ANALYSIS OF MICRODAMAGE IN THERMALLY AGED CF/POLYIMIDE LAMINATES

Microdamage in the layers of CF Thornel (R) T650 8-harness satin-weave composites with a thermosetting NEXIMID (R) MHT-R polyimide resin, designed for high service temperatures, is analyzed. After cooling down to room temperature (RT), a multiple intrabundle cracking due to tensile transverse thermal stresses was observed in the [(+45/-45)/(90/0)](2s) laminates studied. Then, the composite was subjected to two ramps of thermal cycling quantifying the increase in crack density in its layers. A comparison of two ramps with the same lowest temperature showed that the highest temperature in the cycle where thermal stresses were low had a significant detrimental effect on the thermal fatigue resistance of the composite. The effect of holding it at 288 degrees C for 40 days was also studied: many new cracks formed in it after cooling down to RT. During the time at the high temperature, the mechanical properties degraded with time, and the crack density versus aging time was measured at RT. Then, both aged and nonaged specimens were tested in uniaxial quasi-static tension quantifying the damage development in layers of different orientation. Cracking in the layers was analyzed using fracture mechanics arguments and probabilistic approaches: a) a simple one, not considering crack interaction; b) Monte Carlo simulations. It is shown that cracking in the off-axis layers which are not in contact with the damaged 90 degrees-layer can be predicted based on the Weibull analysis of the 90 degrees-layer, whereas in the off-axis layer contacting the 90 degrees-layer, the crack density is much higher due to the local stress concentrations caused by cracks in the 90 degrees-layer. The thermal treatment degraded the cracking resistance in the surface and adjacent layer, whereas the composite close to the midplane was not changed.