Strength and deformability of polycrystalline metals are determined by phenomena at various length scales. At a scale which we would label "nanoscopic", dislocations are created and pushed forward by the applied stress, achieving plastic deformation at the nano-scale. At the micro-scale, large numbers of moving dislocations interact and organize themselves in complex patterns. Still at higher scales, massive collective behaviour of these dislocations and patterns allow the plastic deformation of entire grains. On their turn they interact with each other finally leading to a certain mechanical response of the material at the macroscopic scale (i.e., the smallest scale at which it can be looked upon as a continuous medium). The response of the material on applied stresses depends on all these phenomena. Events on the various length scales may also cause important changes in the material, such as microstructure, internal damage and mechanical properties (strength and ductility). Note that the role of dislocations can partially be taken over by mechanical twinning or other stress-induced phase transformations.
All this has been extensively studies on all these length scales. These studies certainly do have there merits, and have led to important experimental observations and theoretical understanding of the material behaviour at these length scales. However, in recent years it has become strikingly clear the events of each length scale do influence the events on other length scales, and that more significant progress in the understand (and modelling) of the material behaviour may be achieved by studying these relations than by further refining knowledge on each relevant length scale separately. Finally, strong size effects occur when structural dimensions such as for instance film thickness or grain size starts interacting with the dislocation mean free path or the dislocation cell size, revealing a completely new and almost unexplored physics.
On first sight, polymer based, fibre reinforced composite materials seem to belong to a quite different world than polycrystalline metals. And yet, much of what has been said above also applies to these materials. Strength and residual deformability depend on the initiation and further development of damage at the micro-scale. Also here the microscopic events have been seriously studied and modelled, but the study of the coupling with the material behaviour at the macroscopic level (called meso-scopic level by experts in composite materials) is still in its infancy. It did not yet lead to satisfactory generic understanding (and modelling) for the case of general multi-axial straining, so the presents applicants believe that important synergy can be achieved doing the research on metals and composite materials in a collaborative way. Much has to be gained from transferring, when physically relevant, methods and models developed in the metal area to the composite polymer area, and vice-versa.
The engineering motivation for looking at these phenomena involves the development of higher performance materials, the optimisation of the manufacturing operations, and the improvement of the design and integrity assessment methods for both traditional (transport, energy) and emerging (MeMS, multifunctional active panels) structures.