Thermal Fatigue Cracking of an IM7/5250-4 Cross Ply Laminate: Experimental and Analytical Observations

Coupons of a balanced and symmetric cross ply laminate with polished edges made from IM7/5250-4 were thermally cycled between room and liquid nitrogen temperatures to induce cracks in the matrix. Edge crack densities for each symmetric lamina pair were recorded as a function of the number of cycles. Coupons with an epoxy coating on the polished edges experienced minimal cracking in all interior plies compared to significant cracking for uncoated specimens. This suggests that edge effects play a dominant role in crack initiation. Crack counts were quite different than recorded by another research group for nearly identical tests. Thermal shock was eliminated as a possible significant contributor to the overall physics driving crack formation. Stress analysis was used to support discussions that explain some of the observed trends in crack densities in the various ply pairs. The number of thermal cycles reached 2600 without a clear indication of crack saturation in any of the plies.