Anisotropy and non-linear effects in SMC composites : from material data to FE-simulation of structures

Abstract

In the design of composite structures for the automotive industry it is vital to have good tools because development time is very critical. Normally simulation of mechanical properties of sheet moulding compound (SMC) composite structures is stiffness based and limited to isotropic material model. Design guidelines and the designers experience are common tools to estimate that the strength of the part is sufficient. With better design tools an optimal design could be found more quickly, giving improved cost efficiency and lower weight. The research question in the present thesis is therefore an advanced simulation in design of SMC composite structures. The approach has been: a) to perform material analysis of two different types of SMC using mechanical tests and in-situ microscopy; b) to derive material models based on these results and to implement them in a commercial FE-programme. A typical feature in SMC structures normally not considered when simulating the stiffness is anisotropy due to production of the prepreg or during the moulding of the structure. It is shown in thesis how to include this effect with a material model based on micromechanics, for determination of local stiffness as function of fiber orientation distribution. The model has been validated for a SMC with 30 weight-% glass fibers and 45 weight-% CaCO3 filler and successfully been implemented in the FE program ABAQUS through a subroutine. The subroutine performs orientation averaging of fiber orientation distribution described with a second order fiber orientation tensor. In all SMCs studied, significant modulus reduction was observed with increasing strain due to extensive damage. A critical modulus reduction is suggested as a failure criterion rather than strength. Simulation not only of stiffness, but also to use a design prerequisite for allowed load is an interesting new approach to improve the accuracy of the design. For this goal it is important to have an accurate material model. Viscoelastic effects were studied in cyclic loading test and creep test. A material model that considers the SMC composite as linear-viscoelastic material with evolving damage was suggested to explain the nonlinear stress-strain behaviour and the observed damage accumulation with increasing stress levels. Research results on short fiber composites with random fiber distribution considering models for both viscoelasticity and damage evolution are not available. The simplifying assumption in the model is that damage development may be considered as an elastic process and hence depends only on the maximum stress experienced by the material. This allows for damage quantification in terms of stiffness reduction in quasi-static tensile loading and unloading testing. Then the time- and damage-dependent viscoelastic functions of the composite are described as a product of damage- and time-dependent terms, where the time-dependence of the viscoelastic behaviour is described by creep compliance functions of undamaged composite. Hence, the damage-dependent term serves as a scaling factor. An incremental formulation of the non-linear model usable in FE-simulation is derived, presented and implemented in FE-program ABAQUS. Application to constant strain rate tensile test comparing analytical and FE result prove accuracy of the formulation and the subroutine. Finally a stiffness reduction model for SMC composites with evolving damage is suggested and validated for a standard SMC material.Godkänd; 2004; 20061026 (haneit