Problem

Polymers comprise a wide range of materials, including natural and synthetic materials. The demand for such materials has increased considerably during the last few decades. Applications include, but are not limited to, safety-related parts in cars, coatings, thermal insulation in offshore components, seals and inter-glass layers in laminated windows. The finite element method has only recently been employed as a tool in the design process of parts made of polymers. Therefore, constitutive models for such materials are less mature than for metals. Also for polymers, knowledge about the physical mechanisms governing the thermo-mechanical behaviour is of utmost importance for successful development of constitutive models.  

The research programme will pay attention to elastomers, commodity thermoplastics (like PE and PP), fibre-reinforced thermoplastics and foams made of polymers. Typically, polymers made for commercial purposes contain mineral or rubber particles. Some material models for polymers have been implemented in the SIMLab Tool Box, but the validation process has been limited to constitutive models for ductile thermoplastics. Other research groups have also worked on material modelling for polymers. The literature reveals, however, that a majority of the proposed models involve a rather high number of material parameters, rendering the parameter identification from experimental tests more difficult. Industrial application of a material model usually demands that the calibration of the parameters is straightforward. Prediction of fracture is also a topic of interest for research and industry. The accuracy of fracture criteria is closely related to the quality of the constitutive model because it is essential to have an adequate description of the evolution of stresses, strains and other internal variables during the thermo-mechanical history of the part under consideration.

Objective and scope

The main objective of the programme is to develop and improve material models representing the thermo-mechanical response up to fracture for polymers, i.e. thermoplastics with or without fibre reinforcement, elastomers and foams. The models will be developed for application in an industrial context. Particular attention is paid to validation and efficient identification of the parameters involved in the models.

Research tasks

Two main research tasks are addressed in this programme. The first one is related to the representation of the constitutive behaviour, while the other is concerned with fracture. As in the research programme on Metallic materials, experimental and numerical investigations on the micro-scale are important tools to gain the fundamental understanding of the physical mechanisms involved in the deformation process. It follows that experience and some tools from the Metallic materials programme will be useful here as well.

1. Constitutive behaviour: A fundamental issue for all types of polymers is to determine the shares of stored and dissipated energy during deformation. This information provides a differentiation between elastic and inelastic/plastic response, and hence the structure of the material model. On a micro-scale level, it is relevant to study the interaction between the polymer matrix and fibres and/or particles in order to interpret the macroscopic behaviour. Investigations with scanning electron microscopy (SEM) or computer tomography (CT) enable such information to be obtained. Further, the mechanical response for polymers is strongly dependent on the temperature and strain rate. Knowledge about the combined influence of temperature and strain rate on the constitutive behaviour provides valuable information about the appropriate structure of the model, but it may also serve as a useful tool for material characterization. Experimental tests instrumented with conventional and infrared cameras and the use of field measuring techniques, such as digital image correlation and digital infrared thermography, are important in this respect.
2. Damage and fracture: Prediction of damage evolution and fracture is a major concern for the user partners that apply polymer materials in products and structures subjected to severe loading conditions. The presence of reinforcing elements such as fibres or particles has a direct influence on whether the macroscopic fracture is of brittle or more ductile character. The behaviour at the micro-scale may explain the different kinds of fracture response. Prediction of damage and fracture in a numerical model requires that the constitutive behaviour is adequately represented, but also sufficient knowledge of the lower-scale physical mechanisms involved in the deformation and fracture process is essential. Scanning electron microscopy and computer tomography are useful tools in such investigations.