Low Velocity Instrumented Impact Testing of Four NewDamage

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Low Velocity InstrumentedImpact Testing of FourNewDamage TolerantCarbon_poxy CompositeSystems

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D. G. Lance

and A. T. Nettles

(NASA-TP-aO_9i IMpAcT TE_TTNG

CARgON/ffPOXY COMPOSITE

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NASATechnical

Paper3029

1990

National Aeronautics andSpace Administration

Office of Management

Scientific and TechnicalInformation Division

Low Velocity InstrumentedImpact Testing of FourNew Damage TolerantCarbon/Epoxy CompositeSystems

D. G. Lance

and A. T. Nettles

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama

TABLE OF CONTENTS

I. INTRODUCTION ......................................................................................

II. MATERIALS AND EXPERIMENTAL METHODS ..............................................

1. Material ...............................................................................................

2. Impact Testing .......................................................................................

3. Visual Damage ......................................................................................

4. Specimen Cross-Sectioning ........................................................... _ ..........

III. RESULTS AND DISCUSSION ......................................................................

I. Force-Time Plots From Impact Tests .............................................................

2. Absorbed Energy-Time Plots From Impact Tests ...............................................

3. Maximum Force Versus Impact Energy Plots ...................................................

4. Visible Surface Damage ............................................................................

5. Cross-Sectional Visual Damage ...................................................................

IV. CONCLUSIONS ........................................................................................

APPENDIX A - Instrumented Output, Cross-Sectional Photographs, and

Surface Photographs for Each Material ................................................

APPENDIX B - Cross-Sectional Photographs of the IM7/1939

Front Surface Cracks .....................................................................

REFERENCES ..................................................................................................

Page

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35

PRECEDING PAGE BLANK NOT FILMED

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LIST OF ILLUSTRATIONS

Figure

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2.

3.

4.

5.

6.

Title

Maximum load versus Impact energy for T650/1939 ......................................

Maximum load versus impact energy for T650/1962 ......................................

Maximum load versus Impact energy for IM7/1939 .......................................

Maxnnum load versus Impact energy for IM7/1962 .......................................

Maximum load versus impact energy for T300/934 .................. . ....................

Maximum load versus impact energy for all materials ....................................

Page

6

6

7

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iv

TECHNICAL PAPER

LOW VELOCITY INSTRUMENTED IMPACT TESTING OF FOUR NEW DAMAGE

TOLERANT CARBON/EPOXY COMPOSITE SYSTEMS

I. INTRODUCTION

Since foreign object impact is a serious concern with carbon fiber reinforced polymeric materials,

an effort has been made by some manufacturers to produce more damage tolerant resins. Furthermore,

the newer generation of intermediate modulus carbon fibers have a much higher strain to failure, as well

as a higher strength, than the earlier types of carbon fibers. Impregnating the improved fibers with the

new damage resistant resins produces materials that can withstand a larger impact energy before material

damage occurs.

Low-velocity instrumented impact methods have proven to be very useful in the study of damage

tolerance of composite materials (Refs. I-4). Such data as maximum force of impact, force-time graphs,

and absorbed energy of impact can help the researcher characterize different composite materials for

damage tolerance. Cross-sectioning of the specimen through the damage zone has also proven to yield

practical information (Refs. 3,5-9). By utilizing this destructive analysis technique, the impact energies

that cause matrix cracking, delaminations, and fiber breakage can all be identified. Furthermore, a

comparison of the damage tolerance capabilities of various materials can be accomplished by observing

which materials have the most amount of matrix cracking or larger extent of delaminations for a given

impact energy. Comparisons can also be made between the visible surface damage and the internal

damage identified by the cross-sectional examination.

Although a large amount of literature exists on impact damage testing and characterization of

composite materials, data is lacking on the recently developed carbon fiber/epoxy resin systems. In order

for the newer generation composites to be incorporated into high performance applications, much

research needs to be accomplished to better understand the response of these composite materials to

impacts. It is the purpose of this paper to present some preliminary experimental results on four new

composite prepreg systems and compare these to an earlier generation fiber/resin system to see if there

are indeed any improvements in impact resistance, and to characterize those improvements.

II. MATERIALS AND EXPERIMENTAL METHODS

1. Material

Five different prepreg systems were used to construct the panels tested for this study. The T650/

1939 and T650/1962 prepregs were manufactured by Amoco Performance Products. The IM7 carbon

fibers were produced by Hercules Incorporated and impregnated by Amoco with the 1939 and 1962 resin

systems. The T300 fibers were produced by Amoco and impregnated by Fiberite with the 934 resin.

TheT300/394wasemployedasabaselinematerialfor this studybecauseof the largedatabasealreadyin existenceon thismaterial.Theothermaterialswerechosenbecauseof their recentintroductioninto the marketandpromiseof damageresistance.TheT300 is an intermediatemodulusfiber, andthe934 is a standardepoxy resin.The T650and IM7 are intermediatemodulus/high-strengthfibers.The1939and 1962resinsareboth claimedto be damagetolerantepoxies.

Theseprepregswere fabricatedinto 16-ptypanelswith quasi-isotropiclay-up configuration,(0, +45,-45,90)_2. Each material was cured with a hot pressaccordingto the supplier's recom-mendations.TheT300/934rangedin thicknessfrom 1.76mmto 180mm, theT650/1939from !.82 mmto 1.92mm, theT650/1962between1.92mm and2.00 ram, theIM7/1939 wasbetween2.28and2.32mm thick, and the IM7/1962 had a thicknessrangingfrom 1.96 to 2.04 mm.

Specimens10.16cm2 were machinedfrom the compositepanelsfor impact testing.

2. Impact Testing

The specimens were impacted using a Dynatup model 8200 drop weight apparatus. The data was

obtained with a Dynatup 730 Data Acquisition Systern. The impactor had a mass of 1.77 kg with a

I .27-cm diameter hemispherical tup. The specimens were held tightly between two aluminum plates by a

pneumatic clamping mechanism. The plates had holes 7.62 cm in diameter within which the composite

panel was exposed. The specimens were all impacted at their centers.

U

3 Visual Damage

For each level of impact energy on each type of specimen, the visual damage to both sides of the

panel was recorded and photographed.

4 Specimen Cross'Sectioning

For each impact energy level on each type of specimen, a cross-sectional cut was made perpen-

dicular to the outer fibers and through the point of impact with a Buehler diamond wafering blade. The

specimens were then examined and photographed by a Zeiss stereo-optical microscope at magnifications

between 8 x and 20 x. Several of the IM7/1939 specimens were cross-sectionally cut parallel to the outer

fibers and through the front surface crack. These specimens were photographed at magnifications

between, 8 x and 50 x.

III RESULTS AND DISCUSSION

1 Force-Time Plots From Impact Tests

The force-time plots, generated by the data acquisition system, of each specimen damaged from a

drop height which induced fiber breakage showed a sharp drop in force at the maximum recorded load.

For impacttestswhichproducedno fiberbreakagetherewasnosharpdropin force, but ratheragradualreduction.Force-timeplots for eachdrop heightusedon eachmaterialarepresentedin appendixA.

2. Absorbed Energy-Time Plots From Impact Tests

The smooth curves included on the force-time plots in appendix A are absorbed energy-time

plots. A constant percentage of initial impact energy was lost during the impact of all of the materials

tested, until the point of fiber breakage where the the output listed virtually 100 percent of the impact

energy being absorbed. It must not be interpreted that all of the energy loss was released as damage to the

impacted specimen. It has been found that vibrational waves most likely account for a substantial

percentage of the lost impact energy (Ref. 4).

3. Maximum Force Versus Impact Energy Plots

Maximum force of impact values were plotted against their corresponding impact energy levels

for each material. These results are given in figures I through 5. The individual plot for each material

shows a somewhat linear correlation between the maximum force and the increasing impact energy until

the critical impact energy level at which point the maximum force drops slightly and then levels out. The

critical impact energy level is the impact energy corresponding to the peak force from the force-energy

plots and takes place just before fiber breakage occurs. From the comparison of the maximum force

versus impact energy plots for all the materials (fig. 6) it can be seen that the plot actually loosely follows

an inverse parabolic curve.

4. Visible Surface Damage

The visible surface damage was recorded after impact for each specimen. The following results

are given for all five materials tested. In addition, several photographs of the damaged specimens are

presented in appendix A. The apparent fabric pattern surface of the composites in the photographs was

due to the imprint of the Teflon release cloth used during the curing of the specimens. In order to reveal

hard-to-see damaged areas on the front and back sides of the specimen, a fiber optic light source was

placed on the surface of the panel with light angled sharply across the impacted area. This method of

illumination greatly enhanced the photographs as well as enabled the researcher to record minute surface

damage earlier than might otherwise be detected.

The T300/934 plates displayed no damage until a small crack appeared on the back surface of the

specimen at 4. ! J. With the next impact energy level of 5.0 J, the plate showed a significant dent on the

front and a fairly large split on the back. Fiber breakage was first recorded within the front dent at 6.1 J.

A hole was nearly punched through the specimen at 11.3 J.

The T650/1939 plates first displayed damage as a hairline crack along the back surface visible at

6.2 J, but a full split did not occur on the back side until l1.3 J. At an impact energy of 8.2 J, a dent

appeared on the front surface. Fiber breakage was not observed within the dent until 15.2 J. The last

impact, having an energy of 20.3 J, nearly punctured the specimen.

TheT650/1962platesallowednovisibledamageuntil acrackoccurredon thebacksurfaceat animpactof 7.1 J. A minor dentwasproducedwith an impactof 9. t J. Fiberbreakagewasnot observeduntil 15.I J of impactenergy.

The IM7/1939 plates withstood damage until 6. I J when a front surface dent and a rear surface

crack occurred. At an impact of 8.2 J, a small crack appeared beside the impactor dent on the front

surface of the specimen. The crack was perpendicular to the direction of the outer layer of fibers. At 10.2

J, these perpendicular surface cracks were noticed on either side of the impactor dent. The perpendicular

cracks were found on the front surface of all specimens impacted with energies above 8.2 J. In all cases,

the cracks were from 3 mm to 8 mm in length. Larger dents with fiber breakage occurred at the 15.3 J

energy level.

The IM7/1962 plates showed no damage until a hairline crack appeared on the back surface and a

minor dent appeared on the impacted side at the 7.0 J energy Ievell The damage on the back" surface

fluctuated between a hairline crack and several long cracks until the impact energy of 17.2 J when the

surface split and raised. A barely visible dent was noticed at the 12.2 J energy level. At 19. I J, minor

fiber breakage occurred in the front Surface dent.

5. Cross-Sectional Visual Damage

A cross-sectional cut was made perpendicular to the outer fibers for each plate and the specimens

were examined and photographed. Photographs of each material impacted at each energy level are

presented in appendix A.

The T300/934 showed damage on the first impact which had an energy of 1.0 J. Minor delamina-

tions and a small matrix crack were noticed at this energy level. An impact of 4. I J produced a slight

ridge on the back side of the specimen due to the great level of delaminations and matrix cracking. Fiber

breakage occurred at the 6.0 J energy level. Impact energies of 7. I J and above rapidly increased theamount of destruction towards a puncture hole l

The T650/1939 displayed several minor delaminations at an impact of 5.1 J. The next energy

level of 6.2 J produced no damage. At the next impact energy level of 7.1 J, many delminations and

much matrix cracking occurred. Fiber breakage was not noticed until 12.8 J.

The T650/1962 withstood damage until 7. I J when a hairline delamination occurred. There were

no noticeable cracks until the 12.2 J impact. At 15.0 J fiber breakage occurred within the specimen.

The IM7/1+939 showed no damage until the 6, ! J impact which caused delmination. Matrix crack-

ing was first recorded at 8,2 J. Fiber breakage first occurred at the 14.4 J energy level. The impact of

highest force, 19.4 J, produced a near hole in the composite material, For some of the IM7/1939

specimens, a cross-sectional cut was made through the small surface crack on the front surface. For all of

the impacts, the crack occurred only through the outer layer and perpendicular to the fiber direction in

this layer. Under magnification it was clear that, at the site of the crack, the outer fibers were completely

broken. For the more forceful impacts, deliminations were noticed under the topmost layer in the cracked

region. Several cross-sectional photographs of these cracks are provided in appendix B.

4

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The IM7/1962 displayed hairline delaminations on the first impact which had an energy of 4.0 J.

Matrix cracking was first evident at 7.0 J. The damage from the impacts between the 8.2 J and 12.2 J

energy levels were long delaminations with a few matrix cracks. Much matrix cracking was the result of

the 16. ! J impact. At 18.3 J a small amount of fiber breakage occurred within the specimen. With regards

to fiber breakage damage tolerance, the IM7/1962 performed superior to all the other materials. The

1M7/1962 supported a force of almost 800 N more than the second most damage resistant material,

T650/1962, before fiber breakage occurred.

IV. CONCLUSION

This study was designated as an impiricai presentation to benefit the designing engineer by show-

ing the response of four damage tolerant composite systems to a blunt, low velocity impact.

Low velocity impact testing of composite panels can provide essential data on a fiber/resin

system. Force-time plots of the impact event show the maximum force that the composite plate can sup-

port before fiber breakage occurs. Graphs of the maximum Ioad versus impact energy show the critical

energy level at which fiber breakage takes place and extensive tensile strength losses may occur. Surface

photograhic documentation provides a means of identifying damage without the destructive evaluation of

the material. Cross-sectional photography provides a detailed account of the damage modes a specimen

undergoes due to impacts of varying energies.

The damage modes for all the specimens followed in the same order. Minor internal delamina-

tions were the first damage to appear. Soon afterward, matrix cracking began within the specimen. A rear

crack and then a front dent were the next damages to occur. The next damage level was fiber breakage

within the specimen, followed by fiber breakage appearing within the front dent. Fiber breakage was next

seen on the back surface of the panel, followed by a puncture hole.

The T300/934 was employed as a baseline material because of the large data base that exists for

that fiber/resin system. The T300/934 supported a maximuum impact load of 2,367 N. The IM7/1939

upheld a force of 3,894 N, 65 percent greater than the maximum impact load capability of the T300/934.

The T650/1939 supported a maximum load of 4,481 N, 89 percent greater than the maximum force of the

T300/394. The T650/1962 held 5,408 N before breakage which was an increase of 129 percent over the

T300/934. The IM7/! 962 upheld a load of 6,203 N, a 162 percent improvement from the T300/934. The

T300/934 had an impact energy of 9.1 at its maximum load. The IM7/1939's maximum load occurred at

an impact of !1.2 J, the T650/1939 impact of I1.3 J provided its greatest load, the T650/1962 had an

impact energy of 14.2 at its maximum force, and the IM7/1962's greatest load occurred at an impact

energ of 17.5 J.

Throughout the study, the 1962 resin system proved to be more damage resistant than the 1939

resin system. The IM7/1962 prepreg had more superior impact damage resistance than the other three

damage tolerant composite materials.

6500

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Figure I. Maximum load versus impact energy tk)r T650, 1939.

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APPENDIX A

Instrumented Output, Cross-Sectional Photographs and

Surface Photographs of Each Material

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REFERENCES

.

.

.

.

,

.

,

Winkel, J.D., and Adams, D.F.: "Instrumented Drop Weight Impact Testing of Cross-Ply and

Fabric Composites." Composites Vol. 16, October 1985, pp. 268-278.

Aleska, J.C.: "Low Energy Impact Behavior of Composite Panels." Journal of Testing and Evalua-

tion, Vol. 6, May 1978, pp. 202-210.

Sjoblom, P.O., Harntess, J.T., and Cordell, T. M.: "On Low-Velocity Impact Testing of Composite

Materials.'" Journal of Composite Materials, Vol. 22, January 1988, pp. 30-52.

Nettles, A.T.: "Instrumented Impact and Residual Tensile Strength Testing of Eight-Ply Carbon/

Epoxy Specimens." NASA TP 2981, January 1990.

Chang, F.K., Choi, H.Y., and Jeng, S.T." "Characterization of Impact Damage in Laminated" S "Composite.. SAMPE Journal Vol. 26, No. 1 January/February 1990.

Steilbrink, K.' "Examination of Impact Resistance of FRP-Suggestion for a Standard Test Method."

Mechanical Characterization of Load Bearing Fiber Composites and Laminates. Elsevir Applied

Science Publishers, A.H. Cardon and C. Verchery eds., London and New York.

Cantweli, W.J., and Morton, J." "Detection of Impact Damage in CFR Laminates." Composite

Structures, Vol. 3, 1985, pp. 241-257.

. Cantwell, W.J., Curtis, P.T., and Morton, J." "An Assessment of the Impact Performance of DFRP

Reinforced With High-Strain Carbon Fibers." Composite Science and Technology, Vol. 25, No. 2,

1986, pp. 133-148.

9. Wyrick, D.A., and Adams, D.F." "Damage Sustained by a Carbon/Epoxy Composite Material

Subjected to Repeated Impact." Composites, Vol. 19, No. !, January 1988.

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Report Documentation Page

I+ Report No.

NASA TP- 3029

4. Title and Subtitle

I 2. Government Accession No.

Low Velocity Instrumented Impact Testing of Four New

Damage Tolerant Carbon/Epoxy Composite Systems

7. Author(s)

D.G. Lance and A.T. Nettles

9, Performing Organization Name and Address

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

3. Recipient's Catalog No.

5. Report Date

July 1990

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

M-636

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Paper

14. Sponsoring Agency Code

15, Supplementary Notes

Prepared by Materials and Processes Laboratory, Science and Engineering Directorate.

16 ,i_i_si ra_ i-

Low velocity drop weight instrumented impact testing was utilized to examine the damage

resistance of four recently developed carbon fiber/epoxy resin systems. A fifth material, T300/934,

for which a large data base exists, was also tested for comparison purposes. A 16-ply quasi-isotropic

lay-up configuration was used for all the specimens. Force/absorbed energy-time plots were

generated for each impact test. The specimens were cross-sectionally analyzed to record the damage

corresponding to each impact energy level. Maximum force of impact versus impact energy plots

were constructed to compare the various systems for impact damage resistance. Results show that

the four new damage tolerant fiber/resin systems far outclassed the T300/934 material. The most

damage tolerant material tested was the IM7/1962 fiber/resin system.

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

Composite Materials

Epoxy Resins

Damage Tolerance

Instrumented Impact Testing

19. Security Ctassif. (of this report)

Unclassified

18. Distribution Statement

Unclassified-Unlimited

Subject Category: 24

20. Security Classif. (of this page)

Unclassified

IASA FORM1626OCT86For sale by the National Technical Information Service,

21. No. of pages40

22 Price

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Springfield, VA 22161-2171

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