Design of a novel conduction heating based stress-thermal cycling apparatus for composite materials and its utilization to characterize composite microcrack damage thresholds

摘要

The objective of this research was to determine the effect of thermal cyclingcombined with mechanical loading on the development of microcracks in M40J/PMR-II-50, the second generation aerospace application material. The objective was pursued byfinding the critical controlling parameters for microcrack formation from mechanicalstress-thermal cycling test.Three different in-plane strains (0%, 0.175~0.350%, and 0.325~0.650%) were appliedto the composites by clamping composite specimens (M40J/PMR-II-50, [0,90]s, a unitapecross-ply) on the radial sides of half cylinders having two different radii (78.74mmand 37.96mm). Three different thermal loading experiments, 1) 23oC to ??????196oC to 250oC,2) 23oC to 250oC, and 3) 23oC to -196oC, were performed as a function of mechanical inplanestrain levels, heating rates, and number of thermal cycles. The apparatus generatedcracks related to the in-plane stresses (or strains) on plies. The design and analysisconcept of the synergistic stress-thermal cycling experiment was simplified to obtain main and interaction factors by applying 2k factorial design from the various factorsaffecting microcrack density of M40J/PMR-II-50.Observations indicate that the higher temperature portion of the cycle under loadcauses fiber/matrix interface failure. Subsequent exposure to higher stresses in thecryogenic temperature region results in composite matrix microcracking due to theadditional stresses associated with the fiber-matrix thermal expansion mismatch.