Modelling the mechanical and hygrothermal behaviour of composite laminates using a high-order displacement formulation

The response of composite laminates to mechanical, thermal and hygroscopic loads is examined. A third-order displacement formulation, which incorporates transverse normal and shear strain, is employed in order to establish the three-dimentional constitutive relations for a composite laminates. Results are presented for a wide variety of graphite-reinforced materials of moderate thickness including: cross-ply [0/90]and [±30/0] , which process a high ratio of inplane modulus to transverse shear modulus and a high ratio of transverse to inplane thermal, hygroscopic expansion coeficients. A computer program has been developed. The stresses and strains within a laminate due to loads, due only to the changes in temperature and absorption of moisture (hygrothermal effect) are studied. It is shown that the environmental effects in considered composites are very important. Numerical results based on this third-order theory formulation are compared with those obtained by using the Kirchoff hypothesis (classical lamination theory). The results show that surprising discrepancies exist among the present theory and the classical lamination theory, which indicate the importance of incorporating the effect of transverse normal and shear strains in the mechanical and hygrothermal analysis of composite laminates.