Long-term thermo-oxidative aging in composite materials: Failure mechanisms

https://doi.org/10.1016/S0266-3538(97)00123-1Get rights and content

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 GIc (mode I), GIIc (mode II) and Gc (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.

References (35)

  • J.H. Flynn et al.

    A quick, direct method for the determination of activation energy from thermogravimetric data

    Polym. Lett.

    (1966)
  • H.L. Friedman

    Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic

    J. Polym. Sci: Part C

    (1965)
  • J.B. Henderson et al.

    A study of the kinetics of high-temperature carbon-silica reactions in an ablative polymer composite

    Polym. Compos.

    (1983)
  • M.A. Grayson et al.

    On the use of a kinetic map to compare the thermal stability of polymeric materials undergoing weight loss

    (1993)
  • H.T. Lee et al.

    Effect of curing temperature on the thermal degradation of an epoxide resin

    J. Appl. Polym. Sci.

    (1969)
  • M. Arnold et al.

    Problems of the characterization of thermoanalytical processes by kinetic parameters

    J. Thermal Anal.

    (1979)
  • R.C. Hipp et al.

    Accelerated aging and methodology development for polymeric composite material systems

  • Cited by (0)

    View full text