MTL TR 89-12 AD

ENVIRONMENTAL STRESS CRAZINGAND CRACKING OF TRANSPARENTPOLYMERS

k -A205 624

ALEX J. HSIEH and JANICE J. VANSELOWPOLYMER RESEARCH BRANCH

February 1989

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'MAR 2 8 198

H

LADORATORY COMMAND U.S. ARMY MATERIALS TECHNOLOGY LABORATORYfmnwwu MotP uWMTO" Watrown, Masschusett 02172-0001

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MTL TR 89-12

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Final ReportENVIRONMENTAL STRESS CRAZING AND CRACKING

OF TRANSPARENT POLYMERS S. PERFORMING ORG. REPORT NUMER

7 AUTHOR(s) a. CONTRACT OR GRANT NUMBER,

Alex J. Hsieh and Janice J. Vanselow

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18. SUPPLEMENTARY NOTES

i9 KEY WORDS (Conum oumI jid if ece"M wad idevt, by block nwbr)

Fnvironmental tests Crack propagation PolycarbonatesAmorphous materials Fracture mechanics Poly(methyl methacrylate)Crazing Solubility Decontamination

20. ABSTRACT (Conumiw o ,ew j de if necy and identif by block number)

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ABSTRACT

-The resistance to crack and craze growth in polycarbonate and poly(methyl methacrylate) isevaluated with several surface active solvents including a component of the universal chemical war-fare decontaminant, DS2. A static dead weight loading apparatus is used for experimentation,and linear elastic fracture mechanics (LEFM) is used to interpret craze initiation and crackpropagation via compact tension specimens. Results reflect relationships based on solubilityparameters of the solvents and the polymers. Various structural effects due to environmentallyinduced degradation of these amorphous thermoplastics are illustrated.

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INTRODUCTION

Polhmeric matrices and polymer-based composites are finding increased use in militaryapplications. Designing for improved damage tolerance and durability of structures hasbecome an important issue over the last few years. A disadvantage of many polymericmaterials is the tendency to craze or crack when exposed to solvents, including those in achemical warfare (CW) environment. When this failure is enhanced by a simultaneous stress.either mechanically induced or from a residual stress due to manufacturing, the behavior iscalled environmental stress crazing or cracking (ESC). Aggressive surface active agents cancause ductile materials to fracture brittlely.

The purpose of this work is to assess the resistance of the amorphous polymers, polycar-bonate (PC) and poly(methyl methacrylate) (PMMA), to craze initiation and crack propagationin various solvents, including a component of the decontaminant DS2. This includes the inves-tigation of environmentally-induced failure mechanisms.

BACKGROUND

Polycarbonate, known as a ductile thermoplastic, has been widely used for transparentarmor. Despite its good mechanical toughness, PC is susceptible to crazing by many solvents.One of these solvents includes a solution of particular military interest, DS2, which is a chemi-cal warfare (CW) agent decontaminant. DS2 is composed of 70% diethylene triamine(DETA), 28% methyl cellosolve (MECL), and 2% NaOH by weight. In Lee's work, DETAwas shown to be the most active component in DS2 (Figure 1) in causing the most rapid PCweight loss.' The evidence suggests that chain cleavage, as a result of the nucleophilic attackof DETA on the carbonate group of PC, was responsible for the loss of physical integrity.

The critical strain for the resistance to surface crazing of PC has been shown (Table 1)to be higher in MECL than in DETA,1 confirming that PC is more resistant to MECL thanto DETA in surface degradation. The subsequent question is whether the resistance to crackpropagation of PC in MECL follows the same pattern as the resistance to craze initiation.

120

o PC in OETA100 0 0 &' PC in MECL

* PC in MECL * NaOH

J-.

6 0 - a+o a

0 -

240 - 0 0 3

0

0

o I. L A A L ^0 5 10 15 20 25

(timelI /1Z lhrjl'7

Figure 1. Immersion test results of PC inDETA, MECL, MECL + NaOH.

1. i.L-i .ii.. and VANS-L0.ow J ('/etnicalDegradation aid Strcs, Cracking ofPo.,carbonatei iDS2. U.S. Army MatenalsTcchnology Lahorato'y,MI!. TR 87-46. Septcmher 1987.

Table 1. CRAZING RESISTANCE OF POLYCARBONATE

Chemical Critical Strain (%)

DS2 0.32

DETA 0.34

MECL 0.68

EXPERIMENTAL

The polycarbonate (PC) used in this study was Lexan MR5 from GE Plastics, and thepoly(methyl methacrylate) was Plexiglas-G from Rohm and Haas.

The craze/crack growth studies were performed using a static dead-weight loadingapparatus.2 Compact tension (CT) specimens, following ASTM E399, were used.2'3 Thespecimens were initially notched with a slotter, and the notch tip was sharpened with a razorblade. In most of the tests, the specimen was immersed in the solvent. Later, as a meansof reducing the solvent volume, the specimen was subjected to the droplets of solventpumped through a liquid chromatography pump. Craze/crack length measurements were madeas a function of time with eithcr a traveling microscope or a camera.

RESULTS

PC in MECL

Although PC withstands crazing in MECL, it showed poor resistance to crack propagationin this solvent at loads well below fracture. Sample #PC1 broke into two pieces in onehour. The fractured surfaces showed irregular and macroscopic roughness with a hazy whiteappearance. Due to the swelling and possible yielding at the notch while loading, the twobroken pieces did not fit back together. Sample #PC2, which was initially loaded at a lowerload, still fractured at a moderate propagation rate with less roughness and less swelling.However, the hazy white appearance near the notch tip was still clearly seen. This is theresult of extensive crazing which has concentrated on the surface due Abe solvent.

The poor crack resistance of PC in MECL can be attributed to solvent stress cracking.It has been well established that organic agents having solubility parameters, 6, similar to thatof a polymer, can act either as solvents or as cracking agents. 4 As shown in Table 2, thesolubility parameter of PC is 9.5 to 10.5 (cal/cm 3)"/2, which is similar to that of MECL, 10.5(cal/cm 3)1 /'2.1,6 As a result, MECL can be regarded as a good cracking agent for PC understress, even though it does not cause a weight loss of PC as previously ihdicated (Figure 1).

2. VANSELOW, J. 1.. and HSIEH. A]. Evaluation of Transparent Polymers for ChemicallyHardenedArmySystems U.S. Army Materials TechnologyLaboratory, M"L TR 88-24, July 1988.

3. ASTM, Standard Method of Test for Plane-Strain Fracture Toughness of Metallic Materials ASTM E399-83.4. KAMBOUR. R. P. A review of Crazing and Fracture in Thermoplastics. General Electric Report No. 72CRD285, October 1972.5. BPANDRUP, J. Polymer Handbook, 2nd edition, John Wiley & Sons, New York, 1975, p. IV 340 - IV 354.6. JACQUES, H. M., and WYzGOSKI, M. 0. Prediction of Environmental Stress Cracking of'Potycarbonae from Solubility Consideration Joumal or"Applied

Polymer Science, v. 23, 1979, p. 1153-1166.

2

Table 2. SOLUBILITY PARAMETERS OF SOLVENTS AND POLYMERS(References 5 and 6)

3 (cal/cm3 )1" 2 H Bonding Tendency

MECL 10.5 Moderate

Ethanol 13.0 Strong

Hexane 7.3 Poor

PC 9.5- 10.5

PMMA 8.9- 12.7

PC in Ethanol

Ethanol, with a solubility parameter of 13.0 (cal/cm 3) 1/2 , is significantly different from the6 of PC so that ethanol is considered a crazing agent. Figure 2 shows a smaller growth ratein ethanol than in MECL, even at the same load level. This is as expected. The results ofcraze growth profiles in ethanol as a function of load are shown in Figure 3. The conceptof linear elastic fracture mechanics (see Appendix) is utilized to interpret the data. Thehigher the initial stress intensity factor, (Ki), the faster the initial growth rate. A plot ofcraze increment versus time on the logarithmic basis is shown in Figure 4.

20

00 0

4 a in MECL 0 0

16 - 0in Ethanol 6 O 0 0 0 K - 2.5 MPA -mrn

a

16 0

0 oK l=.2MPa-n'0

0 Ol~hni60

j 0

- oC 54 0

C X 00

0

40

0 00

0o 0

0130 0 or00

0 0 10 0 1200 00 40D 10D 200 300 40

Time Imini Time imini

Figure 2. Comparative data for craze growth for Figure 3. Craze growth curves for PC testedpolycarbonate in MECL and in ethanol, in ethanol at various Ki levels.

The latter portion of the curves are linear with the average slope determined as 0.24.The lines in Figure 4 were extrapolated to determine the intercepts. Plotting the interceptsof the curves in Figure 4 versus Ki on a logarithmic scale results in a straight line as shownin Figure 5. The slope, which is a measure of the dependence of the craze increment onthe initial stress intensity factor, was calculated as 1.8. This is similar to 2 in the relationderived by Williams and Marshall 7 for relaxation controlled craze growth behavior:

(craze displacement) a ki2 (-iM) 2 (t) 2m

where t is time, and m is the relaxation parameter. The latter is defined as a measure ofthe time dependent response of the yield stress. Their approach for environmental strcss

7. WILLIAMS J. G., and MARSHALL G. P. Entironmental Crack and Craze Growdi Phenomena in Poymers. Proc. R. Soc., London, A342, 1975, p. 55-77.

3

crazing/cracking was based upon crack opening displacement in conjunction with simple flowanalysis which describes the movement of the liquid environment within the craze. The valueof m, equal to half of the average slope in Figure 4, is determined as 0.12 in this study. Wil-liams and Marshall 7 reported m as 0.04 and 0.11 for PC and PMMA in air at 20'C respectively.

o i - 2.5 MPa m'

* - 1.7 MPamn"2

0 - 1.2MPa.m '2

Slope - -0.18-~r - .~0.99S6

2,, - .-.~ l intercep) • 1.8 (log Kinill, - 1.46

0 00 2 3 4 0 1 2 3 4 5

Log Time (min 0 lOx Og K

Figure 4. Linear regression of the curves Figure 5. Determination of the dependence of the crazefrom Figure 3 on a logarithmic plot, displacement on K for PC tested in ethanol.

For all PC specimens tested in ethanol and MECL, an assembly of crazes was seenprogressing outward from the initial notch tip. At the beginning, many of these crazesadvanced at various angles from the direction of the initial notch, but eventually, after somedistance, they tended to grow parallel to the initial notch. The presence of multiple crazingmight blunt the notch tip and subsequently influence the craze/crack propagation. Forexample, the craze growth of PC in ethanol eventually stopped after some period of timewithout any fracture.

The influence of a sustained load on craze initiation was further studied by choosing arange of lower K i values. The craze increment profiles are shown in Figure 6. As found inFigure 3, the initial craze growth rate increases as Ki increases. For K i < 0.36 MPa*m 1/2

(where m designates meters), there was no obvious craze growth seen. However, forK, > 0.64 MPa*m /2, multiple crazing was seen immediately after ethanol reached the notchtip of the stressed PC specimens. Therefore, a critical value of the initial stress intensity fac-tor, below which no crazing occurs, will be within the range of 0.36 < Ki < 0.64 MPa*m112-

PC in Hexane

The effect of the solvent on craze initiation and growth was further studied with hexane,which has a 6 value of 7.3 (cal/cm 3) 1/2. This has about the same A 6 as that of ethanol withrespect to PC. With the same A 6, the same rate of crazing would be expected. However,the results in Figures 6 and 7 showed faster rates in ethanol than those in hexane. Thefunctional groups in these solvents result in different tendencies. Ethanol has a muchstronger tendency towards hydrogen bonding which results in a greater tendency to craze PC.

4

4 4

& K = 1.05 MPa Min'

o K -0.91 MParn'7 2 0 K 1 05MPa.rn2

0 1 = 0.78MParn M a K 0 91MPa r'

A K = 0.64 MPa rn'0 30

3 K = 0.36MPa m7rn 2

2 0 K = 0.78 MPa n"7

52

a , ao

a 0 0 0 0 0

0 0 0 0 0 0 0 0 ao a a A a

an E ) a --------- 0 -___1__ _0 10 20 300 400 0 100 200 300 4OO

Time tmin) Time (mini

Figure 6. Variation of craze length with time for PC Figure 7. Variation of craze in length with time for PCtested in ethanol at various K levels tested in hexane at various K levels.

In addition to the results of the craze increment profiles, the following observations weremade:

1. The sizes of the deformation zone ahead of the crack tip in hexane were smaller thanthose in ethanol under the same load level (Figure 8).

2. The critical !cad for craze initiation in hexane was higher than that in ethanol.

3. Multiple crazing was still seen, but to a lesser extent, in the crazing of hexane.

Crack Tip K

MPa. m112

-- 0.36

- 0.64 Crack Tip

- -0.78-- -

- -0.91- -

Crack Growth - Crack Growth

In Ethanol Direction in Hexane Direction

Figure 8. Sketch of the craze profiles near the notch chipfor PC tested in ethanol and in hexane.

5

PMMA in Ethanol

The craze growth rate of PMMA in ethanol (Figure 9) was much faster than that of PCat about the same percent K value relative to its critical load of fracture in air, K1 c. Kic [or1,212

PC was determined as 2.7 MPa*m ., and that for PMMA as 1.1 MPa*mt' 2 . The three curvesin Figure 9 of PC craze growth profiles are identical to those in Figure 3. PMMA, at a K,of 65% of its K1c, and PC at a K, of 93% of its K1c, h::d about the same initial growth rate.but the rate of growth of PC fell continuously to zero, while the PMMA specimen eventuallybroke into two pieces. This may be due to the presence of multiple crazing in PC, whichblunts the crack tip and slows down the craze growth.

_ , A M, _ K - 65% K

t = • • •

P C . K - 9 3 % K a :

• • o PC. K - 63% Kc

t. • rU PC. K - 44% Kc

4a

0a u

0 0 0~3%4

to o 2 0 3 4W

rime (min)

Figure 9. Comparative data for craze growthfor PC in PMMA in ethanol.

CONCLUSIONS

By using selected surface active agents with PC, solubility parameters can be used toattempt to predict the initiation and propagation of crazes and cracks. The role of the dif-ferent functional groups of the solvents must be considered. The results of this work haveshown that the higher the initial load, the faster the craze/crack propagation rate in both PCand PMMA. There was no obvious craze growth seen for PC in either ethanol or hexanc atstresses below some critical value. At stresses above that critical value, the rate of crazeinitiation and propagation of PC in ethanol is higher than that in hexane. This suggests thathydrogen bonding plays a significant role in solvent crazing. The presence of multiple craz-ing, which causes blunting at the crack tip, is responsible for the slower crack growth of PCcompared to that of PMMA in ethanol. Continued work in this area should include the useof solvents with solubility parameters of greater variation from the polymer than thoseselected here.

-4 6

APPENDIX

In LEFYM. it single parameter. K. the stress intensity factor, is used to relate the intensityof the elastic stress-strain field neat the crack tip to the loading and geometry of the body.Crack extension will occur when K reaches a critical value, K1t. The K1c, named plane strainfracture tougzhness, is expected to he a material parameter, provided that the plastic zone sizeadjacent to the crack tip is small compared to the relevant specimen dimensions of cracklcngoth, thickness, and width.

The plane strain fracture toughness can be determined according to the relationshipbetw-een the maximum load and the geometry of the compact tension specimens given in\STNI E399:

K, = P (Bw' 2 9((w)'2 185.5(a/w) + 655.7(a/w) - 1017(a/w) + 6SM.7(av,)"2

where P is the maximum applied load (kN), B is the thickness of the specimen (cm). a is the,.wick length (cm), % is the width of the specimen (cm), and Kic is in lPa (mi.

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I ATTN: AMXSY-CR (Mrs. F. Liu) APO New York 09189-2737

1 AMXSY-GC (Mr. F. Campbell) I ATTN: AEUSA-UN

1 AMXSY-MP (Mr. H. Cohen)Commander, U.S. Military Academy, Department of Physics,

Commander, U.S. Army Toxic and Hazardous Materials Agency, West Point, NY 10996-1790Aberdeen Proving Ground, MO 21010-5401 1 ATTN: MAJ Decker

I ATTN: AMXTH-ESI AMXTH-TE Headquarters, Wright Patterson Air Force Base, OH

45433-6503

Commander. U.S. Army Environmental Hygiene Agency, I ATTN: AFWAL/FIEEC

Aberdeen Proving Ground, MD 21010-5422 1 ASD/AESD

I ATTN: HSHB-O/Editorial Office I AAMRL/HET

I FTD-TQTR, Wright Patterson Air Force Base, OH 45433-6508

No. of No. ofCopties To Copies To

I AFWAL,'FIES/SURVIAC. Wright Patterson Air Force Base, Oil Commander, U.S. Army Foreign Science and Technology Center45433-6553 220 Seventh Street, NE. Charlottesville, VA ?2901-5396

1 ATTN: AIAST-CW2I AAIRL/TID. Wright Patterson Air Force Base, Oil 45433-6573

Director, Aviation Applied Technology Directorate,ommandant, U.S. Army Field Artillery School, Fort Sill, Fort Eustis, VA 23604-5577

OK 73503-5600 I ATTN: SAVRT-ATL-ASVI ATTN: ATSF-GA

Commander. U.S. Army Training and Doctrine Coimmand,Commander, Naval Air Development Center, Warminster, Fort Monroe, VA 23651-5000PA 18974-5000 I ATTN: ATCD-N

I ATTN: Code 60332 (D. Herbert)1 HO TAC/DRPS, Langley Air Force Base, VA 23665-5001

Commandant. U.S. Army Academy of Health Sciences,Fort Sam Houston, TX 78234-6100 Commander, U.S. Army Logistics Center, Fort Lee. VA

I ATTN: HSHA-CDH (Dr. R. H. Mosebar) 23801-6000"I HSHA-CDS (CPT Eng) I ATTN: ATCL-MGFI HSHA-IPM

Commander, U.S. Air Force Wright Aeronautical Labs,Headquarters, Brooks Air Force Base, TX 78235-5000 Wright-Patterson Air Force Base, OH 45433

1 ATTN: HSD/RDTK 1 ATTN: Dr. H. GrahamI HSD/RDS I Dr. R. RuhI USAFSAM/VNC 1 AFWAL/MLLP, Mr. D. Forney

I AFSC/MLLM, Dr. A. KatzCommander. U.S. Army Dugway Proving Ground. Dugway. I Aero Propulsion Labs, Mr. R. MarshUT 84022-5010

1 ATTN: STEDP-SO (Dr. L. Salomon) AVCO Corporation, Applied Technology Division, Lowell

Industrial Park, Lowell, MA 01887Commander, U.S. Army 3ugway Proving Ground, Dugway, 1 ATTN: Dr. r. VasilosLT 34022-6630

I ATTN: STEDP-SD-TA-F (Technical Library) Materials Research Laboratories, P.O. Box 50, Ascot tale,AIC 3032. Australia

HO, Ogden Air Mater1a Area, Hill Air Force Base, ATTN: Dr. C. W. WeaverUT 34056-5609

1 ATTN: MMWIM Case Western Reserve University, Macromolecular Science

Department. Cleveland, Ohio 44106Director, U.S. Army Comunications-Electronics Command, 1 ATTN: Dr. J. KoenigNight Vision and Electro-Optics Directorate, Fort Belvoir,VA 22060-5677 Southern Research Institute, 2000 Ninth Avenue South,

1 ATTN: AMSEL-NV-O (Dr. R. Buser) Birmingham, AL 35255I ATTN: R. B. Spafford

Commander, Marine Corps Development and Education Command,

Quantico. VA 22134-5080 PDA Engineerinq, 3754 Hawkins NE, Albuquerque, NM 371091 ATTN: Code D091, SPWT Section I ATTN: R. E. Allred

Deputy Director, Mari-e Corps Institute, Arlington, Swedlow, Inc., 12122 Western Avenue, Garden Grove,VA 22222-0001 CA 92641

1 ATTN: NBC CO; C002 1 ATTN: M. W. Preus

Commander, U.S. Army Nuclear and Chemical Agency, Strainoptics Technologies, Inc., 108 W. Montgomery7500 Backlick Road, Fldg. 2073, Springfield, VA 22150-3198 Avenue, North Wales, PA 19454

1 ATTN: MONA-CM 1 ATTN: A. S. Redner

Chief of Naval Research. 800 N. Quincy Street, Arlington, Polysar Inc., Plastics Division, 0.0. Box 90, 29 FullerVA 22217 Street, Leominster, MA 01453

1 ATTN: Code 441 1 ATTN: P. R. Cowley

Commander, U.S. Army Materiel Command, 5001 Eisenhower Loral Systems Group, P.O. Box 85, Litchfield, AZ 35340Avenue, Alexandria, VA 22333-0001 1 ATTN: J. Uram

I ATTN: AMCCNI AMCSF-C McDonnell Aircraft. P.O. Box 516, St. Louis. MO 631661 AMCLD I ATTN: Mr. J. Meador, Dept. 357

Commander, Defense Technical Information Center, CameronStation, Bldg. 5, 5010 Duke Street, Alexandria, VA Director, U.S. Army Materials Technology Laboratorv,22304-1145 Watertown, MA 02172-0001

2 ATTN: OTIC-FDAC 2 ATTN: SLCMT-TML2 Authors

Commander, Navul Surface Weapons Center, Dahlgren, VA 22448I ATTN: Code E4311I Code G51 (Brumfield)

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