Residual Stresses in Short-Fibre Reinforced Injection-Moulded Thermoplastic Parts

Abstract

Having the prediction of the warpage and shrinkage behaviour of injection-moulded short-fibre reinforced thermoplastics in mind, a model is developed to calculate the residual stresses. In accordance with the structure and the behaviour of these materials, a layerwise orthotropic, viscoelastic constitutive equation is used. As a cause for the stress development, a process determining the history of cooling under pressure is considered. The volume expansion of the matrix is described using a time-dependent viscoelastic free volume concept. From this, the viscoelastic functions for thermal expansion are derived as well as a time-dependent temperature shift factor. The material law of the composite is developed with the help of micromechanical considerations assuming viscoelastic matrix and elastic fibre behaviour. The residual stresses are calculated for a specially designed testing device which possesses regions of different fibre orientation. Measured fibre orientation tensors are used as simulation input. In the following, the calculated stress profiles were compared with measured values, which were determined using the layer removal technique. The calculated and measured profiles are in good agreement and show a strong influence of the fibre orientation state. The highest occurring stresses are in the same order of magnitude as the long-term strength of these materials.