Predictions of manufacturing induced shape distortions high performance thermoset composites

Abstract: High performance composites usually consist of continuous fibres and a thermoset matrix. A well-known example is carbon fibre epoxy composites. When this kind of material is cured residual stresses and/or shape distortions are produced owing to thermally and chemically induced volumetric strains. The cure means the manufacturing step where the thermoset matrix is transformed from a liquid to a solid material. It is a quite complex thermal- chemical- mechanical process that in addition to volumetric strains, involves heat generation and dramatic changes in mechanical properties. For manufacturing of parts with high shape tolerances, such as aircraft components, the geometry of the mould is compensated to accommodate for shape distortions. Today this is made based on thumb rules and experience followed by trials. This is time consuming and expensive. Development of a tool for prediction of shape distortions and residual stresses is therefore an important step towards more optimised manufacturing of composites. The present thesis, consisting of five papers, describes the development and validation of a simulation tool for prediction of shape distortion and residual stresses. In the first paper a typical material and manufacturing process for high performance composites was used to experimentally investigate the effects from the cure temperature on spring-in of angle sections. The experimental results were interpreted in terms of mechanisms responsible for shape distortions. Based on the observations, a process model including a new mechanical constitutive model for predictions of residual stresses and shape distortions was proposed and implemented in a general purpose FE-program, as presented in the second paper. In the third paper, the model was validated by comparing spring-in predictions with the experimental results of the first paper complemented by same new experiments. The third paper also embraces a numerical investigation of the effect from the mechanical boundary conditions during cure. So far (in the three first papers), the curing conditions were kept isothermal. When a thick component is cured, the conditions are no longer isothermal owing to heat generated by the exothermal cure reaction. Hence, in the fourth paper the process model was validated against experimental results for a non-isothermally cured component. Finally, in the last paper shape distortions of a complex aircraft component was studied. This was made to both get further validation of the process model as well as investigate the feasibility to simulate large parts of complex shape.