Thermal Microcracking in Celion 6000/PMR-15 Graphite/Polyimide
Six laminate configurations were subjected to five different thermal exposures in the temperature range 78K to 603K (−320°F to 625°F), and then studied using microscopy and x-ray to determine the characteristics of microcracks formed during the thermal loadings. The laminates studied were: s, [02/902]s, [(0/90)3]s, [45/−45/0/90]s, [0/45/90/−45]s, and [0/60/0/−60]s. The material system investigated was found to be free of cracks after curing, but microcracks did develop in most laminates when cooled from 603K (625°F) by quenching in ice water or liquid nitrogen. Crack density was dependent on laminate configuration and rate of cooling. Microcracks present at free edges extended across the entire width of the specimens. The [45/−45/0/90]s laminate proved to be very resistant to microcracking for all thermal loadings. The thermal load required to initiate microcracking, determined using laminate analysis with stress and temperature dependent material properties, compared reasonably well with experimental results.
KeywordsResidual Stress Thermal Loading Free Edge Crack Density Thermal Exposure
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