Viscoelastic-viscoplastic model to predict creep in a random chopped mat thermoplastic composite

Random glass-mat thermoplastic (GMT) composites are widely used in automotive applications due to their mechanical properties and relatively low processing cost. However, there is an inherent issue with these materials in that the thermoplastic matrices exhibit viscoelastic behaviour. In order for manufacturers to have confidence in their products, it is important to be able to predict the long-term behaviour of these materials.;One day creep experiments indicated the presence of viscoplastic damage accumulation in the composite, which was verified using in-situ microscopy. The creep deformation in this GMT material has been modeled using a viscoelastic-viscoplastic model. To verify the model at the limit stress level of 50 MPa, two separate creep tests of 33 day duration were conducted. The test results agreed well with the model. The temperature effects and applicability of time-temperature superposition (TTS) principle on the chopped fibre composite have been investigated over a temperature range of 25°C to 90°C. A stress range of 20 MPa to 40 MPa was studied at each temperature level and it was found that the increase in creep compliance with temperature is similar for all stress levels. However, the variation in the creep compliance values increased by 3 to 7% on average at higher temperatures. Parametric studies conducted suggest that the failure modes for chopped fibre composite become matrix dominated at temperatures higher than the secondary glass transition of 60°C. Through the development of a master curve based on 20 MPa data and comparisons to long-term verification experiments at 20MPa and 50 MPa at room temperature, it was shown that TTS is applicable to the composite.;Short-term tests indicated that the material response of chopped fibre mat composites is far too random to be meaningfully quantified and this is further exacerbated at higher temperatures. Simply, the long-term creep behavior of these materials is not sufficiently repeatable to consider the use of a complex viscoelastic-viscoplastic model and therefore there is no practical reason to pursue development of such a model for the short fibre composite.;From 1 day creep tests at various temperatures, it was seen that temperature appears to increase plastic strain in the material exponentially at each stress level. By comparing the results from these tests to micrographs of the material, it showed that above the secondary glass transition, 60°C, bulk deformation of the matrix phase in the composite is dominant due to matrix softening. Deformation of the matrix phase accelerates fibre-matrix debonding and therefore the progressive failure process. It suggests then that bulk plastic deformation of the matrix phase is a major contributor to residual strains measured in this work.;In this work, chopped glass fibre mat reinforced polypropylene was studied over a stress range of 5 -50 MPa at room temperature. An upper limit of 50 MPa was used because experiments at 60 MPa resulted in a high percentage of failed specimens. Through shortterm creep (30 minutes) experiments, a material variability of +/-18% was determined. Using statistics, the shortterm test data also indicated that the material was only slightly nonlinear above 45 MPa. Since the nonlinearity was within the margin of experimental scatter, a linear viscoelastic model was used.;Overall, there is strong evidence from this extensive experimental program that even for materials with inherently high property scatter, it is possible to identify the effects of nonlinearities arising from external factors such as stress and temperature using short-term creep tests on single specimens. It is, however, more difficult to develop an accurate generalized long-term model that can account for stress and temperature conditions because of the wild experimental scatter.