Residual stress and thermal expansion of graphite epoxy laminates subjected to cryogenic temperatures

In composite structures understanding and quantifying the existence of residual stresses is a very important issue. The presence of these residual stresses can lead to unexpected failure of structures as seen in the fuel tank of the X-33 reusable launch vehicle. A study into the development of residual stresses as a function of temperature was performed to help provide insight into this situation. A recently developed optical technique, the cure reference method, and a modification of traditional strain gage techniques were used in this pursuit. Diffraction gratings and strain gages were applied to IM7/977-2 composite panels of differing stacking sequences. Strains were measured under no load while temperature was varied. Consequently stresses developed between the plies and a method to calculate them was proposed. Residual stress levels were dependent on stacking sequence and quasi-isotropic laminates perform poorly when exposed to cryogenic temperatures. Laminates should be optimized to account for both mechanical and thermal loading.