Determination of the effect of seawater on the interfacial strength of an interlayer E-glass/graphite/epoxy composite by in situ observation of transverse cracking in an environmental SEM

In an effort to understand the failure behaviour and to determine the effects of seawater on composites, a program has been developed to determine the interfacial strength (normal to the fiber) of an interlayer hybrid composite which has been exposed to seawater for two different lengths of time. Specimens were tested in transverse tension in an environmental scanning electron microscope. The specimens were tested in the as-received condition, after they reached saturation, and 8 months after they reached saturation. Observations revealed that damage was initiated at the boundaries of resin-rich regions, regardless of the conditioning process. Analytical results obtained by linear superposition to determine the stress at the fiber/matrix interface revealed several interesting findings. For example, as moisture is added to a composite, the hydrothermal residual stresses in resin-rich inhomogeneities change from tensile to compressive. Assuming a constant interfacial strength, this should make it more difficult to initiate damage in conditioned specimens. As this was not the case, moisture appeared to have a slight degrading effect on the interfacial strength. However, propagation of damage away from the resin-rich regions requires a higher stress than that required to initiate this damage. Moisture actually helped to arrest damage growth causing the ply stress required to cause transverse cracking to increase with increased moisture content and with increased aging time.