Stretch forming of thermoplastic fibre-metal laminates

Abstract

Fibre--metal laminates are sandwich materials comprised of altering layers of fibre-reinforced composites and metal alloys. These materials can offer superior properties compared to the monolithic constituents such as superior specific strength compared to metals and better impact and fatigue resistance than composite materials. The use of fibre--metal laminates is currently restricted to specialised applications where the superior properties justify the high cost. This is due to the increased manufacturing time and cost over conventional materials. A method for mass production of fibre--metal laminates would allow them to be integrated more easily into existing production facilities and greatly reduce the cost associated with their use. This thesis investigates the stamp formability of fibre--metal laminates using two distinct materials; one laminate based on a self-reinforced polypropylene composite and the other based on a glass-fibre reinforced polypropylene composite. Specimens of varying geometry were stretched over a hemispherical punch to elicit different deformation modes in the fibre--metal laminates and a non-contact optical measurement system was used to measure the surface strain during deformation. These experiments analysed the effect of the deformation mode on the formability of the laminates. The results from the experimentation were used to assess the deformation behaviour of the fibre--metal laminates and to identify the safe forming limits of the materials. It was found that the fibre--metal laminates can be formed in a similar manner to monolithic metals. The self-reinforced polypropylene laminate was found to exhibit superior formability to monolithic aluminium whereas the glass-fibre reinforced polypropylene laminate showed reduced formability. In addition, the effect of temperature on the formability of the laminates was investigated. The temperature did not have a significant effect on the deformation behaviour during the forming process in either fibre--metal laminate and no increased formability was exhibited by the glass-fibre reinforced polypropylene based laminate. However, the self-reinforced polypropylene based laminate showed improved formability at elevated temperatures. Two significant findings were identified; the friction interaction between the specimens and the tooling has a major effect on the forming of the laminates, and the forming limits of the aluminium are improved when bonded to self-reinforced polypropylene composite. The finite element analysis software ABAQUS/Standard was chosen for simulation of fibre--metal laminate forming. Tensile tests were performed to obtain the mechanical behaviour of the constituent materials, where the composites were simulated using non-linear elastic orthotropic material models and the aluminium using an elastic-plastic model. The experimental forming results were compared to the simulation and it was found that the simulation could accurately represent the general forming behaviour of the laminates. There was difficulty in matching some of the glass-fibre reinforced polypropylene laminates due the non-homogeneous behaviour of the composite. Results from the simulated specimens were used to assess the deformation of the composite, which could not be directly observed in the experiments, and to determination a preliminary failure condition of the composite experiencing stretch forming using the predicted strain in the failure region.