A novel method to measure residual stresses in laminated composites

A non-destructive method, called the Cure Referencing Method (CRM), was developed to measure the residual stresses in laminated composites. The method incorporates the use of moire interferometry (a laser based optical method) and replication of diffraction gratings on the surface of laminates in the autoclave during cure. This allows the strains due to resin shrinkage and thermal expansion to be referenced back to the stress free state. For thermoset matrix composites, the stress free state exists just prior to matrix solidification. To determine the residual stress in multidirectional laminates, the strain difference from cure temperature to room temperature was first measured. Then, by doing the same for a unidirectional panel, that was cured simultaneously with the multidirectional laminate, the free thermal expansion of the unidirectional lamina could be defined for the multidirectional case. Using laminate theory, the residual stresses in each ply of the multidirectional laminate were calculated accordingly. The method was evaluated by measuring the residual stresses in balanced symmetric cross-ply, unbalanced symmetric cross-ply and symmetric angle-ply AS4/3501-6 graphite/epoxy laminates. To determine the repeatability of the method, five separate autoclave runs were performed, each with the three multidirectional panels, and the unidirectional panel.