Environmental factors that affect matrix cracking in composite laminates include temperature, moisture content, and hygrothermal history. Matrix cracking in laminates exposed to aircraft environments was modelled using a fracture mechanics approach based on strain energy release rates. Residual stress relaxation due to hygrothermal history was found to alter the static mechanical strain necessary to cause matrix cracking in experiments. Depending on material type and hygrothermal history, this effect was shown to either increase or decrease the resistance to matrix cracking. Close agreement between predicted and measured strains at the onset of matrix cracking was obtained using a modification to the fracture mechanics approach. Viscoelastic stress relaxation was modelled as shifts in the stress free temperature. This approach seems feasible providing that matrix crack growth is an elastic phenomena. Matrix crack growth in laminates subjected to creep loads and severe environments may require the development of viscoelastic failure criteria.

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