NASA

Technical

Paper

2520

AVSCOM

Technical

Report

85-B-7

November 1985

Abrasion Behavior of Aluminum

and Composite Skin Coupons,

Stiffened Skins, and Stiffened

Panels Representative of

Transport Airplane Structures

Karen E. Jackson

NASA

Technical

Paper

2520

AVSCOM

Technical

Report

85-B-7

1985

Nabonal Aeronautqcs

and Space Adm_nmlratlon

Scientific and Technical

Information Branch

Abrasion Behavior of Aluminum

and Composite Skin Coupons,

Stiffened Skins, and Stiffened

Panels Representative of

Transport Airplane Structures

Karen E. Jackson

Aerostructures Directorate

USAAR TA-A VSCOM

Langley Research Center

Hampton, Virginia

Summary

A three-phase investigation was conducted to determine the friction and wear behavior of aluminum

and composite materials under conditions similar

to the loadings experienced by skin panels on the

underside of a transport airplane during an emergency belly landing. In the first set of experiments,

small skin coupons of aluminum and graphite-epoxy

(Gr-Ep) were abraded in the laboratory. An abrasion test apparatus was designed which used a standard belt sander to provide the sliding surface. The

test rig was equipped with a load cell to measure

the frictional forces developed during abrasion. The

skin-coupon specimens were abraded over a range of

pressures (2 to 5 psi), belt velocities (16 to 50 mph),

and belt surface textures (0.01 to 0.02 in.). The parameters chosen fall within the range of conditions

considered typical of an airframe sliding on a runway

surface. The effects of pressure and velocity on the

wear rate and coefficient of dynamic friction were determined, and comparisons were made between the

Gr-Ep and aluminum. Results of the laboratory tests

indicate that Gr-Ep skin coupons have wear rates

four to five times higher than aluminum and a coefficient of friction of about half that of aluminum.

The second phase of the investigation involved

abrading more representative skin structures, consisting of I-beams with attached skins constructed

of aluminum, Gr-Ep, and glass hybrid composite.

These stiffened skins were abraded on an actual runway surface over the same range of pressures and

velocities as in the laboratory skin-coupon tests.

While the trends in the wear and friction behavior

of the stiffened skins on runway surface were substantially the same as those observed in the skincoupon tests, the magnitude of the wear rate decreased considerably for the Gr-Ep material. The

coefficient-of-friction data for the two tests were in

good agreement.

In the third phase of the investigation, large

Gr-Ep stiffened panels which closely resembled the

structure of a transport fuselage skin section were

abraded oil a runway surface at a pressure of 2.0 psi

over a range of velocities. The data from these tests

tended to correlate the stiffened-skin results.

Introduction

Friction and wear behavior of fuselage skins can

be an important consideration in the design of transport aircraft, especially in the event of an emergency

sliding (belly) landing on a runway surface. A review of the National Transportation Safety Board

(NTSB) accident records and the Federal Aviation

Administration (FAA) difficulty reports show that

21 accidents or incidents involving interference between the main landing gear tire and door have been

reported since 1965, 10 of which resulted in gear-up

landings (ref. 1). Reference 2 puts the number of

emergency sliding landings at roughly a dozen such

incidents involving transport aircraft ill the last 5

years. Typically, these aircraft slide 4000 to 5000 ft

with touchdown velocities of approximately 140 mph.

Resulting abrasion damage to the aircraft fuselage is

often quite substantial.

Composite materials are currently being used for

secondary structural components and are being considered for use as primary structural components of

transport aircraft. The trend in the aircraft industry

towards increased application of composite materials raises the question of how these materials would

behave under the conditions of a belly landing as

compared with current aluminum construction.

This paper describes a three-phase investigation

to study the friction and wear behavior of composite

materials and aluminum under abrasive loading conditions similar to those experienced on the underside

of a transport airplane during a belly landing. In

the first phase (ref. 3), small skin-coupon specimens

of aluminum and various advanced composite materials, including graphite-epoxy (Gr-Ep), Kevlar, and

toughened-resin composites were abraded in the laboratory using a standard belt sander to provide the

sliding abrasive surface to simulate a runway. The

aluminum and composite skin coupons were abraded

over a range of pressures (2 to 5 psi), belt velocities (16 to 50 mph), and belt surface textures (0.01

to 0.02 in.). The parameters chosen fall within the

range of conditions considered typical of an airframe

sliding on a runway surface. The effects of these

test variables on the wear rate and coefficient of friction were determined, and comparisons were made

between the aluminum and composite materials.

The second phase of the investigation (ref. 4) involved testing more representative skin structures

on an actual runway. The test specimens consisted

of I-beams with attached skins constructed of aluminum, graphite-epoxy, and glass hybrid composite. These stiffened skin specimens were abraded

on the Langley Air Force Base north-south runway

over the same range of pressures and velocities as

in the laboratory skin-coupon tests. The effects of

pressure and velocity on wear and friction behavior

were determined for the stiffened skins and comparisons were made between the aluminum and composite materials.

In the final phase of the investigation, three large

stiffened panels constructed of graphite-epoxy composite material were abraded on a runway surface at a

pressure of 2.0 psi over the same range of velocities as
in previoustests.Thestiffened-panel test specimen

mostcloselyresembledthe construction of a transport fllselage skin section, and these tests were performed to correlate the results from the skin-coupon

tests and the stiffened-skin tests.

This paper presents results from each phase of the

project and shows comparisons between the friction

and wear behavior of the aluminum and graphiteepoxy composite material. The paper is limited to

graphite-epoxy simply because that particular composite material was tested in each phase of the investigation. More complete information oil the first two

phases of the project, which includes other material

systems and data from temperature-time histories,

may be found in references 3 and 4.

Test Apparatus and Procedures

Phase I--Skin-Coupon Tests

Specimens. A schematic of a typical skin-coupon

test specimen is shown in figure 1. Thicknesses of

the skin coupons varied, depending on the material,

and ranged from 0.20 to 0.30 in. A 45 ° chamfer

on the front edge of the specimen helped to smooth

the initial contact of the specimen to the abrading

surface. Figure 1 also lists the types of aluminum

and composite materials tested, giving the lay-up of

each of the composite skin-coupon specimens. Aluminum 7075-T76 is a readily available stock aluminum. The T300/52081 is a standard commercial

graphite-epoxy composite in wide use today. Kevlar

49/9342 is a popular aramid-epoxy composite, also

commercially available. Three additional graphiteepoxy materials (T300/BP907, T300/Fibredux 920,

and T300/Ciba 4 a) chosen for testing are toughenedresin composites. As mentioned previously, although

several composite-material systems other than Gr-Ep

were tested, this report presents only the T300/5208

Gr-Ep and aluminum data for comparison with the

stiffened-skin and stiffened-panel tests.

Apparatus. The apparatus used to perform the

skin-coupon tests is shown in figures 2 and 3. A

belt sander fitted with a 6-in. by 48-in. aluminum

1 Thornel 300 (T300) graphite fiber is manufactured by

Union Carbide Corporation; 5208 epoxy resin is manufactured

by Narmco Materials, a subsidiary of Celanese Corporation.

2 Kevlar 49 aramid fiber is manufactured by E. I. du Pont

de Nemours & Co., Ine; 934 epoxy resin is manufactured by

Fiberite Corporation.

3 BP-907 epoxy resin is manufactured by American Cyanamid Corporation; Fibredux 920 and Ciba 4 epoxy resins

are manufactured by Ciba Geigy Co. Ciba 4 is a specially

prepared epoxy resin not available commercially.

2

oxide belt provided the sliding, abrasive surface.

Skin-coui)on test specimens were held in place by

a specimen holder which was attached to the belt

sander by a parallelogram arrangement of mechanical

linkages. The linkages were pivoted about a back

upright such that the specimen holder could be raised

and lowered parallel to the abrading surface. The

skin coupons fitted into a recess in the specimen

holder and were held securely in place by a vacuum

created behind tlw specimen by the vacuum pump

(fig. 2).

Figure 3 shows a detailed sketch of the test apparatus in the locked (upper position) and test (lower

position) ('onfigurations, where the upper position is

shown by dashed lines. The specimen holder remained perpendicular to the abrading surface because of the parallelogram linkage arrangement. As

the specimen wore, this arrangement kept the load

normal to tile abrading surface. Loads were applied

to the specimen by placing lead weights on a rod attached to the specimen holder. A counterweight was

used to offset any load applied to the specimen by

the weight of the linkages and the specimen holder.

Instrumentation. The test apparatus was instrumented with a load cell located in the lower linkage

arm (fig. 3). During abrasion testing the frictional

force developed between the skin coupon and the belt

produced a tensile force in the lower arm. The strain

induced by this tensile force was converted by the

load cell into an electrical signal which was amplified

and filtered through a 10-Hz low-pass filter. The signal was then fed to a two-channel strip-chart recorder

to provide a force data trace. This force measurement was used to calculate the friction and normal

forces from a static analysis of the specimen holder

given the applied load, the angle of inclination of the

linkage arms, and the holder dimensions.

Tile test apparatus was also instrumented with a

limit switch (fig. 2), The limit switch triggered an

event marker on the strip-chart recorder when the

test specimen was lowered to the abrading surface

at the start of a test run. When the run was

complete, the test specimen was raised, the limit

switch was released, and the event marker returned

to its original position. The test run time was then

determined by ineasuring the distance between the

two event marks on the strip-chart recorder and

dividing that value by the chart speed.

Parameters. Typical loading conditions on the

skin panels of a transport airplane during a belly

landing would fall in the pressure range of 2.0 to

5.0 psi. The effect of pressure on the friction and

wear behavior of the aluminum and composite skin
couponswas,therefore,determinedby performing

separatetestsat 2.0, 3.2,and 4.8psi. Thesetest

pressureswereachievedby placing5-,8-, and12-1b

leadweightson the rod abovethe specimenholder

normalto thetestspecimen.

Standard6-in. by 48-in.aluminumoxidebelts

with grit sizesrangingfrom No.36to No.60were

usedto simulaterunwaysurfaces.This rangeof

grit sizeswasselectedbasedonthe averagesurface

texturedepthsof thesebeltsasmeasuredwith the

greasesampletechnique(ref.5), whichhasevolved

as a methodof classifyingrunwaysurfaces.This

technique,illustratedin figure4, involvesmarking

a constantwidth on the surfaceto be testedand

spreadinga knownvolumeof greaseevenlywithin

the markedregion,filling thecrevicesandcovering

asmuchof the surfaceaspossible.The volumeof

greaseuseddividedby the surfaceareacoveredis

theaveragesurfacetexturedepth.

Figure5 (fromdatain ref. 5) showsthevarious

surfacetypesandclassesofrunwaysandtheaverage

surface-texturedepthsforrunwayswithineachclass.

The texturedepthrangeof 0.01to 0.02in., shown

astheshadedregionin figure5,isconsideredtypical

forrunwaysurfaces.Therefore,to simulatearunway

surface,abrasivebeltswithtexturedepthsinor near

this rangeweredesired;asshownin figure6, belts

havingsurfacetextureswithin or nearthe0.01-to

0.02-in.rangewerethosewithNo.36,No.40,No.50,

andNo.60gritsizes.Theeffectofvaryingthesurface

texturedepthontilefrictionandwearbehaviorofthe

skincouponswasinvestigatedin reference3. The

test resultsfor the skin couponspresentedin this

paperwereall performedonNo.36grit belts.

Typical touchdownvelocitiesof transport airplanesareapproximately140to 160mph.Thishigh

velocityrangewasunattainablewith themotordrive

systemof thebelt sander.However,by alteringthe

pulleyratios,a rangeof velocitieswasachievedfor

testing.Testsat 16.0,36.4,and52.0mphwereperformedonthealuminumandcompositeskin-coupon

specimensat a pressureof 3.2psiusinga No.36grit

belt.

Procedure. Prior to testing, all pertinent data

such as skin-coupon initial mass and test parameters

such as load, belt velocity, and belt surface-texture

depth (indicated by grit size) were recorded. Skin

coupons were abraded for approximately 5 seconds.

This length of time was sufficient to get an adequate

force data trace, yet short enough to prevent clogging of the belt with debris. Following the test, the

skin-coupon mass was measured and recorded. The

specimen holder and linkage assembly was then adjusted laterally to allow for another test run with a

new specimen on an unused track of the belt. In this

manner, three abrasion tests were performed per belt

with each test being run on a new belt surface.

Phase II--Stiffened-Skin Tests

Specimens. Figure 7 shows an aluminum specimen and a graphite-epoxy I-beam, stiffened-skin

specimen with their typical dimensions, the specific

skin materials, and, in the case of the Gr-Ep, the ply

lay-up. The composite specimens were all fabricated

with a ski front (fig. 7). The purpose of the ski was to

help smooth the initial contact of the specimen with

the runway surface. The aluminum stiffened skins

were also outfitted with skis. Twenty stiffened-skin

specimens (five each of four different material types)

were fabricated. More information on this phase of

the investigation is given in reference 4.

Apparatus. The apparatus used to perform abrasion tests on the I-beam stiffened skins is shown in figures 8 and 9. The integrated tire test vehicle (ITTV)

was used to tow the runway abrasion test trailer with

the test apparatus mounted on it (fig. 8). Figure 9

is a detailed view of the runway abrasion test trailer

and apparatus. The design and operation of the apparatus to conduct the stiffened-skin tests is similar

in concept to that used for the test apparatus for the

skin-coupon tests. The specimens were held in place

by securing the top flange to a holder which was attached to a parallelogram arrangement of mechanical linkages consisting of a large top beam, a rigid

central support, a lower linkage arm, and the specimen holder. These four members were connected by

pinned joints so that the specimen holder could be

raised and lowered to the runway surface by pivoting the top beam. The specimen holder remained

perpendicular to the runway surface because of the

parallelogram linkage arrangement. As the skin of

the I-beam specimen wore, this arrangement ensured

that the load remained normal to the abrading surface. Loads were applied to the stiffened-skin specimen by placing lead weights on the specimen holder

(fig. 9).

A hydraulic system (fig. 9) was mounted to the

trailer to control the action of the mechanical linkages in lowering and raising the test specimens to the

runway surface. The system consisted of a pump,

an accumulator, a four-way valve, and a hydraulic

cylinder. To lower the stiffened skin, the hydraulic

system was actuated such that the hydraulic cylinder extended, thereby releasing the cable attached

through a pulley to the top beam of the linkage assembly. The system worked in reverse to raise the

test specimen.

3
Operationof the abrasiontestswasautomated

througha controlcircuit which,wheninitiatedby

anoperatorin tile ITTV cab,progranmmdthe hydraulicsystemto lowertile stiffened-skinspecimen

to therunwaysurfaceandstart thedatarecording

devicesat specificintervals.Thetestspecimenswere

abradedfor approximately6 seconds,at whichtime

thehydraulicsystemautomaticallyraisedthemfrom

the runwaysurfaceandshutoff thedatarecorders.

ThecontrolcircuitandalldatarecordingdevicesoperatedofftheonboardpowergeneratoroftheITTV.

Instrumentation. The test apparatus was instrumented with a load cell located in tile lower linkage

arm (fig. 9). During abrasion testing the frictional

force developed between the stiffened skin and the

runway produced a tensile force in the lower arm.

The signal generated from the load cell was amplified

and filtered through a 2-Hz low-pass filter. The signal was then fed to a two-channel strip-chart recorder

to provide a force data trace. This force measureinent was used to calculate the coefficient of dynamic

friction in the same manner as in the skin-coupon

tests. The test apparatus was also instrumented with

a limit switch (fig. 9). The linfit switch triggered

an event marker on the strip-chart recorder to signify when the stiffened skin was in contact with the

ground. The run time of the test was determined by

measuring the length between the two event marks

on the strip-chart recorder and dividing that value by

the known chart speed. The strip-chart recorder, filter, amplifier, and other data recording devices were

mounted to the test trailer as shown in figure 9.

Parameters. The effect of pressure on specimen

wear behavior and coefficient of fl'iction was determined for the alunfinum and Gr-Ep stiffened skins

at pressures of 2.0, 3.2, and 5.0 psi. These tests were

conducted at a test velocity of 32.5 mph such that

pressure was the only variable in the series of tests,

except for the natural variations in the runway surface. These conditions are similar to those used in

the skin-coupon tests to determine the effect of pressure on friction and wear behavior.

Unlike the skin coupon tests, which were performed in tile laboratory using a belt sander to simulate a runway surface, the stiffened-skin tests were

conducted on an actual runway surface. The tests

were performed on the Langley Air Force Base northsouth runway located in Hampton, Virginia. This

runway is a concrete surface and has a measured

surface texture depth of 0.011 in., which is typical

of heavily textured concretes and the majority of

harsher types of asphalt (ref. 5).

The high velocity range (140 to 160 mph) required

to accurately simulate the touchdown velocities of a

transport airplane was unattainable with the ITTVtowed al)rasion trailer and test apparatus. However,

a range of velocities was chosen for testing purposes.

Tests at 16.0, 32.5, and 45.0 mph were performed on

the aluminum and Gr-Ep stiffened-skin specimens at

a pressure of 3.2 psi. These conditions are similar to

those used in _he skin-coupon tests to determine the

effect of velocity on friction and wear behavior.

Procedure. All pertinent data," such as testspecimen dimensions and mass, and test parameters.

such as load and velocity, were recorded prior to testing. The weight necessary to achieve the desired test

pressure was attached to the specimen holder. The

test operator activated the control circuit to begin

the test from a remote switch inside the ITTV cab,

once the test velocity had been achieved. The driver

of the ITTV maintained this speed for approximately

15 seconds to ensure constant velocity during the entire run time of the test. Following the test, the

stiffened-skin mass was recorded, and the force data

trace was removed and labeled. This procedure was

repeated for each test.

Phase III--Stiffened-Panel Tests

Specimens. The third phase of the investigation

involved abrading three stiffened panels constructed

of Gr-Ep composite material. A typical panel is

shown in figure 10. The panel consisted of a 24-in.

by 24-in. skin constructed of AS4/35024 Gr-Ep composite material with a lay-up (fig. 10). The panel

was stiffened in the sliding direction by three, 1-in.

high Z-s_ ringers and was stiffened in the perpendicular direction by two, 5- in. high Z-frames. These tests

were performed to correlate the results from the two

previous phase_ and to provide the most realistic simulation of the transport skin structure for wear anti

friction t,est ing.

Apparatus and procedure. The apparatus used

to perform the stiffened-skin tests in phase II of

the investigation was modified to accommodate the

larger panel test specimens. The specimen holder

was altered so that the stiffened panels were held to

it by the top flanges of the Z-frames. Also, a higher

capacity load cell was installed in the lower linkage

arm in anticipation of the higher loads caused by the

larger panel size. The same data recording system

and hydraulic control system were used to conduct

4 AS4/3502 graphite-epoxy composite is a prepreg manufactured by H_'rmfies Incorporated.
thetests.Thetestprocedurewasthesameasused

in thestiffened-panel tests.

The stiffened-paneltestswereperformedat a

pressureof 2.0psiandat velocitiesof 16.0,32.5,and

45.(}mph. Exceptforthefactthat a lowerpressure

levelwasusedfor thesetests,theseconditionsare

similarto thoseusedto determinetheeffectofvelocity onwearandfrictionbehavioroftheskincoupons

andstiffenedskins.Thelargeloadsintroducedinto

thetestapparatusby applyingtheamountof weight

requiredto obtainthe 3.2-psipressurelevelusedin

the skin-couponand stiffened-skintestswereprohibitiveforthestiffened-panel tests.

Results and Discussion

Abrasion Surface Description

Skin coupons. Tile general appearance of the

abraded wear surface and the wear debris of the

skill coupons are shown in figure l l. The wear

surface of the aluminum specimen contained thin,

evenly spaced grooves along the direction of sliding.

Aluminum wear debris consisted of small particles

having a powder-like texture. The graphite-epoxy

specimen exhibited a wear surface similar to the

alumimnn specimen, although tile Gr-Ep surface was

smoother and the grooves were not quite as deep.

Wear debris from these specimens consisted mainly of

fine particles interspersed with some pieces of broken

fibers. The abraded surface of a Kevlar specimen is

also depicted, though the Kevlar skin-coupon results

are not presented in this paper.

Stiffened skins. Figure 12 shows the general appearance of the abraded surface for a typical aluminum specimen, a graphite-epoxy specimen,

and two glass hybrid composite stiffened-skin specimens. The wear surface of the aluminum specimen contained rough, jagged grooves whieh were

fairly regularly spaced along the direction of sliding. The grooves were more widely separated and

more shallow than those observed in the aluminum

skin coupons. The Gr-Ep stiffened skins exhibited a

wear surface with similar long groove marks. These

grooves were smoother than for the aluminum specimen, but were more irregularly spaced and less deep

than those of the Gr-Ep skin coupons. In both the

skin-coupon and stiffened-skin tests, the wear appeared to be fairly uniform over the skin area.

Stiffened panel. The wear surface for one of the

stiffened panels is shown in figure 13. The .... rface

contained irregularly spaced long and short gouges

which were wider than those in either the Gr-Ep skin

coupons or stiffened skins. The wear was heaviest

towards the front of the specimen, nearest the ski.

Also, the runway appeared to be undamaged from

the stiffened-skin and stiffened-panel abrasion tests.

Wear Behavior

In the following sections, tile effects of independently varying the pressure and velocity on the wear

behavior of the skin coupons, stiffened skins, and

stiffened panels are discussed, and comparisons are

made between the aluminum and Gr-Ep composite

material. In particular, the discussions are centered

on how the specimen wear rate and wear index are affected by the test variables. The wear rate is defined

as the average reduction in specimen thickness per

unit of run time and is calculated fi'om the following

equation:

Wear ratem i - mf

plw( tr )

where

m i initial mass

rnf final mass

p density of skin material

l specimen length

w specimen width

tr run time

Thus, wear rate is computed in dimensions of inches

per second. A means of nondimensionalizing the results is to divide the wear rate by the test velocity.

This parameter is called the wear index and is computed from the following equation:

Wear indexWear rate

Velocity

Effect of pressure. The wear rate as a function of

normal pressure is shown in figures 14 and 15 for the

aluminum and Gr-Ep skin specimens, respectively. A

least-squares linear curve fit was made through the

data points, since a linear relationship appeared to

best represent the trends in the data. The data for

the skin coupons, stiffened skins, and stiffened panels

are given in tables I, II, and III, respectively. Each

data point in table I represents the average of 2 to 3

separate tests, whereas the data in tables II and III

for the stiffened skins and stiffened panels represent

a single test.

The wear rate increased as a linear function of

load for both the aluminum and the Gr-Ep specimens. In tile case of the aluminum (fig. 14), the
stiffenedskinsshoweda 20-to 40-percentdecrease

in wearrate at eachpressurelevel. However,the

Gr-Epstiffenedskinsexhibiteda muchgreaterdecrease,approximately75percent,at eachpressure

level.Thisdramaticdifferenceisdepictedin thebar

chart of figure 16. At each pressure level, the Gr-Ep

skin-coupon wear rate is several times greater than

the wear rates for the other test specimens.

The large decrease in wear rate between the GrEp skin coupons, stiffened skins, and stiffened panels

is probably the result of a combination of two factors.

First, the skin-coupon specimens were abraded on

aluminum oxide abrasive belts having a surface texture depth similar to a typical runway. However, the

surface texture depth measurement does not indicate

in any way that the roughness characteristics of the

two surfaces are similar. In fact, they are very different. The aluminum oxide belt was a uniform, sharp,

jagged surface which wore the specimens in very fine,

regularly spaced grooves. The runway was a highly

nonuniform surface with irregularly spaced rocks and

small gravel imbedded in tile concrete surface. This

resulted in the more shallow and irregularly spaced

groove patterns on the stiffened-skin specimens. The

difference in roughness and surface quality between

the belt surface and the actual runway may account,

in part, for the decrease in wear rate between the

Gr-Ep skin coupons and stiffened skins. This factor

may also account for the difference in the behavior

of the aluminum specimen; however, the aluminum

appears to be much less sensitive to the difference in

surface quality than the Gr-Ep.

Effect of velocity. The effect of velocity on wear

rate is shown in figures 17 and 18 for the aluminum

and Gr-Ep skin specimens, respectively. A linear

least-squares curve-fit technique was used to plot

trends in the data. The wear rate increased with

velocity for both the aluminum and Gr-Ep skin speeimensl however, for both materials, the skin coupons

exhibited the greatest rate of increase of wear rate

with velocity. As the skin area increased, the wear

rate became less sensitive to changes in test velocity. In fact, the wear rate of the Gr-Ep stiffened panels remained almost constant throughout the velocity

range. In general, the aluminum specimens had wear

rates 2 to 5 times less than their Gr-Ep counterparts.

This difference is shown graphically in the bar chart

of figure 19. The Gr-Ep skin-coupon wear rate was

several times that of the other test specimens. This

salne trend is seen in the plot of wear rate versus

pressure (fig. 16).

Figures 20 and 21 are plots of the wear index versus velocity for tile aluminum and Gr-Ep test specimens, respectively. For the skin coupons, both aluminum and Gr-Ep, the wear index increased with

velocity. The stiffened skins and stiffened panels exhibited the opposite behavior and tended to decrease

with velocity. The differences between the wear index at each velocity of the various test specimens are

depicted in a bar chart in figure 22. The Gr-Ep skincoupon wear index is several times greater than that

of the other test specimens because of its higher wear

rate at each velocity.

The second factor which may have influenced the

wear behavior is a combination of specimen size and

the resulting problems of uniform wear and loading.

Skin area increased by an order of magnitude from

the skin-coupon specimens to the stiffened skins and

again from the stiffened skins to the stiffened panels. Increasing the specimen size inade obtaining

even pressure and uniform wear more difficult. Adjustments were made to test apparatus for the skincoupon specimens to insure uniform wear and loading. Adjustments were also made to the test apparatus used for the stiffened-skin and stiffened-panel

tests. However, for the stiffened-panel tests, the specimen would not sit fiat because of the curvature of

the runway. This condition may have contributed to

the decrease in wear rate with specimen size. Since

an aluminum stiffened panel was not tested, the size

effect cannot be fully determined. In observing the

wear patterns of the Gr-Ep specimens, it is obvious

that the stiffened panels did not have uniform contact with the runway surface.

6

Coefficient-of-Friction Data

The frictional forces developed between the test

specimen and the sliding abrasive surface (belt or

runway) were calculated from a static analysis of the

specimen holder (sketch A) given the applied load P,

the angle of inclination of the linkage arms 0, and the

force output measured from the load cell F L. The

coefficient of friction # is derived from the computed

frictional force based on the measured force in the

lower linkage arm. Summation of the moments yields

,:

Suummtion of the forces in the horizontal and vertical directions yields

N = P + FL(sinO ) (1--_ °)
p

I

F

t

N

Sketch A

--V

a

I J

I

b

t

Therefore, the coefficient, of friction is given by

F

#-N

l)

In the following sections, the effect of pressure and

velocity on the coefficient of friction is presented.

The data for the skin coupons, stiffened skins, and

stiffened panels are given in tables I, II, and III,

respectively.

Effect of pressure. The effect of normal pressure

on the coefficient of friction for the aluminum and

Gr-Ep test specimens are plotted in figures 23 and

24, respectively. A least-squares linear curve fit

was made through the points. The data indicate

that there are no clear trends in the behavior of

the coefficient of friction as a flmction of pressure.

For the stiffened-skin tests, the coefficient of friction

increased with pressure for both the aluminum and

Gr-Ep materials. However, it tended to decrease

slightly for the skin-coupon tests. Figure 25 shows

the coefficient-of-friction data presented as a bar

chart. This figure indicates, perhaps better than the

graphs, that the aluminum specimens tended to have

coefficients of friction of about 0.20 and that the GrEp specimens tended to have coefficients of friction

in the range of 0.10 to 0.15, or approximately half

that of aluminum. These data suggest that during

an airplane belly landing, a transport with a Gr-Ep

composite skin may slide twice as far as a similar

transport with an aluminum skin.

Effect of velocity. Figures 26 and 27 show the

variation in coefficient of friction with velocity for

the aluminum and Gr-Ep test specimens. As with

the plots of coefficient of friction versus pressure, no

consistent trends in the data are apparent. Again,

the data indicate that the aluminum coefficient of

friction is approximately 0.20 and that the Gr-Ep

coefficient of friction ranges from 0.10 to 0.15. This

point is emphasized graphically in figure 28.

Concluding Remarks

The objective of this investigation was to compare the friction and wear response of aluminum and

graphite-epoxy (Gr-Ep) composite materials when

subjected to loading conditions similar to those experienced by the skin panels on the underside of a

transport airplane during an emergency sliding landing on a runway surface. A three-phase experimental

program was conducted to simulate these conditions.

The first phase involved a laboratory test which used

a standard belt sander to provide the sliding abrasive surface. Small skin-coupon test specimens were

abraded over a range of pressures and velocities to

determine the effects of these variables on the coefficient of friction and wear rate. The second phase

involved abrading I-beam stiffened skins on an actual

runway surface over the same range of pressures and

velocities used in the first phase. In the third phase,

large stiffened panels, which most closely resembled

transport fuselage skin construction, were abraded

on a runway surface.

Comparisons were made between the aluminum

and the Gr-Ep composite materials and between the

laboratory controlled tests and those conducted on a

runway surface. Major findings of this investigation

include:

1. Wear rate for both the aluminum and graphiteepoxy materials was a linearly increasing function of

load and velocity.

2. For each specimen type, skin-coupon and

stiffened-skin, the Gr-Ep specimens had wear rates

two to five times higher than their aluminum

counterparts.

3. The coefficient of friction for the Gr-Ep specimens was approximately half that of aluminum.

4. Wear behavior of the skin-coupon tests performed in the laboratory on abrasive belts to simulate a runway surface did not correlate well with the

wear behavior of the stiffened-skin or stiffened-panel

tests performed on an actual runway surface. Wear

under laboratory test conditions was several times

greater than that. experienced on the runway.

NASA Langley Research Center

Hampton, VA 23665-5225

September 9, 1985

7
References

1. NTSB Incident File, Miami, Florida. Boeing B-727-200,

N8831E, Feb. 15, 1983, MIA-83-I-A075.

2. Passengers Praise Pilot for Actions in Accident. TimesHerald (Newport News, Va.), 83rd year, no. 71, Mar. 24,

1983. p. 13.

3. Jackson, Karen E.: Friction and Wear Behavior of Aluminum and Composite Airplane ,S'kin.s. NASA TP-2262.

AVSCOM TR 83-B-7, 1984.

,i. Jaeks()n. Kar(.n E.: Friction and Wear Behavior of Alunm_._m_ [_nd ('omposite l-Beam Stiffened Airplane Skins.

NASA TM-86418. USAAVSCOM TM 85-B-2, 1985.

5. Yager. Th(mla._ .l.: Phillips, W. Pelham; Horne, Walter B.;

and Sparks. Howard C. (appendix D by R. W. Sugg):

A CoTnpari.son of Aircraft and Ground Vehicle Stopping

Pe:rforTrmnee on Dry, Wet, Flooded, Slush-, Snow-, and

lee-Cc)_'rre_t flf_r_ways. NASA TN D-6098, 1970.
ZD

09

Z

0

D

0

0

0

r_

r]3

m

_2

i

X

.._o_ _

I

X

. .__. __

m_
OQ

p.l

OO

pa

OO

pa

<

p_

©

_Q

OQ

pa

oO

pa

pa

0

o _IZ

'_x

_. -._. x

_._ g__

_: Io ._._.._

i

_ ©

<

r._ "" :-_. X

pa

z

..

© >

m d

_ "_ o_.o

[_

pa

10
TEST MATERIALS

Material Lay-up

Aluminum:

2024-T4

Composite:

T300/5208

Kevlar 49/934

T300/BP-907

T300/Fibredux 920

T300/Ciba 4

(_45/0/90/_45/0/90)3S

(+--45/02/+--45/02/+--45/0/90) 2S

Figure 1. Schematic of a typical skin-coupon abrasion test specimen and a list of types of aluminum and

composite materials used in abrasion tests. Drawing dimensions are in inches unless otherwise noted.

11
i

_C

12
!:!!

J.

+_

o

o

°_

13
14

r_

b_c

o_

c_

r_

o_©

r_

_c
SURFACE TYPE

0

z_

TYPE A

MAI NLY

VERY SMOOTH

CONCRETE RUNWAYS,

SOME SMOOTH

ASPHALT RUNWAYS

TYPE B

TYPI CAL OF MOST

LI GHTLY TEXTURED

CONCRETE AND

MOST SMALLAGGREGATEASPHALT

TYPE C

HEAVI LY TEXTURED

CONCRETES AND

THE MAJORITY OF

HARSHER TYPES

OF ASPHALT

TYPE D

SHALLOW GROOVING

TYPE E

DEEP GROOVED

SURFACES,

OPEN TEXTURED

INOT TYPICAL OF

' CONVENTIONAL

D

D

0

0

0

0

[]

13

[3

L3

(

RUNWAYS

)

O

0

[3

0

0

[]

0

[]

0

%

[]

[i]

O- CONCRETE SURFACES

O-ASPHALT SURFACES

Q-GROOVED SURFACES

_- METAL SURFACES

[]

I i I ! I I I !

2 3 4 56789 2 3 4

10.3 I0-2

[]

D

r-i

I I I •

56789

10-1

TEXTURE DEPTH, IN.

Figure 5. Classification of runway surfaces. Texture depths measured by grease or sand patch methods. (From

data in rcf. 5.)

15
0

Belt

grit

size

4O

2O

0

Typicalrunway

textures

I I I

-2 0 1 2 3XlO

Surface-texture depth, in.

Figure 6. Results of performing grease sample technique on alumintun oxide belts. Belts lying in the typical

range were used in abrasion testing.

16
20.0 in.

Aluminum 7075-'1"76

AS 4/3502

90, 45, O,-45, 90, 45, O) S

L-85-150

Figure 7. I-beam stiffened-skin test specimen.

trailer

L-85-151

Figure 8. Abrasion test trailer towed by integrated tire test vehicle (ITTV) on Langley north-south runway.

17
18

r_

c_

c_

c_

_C

o_
C

C

o_

@

s_

vm_

19
Kevlar Graphite-Epoxy Aluminum

\

: ..... L:S3-2468

Figure ll. General appearance of wear surface and wear debris particles for typical Kev]ar, graphite-epoxy.

and almnimml skin-coupon test specimens. Wear debris shown is not indicative of volume of wear for each

spechnen type.

2O
Aluminum Gr-Ep Hybrid I

Hybrid

i::!j" ;

Figure 12.

L-84-13,735

Typical abraded surfaces of stiffened-skin test specimens.

21
t

Front

Figure 13. Typical abraded surface of stiffened-panel test specimen.

L-85-3499

22
Wear rate,

in/sec

-3

x 10

o_ Skin coupon

n--- Stiffened skin

0

o_ ,.."5"

f If

[_,"

I I I I I

0 I 2 3 4 5 6

Pressure, psi

Figure 14. Wear rate of aluminum test specimens as a function of normal pressure for a test velocity of

32.5 mph.

Wear rate,

in/sec

15

10

-3

x 10

] I

0 1 6

o_ Skin coupon

n--- Stiffened skin

Z_

1_ 1 I i

2 3 4 5

Pressure, psi

Figure 15. Wear rate of graphite-epoxy specimens as a function of normal pressure for a velocity of 32.5 mph.

23
I:::a AI skin coupon

r-rl AI stiffened skin

15

10

Wear rate,

inlsec

5

x 10-3

Gr-Ep skin coupon

Gr-Ep stiffened skin

I_ Gr-Ep stiffened panel

p

I

2.0 3.2 5.0

Pressure, psi

Figure 16. Comparisons of wear rate versus pressure for aluminum and graphite-epoxy (Gr-Ep) composite skin

specimens for a test velocity of 32.5 mph.

Wear rate,

in/sec

-3

x i0

3-

2-

I0

o_ Skin coupon

[3--- Stiffened skin

0

0

[3

I

10

I I I I I

20 30 40 50 60

Velocity, mph

Figure 17. Wear rate of aluminum specimens as a function of velocity for a test pressure of 3.2 psi.

24
Wear rate,

in/sec

-3

x 10

15-

10-

5-

l

0 10

o_ Skin coupon

n--m Stiffened skin

Am- Stiffened panel

Pressure = 3.2 psi

___. __ ._l.,_.__..t_ _ ._--_ Pressu_e

20 30 40 50 60

Velocity, mph

= 2.0 psi

Figure 18. Wear rate of graphite-epoxy composite specimens as a function of velocity for a constant test

pressure.

[] Aluminum skin coupon

I-1"3 Aluminum stiffened skin

-3

x 10

15-

Wear rate, 10 -

in/sec

5-

0 16.0

IZ_ Gr-Ep skin coupon

Gr-Ep stiffened skin

32. 5

Velocity, mph

7";

#'j

v.

fJ

fJ

fJ

fJ

¢,o

fJ

r,

fJ

Z

50.0

Figure 19. Comparisons of wear rate versus velocity for aluminum and graphite-epoxy (Gr-Ep) composite skin

specimens for a test pressure of 3.2 psi.

25
Wear index,

in/in

x 10-6

4-

3-

2

io-- Skin coupon

[]-- - Stiffened skin

0

0

I I I ! I I

0 10 20 30 40 50 60

Velocity, mph

Figure 20. Wear index for ahmfinun_ specimens as a flmction of velocity for a test pressure of 3.2 psi.

Wear index,

in/in

x 10-6

15-

10-

5-

I

0 10

o-- Skin coupon

[]--- Stiffened skin

z_--- Stiffened panel

")-- Pressure = 3. 2 psi

/

/--Pressure = 2. 0 psi

_'-T-----I "_' -'-T- -_ I I

20 30 40 50 60

Velocity, mph

Figure 21. Wear index for graphite-epoxy specimens as a function of velocity for a constant test pressure.

26
F:=lAluminum skin coupon

1-r'l Aluminum stiffened skin

x 10-6

15-

Gr-Ep skin coupon

Gr-Ep stiffened skin

Wear index,

in/in

10

r.#

fJ

r,

7,

16.0

/

j-

/

f

¢

¢

/

/

j32.5

Velocity, mph

Figure 22. Wear index versus velocity for aluminum and graphite-epoxy (Gr-Ep) skin specimens for a test

pressure of 3.2 psi.

.25

.20

Coefficient .15

of friction

• 10

.05

o-- Skin coupon

n--- Stiffened skin

,I

j,

Q 1"

I I I I I l

0 1 2 3 4 5 6

Pressure, psi

Figure 23. Coefficient of friction for aluminum specimens as a function of pressure for a constant test velocity

of 32.5 mph.

27
Coefficient

of friction

.25

.2O

• 15

.10

• O5

o_ Skin coupon

[]--- Stiffened skin

A Stiffened panel

,z]

f

13./

A t

f

[31

I 1 I 1 I

0 1 2 3 4 5 6

Pressure, psi

Figure 24. Coefficient of friction for graphite-epoxy specimens as a [unction of pressure for a constant test

velocity of 32.5 mph.

F:::] Aluminum skin coupon

VII Aluminum stiffened skin

Coefficient of

friction

Gr-Ep skin coupon

Gr-Ep stiffened skin

m Gr-Ep stiffened panel

.25

.2o -

.15

•I0

•05 _= ._ 1

.e

0 2.0 3.2 5.0

Pressure, psi

Figure 25. Coefficient of friction versus pressure for almnimun and graphite-epoxy (Gr-Ep) skin specimens at

a constant test veh)city of 32.5 ,nph.

28
Coefficient

of friction

.25

.20

.15

• 10

. O5

o-- Skin coupon

o--- Stiffened skin

[]

E]Os._._..v ¢"'_" [] 0

0

I I I I I 1

0 10 20 30 40 50 60

Velocity, mph

Figure 26. Coefficient of friction for aluminum specimens as a function of velocity for a constant test pressure

of 3.2 psi.

Figure 27.

.25

.20

Coefficient

.15

of friction

• 10

.05 -

o-- Skin coupon

[]--- Stiffened skin

zx----- Stiffened panel

/i--Pressure = 3. 2 psi

[] []

o

z_ re

o= 2.0 psi

I I I I 1 I

0 lO 2(} 30 40 5(} 60

Velocity, mph

Coefficient of friction for graphite-specimens as a function of velocity for a constant test pressure.

29
[] Aluminum skin coupon

IT1 Aluminum stiffened skin

.25

.2O

Coefficient of .15

friction

.10

.O5

Gr-Ep skin coupon

k-_ Gr-Ep stiffened skin

Velocity, mph

Figure 28. Coefficient of friction versus velocity for aluminum and graphite-epoxy (Gr-Ep) skin specimens for

a test pressure of 3.2 psi.

3O

1. Report No.

NASA TP-2520 AVSCOM TR 85-B-7

!2. Government Accession N(,. -.... T3

• Recipient's Catalog No.

4. Title and Subtitle

AI)rasion Behavior of Ahlminum and Composite

Skin Coupons, Stiffened Skins, an(l Stiffened Panels

Representative of Transport Airplane Structures

7. Author(s)

. Report Date

November 1985

6- _O_ganization Code

Karen E. Jackson

9. Performing Organization Name and Address

Aerostructures Directorate

USAARTA-AVSCOM

Langley Research (?enter

Hampton, VA 23665-5225

12. Sponsoring Agency Name and Address

National Aeronautics and Space A(hninistration

Washington, DC 20546-0001 and

U.S. Army Aviation Systems Command

St. Louis, MO 63120-1798

i 505-33-53-09

F

8. Performing Organization Report No.

L-16018

10. Work Unit No.

11. Contract or (]rant No.

13. Type of Report and Period Covered

Technical Paper

14. Army Project No.

1L161102AH45

15. Supplementary Notes

Karen E. Jackson: Aerostructures Directorate, USAARTA-AVSC()M.

16. Abstract

A three-phase investigation was conducted to compare the friction aJ_d wear response of aluminum and

graphite-epoxy composite materials when subjected to loading conditions similar to those experienced by

the skin panels on the underside of a transport airplane during an emergency belly landing on a runway

surface. The first phase involved a Laboratory test which used a standard belt sander to provide the sliding

abrasive surface. Small skin-coupon test specimens were abraded over a range of pressures and velocities

to determine the effects of these variables on the coefficient of friction and wear rate. The second phase

involved abrading I-beam stiffened skins on an actual runway surface over the same range of pressures and

velocities used in the first phase. In the third phase, large stiffened panels which most closely resembled

transport fuselage skin construction were abraded on a runway surface. This report presents results from

each phase of the investigation and shows comparisons between the friction and wear behavior of the

aluminum and graphite-epoxy composite materials.

17. Key Words (Suggested by Authors(s))

Abrasion of composites

Friction and wear of metal and composites

Composite airplane skin

Toughened-resin composites

18. Distribution Statement

Utlclassifie(1 I Jnlimited

Subject Category 05

19. SecuritYunclassifiedClassif.(ofthis report) 20. SecuritYunclassifiedCla_ssif.(of this page) 121.31N°' of Pages 22.A03Price !

For sale by the National Technical Information Service, Springfiold. Virginia 22161

NASA-Langley, 1985