The Effect of Seawater Exposure on Mode I Interlaminar Fracture and Crack Growth in Graphite/Epoxy

Abstract

The effects of seawater exposure on the mechanics of Mode I fracture and crack growth in unidirectional AS-4/3501 graphite/epoxy were investigated utilizing the previously developed compliant load-frame (CLF) technique. Mode I double-cantilever- beam (DCB) specimens were exposed to fresh flowing seawater under both static and fa tigue loading conditions. Long-term fatigue loading in air resulted in extensive bridging of reinforcing fibers across the crack tip, increasing the apparent fracture resistance of the material. The primary effect of seawater was to weaken the fiber-to-matrix bond in the composite, causing an increase in the number of bridged fibers. Fiber bridging increased the fracture resistance of the DCB test specimens and reduced the rate of crack growth to near zero. An increase in the fracture resistance of specimens soaked in seawater but not loaded was also attributed to a weakened fiber-to-matrix bond. The compliant load-frame (CLF) technique was found to be a simple and cost-effective method for studying the mechanics of crack growth under environmental exposure conditions. However, linear- elastic fracture mechanics (LEFM) relationships derived previously for the CLF become invalid when the fracture resistance of the material increases during crack growth, e.g., during fiber bridging.

Keywords

fiber bridging fracture-mechanics environmental cracking stress- corrosion corrosion-fatigue graphite/epoxy composites seawater.

Get full access to this article

View all access options for this article.

References

Carlsson, L.A. and R.B. Pipes. 1987. Experimental Characterization of Advanced Composite Materials. Prentice-Hall.

Colins, J.A. 1981. Failure of Materials in Mechanical Design NY: Wiley.

Dharan, C.K.H. 1978. "Fracture Mechanics of Composite Materials," J. of Engrg. Matls. and Tech., 100:233-247.

Friedrich, K., ed. 1989. Application of Fracture Mechanics to Composite Materials, Composite Material Series, No. 6. NY: Elsevier .

Garg, A. and O. Ishai. 1985. "Hygrothermal Influence on Delamination Behavior of Graphite/Epoxy Laminates." Engrg. Fracture Mechanics, 22:413-427.

Hojo, M. , K. Tanaka , C.G. Gustafson and R. Hayashi. 1987. "Effect of Stress Ratio on Near-Threshold Propagation of Delamination Fatigue Cracks in Unidirectional CFRP," Composites Science and Technology, 29:273-292.

Johnson, W.S. and P.D. Mangalgiri . 1987. "Investigation of Fiber Bridging in Double Cantilever Beam Specimens," J. of Composites Tech and Research, 9:10-13.

Kaelble, D.H. , P.J. Dynes and E.H. Cirlin. 1974. "Interfacial Bonding and Environmental Stability of Polymer Matrix Composites," J. of Adhesion , 6:23-48.

Kaelble, D.H. , P.J. Dynes , L.W. Crane and L. Maus. 1974. "Interfacial Mechanisms of Moisture Degradation in Graphite-Epoxy Composites," J. of Adhesion , 7:25-54.

10.

Kinloch. A.J. 1985. "Mechanics and Mechanisms of Fracture of Thermosetting Epoxy Polymers," Epoxy Resins and Composites, Advances in Polymer Science Series, No. 72 , I. K. Dusek, ed., New York: Springer-Verlag , 1:45-67.

11.

Macander, A. , M. Silvergleit and H.P. Edelstein. 1979. "Marine Durability of a Graphite/Epoxy Composite Subjected to Static and Fatigue Loading," Mechanical Behavior of Materials. New York: Pergamon Press, pp. 333-343.

12.

May, C. A., ed. 1988. Epoxy Resins: Chemistry and Technology. New York: Marcel Dekker.

13.

Ramkumar, R.L. and J.D. Whitcomb. 1985. "Characterization of Mode I and Mixed-Mode Delamination Growth in T300/5208 Graphite/Epoxy," Delamination and Debonding of Materials, ASTM STP 876, pp. 315-335.

14.

Reifsnider, K.L. , K. Schulte and J.C. Duke. 1983. "Long-Term Fatigue Behavior of Composite Materials," Long-Term Behavior of Composites , ASTM STP 813, pp. 136-159.

15.

Russell, A.J. and K.N. Street. 1985. "Moisture and Temperature Effects on the Mixed-Mode Delamination Fracture of Unidirectional Graphite/Epoxy," Delamination and Debonding of Materials, ASTM STP 876, pp. 349-370.

16.

Russell, A.J. and K.N. Street. 1988. "A Constant ΔG Test for Measuring Mode I Interlaminar Fatigue Crack Growth Rates," Composite Materials: Testing and Design (Eighth Conference), ASTM STP 972, pp. 259-277.

17.

Sandifer, J. 1982. "Effects of Corrosive Environments on Graphite/Epoxy Composites," Progress in Science and Engineering of Composites , T. Hayashi, K. Kawata and S. Umewaka, eds., ICCM-IV , Tokyo, pp. 979-986.

18.

Sloan, F.E. and R.J. Seymour. 1990. "Environrnental Testing Using a Compliant Load-Frame." J. of Composite Materials , 24:727-738.

19.

Sumsion, H.T. 1976. "Environmental Effects on Graphite-Epoxy Fatigue Properties," J. of Spacecraft, 13:150-155.

20.

Tsotsis, T.K. and Y. Weitsman. 1990. "Energy Release Rates for Cracks Caused by Moisture Absorption in Graphite/Epoxy Composites," J. of Composite Materials, 24:483-496.

21.

Whitney, J.M. , C.E. Browning and W. Hoogsteden. 1982. "A Double Cantilever Beam Test for Characterizing Mode I Delamination of Composite Materials," J. of Reinforced Plastics and Composites, 1:297-313.

22.

Zaikov, G.E. 1988. Diffusion of Electrolytes in Polymers Ultrecht, The Netherlands: VSP.