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 det ermine the interfacial strength (normal to the fiber) of an interlayer hybrid composite which has been exposed to seawater for two different lengths of rime. Specimens were tested in transverse tension in an en vironmental scanning electron microscope. The specimens were tested in the as-received condition, after they reached saturation, and 8 month s 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 super-posi tion to determine the stress at the fiber/matrix interface revealed se veral interesting findings. For example, as moisture is added to a com posite, the hydrothermal residual stresses in resin-rich inhomogeneiti es change front tensile to compressive. Assuming a constant interfacia l strength, this should make it more difficult to initiate damage in c onditioned specimens. As this was not the case, moisture appeared to h ave a slight degrading effect on the interfacial strength. However, pr opagation of damage away front the resin-rich regions requires a highe r stress than that required to initiate this damage. Moisture actually helped to arrest damage growth causing the ply stress required to cau se transverse cracking to increase with increased moisture content and with increased aging time. (C) 1997 Elsevier Science Limited.