Process Simulated Laminate (PSL) : A Methodology to Internal Stress Characterization in Advanced Composite Materials

Solidification, under nonisothermal conditions and pressure gradients present during processing, is shown to be a major contributor to the generation of internal stresses in advanced composite materials. These stresses may be amplified in composite materials with significant anisotropic characteristics. Using the process simulated lami nate (PSL) technique, a methodology was developed for the evaluation of the residual stress distributions induced during processing. The PSL contained separation films placed between certain layers of the laminate, enabling separation of the laminate after process ing. Using a strain gage attached to the laminate, the residual stress distribution over the laminate was calculated. The detached laminates served as specimens for the evaluation of through-thickness morphological and property distributions. The direct relationship be tween the residual stresses and the investigation of stress releasing phenomena such as voids, microcracks and/or fiber buckling was important for a complete evaluation of pro cess induced effects. Different techniques for residual stress evaluation were presented and compared for pressformed 40-ply unidirectional composites cooled at a rate of 20-50°C/s. A pronounced skin-core stress profile was observed in unidirectional PEEK/AS4 compos ites demonstrating compressive stresses at the surface and tensile stresses at the center of the laminate.