Stamp forming of composite materials : an experimental and analytical study

Abstract

Composite materials are fast gaining popularity as an alternative to metals for structural and load bearing applications in aerospace, automotive, alternate energy and consumer applications industries. With the advent of thermoplastic composites and advances in recycling technologies, fully recyclable composites are gaining ground over traditional thermosetting composites. For the widespread use of composite material, the cost of manufacturing is one of the key challenges. Stamp forming has been proven to be effective for mass production of components. This thesis investigates the feasibility of using stamp forming technique for the processing of thermoplastic, recyclable composite materials. The investigation includes a detailed experimental study based on the strain measurements using a non-contact optical strain measurement system in conjunction with the stamping equipment to record and measure the formability of the thermoplastic composites in real time. A statistical analysis approach based on Design of Experiments and ANOVA was used to identify the influence of process parameters on the forming behaviour of the composite materials. Significant changes in the forming modes, including increased biaxial stretch and plane strain, experienced by the composite blanks were analysed. The research on the forming modes show that the composite materials can be stamp formed for mass production. To develop a finite element material model to elucidate the response of the material to temperature, an experimental characterisation was undertaken for the two composite materials used. This formed the basis of a temperature dependent, non-linear elastic orthotropic material model which was incorporated into LS-Dyna explicit finite element code. The results from the finite element simulations showed a high degree of correlation with the experimental results. Significant improvement was made over the existing constitutive models of the material library by including the thermal effects on the stress-strain behaviour of the composite materials. This improvement is a world's first in implementing thermal dependency with non-linear orthotropic behaviour for composite materials. This temperature dependency is vital for accurate simulation of stamp forming of heated composite materials. The analysis of the strain measurements and forming modes indicate that significant reduction in compressive strains can be achieved with the application of temperature and blank holder forces. This finding has important ramifications to real world manufacturing where any failure in the flange region could propagate to the areas of interest in a processed part. A through the thickness strain estimate has been studied to illustrate the effect of the forming parameters. An important conclusion that resulted from this investigation is the need for a judicial choice of the process parameters which are paramount to the quality of the manufactured part. These results indicate that composite materials have the potential to exhibit superior forming behaviour compared to aluminium. Currently, aluminium is considered as an attractive, lightweight alternative to steel in the body panels of automobiles. The results from this study indicate that thermoplastic composites have superior forming characteristics to aluminium and hence, have a potential to be the material of choice to reduce the weight of automobiles. - provided by Candidate.