Objective
Progressive plastic deformation and fracture of advanced aerospace materials in quasistatic tension and dynamic tension, compression and torsion are studied. The test materials are aluminium lithium metal matrix composites and aluminium based metal matrix composites. The following areas are investigated:
the micromechanisms of plastic deformation and fracture in both classes of material;
the application of macroscopic shear banding to flow localisation in the materials and the development of the Bai-Dodd shear banding model for multidimensional stress states;
examination of the dilatational strain energy density criterion (T-criterion) for specification of the type of fracture under various strain states and strain rates;
development of forming limit diagrams as functions of imposed strain rate and use of strain rate in metal forming processes to make fabrication of the 2 types of material more economic (incorporation of the forming limit loci in computer aided design and computer aided manufacture (CAD and CAM) programs will make the outcome of this work particularly valuable.
It has been shown that the materials have different fracture modes depending on the type of loading. For the aluminium lithium alloy and its composites, it has been shown that the fracture strains increase with increasing strain rate. However, the stress levels do not change significantly with strain rate. It has also been found that the Johnson-Cook equation is accurate enough to be used as a constitutive equation for all the materials.
A 3-dimensional shear banding theory has been developed and successfully applied to forming limits in cold forming.
It has been found that the failure of all materials could be described by the T-criterion which is a dilatational strain energy density criterion.
The microstructure and composition of each constituent of the materials have been well defined.
This work has helped the understanding of how best to cold plastically shape these materials and 2 techniques for plastic forming have been developed. These are cold forging in the overaged condition with, if necessary, intermediate anneals, and deep drawing under a superimposed hydrostatic pressure.
Because of the relatively high materials costs, the main applications will be in small parts in automotive engines which have a major impact on efficiency and performance.
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RG6 2AY Reading
United Kingdom