THE EFFECT OF SEAWATER EXPOSURE ON MODE-I INTERLAMINAR FRACTURE AND CRACK-GROWTH IN GRAPHITE EPOXY

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 fatigue 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.