Hygromechanical coupling in laminate composites and its effect on interlaminar failure

Graphite-Phenolic (GPh) laminates are used as structural parts under mechanical and hygrothermal loadings. High shrinkage strains during drying may cause premature interlaminar failure which originates from the hygromechanical coupling. In this study, modified coupled hygromechanical governing equations which include non-constant coefficient of moisture expansion (CME), were derived. The model was experimentally validated by measuring transient weight and strain between various equilibrium states of humidity, obtaining relevant material parameters. Finally, a set of three point bend specimens was used to obtain the strength profile during drying, which revealed a minimal value at a characteristic time. Using a non-local failure criterion it was possible to associate the temporal strength profile with the build-up of hygromechanical stresses.