A constitutive model for non-linear behavior of SMC accounting for linear viscoelasticity and micro-damage

An approach for modeling sheet molding compound (SMC) composites as viscoelastic damageable material is presented. Continuum damage mechanics theory by Chow and Wang (Int J. Fract., 33, 3 (1987)) was used in combination with linear viscoelasticity. The model was applied to a modern SMC composite material containing both hollow glass spheres for low density and toughening additive for improved impact resistance. Tensile tests and uniaxial creep test were employed to build the constitutive model. Validation was done by comparing test data with simulations of uniaxial creep on material with different degrees of damage. The model has good accuracy at moderate damage levels under controlled time-dependent crack propagation. Tensile testing at two different fixed strain rates was simulated using quasi-elastic method to calculate relaxation modulus. The model predicts the stress-strain curve with good accuracy until the region is close to failure, where new mechanisms not accounted for are taking place. Finally, a simulation of a cyclic tensile test with increasing maximum strain per cycle was performed, and since both damage and viscoelasticity are included in the model, the slope change, accumulation of residual strain, and hysteresis in the stress-strain, loading-unloading curve are predicted. POLYM. COMPOS., 26:84-97, 2005. (C) 2004 Society of Plastics Engineers