Time-dependent deformation and damage growth in a rubber-toughened fiber composite

Time-dependent deformation and damage growth was studied in a rubber-toughened carbon fiber composite. Cyclic creep/recovery loading was performed on unidirectional off-axis coupons to derive the transverse and shear moduli. Constant stress-rate experiments were also performed using modal acoustic emission monitoring. This monitoring provides indirect evidence of what is believed to be the primary damage mechanisms in the material studied, matrix cracking and fiber/matrix debonding. Three significant findings from this study are emphasized in this paper. The first is the viscoelastic and viscoplastic behavior of the material with growing damage. Although a complete characterization has not been performed, emphasis is on material behavior believed to be not previously reported in the literature. The second is a new Damage Effect Study that quickly identifies the material parameters in a nonlinear viscoelastic constitutive theory that are affected by damage. This study can assist in streamlining the characterization process. Finally, a simplified material model for the microstructure is developed based on AE monitoring of damage growth. This results in damage evolution equations based on AE data and viscoelastic fracture mechanics. The damage equations correlate AE data for different loading histories. The use of direct monitoring to develop damage evolution equations and the Damage Effect Study reflect a new approach to characterization testing of time-dependent materials.