGLE

Figure 21. Momentum distribution obtained using the momentum device.

33
Z-32

0.25" Projectile

7.2 km/s

0.08" Bumper

4

2

1 -

kgcm/s

- p(

cm

-4O -2O 0 2O 4O

ANGLE

Figure 22. Momentum distribution obtained using the momentum device.

34
0.3 13" Projeciile

6 km/s

O. 04" Bumper

8- P(

_

4-

2 -

kgcm/s

I

cm

I I 0 I I

--40 --20 0 20 40

ANGLE

Figure 23. Momentum distribution obtained using the momentum device.

35
Z-44

0.313" Projectile

6 km/s

0.04" Bumper

8

7

5

• 4 -

3

2

I -

kgcm/s

P( . )

cm

I I 0 I i

-40 -20 0 20 40

ANGLE

Figure 24. Momentum distribution obtained using the momentum device.

36
Z-56

0.313" Projectile

7.2 km/s

O. 04" Bumper

P(

6-

5

4

3

2

kgcm/s

I

cm

)

i J 0 I j

-40 -20 0 20 40

ANGLE

Figure 25. Momentum distribution obtained using the momentum device.

37
Z-46

0.313" Projectile

7.2 km/s

0.04" Bumper

kgcm/s

P(

6-

5 -

@

4 -

5 -

2

I

i l 0 l i

-40 -20 0 20 40

2

cm

@

ANGLE

)

Figure 26. Momentum distribution obtained using the momentum device.

38
Z-42

0.313" Projectile

6 km/s

0.08" Bumper

kgcm/s

P( , )

cm

6 -

5 -

4 -

5 - •

2 -

1 -

A I

I I 0 I v

-40 -20 0 20 40

ANGLE

Figure 27. Momentum distribution obtained using the momentum device.

39
Z-54

0.313" Projectile

6 km/s

0.08" Bumper

5

4

2

1 -

kgcm/s

P(

cm

, i 0 i r

-40 -20 0 20 40

ANGLE

Figure 28. Momentum distribution obtained using the momentum device.

40
Z-45

0.313" Projectile

7.2 km/s

0.08" Bumper

kgcm/s

P(

8-

7-

6-

5-

4-

3-

• 2-

I -

i i 0 j I

-40 -20 0 20 40

cm

ANGLE

Figure 29. Momentum distribution obtained using the momentum device.

41
Z-55

0.313" Projectile

7.2 km/s

O. 08" Bumper

12 -

10 -

8

6-

4-

2 -

kgcm/s

P( ,, )

eel

i i 0 I i

-40 -20 0 20 40

ANGLE

Figure 30. Momentum distribution obtained using the momentum device.

42
Group

Table i. Summary of shots conditions

Shot ID Proj Size (in) Vel (km/s) Bumper Thick (in)

A

Z-26

Z-50

Z-24

Z-47

Z-31

Z-38

0.25

0 25

0 25

0 25

0 25

0 25

0 25

B

5 0.04

5 0.04

6 0.04

6 0.05

6 0.05

7.2 0.04

7.2 0.04

* 0.25 5 0.08

Z-39 0.25 6 0.08

Z-48 0.25 6 0.08

Z-51 0.25 6 0.08

* 0.25 6.5 0.08

* 0.25 7 0.08

Z-33 0.25 7.2 0.08

Z-32 0.25 7.2 0.08

* 0.313 6 0.04

Z-44 0.313 6 0.04

Z-56 0.313 7.2 0.04

Z-46 0.313 7.2 0.04

Z-42 0.313 6 0.08

Z-54 0.313 6 0.08

Z-45 0.313 7.2 0.08

Z-55 0.313 7.2 0.08

* Early shots have no _D assigned to them,

integrating the momentum density as follows.

From the momentum profiles obtained experimentally, we obtain values for

P' and ro. The integration of the momentum density from 0 to ro gives:

i"

Ptot " J 2_r(P'-Br)dr [i0]

O

where r is the radial distance from normal incident and B is the slope of the

line connecting ro and P'.

B - F'/r o [ii]

Substituting equation [ii] into [I0] and completing the integration yields:

Ptot - _P'ro2/2 [12]

43
WherePtot represents the total momentumof the debris cloud. The above

calculations assume that the momentum distribution is symmetrical with a

dependance on radial distance. This is true for a normal incident impact.

This total momentum calculation was applied to a series of four

experiments with small projectiles, as given in Table 2 which lists the values

for P' and ro for each test. These four tests represent two bumper

thicknesses and two velocities. More data are needed to facilitate similar

calculations for' the impacts with larger projectiles due to the discrete

characteristics of their debris cloud. Table 3 summaries the results from

these calculations. It includes the projectile velocities (vo) , the initial

momentum from the projectile (movo), the calculated momentum multiplier. It

is evident from these tables that the bumper impact process results in a

multiplication of the momentum by a factor of approximately 2.3 to 3.0. This

momentum gain does not violate the conservation of momentum since the primary

impact also generates backscattered debris which is not measured in this

study. Similar results were obtained from the Southwest Research Institute

where multiplication factors between 1.44 and 1.76 were obtained. The factors

observed in our study were higher since the SoRI study measured momentum

resolved in the incident direction only whereas our study integrates the

momentum vectors in the debris cloud in all directions. In addition, the

experimental conditions were different between the two investigations.

Nevertheless, the presence of a momentum amplification factor implies that the

bumper does not reduce the momentum of the secondary debris cloud although it

does result in a net loss in energy from the initial impact. Therefore the

reduction in the damage on the pressure wall in such a concept must be due to

a spreading of the momentum int he presence of the bumper. The exact

44
dependanceof this factor on impact and material parameters can not be

determined at this time due to the limited amount of data.

Table 2.

Shot ID

Summary of momentum distribution

P' (kg-cm/s-cm 2) ro (cm)

Z-38 11.2 6.68

Z-33 4.1 9.79

Z-47 10.2 5.71

Z-51 4.5 8.38

Table 3. Summary of momentum multiplier

Shot ID Proj. Velocity Pproj Ptot (debris)

(km/s) (kg-cm/s) (kg-cm/s)

Momentum

Multiplier

Z-38 7.2 260 785 3.0

Z-33 7.2 260 618 2.4

Z-47 6 217 522 2.4

Z-51 6 217 622 2.9

In summary, a system was designed by which the momentum profile through

the debris cloud produced during hypervelocity impact may be monitored and

analyzed. The objectives were to construct a reliable system which could be

utilized several times without major adjustment, to obtain momentum profiles

for a given set of test conditions, and to calculate the total momentum

present in the debris cloud using the experimentally obtained debris momentum

profiles. The system proved to be both reliable and durable. Momentum

profiles were obtained and indicated that he data fitting could be

accomplished using a triangular approximation. The total momentum calculated

revealed a momentum amplification with a multiplier of between 2 and 3.

Future work will be done using different projectile sizes and velocities to

determine the possible effects of material properties and other variables upon

momentum distribution.

45
Development _or Ener2v Monitorln2

An initial design was made for an energy measurement system without any

moving components and a device was fabricated. A flat circular plate for

catching debris was attached onto a short thin wall cylinder which was in turn

rigidly attached to a strong angle brace. The 4" diameter cylinder had a wall

thickness of 0.i" and its height was i". A group of solid state strain gages

(minimum of four at 90 ° ) was mounted onto the outside surface of the short

cylinder at regular angular interval. During the impact-debrls forming

process, the debris hitting the target at selected angle will result in

measurable strains in the strain gages. Since we are interested in the

angular distribution of the energy in the debris cloud, a circular aperture

would be appropriate in this geometry. If the momentum distribution of the

debris being detected is not symmetrical, then the different strain gages will

yield different values. In certain cases, it may even be tensile in nature.

The signals from the strain gages will be either collected via a group of

strain gage controller or to a group of oscilloscopes. A prototype detector

was made but it was quickly realized that results from this device were

significantly convoluted so that a precise determination of the energy

impacted at a certain angle is almost impossible. Furthermore, this design

only allowed one measurement to be made during every shoot. This is very

inefficient since only a limited number of shoots can be made for this

project. For example, to properly characterize the debris, it is necessary to

make at least four shoots. That means, only a maximum of two sets of data can

be obtained using eight shoots.

A similar concept was developed which measured energy deposition as a

function of scattering angle also using strain gages. We came up with a new

46
and simpler idea. Individual catcher made of aluminum/steel rod of 1/4"

diameter with a larger circular head (3/8") were employed. On each rod, the

recoil energy was to be recorded using strain gages. During the impact

process, the data will be recorded on an array of oscilloscopes (four

oscilloscopes with two channels each). An experiment was conducted to test

the sensitivity of various strain gages in terms of response time and gage

output. In this experiment, strain gages were attached to aluminum rods. The

impact on the rod was simulated by the impact of a hammer. Results from the

study shows that the response time may be adequate. In this unit the energy

imparted to the sensor can be determined by measuring the response of a set of

strain gages mounted on the sensor rod. It is necessary to emphasis that the

initial projectile has speeds in excess of 5 km/s, however the debris cloud

formed by the initial impact is known to be more spread out in terms of time.

Furthermore, even though the debris that the strain gages measure travel at

high velocities, the signals from the gages are much slower in time. This is

because the speed of these signals corresponds to the natural frequency of the

entire recoil device. The response time was found to be in the order

milliseconds. This means that a normal strain gage with time resolution in

the kHz range is sufficient to resolve the debris energy.

Two types of mounting rods, steel and aluminum, were investigated.

These two materials provide different moduli, different yield strength and

different work hardening rates. Tests were conducted using different

activation processes which include varying forces and durations. Results from

the single strain gage in conjunction with a strain gage bridge were very

encouraging. The amplitude and the characteristics of the signals from the

unit were indeed a function of the impact loading. It was also apparent that

47
more complex solid state strain gages are not necessary. It was found that

the energy measurement devices could be used in two different modes: one based

on the elastic behavior of the rod and the other on the plastic deformation.

The former method requires a fast response of the sensor as well as the data

acquisition system. A set of strain gage amplifiers and an elght-channel data

recording unit were temporarily transferred from NASA for this purpose. The

plastic deformation method on the other hand needs no real time acquisition.

However, due to the none recovery deformation induced by the impact, each

sensor has a certain lifetime (in terms of number of impact). For the 1/4 _-

diameter aluminum rod, it was found that an impact energy of approximately 15J

resulted in a permanent strain of approximately 1600 micro-strain. Results

from the experiment on aluminum show that even in this soft material, a linear

response (and reproducible as well) can be obtained even up to I0 impacts

making this time-independent method very attractive. In addition, the

deformation response of the rod behaved linearly with energy as expected. A

set of experiments was also conducted using steel rods. Under similar impact

condition, the strain response of steel was approximately one quarter that of

aluminum.

Four types of strain gage configurations were tested: (i) four

orthogonally mounted and independent gages, (2) four orthogonally mounted

gages which formed an inherent bridge configuration, (3) two orthogonally

mounted and independent gages and (4) single strain gage. Each one of these

configurations yield different information. Of course configuration (i) would

yield the most information but also possesses the most experimental

complexity. In the performance tests, it was quickly found that if the energy

impacted on the sensor rods was radially symmetrical (that is there was no net

48
momenton the rod), then all configurations would yield the same information.

Results described in the last paragraph were obtained under this condition in

which case the simplest configuration (4) would be preferred. However,

realistic impact of the debris onto these rod would not be symmetrical in

which case bending would occur. In that more complex case, the configuration

(i) which consists of four orthogonally mounted gages with independent

monitoring is required. Results from off centered shots (asymmetrical impact)

confirm this requirement. This more complex configuration will be used in the

final device. In all case a pusedo full bridge configuration was used in the

strain gage setup. Three external high precision resistors were used to form

a full bridge configuration. This, in conjunction with the high gain

amplifiers, provides the sensitivity and flexibility needed for the device.

It is apparent that more complex solid state strain gages are not necessary.

Tests were conducted to determine the ideal method to extract the

information regarding energy density from these unit. Results show that even

for steel rods, unless the energy deposition is very low, the large portion of

the signal appears in the form of plastic deformation. Based on this

information, we decided to concentrate our effort on the determination of

energy density of monitoring the permanent strain induced in the sensor. A

prototype device consisting of five sensors made of 1/4" diameter steel rods

was designed and submitted for machining. A circuit was designed and built to

manually zero each individual strain gages.

Energy distribution can also be detected by measuring temperature

increase due to impact. Ultimately, energy deposited on a material would be

converted to heat. The feasibility of using an infrared thermographic system

to measure the surface temperature profile of composite materials under

49
ballistic impact was demonstrated. Studies have been conducted to

characterize the energy absorption mechanisms in composite systems during

impact. Infrared thermographic (IR) techniques have been used for nondestructive evaluation of composite materials. The contrast in the

thermograph arises from the difference in conductivity and specific heat

between the material and the defects. Cielo et.al, were able to distinguish a

0.5 mm thick Teflon sheet in a i0 mm thick graphite-epoxy composite. However,

the application of IR thermographic system in determining temperature profile

and heat deposition is still very limited. Roberts was able to determine the

temperature profile during low velocity impact. However, the profiles

obtained were not converted into readable temperature scale using appropriate

calibration. In this research, an infrared thermographic system was used to

measure the surface temperature profile of composite materials under impact

condition similar to that in the debris cloud.

The helium gas gun system was used to provide a velocity of 400 m/s for

a projectile of 5 grams in mass. Upon activating the fast acting valve, the

charge of helium gas in the gas reservoir propelled the projectile through the

barrel of the light gas gun. Composites panels were fabricated using three

types of reinforcing fibers (PE, Kevlar and graphite) in an epoxy matrix. The

matrix system was epoxy 507 and hardener 956 by Ciba-Geigy. The composite

panels were molded in a 216 mm square mold. Each panel was cured under 333 K

(60°C) for an hour in the compression molding machine. A ram force

corresponding to a pressure of about 0.03 MPa was used. The panels were then

cut into lOOmmx20Omm specimens.

The infrared (IR) thermographic system is shown schematically in figure

31. The temperature distribution was monitored using an IR thermographic

50
scanner by Inframatrics. The scanner was directed at the center of the sample

holder in the gas gun system, as shown in figure 32. Temperature distribution

of a line across the point of impact was measured. The temperature

distribution data acquired using a Hewlett Packard model 320 computer. The

temperatures were measured every 0.25 s until the heat deposited in the

composite had completely dissipated. The infrared system was also capable of

measuring temperature distribution over an area of the target. This feature

would be necessary in the case of non-symmetric sample or impact

configurations. However since all the experiments in this present study

possessed cylindrical symmetry, the simpler line scan technique was found to

be adequate. The operating wavelength of the IR system was I0 pm, which

provided the optimum sensitivity at near ambient temperature.

Both baseline scans and calibrations were performed before the

experiments were conducted. In the baseline scan, the gas reservoir was

discharged without a projectile. The thermographic data were taken for each

type of composites. The difference between the actual data upon shooting and

the baseline is then the effective temperature profile. All the calculations

of heat deposition were based upon the normalized effective temperature

profiles.

Calibration was conducted to correlate the intensity of the IR

thermographic signals to temperature. As the emissivities of the composites

were different, calibrations were performed for all three types of composites.

A thermocouple was used to measure the temperature of the composites during

calibration. At above room temperature, the samples were heated slowly on a

hot plate and readings (both thermocouple and IR intensity) were measured

during the heating and cooling processes. For calibration below the ambient

51
temperature, the samples were first chilled in a refrigerator and the readings

were taken during warming. The calibration curve for Spectra-900, Kevlar-49

and graphite composites are shown in figures 33a to 33c respectively. The

calibrations were fitted to a linear function and the slope and intercept for

the three materials are given in their respective graphs.

Figure 34a shows the typical raw data obtained from the IR thermographlc

system. All the data were converted using the calibration curves to yield the

temperature profiles such as one shown in figure 34b. Note that the "nolse n

in the curve is a result of difference in thermal conductivity between the

fiber and the matrix. The typical error of temperature data is ±I°C.

The expected variation of temperature with time is shown schematically

in figure 35. From to to tl, or prior to the impact event, the material was

at equilibrium at room temperature. The process between tI to t2 was due to

cooling of the target material by the gas in front of the projectile.

Beginning from t2, the conduction heating from bulk to surface occurred until

t3 where the maximum temperature was reached. The temperature then decreased

as a result of heat dissipation. Since the length of the projectile was 5.1

cm and its speed was 260 m/s, the entire penetration process took 0.16 ms.

The IR scan was incapable of acquiring data at this rate. The maximum

temperature in the temperature versus time plot occurs at approximately 2.5

seconds after the penetration process. This maximum is therefore due to a

dynamic balance between the gas cooling effect and the heat conduction from

the bulk to the surface.

The temperature versus distance profile across the surface of the sample

through the impact point at different times for Kevlar-49 composite impacted

by a blunt projectile is shown in figure 36. In figure 36a the sample at

52
equilibrium was cooled by the gas discharged from the gun barrel. This

process corresponds to the interval between tI to t2 as shown in figure 35.

During conduction heating from bulk to surface, the measured peak temperature

increased, as shown in figure 36b. This process corresponds to t2 through t3

in figure 35. Upon heat dissipation, the peak became broader and lower, as

shown in figure 36c. This corresponds to the dissipation process from t3 to

t4 in figure 35.

The surface of the target materials was cooled by the gas stream during

impact. Background temperature profiles were obtained by discharging the

helium gas without a projectile. Figure 37a shows the background temperature

curves for Spectra PE composite at different times. The effective temperature

curve could be obtained by superimposing the background on the raw data, as

shown in figures 37b to 37d. By subtracting the two curves, the effective

temperature profiles can be determined. Figure 38 shows the effective

temperature profiles at 1.25 s into the impact process for PE, Kevlar and

graphite composites respectively with the gas cooling effect subtracted out.

An experiment was also conducted with the IR aimed at the back side of the

target (where the projectile exited the target) to examine the difference in

the temperature distribution of the two surfaces. The initial cooling of the

target by the discharging gas stream prior to impact of the projectile was

found to be smaller on the back surface. However, when the background due to

this gas cooling was subtracted from the raw signals, the temperature profiles

for the two surfaces were very similar. This indicates that the temperature

gradient in the thickness direction is unimportant and supports the validity

of employing only one IR scanner to monitor the temperature distribution.

53
INFRARED THERMOGRAPHIC SYSTEM

VIDEO TAPE RE_DER

I-I

, 'J CONTROL I

ELECTRONI(

SCANNER CA!

1%/ MONITOR

m

SCANNER BA_'ERY

OR POWER SUPPLY

COMPUTER

Figure 31. The experimental configuration of the infrared thermographic

system for monitoring temperature distribution during impact.

54
ENTRANCE

COIL_ EXIT COl/L/

SCANNER _

SAMPLE

Figure 32. The position of the IR scanner with respect to the gun and the

target.

55
(a) CALIBRATION FOR PE COMPOSITE

300 T=A+BI f

,-. A=289.3 ^/._-

B=0.059:5 o

290

| J

0 1O0 200

INTENSITY (rnV)

(b) CALIBRATION FOR KV COMPOSITE

300

:=¢

la.I

n,-

"' 290 G.

_E

LLI

I'-

T=A+BI

A=28B.9 _ -

B=O_

/o °

I I |

0 1O0 200

INTENSITY (mY)

(C) CALIBRATION FOR GR COMPOSITE

'_" 300 T=A+BI .g.'o

,-." A=287.1 ...._-

290

i,-

| ,. i , I

0 I O0 200

INTENSITY (mV)

Figure 33. Intensity-temperature calibration for (a) 20-layered Spectra-900

PE composite, (b) 20-1ayered Kevlar-49 composite and (c) 20-

layered graphite composite.

56
240

>

E

160

>-

I--

(/)

Z

,., 80

I--

Z

0

(a) RAW DATA AT 1.25 S.

I !

, i i i ,

0 80 160 240

DISTANCE (PIXELS)

v

LLJ

12C

I--

W

O300290

(b) CONVERTED RESULT

l

, I . i *

0 40 80 120

DISTANCE (mm)

Figure 34. Conversion of (a) raw data in unit of intensity into (b)

temperature-distance curve. This set is taken from a PE composite

at 1.25 second.

57
_, 300

2

290

i--

280

TIME (s)

Figure 35. The temperature history showing Kevlar-49 composite impacted by

blunt proj ectiles.

58
(a) COOLING BY GAS, tl-f 2

V

'" 300 iv"

D

I--

290

LIJ

I--

0 120

0 $.

0.25 s.

I * I

40 80

v

V

'" 300

:D

I---

laJ

=- 290

_E

laJ

I--

v

V

"' 300

:D

I---

laJ

a. 290

_J

I--

(b) IMPACT HEATING, t2-I 3

i

1.25 s.

f_

0.75 s.

i ! • I

0 40 80 120

(c) HEAT DISSIPATION, t3-t 4

i

3.75 s.

_;-._/20 s.

! , |

0 40 80 120

DISTANCE (rnm)

Figure 36. Variation of temperature with time for Kevlar composite impacted

by a blunt projectile. (a) initial cooling by air, (b) impact

heating and (c) heat dissipation.

59
(a) BACKGROUND FOR PE COMPOSITE (b) PE COMPOSITE

v

V

"' 300

a::

:D

L_

" 290

_E

LIJ

I--.

el_ile_. 5 s" .,_

l_,_,,.,,_,,.,._,._7_ ......

I__-_ 2.s ,._}/

0.25 s.

v

V

"' 300

n,.

::3

a.. 290

=E

i.i

I-.-

I •

• I I ,

0 '40 80 120 0 40 80 120

DISTANCE (rnm) DISTANCE (ram)

v

V

"' 300

:3

I---

ILl

a. 290

i.i

I--

(c) KV COMPOSITE

v

V

'" 300

:3

i=-

n,,

a. 290

_E

i--

(d) GR COMPOSITE

i . | . 1 . •

O 40 80 120 0 40 80 120

DISTANCE (ram) DISTANCE (rnm)

Figure 37. (a) The background curve showing gas cooling for a PE composite.

(b), (c) and (d) are overlaid curves for the temperature profiles

and the corresponding background for PE, Kevlar and graphite

composites respectively at a time of 1.25 second.

60
(a) PE, BACKGROUND SUBTRACTED

i

3(

'" 300 rt,,

:D

I--

,,,(

tY

W

n 290

i,i

I--

]

• i

O 40 80 120

(b) KV, BACKGROUND SUBTRACTED

V

'" 300

rv

:D

I---

la.I

a. 290

_E

I.=J

I--

0

, I = I

40 80 120

(C) GR, BACKGROUND SUBTRACTED

v

V

"' 300

rv,

i-.

lad

a. 290

_E

L,J

i---

0

,'a

• _ ' _ ,4, ;.,,. .,.'7_',,,,,,;,_<' _S'v_ ,<I ,,',("";'_,'.', .",'i'.

i

i I I

40 BO 120

DISTANCE (mm)

Figure 38. Effective temperature profiles with the background subtracted for

(a) PE Spectra-900, (b) Kevlar and (c) graphite composites

respectively.

61
CHARACTERIZATION OF ENERGY LOSS _N ADVANCED MATERIALS FOR SECONDARY PROTECTIO_

It has been demonstrated that energy can be effectively dissipated in

aumented configurations using advanced composites. The objective of this

study is to determine the energy dissipation processes in polymer-matrix

composites during impact conditions similar to that experienced by the

pressure wall due to the debris cloud. These processes include heat, fiber

deformation and breakage, matrix deformation and fracture and interfacial

delamination. In this study, experimental measurements were made, using

specialized specimen designs and test methods, to isolate the energy consumed

by each of these processes during impact in the ballistic range. Using these

experiments, relationships between material parameters and energy dissipation

were examined. Composites with the same matrix but reinforced with Kevlar, PE

and graphite fabric were included in this study. These fibers were selected

based on the differences in their intrinsic properties. Matrix cracking was

found to be one of the most important energy absorption mechanisms during

impact, especially in ductile samples such as Spectra-900 PE and Kevlar-49

reinforced polymer. On the contrary, delamination dominated the energy

dissipation in brittle composites such as graphite reinforced materials. The

contribution from frictional forces was also investigated and the energy

partitioning among the different processes evaluated.

Three types of such materials were selected for this research: PE

Spectra-900 for its high strength, high ductility, low modulus and low

specific gravity; Kevlar-49 fiber for its high strength, medium ductility and

specific gravity and low modulus; and graphite for its high tensile modulus

and low ductility. Only one resin material (Epoxy 507 and hardener 956 by

Ciba-Geigy) was used because of its availability and ease of processing. The

62
diameter of the PE fibers was 38 _m, that of Kevlar was 12 _m and that of

graphite was 7 _m. The numbers of fibers per strand were 118, 768 and 3000 in

PE, Kevlar and graphite respectively.

All the composite panels were molded in a 216 mm square mold. Each

panel was cured under 333 K (60°C) for an hour in the compression molding

machine. A ram force, corresponding to a pressure of about 0.03 MPa, was

used. The panels were then cut into 100m x 200mm specimens. In the

experiments to study the effect of delamination, a stacked single layer

geometry was used to simulate a delaminated composite structure. In this

case, single layer sheets were made by inserting polypropylene sheets in

between layers. The panels were then separated into individual composite

sheets. The difference in total energy absorption between the composite

panels and the corresponding separate stacked sheets was assumed to be the

contribution from the delamination process.

Samples for fiber tests were made by winding each end of the fiber

strand around an aluminum tab. Adhesives were applied along the fiber strands

on the aluminum tabs. Care was taken to ensure the gauge length was 63.5 nun

(2.5 inches). It was found that fiber samples prepared in this manner

provided less slippage and hence more accurate gauge length than other sample

configurations. In these fiber experiments, a special projectile tip was used

to eliminate slippage of the projectile in between yarns. The number of

fractured fiber strands were evaluated by a post-mortem examination of the

samples. The energy absorbed by the fibers was calculated by multiplying the

energy absorbed by an individual strand by the number of broken fibers.

Frictional forces between the projectile and the material is the source

of energy dissipation after the target has been perforated. This contribution

63
is usually a secondary effect. Nevertheless, its contribution is not

negligible in elastic materials. A sample holder bracket was designed to hold

the specimen in exactly the same position. The energy loss of the projectile

passing through the same hole a second time is directly related to this

frictional energy. The data obtained from this test were used to compare with

the frictional loss measured from the original tests. The contribution of

friction to the total energy absorption of each composite system was then

evaluated.

The volume fractions of the fiber in the composites were determined

using quantitative optical microscopy. The fiber volume fractions for

graphite and Kevlar-49 composites were found to be 54.7_ and 55.4_

respectively. Because polyethylene fiber has a much higher ductility than the

matrix system, the polished surface revealed only broken fiber ends without

any matrix. Consequently quantitative microstructural analysis was not an

appropriate method of determining the volume fraction of PE fiber in the

composite. Since all the composite panels were molded with the same pressure

and all the fibers used were in the same (plane weave) form, it is reasonable

to assume the fiber volume fraction for PE composite to be 55_. Density

measurements of all the composites (including PE composites) were also made

and their results agreed well with the optical measurements.

In all the tests, a steel projectile tip was used for the projectile to

ensure minimal plastic deformation so that energy loss measured can be

attributed to target deformation. A conical shape with an angle of 50 ° was

used for the tip. A projectile velocity of 280 m/s was used in all the tests.

The most important issue to be addressed is the total energy absorption

of the composites during the penetration process. The total energy absorption

64
qualifies a composite material for ballistic impact applications. The major

energy absorption mechanisms include fiber deformation and breakage, matrix

cracking, delamination process and frictional loss. The contribution from

each of these mechanisms was investigated and determined quantitatively.

Figures 43, 44 and 45 show the energy profiles of PE, Kelvar and

graphite composites and are representative of all the materials examined.

Each figure contains data from two shots, denoted by open and filled circles

respectively. As shown in these figures, total energy loss values obtained

from these experiments are very consistent between shots. It is important to

note that the data in figures 43 to 45 represent the positional dependence

energy loss as the projectiles traverse through the targets. The range at

which energy is lost occurs over a longer distance than the thickness of the

target. This is because the energy dissipation process begins at the point of

contact and ends when the projectile exists material. The total energy loss

process can be divided into two stages. The initial rapid energy loss is due

to the fracture of the target. This starts at the moment of contact and

finishes when a hole is formed. This is followed by a more gradual loss due

to friction. Polyethylene reinforced composites exhibited a smoother behavior

than graphite composites because the total energy loss in the former is

larger. With the same absolute error value, the percentage error in the PE

composite is proportionally smaller than the graphite counterpart.

Figure 46 summarizes the data obtained from Spectra-900 PE, Kevlar-49

and graphite composites respectively, each composed of i0, 20 and 30 layers.

The total energy absorption was found to be approximately proportional to the

thickness of the composite (or the number of layers). This implies that the

energy loss rate is independent of the thickness of the target, at least

65
within the range of thicknesses investigated. It appears that this linear

relationship applies regardless of the fiber properties, although graphite is

a high strength but brittle fiber, whereas Kevlar and PE have lower moduli but

higher ductility. Figures 47a and 47b show the data normalized to sample

thickness and weight instead of the simple absolute energy loss values. The

Spectra-900 PE composites possess the best energy absorption density. This is

because PE fiber has the lowest specific weight and highest ductility.

Nevertheless, the full potential of the ductility of PE and Kevlar fibers was

not realized due to the restriction imposed by the matrix. This restriction

limits the deformation of the ductile PE and Kevlar fibers to regions in the

vicinity of the impact. The damaged region could be expanded by using

unidirectional fiber reinforced composites. The stress wave will travel over

a greater region in this type of composites. However, the degree of fiber

breakage will decrease because the shear force acting on the fiber by the

projectile is lower.

The energy absorption process can be divided into two parts, namely

energy of penetration and frictional energy. The energy of penetration

includes contributions from fiber deformation and breakage, matrix cracking,

delamination and debonding. The average penetrating force can be determined

from the slope of the energy-position curve. The average force required to

penetrate the composites increases with increasing target thickness as shown

in Figure 48. There is a near linear relationship between the penetrating

force and sample thickness for graphite composite, but the PE and Kevlar

composites exhibit a positive deviation from linearity. The average force of

penetration can be simulated by a parallel array of sprlng-dashpot systems.

The single-layered composite can be represented by a model of spring constant,

66
k, and dashpot constant (damping coefficient), c. The equation of motion can

be written as cx+kx-F I where x is the displacement vector and x the velocity

vector. Hence, the force required to penetrate n layers, Fn, can be written

as n(cx+kx)-F n. Combining the two equations, we have Fn-nF I. This Maxwell

model predicts a linear relationship between the average force of penetration

and the number of layers. For a brittle composite such as graphite, the

plastic deformation is negligible, the elastic model thus accurately predict

the behavior of the material. However, for ductile materials such as Spectra900 PE and Kevlar-49, the plastic deformation is not negligible. The damping

coefficient becomes progressively higher in thicker samples resulting in a

positive deviation from linearity in the force-thickness relationship observed

in these materials.

A series of experiments were conducted to determine the intrinsic

properties of the fibers used in this study in order to determine the fiber

contribution to energy absorption. The samples for all the fiber tests were

prepared in the same manner to ensure consistent testing conditions. The

number of strands in each sample were 20, 15 and I0 for graphite, Kevlar-49

and Spectra-900 PE fibers respectively. The number of fiber strands used in

this experiment was the maximum amount of fibers that the projectile could

interact with. The fiber strands were twisted 2 turns/cm (5 turns/inch) to

form a bundle. This sample configuration helped prevent slippage during

impact. Only those samples in which all fibers broke at the center were

included in the analyses. Data from samples with fracture away from the gauge

length or partially broke were not included. A projectile velocity of 280 m/s

was used.

The energy loss values for Spectra-900 PE, Kevlar and graphite fibers

67
under this high velocity test condition were 0.7, 0.7 and 0.4 J/strand

respectively. Normalizing these values to the volume of the fiber yielded

energy absorption densities of 82.4, 127 and 54.5 J/cm 3 for the three

respective fibers. These values are related to the toughness of the fibers

which in turn is proportional (upper limit) to the strength-ductility product

of the materials. According to the manufacturers' specifications, the

toughness values of the three fibers of interest are 90.6 J/cm 3 for PE, 78.4

J/cm 3 for Kevlar and 37.2 J/cm 3 for graphite. A direct comparison of these

calculated toughness values with the energy absorption densities shows that

the Kevlar and graphite fibers are capable of absorbing more energy under the

impact condition than expected. An inspection of the broken fibers reveals

that the elongation observed is higher than expected. The elongation was

about 5_ in the Kevlar fibers and about 2_ in graphite implying a strain rate

sensitivity of the deformation process.

The total energy absorption of the fibers can be calculated by

multiplying the measured energy density values by the volume of the fibers in

the fracture zones of the composites. The average size of the fracture zone

was 9.53 nun. The thickness of 20-1ayer thick composites was 8.4 mm, 5.5 mm

and 5.4 mm for PE, Kevlar and graphite composites respectively. Using the

volume fraction of the fibers and the thickness of the 20olayer composite, the

energies consumed to damage the fibers within the fracture zone were

determined to be 23.7 J for PE, 23.9 J for Kevlar and i0.i J for graphite.

The contributions from fiber energy in the total energy absorption of the PE,

Kevlar and graphite composites were 35_, 58_ and 19_ respectively.

In this analysis, the energy dissipated by fiber pull-out (which is

affected by fiber size) and the energy dissipated by those fibers that

68
deformed outside the fracture zone but not break were ignored. The implies

that the analysis underestimate the total amount of energy dissipated. To

successfully isolate the contribution from fiber pull-out, experiments using

controlled interfaces and different fiber sizes are needed. This is beyond

the scope of the present investigation. Post-mortem examination of the

fracture zones in all three composites reveals that most of the fibers that

participate in the energy absorption process are broken. This indicates that

under ballistic impact condition, the energy dissipated in straining the

fibers that do not break is insignificant.

Initially, the energy loss due to matrix cracking was studied using pure

resin materials (without fibers). It was determined that the damage

characteristics from this unreinforced material did not resemble those

observed in composites. The extent of matrix cracking in a composite is

different from that in the pure resin. Hence, the energy absorbed for matrix

cracking depends on fiber reinforcement and is not the same in the three

composite systems. A new test methodology was then developed to measured the

fiber contribution using composite samples with different volume fractions of

matrix material. This was accomplished by fabricating and testing composites

with identical amount of fabric but different amounts of resin. The amount of

excess resin was small enough so that the flexural rigidity of the composites

was not altered in a significant manner. Effort was concentrated on the PE,

Kevlar and graphite composites each with 20 layers of fabric. One set of

samples were fabricated in a normal fashion whereas excess resin was

intentionally added to the thicker samples. Care was taken to introduce the

excess resin uniformly in between layers. The thick samples have a higher

volume fraction of matrix than the standard samples have. The difference in

69
energy absorption between the two is then the contribution from the matrix and

the results were normalized to reflect only the matrix contribution. The

graphite, Kevlar and PE materials have normalized matrix cracking energies of

56, 66 and 71 J/cm of resin respectively. Using the fracture zone morphology

of the three types of samples, these energies correspond to energy absorption

densities of 78.5, 92.5 and 99.5 J/cm 3 respectively for graphite, Kevlar and

PE composites. For comparison, the energy absorption density of pure resin

(without fibers) was found to be 21 J/cm s. This implies that the presence of

fiber strongly enhance the energy absorption capability of the resin due to a

complete change in the damage morphology in the composite structure as opposed

to a homogeneous resin system.

The energy of matrix cracking in a composite can be evaluated from

multiplying the normalized matrix cracking energy by the volume of the resin

in the composites. This requires evaluating the volume fractions of the

fibers and the matrix. Using the approach similar to that given in

determining fiber volume fraction of the composites, the equivalent resin

thickness for 20-1ayer thick composites were determined. The energy

absorption of resin was then calculated to be 26.8 J for PE, 16.4 J for Kevlar

and 13.5 J for graphite composites. The contributions from matrix cracking in

total energy loss were 40%, 40% and 25% for PE, Kevlar and graphite composites

respectively. However, the increase in energy absorption as a result of

thicker resin cannot be completely attributed to matrix cracking. The effect

of higher frictional loss in the thicker samples must also be taken into

consideration. The frictional partitions in total energy absorption for 20-

layer thick composites were determined to be 16.4%, 11.1% and 19.6% for

Spectra-900 PE, Kevlar-49 and graphite composites respectively. This

70
frictional contribution will be discussed later in the paper. Thus, the

contributions from matrix cracking, excluding frictional component, were 33%,

35% and 20% for Spectra-900 PE, Kevlar and graphite composites respectively.

The effect of delamination was investigated by comparing the response of

normally fabricated composites where delamination can occur with materials

where delamination was eliminated from the energy absorption process. The

latter was achieved by stacking individual layers of the composite sheets

together, which simulated the delaminated structure of the composites. Data

from these experiments are shown in Figure 49. The difference between the

energy absorbed by the two configurations yields the energy consumed by

delamination. The effects of delamination appear to be minor in the Kevlar

and PE composites, whereas the delaminated graphite-based materials absorb

less energy than their composite counterparts. The stacked composite sheets

were found to be thicker than the composite panels of the same number of

layers. Since both types of samples used had the same number of fibers, the

difference in thickness could be attributed to the difference in resin

thickness. The energy loss of the delaminated samples can then be normalized

by subtracting the contribution of the excess matrix energy from the total

energy loss. The data listed in Table 4 are the corrected delamination

energies. These values suggest that the delamination process is the governing

energy absorption mechanism for graphite composites. This accounts for 62% in

total energy loss for 20-1ayer thick graphite composite and over 46% in the

30-1ayer material. The PE reinforced composites exhibited the least energy

loss by this mechanism (5.1% in 30 layers to 1.5% in 20 layers). The -3J

observed in the lO-layer PE is due to a combination of the low energy

absorption by delamination and systematic error of the measurement (±3J). The

71
total delaminatlon area can be evaluated by dividing the normalized

delamination energy by the shear-mode fracture toughness, Giic, of the

composites. These values correspond to 0.15 J/cm 2 for graphite composites,

0.09 J/cm 2 for Kevlar and 0.014 J/cm 2 for PE. The total delamlnatlon area was

found to be 243 cm 2, 231 cm z and 357 cm 2 for 30-1ayer thick graphite, Kevlar49 and Spectra-900 PE composites respectively. Hence, the average diameter of

delamination, assuming a circular shape, was 3.3 cm, 3.2 cm and 3.9 cm for

graphite, Kevlar-49 and Spectra-900 PE composites respectively. These

delamination sizes agree with the damage zone observed in the impacted

materials. Results from these delamlnatlon energy experiments suggest that

the delaminatlon process is the most important mechanism for graphite

composite followed by Kevlar and PE composites. This is due to the low energy

dissipation factor in the inplane shear mode in the PE rather than the ability

of the material to generate delamination. It is controversial whether

delamlnation is the governing mechanism of a composite under impact

penetration conditions. Some researchers have suggested that strain energy at

failure of the fibers is the controlling factor of the impact response of

composites, whereas Wrzesian remarked that, UPanels with good penetration

resistance were heavily delamlnated, indicating that a considerable amount of

energy was absorbed by the delamination n . Results from this research suggest

that the effectiveness of delamlnatlon as a means to dissipate energy is a

function of the fundamental properties of the composites, such as the

ductilities of the fibers and the matrix. Wrzesian's position is therefore

valid only for graphite composites and not for the other two systems.

The contribution from frictional loss during high velocity impact was

determined from the frictional component of the energy-position curves. The

72
frictional process starts with the point of perforation and ends with the

complete passage of the projectile. The energy loss during this interval was

taken as the frictional loss.

The frictional loss is proportional to the normal force and the

coefficient of friction between the two contacting materials. The frictional

energy loss in each type of material was thus quite different. Since graphite

composites fracture by fragmentation, the area of contact between the two

materials is very limited and unstable, especially for thinner samples.

Postmortem examination of the graphite samples reveals that part of the target

material forms debris and disintegrates upon impact. Spectra-900 PE

composites, on the other hand, fracture in a mode involving the growth of a

ductile zone. This ensures a constant contact between the target and the

projectile materials. As a result, a well behaved trend in frictional

behavior was observed in Spectra-900 PE composites, whereas greater

fluctuations were encountered in the graphite materials.

The contributions from frictional energy in each type of material are

listed in Table 5. Results from these experiments reveal that the frictional

component accounts for 20-21% of the energy absorption in graphite composite,

11% in Kevlar-49 composite and 14-16% in Spectra-900 PE composite.

Table 6 summaries the fractional contributions from each of the four

different energy dissipation processes determined from the experiments

conducted in this study for the 20-1ayer and the 30-1ayer composites. The

delamination process was found to be the most important mechanism in graphite

composites. For Kevlar-49 and Spectra-900 PE composites, the fiber breakage

and matrix cracking are the governing energy absorption mechanisms. The

experimental error in measuring the energy loss is ±3.5 J. The cumulative

73
error range in summing up the four mechanisms is therefore ±14 J. The error

in estimating the energy contributions thus decreases with increasing total

energy. The methodology developed in this study of partitioning the energy

absorption in a composite system is very useful. The relative contribution

from each energy absorbing mechanism can be determined. Higher precision can

be achieved by improving the sensor and/or the sensitivity of the microvelocity sensor.

The total energy loss can be written as

E - Ezlb, r + E_trl = + Ed.l.lnati_ + Efzlcti _ [13]

As a result of the present findings, it is now possible to quantify these

energy loss processes. The sum of the four individual contributions does not

equal to unity since the fractional contributions were determined by

independent experiments. The deviation from unity arises from the intrinsic

error associated with the experiments and the mutual interaction between the

various components that the study is unable to account for. According to

Table 6, the sum of the individual energy absorption factors ranges from 87%

to 124% of the total energy loss measured. These values are within the

systematic error of the measurement scheme.

It is impossible at this stage to analyze the energy partitioning

behavior from a first principle approach. Nevertheless several

characteristics are evident from the data obtained in this study. It appears

that in brittle composites (graphite), the single most dominating energy

absorption process is delamination. However, this is due to the large energy

release density and not because of the ability of the graphite composite to

delaminate. On the contrary, composites reinforced with ductile fibers tend

to dissipate energy via fiber deformation and matrix cracking as expected. In

74
addition, generation of friction during the passage of the projectile also

contributes significantly to the energy absorption process in all three

composites.

In summary, energy loss during the penetration of ballistic projectiles

was experimentally measured for composites with different reinforcements

(including Spectra-900 PE, Kevlar-49 and graphite). The partitioning of this

energy among four distinctive modes was determined. These modes include fiber

deformation and breakage, matrix fracture, delamination and friction. Results

indicate that the energy partitioning behavior is a complex and interactive

function between the fibers and the matrix. A simple correlation of the

toughness of the individual components (fiber and matrix) does not exist for

the composite structure. Delamination plays an important role in brittle

composites as a result of the large energy release from such a process. For

ductile materials, the contributions from the fibers and the matrix are more

important. This information can be incorporated to aid the design of more

effective aumented bumpers.

Table 4.

I0 Layers

20 Layers

30 Layers

Corrected Delamination Energy

Kevlar-49

Composite

Graphite

Composite

12 J OJ

33J 8J

37 J 21 J

Spectra-900 PE

Composite

-3J

IJ

5 J

75
Table 5. Energy Absorbed by Friction in 20-Layer Composites

Graphite

Composite

Kevlar-49

Composite

Spectra-900 PE

Composite

19.6 % II.I % 16.4

Table 6. Summary of the Cont_ibution from each Mechanism to Total Energy Losq

2O

Graphite

Composite

Layers

Kevlar-49

Composite

58.3 %

Spectra-900 PE

Composite

Fiber 19.1% 35.4 %

Matrix 20.5 % 35.5 % 33.4 %

Delamination 62.3 % 19.5 % 1.5 %

Friction

Total

11.2 %

124 ± 36 %

19.6 %

121 ± 27 %

16.4 %

87 ± 21%

3O

Graphite

Composite

20.3 %

Friction

Total

myers

Kevlar-49

Composite

47.8 %

Spectra-900 PE

Composite

Fiber 35.7 %

Matrix 19.3 % 27.2 % 32.5 %

Delamination 46.3 % 28.4 % 5.1%

21.3 % 10.8 % 13.5 %

107 ± 17 % 114 ± 19 % I 87 ± 14 %

76
PE COMPOSITE, 20 LAYERS

80

CO

LM

-J 60

C)

O

40

CO

20

...J

o

CE

w -20

Z

LU

-400

-, J

D

I I I

5 10

POSITION (cm)

Figure 43. Energy loss versus position curve for a Spectra-900 PE composite.

77
KEVLAR COMPOSITE, 30 LAYERS

90

o0

w

._J

60

30

q

0

rr"

UJ

Z

0

w _300 I I , I 5 10

POSITION (cm)

Figure 44. Energy loss versus position curve for a Kevlar-49 composite.

78
GRAPHITE COMPOSITE, 20 LAYERS

O9

UJ

._1

O

-.)

V

O9

O9

O

.._1

>-

(.9

rr

uJ

Z

uJ

50

40

30

20

10

0

-10

_200 I

POSITION (cm)

10

Figure 45. Energy loss versus position curve for a graphite composite.

79
GRAPHITE KEVLAR-49 ._PECTRA-900

LAYERS LAYERS LAYERS

Figure 46. Total energy loss for PE, Kevlar-49 and graphite reinforced

composites as a function of number of layers when impacted by a

high velocity projectile.

8O
(a) TOTAL ENERGY LOSS

A

-3

v

80

O3

0

_..I

>" 4O (..9

n.-

U.I

Z

uJ I

0 40

G,R, , , , ,

10 20 30

NUMBER OF LAYERS

(b) NORMALIZED BY AREAL DENSITY

80

40

z 0 10 20 30 40

NUMBER OF LAYERS

Figure 47. Energy absorption characteristics of the three types of fiber

reinforced composites as a function of (a) number of layers and

(b) normalized to the areal density.

81
Z

uJ

O

rr

O

U_

Z

nI.U

Z

W

n

7

w

5-

4-

3-

2-

1-

I I I

o GR Composite

• KV Composite

v PE Composite

0 I I !

0 10 20 3O 40

NUMBER OF LAYERS

Figure 48. The average force of penetration calculated from the slope of the

energy loss curves for Spectra-900 PE, Kevlar-49 and graphite

composites.

82
GRAPHITE

140

03 120

1oo

.._1

8o

n"

LU 60

Z

UJ

_j 40

_<

O

I I I

o COMP.

-e SHEET

m

20

KEVLAR-49

140 , , ,

o COMP.

• SHEET

m

- 120

100

80

60

- 40

- 20

- 120

100

80

SPECTRA-900

140 , , , ,

_o COMP. _

• SHEET

6O

- 40

- 20

°o _'02'o3'040 °o 1'02'o3'040 °o _'02'03'040

LAYERS LAYERS LAYERS

Figure 49. A comparison of the total energy loss between the composites and

the stacked sheets configuration illustrating the contribution

from delamination.

83