Fatigue-induced stress-softening in cross-linked multi-network elastomers: Effect of damage accumulation

Cross-linked elastomers are reinforced elastomers with high stretchability and toughness. However, most elastomeric materials lose their toughness due to the accumulation of damage under cyclic loadings during their lifetime. While recent advances in the process and characterization of the multi-network elastomers have led to significant improvements in their properties, our understandings of the accumulated damage mechanisms within the material remain sparse and inconclusive. Here, a physically motivated constitutive model is presented for multi-network elastomers subjected to a high number of cyclic deformations. The model can be particularly used to elucidate the inelastic features, such as permanent damage during the deformation of each cycle. The observed damage may be induced from the chain scission, chain slippage, polymer relaxation, or fatigue detachment of polymer chains. Therefore, irreversible chain detachment and decomposition of the network due to its highly cross-linked structure are explored as the underlying reasons for the nonlinear progressive stress softening phenomenon. The model is validated against the uni-axial tension cyclic experimental tests. The results show that progressive breakage of the chains is responsible for damage accumulation in the material. The model with a few physically motivated material constants is expected to provide a tool to understand the complex nature of constitutive behavior in the multi-network elastomers.