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A micromechanically motivated phase-field damage model is proposed to investigate the fracture behaviour in crosslinked polyurethane adhesive. The crosslinked polyurethane adhesive typically show viscoelastic behaviour with geometric nonlinearity. The finite-strain viscoelastic behaviour is modelled using a micromechanical network model considering shorter and longer chain length distribution. The micromechanical viscoelastic network model also consider the softening due to breakage/debonding of the short chains with increase in deformation. The micromechanical model is coupled with the phase-field damage model to investigate the crack initiation and propagation. Critical energy release rate is needed as a material property to solve phase-field equation. The energy release rate is formulated based on the polymer chain network. The numerical investigation is performed using finite element method. The force-displacement curves from the numerical analysis and experiments are compared to validate the proposed material model.

Crosslinked polyurethane adhesives show large deformation viscoelastic behaviour and age under the moisture influence because of their hygroscopic behaviour. The viscoelastic behaviour of the material is modelled with the micromechanical network model. The micromechanical model considers the shorter and longer chains with a probability distribution function. The network evolution concept is used to model softening of material due to the breakage of the shorter chains with an increase in deformation. The moisture diffusion in the polyurethane adhesive is behaviour, therefore Langmuir-type diffusion model is used to model moisture diffusion. The transported moisture in the material leads to an exponential decay in the mechanical properties causing the ageing of the material. The micromechanical model needs to be coupled with the Langmuir-type diffusion model to analyse the ageing process, where the mechanical properties are considered as the function of the local moisture concentration.