Non-linear Behavior of Polymer Based Composite Laminates under Cyclic Thermal Shock and Its Effects on Residual Stresses

The residual stresses in composite laminates depend on several factors includingthe conditions of fabricating process, property of materials, direction andlay-ups of the layers. However the thermo-elastic behavior of composite isone of the most important parameters affecting the magnitude of residual stresses. Theelastic properties and residual stresses do not remain constant under cyclic thermalloading and vary in a non-linear manner. The study of these non-linear variations ofmodulus and strength and their effects on residual stresses in laminates while experiencingthermal load cycles is the main goal of this paper. An experimental deviceis designed and manufactured conveniently to exert defined thermal load cycles withdifferent temperatures and cycle time on composite laminates. Then orthotropic glassepoxy composite laminates made by hand lay-up are tested under cyclic thermal loading.The elastic modulus and fracture strength of the samples are measured beforeand after experiencing defined number of thermal shocks. The residual stresses incomposite laminates are calculated and compared based on the actual behavior ofcomposite and by means of a modified classical laminate theory. The results of thisstudy demonstrate that the non-linear behavior of composites influences the residualstresses significantly. In addition, if the sample becomes more brittle, the residualstrains remain constant; while the elasticity modulus and residual stresses decrease