Thermal/mechanical Response and Damage Growth in Polymeric Composites at Cryogenic Temperatures

In order to increase the reliability of the next generation of space transportation systems, the mechanical behavior of polymeric matrix composite (PMC) materials at cryogenic temperatures must be investigated. This paper presents experimental data on the residual mechanical properties of a carbon fiber polymeric composite, IM7/PETI-5 both before and after aging at cryogenic temperatures. Tension modulus and strength were measured at room temperature, –196°C, and –269°C on five different specimen ply lay-ups, [0 ] 1 2 , [90]12, [±45]3S, [±25]3 S and [45,903,-45,03,45,903,45]. Specimens were preconditioned with one set of coupons being isothermally aged for 555 hours at –184C in an unloaded state. Another set of corresponding coupons were mounted in constant displacement fixtures such that a constant uniaxial strain was applied to the specimens for 555 hours at –184C. The measured lamina level properties indicated that cryogenic temperatures have an appreciable influence on behavior, and residual stress calculations based on lamination theory showed that the transverse tensile ply stresses could be quite high for cryogenic test temperatures. Microscopic examination of the surface morphology showed evidence of degradation along the exposed edges of the material due to aging at cryogenic temperatures.