In this paper, a novel numerical-experimental methodology is outlined to predict delamination in pristine as well as isothermally aged (in air) polymer matrix composites. A rate-dependent viscoelastic cohesive layer model was implemented in an in-house test-bed finite element analysis (FEA) code to simulate the delamination initiation and propagation in unidirectional polymer composites before and after aging. This unified model is fully rate-dependent and does not require a pre-assigned traction-separation law. The actual shape of traction separation law depends on: (a) the strain rate via the viscoelastic constitutive relationship, (b) the degree of thermo-oxidative aging via the changes in the experimentally measured creep compliance due to oxidation, and (c) the evolution of the internal state variable defining the state of damage. To determine the model parameters, double cantilever beam (DCB) experiments were conducted on both pristine and isothermally aged IM-7/bismaleimide (BMI) composite specimens. The J-Integral approach was adapted to extract cohesive stresses near the crack tip. A principal-stretch dependent internal damage state variable defines the damage in the cohesive layer. Within the cohesive layer, pristine and cohesive stresses were compared to estimate the damage parameters. Once the damage parameters had been characterized, the test-bed FEA code employed a micromechanics based viscoelastic cohesive layer model to simulate interlaminar delamination. From a numerical stability standpoint, the viscous regularization effect of the viscoelastic constitutive equations in the cohesive layer helps mitigate numerical instabilities caused by elastic energy released due to crack growth, thereby enabling the FEA model to simulate the load-deflection response of the composite structure well beyond peak load. The present cohesive-layer based FEA model was able to accurately predict not only the macro level load-displacement curve, but also the micro level crack growth history in IM-7/BMI laminate before and after thermal aging, using only three parameters.

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