Thermal fatigue induced microcracking in graphite-epoxy laminated composites

Four experiments were conducted in order to determine micro-crack densities in a CFRP cross-ply laminate subject to cryogenic thermal cycling in liquid nitrogen. In the first experiment, cross-ply [0/90]2 S IM7/5250-4 coupons with free-edges were cycled 2600 times. Cracks formed early in all plies except 3 and 6 and accumulated throughout cycling with the most damage occurring in plies 2 and 7. In the second experiment, similar coupons with laminate edges coated in epoxy were cycled 1600 times in order to simulate a material whose fibers ends are not directly exposed. These coupons had delayed crack formation and all interior plies had much lower crack densities when compared to the free-edge coupons. A third experiment used nitrogen gas to cool the coupons prior to full submersion in liquid nitrogen to test the theory that thermal shock and associated high temperature gradient may play a role in microcracking. Cycling to 325 cycles yielded results similar to the free-edge experiment. For the fourth experiment, free-edge coupons were soaked in a water bath for 10 days prior to thermal cycling. The resulting moisture content was 1.54%. A total of 1600 thermal cycles were then conducted in a controlled high (80-90%) relative humidity environment to maintain the moisture content. The outer plies resisted cracking until after 400 cycles and the crack density remained low compared to the free-edge results. No cracking was observed on any of the interior plies indicating that moisture content is a significant microcracking parameter.;Three finite elements models were used to further interpret the test data. The first model simulated a worst-case thermal gradient by modeling the outer ply at liquid nitrogen temperature and keeping the inner plies at room temperature. The second model was an unsymmetric laminate strip and aided in establishing an estimate for the stress-free temperature of the laminate. A third model simulated periodic cracks in all plies so that the nominal stress field between cracks as a function of crack density could be estimated.