Calculation of Residual Stresses of Cross-Ply Laminates

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Residual stresses in composite laminates are primarily caused by the difference between the volume change in the polymer matrix and the reinforcing fibres. In resins, volume changes are produced by the chemical shrinkage during the cure and by thermal expansion through the whole manufacturing process. In the laminate plane the resin volume change is restricted, which causes quite large dimension change in thickness direction. In addition, global and local structures of the laminate are engaged such that they together determine the laminate temperature, degree of cure and dimension change histories. Unlike most existing work, which model the laminates either in global or in local level only, a calculation procedure of the residual stresses is developed using both the global and the local model. A perfect coupling between the models has not been reached, but the residual stresses in the local structure are obtained by using the results of the global structure as boundary conditions. The computations address that the local stresses in the resin and fibres differ remarkably from the macro scale stresses in the laminate layers. Also, it is necessary to model the laminate in three dimensions, because the residual stresses in the resin are closely related to the changes in the laminate thickness.