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Viscoelastic constitutive models for evaluation of residual stresses in thermoset composites during cure Zobeiry, Nima

Abstract

A particularly important aspect in the behaviour of thermoset matrix composite materials during the manufacturing process is the development of mechanical properties of the matrix and the resulting buildup [sic] of stresses. The behaviour of the matrix is generally acknowledged to be viscoelastic, and as both temperature and degree of cure vary with time, the characterization and representation of the behaviour is both critical and complex. Different approaches have been suggested for modeling this behaviour. The common approaches that invoke the simple linear elastic cure hardening model have been shown to provide good predictions but have not been studied for their accuracy and applicability. More sophisticated representations of viscoelastic behaviour are the Prony series of Maxwell elements implemented in finite element codes in 3D hereditary integral forms. In this thesis, different constitutive models are considered and their suitability for representing the behaviour of composite materials during cure is studied. The presented models provide the user with a range of options depending on whether costs or accuracy of solutions are of primary concern. For elastic hardening models, it is shown that the full viscoelastic formulations can be progressively simplified, and that these simplifications are valid for the typical cure cycles. It is shown that in general if these models are property calibrated they are valid and efficient pseudo-viscoelastic models. It is also noted that these models are not always applicable and an efficient viscoelastic model is needed. For such cases, viscoelastic behaviour of the polymer is represented using a differential form approach. It is shown that this form is equivalent to the more common integral form, but has significant benefits in terms of extension to more general descriptions, ease of coding and implementation, and computer runtimes. This formulation is extended to composite materials, using an appropriate micromechanical approach, and to 3D behaviour with finite element implementation such that it can be used with an existing code. Some important features are included, such as time-variability of all material properties, methods for calculating polymer and fibre stresses, and considering thermoelastic effects. Several case studies are presented for verification/validation purposes and to highlight various features of the models.

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