Characterization of microcrack development in BMI-carbon fiber composite under stress and thermal cycling

The objective of this research was to determine the effect of thermal cycling on the development of microcracks in bismaleimide (BMI)-carbon fiber composites (5250-4 RTM/1M7 4-harness satin weave fabric). By clamping composite specimens on the radial sides of half cylinders having two different radii (78.74 and 37.96 mm), two different strain conditions with respect to the neutral axis (-0.406 to 0.406% and -0.843 to 0.843%) were applied to the composites. Three different thermal cycling experiments: (1) -196 to 250degreesC, (2) 23degreesC to (i) 150degreesC, (ii) 200degreesC, (iii) 250degreesC, and (3) -196 to 23degreesC were performed as a function of stress, number of thermal cycles, heating or cooling rate, and humidity conditions. An in situ monitoring microscope was used to observe the microcrack development during the experiment. The results suggest that there is a higher probability of microcracking with increasing number of thermocycles, higher prestrain, and humidity. The principle findings are that the full cycles from -196 up to 250degreesC cause the most significant microcrack development. Observations indicate that the high-temperature portion of the cycle under load causes fiber-matrix interface failure. Subsequent exposure to higher stresses in the cryogenic temperature region results in composite matrix microcracking due to the additional stresses associated with the fiber-matrix thermal expansion mismatch.