Issue 19

M. Paggi, Frattura ed Integrità Strutturale, 19 (2012) 29-36; DOI: 10.3221/IGF-ESIS.19.03 29 Structural integrity of hierarchical composites Marco Paggi Politecnico di Torino, Department of Structural and Geotechnical Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy marco.paggi@polito.it A BSTRACT . Interface mechanical problems are of paramount importance in engineering and materials science. Traditionally, due to the complexity of modelling their mechanical behaviour, interfaces are often treated as defects and their features are not explored. In this study, a different approach is illustrated, where the interfaces play an active role in the design of innovative hierarchical composites and are fundamental for their structural integrity. Numerical examples regarding cutting tools made of hierarchical cellular polycrystalline materials are proposed, showing that tailoring of interface properties at the different scales is the way to achieve superior mechanical responses that cannot be obtained using standard materials K EYWORDS . Hierarchical composites; Fracture mechanics; Finite element method; Cohesive zone model. I NTRODUCTION significant advancement in the field of strength of materials has been achieved with the advent of composites. Combining different materials together allows us to realize structures with enhanced mechanical properties. Fiber reinforced materials are just one of these successful examples. The matrix contributes to the toughness and the density of the material, whereas fibers significantly increase the strength. Notable applications regard metal matrix composites used for aerospace applications, as well as fiber reinforced concrete for civil engineering purposes [1]. Similar strategies are accomplished with laminates and sandwich structures, where superior mechanical properties are achieved through the suitable combination of the individual material constituents [2]. In this context, the mechanical behaviour and the overall performance of composites are usually not limited by bulk properties, but by the interface characteristics. Debonding between matrix and reinforcement develops from early stage of deformation under monotonic and cyclic loading [3]. This damage affects the tensile strength, the fatigue strength, the fracture toughness, as well as the main mechanical properties. Therefore, to understand the effect of the interface properties upon the mechanical response, several theoretical, numerical and experimental studies have been put forward in the last decades. Although research progresses are evident, especially from the computational point of view, a lot of work has still to be done to understand the mechanics of interfaces and their effect on the global structural response. In general, interfaces are commonly considered as defects, i.e., weak points of the material microstructure that limit the achievement of the maximum theoretical strength. This way of thinking, in conjunction with the difficulty of defining appropriate physical and mathematical models for interfaces, leads to a passive design approach. The attention is therefore focused on preserving the structural integrity by remaining in the elastic regime, covering all the modelling uncertainties with severe safety coefficients. A different approach, leading to an active design, could however be pursued. Once suitable models are developed for characterizing the mechanics of interfaces, then structural analysis should pay attention to the failure modes, optimizing the material microstructure and the structural component performance through a suitable tailoring of the interface properties. A