Determination of residual stress and thermal history for IM7/977-2 composite laminates

As graphite/epoxy composites become more popular in advanced structural design, there is a need to better understand their mechanical behavior in extreme temperature environments. Composites offer many advantages over traditionally used materials; however, they tend to be susceptible to residual stress-induced failure. Residual strains, residual stresses, and thermal history of a composite panel are determined over a broad temperature range by using a combination of strain gages and an optical technique called the Cure Reference Method (CRM). CRM is an accurate, full field, method used to determine the strain on the surface of a composite via Moire Interferometry. CRM also enables the accurate determination of the chemical shrinkage of the epoxy matrix during cure. Classical Laminate Theory (CLT) is widely used to predict stress and strain in composite panels, yet it does not account for the chemical shrinkage of the epoxy. The paper describes a method to characterize multidirectional laminate behavior based on a few tests carried out on a unidirectional laminate. The purpose is to eliminate testing of each desired lay-up, by modifying CLT to predict the behavior of a multidirectional laminate based solely on the behavior of a unidirectional sample of the same material system. Testing various lay-ups and comparing analytical to experimental results is used to validate the model.