Abstract
This work, an earlier study (Tsotsis, T. K., J. Compos. Mater., 1995, 29(3) 410–422) and a companion paper (Tsotsis, T. K., J. Compos. Mater., in press) constitute a comprehensive analysis of the effects of thermo-oxidative aging (177 °C in air for up to 10 000 h) on the thermal, fracture and compressive behavior of two carbon-fiberreinforced epoxy composites: G30-500/R922-1 and G30-500/R6376. The two materials studied were not designed for long-term use at 177 °C but were chosen as model systems for establishing a meaningful aging test methodology. This paper focuses on the mechanisms controlling property degradation to determine the roles of matrix, fiber and interface on the aging behavior observed. Delamination-critical strain-energy release rates G Ic (mode I), G IIc (mode II) and G c (edge delamination) clearly showed the effects of matrix and, especially, interface degradation as a consequence of aging. Compressionafter-impact (CAI) strengths exhibited a sharp drop during initial aging because of such effects and then leveled off at longer aging time as extensive delamination induced other failure modes (e.g. transverse cracking and fiber breakage) to limit the damage zone growth. Examination of aged specimens by means of scanning electron microscopy revealed pronounced degradation regions close to composite surfaces or edges, indicating that the oxidative diffusion mechanism dominated the degradation process. Residual stresses arising from aging-induced differential resin shrinkage and interaction between plies of different orientations were found to have a strong effect on the degradation process for plies close to the surface and, especially, near free edges. Comparison between the toughened (R6376) and standard (R922-1) resins suggests that resin fracture toughness contributed to thermo-oxidative resistance by suppressing resin cracks which would create additional paths for oxygen diffusion. This study revealed the importance of matrix and interface degradation, matrix toughness, ply interaction, and edge effects in the thermo-oxidative aging of polymeric composites.
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