Hardening Cohesive/Overlapping Zone Model and Fractal Approach to Ductile Fracture
Most of fracture parameters (K, G, J-integral, COD) break down, especially for ductile materials and large crack extensions. In addition, even at small scale yielding, K, G, J- integral and COD are limitedly used to distinguish the onset of unstable fracture, without any concern regarding the pre-onset and post-reaction. On the other hands, the specimen size and geometry dependences of K/G/J-integral/COD are beyond the capability of the classical Fracture Mechanics where most of methodologies rely on only a single-parameter to characterize the structural integrity. In essence, K/G/J-integral/COD are based on the same underlying idea of energy released per crack extension. Thus, when crack extension is accompanied by the other irreversible energy dissipation, neither J-integral nor COD/CTOA is able to describe the failure processes (crack onset, growth and catastrophic fracture). Moreover, K/G/J-integral/COD are by definition global parameters in nature. Their average field characters are unable to describe some mechanical behaviour inherent to material heterogeneity and disorder. As a result, it is no wonder that the applications of K/G/J-integral/COD are severely limited, and even invalid as observed in substantial experiments. The cohesive zone model and fractal approach are the cornerstone in this thesis. It is essential that a proper fracture theory should basically take into both the stress intensification and material disorder at different scales into account. A Hardening Cohesive/Overlapping Zone Model is proposed for the analysis of complex mechanical phenomena in fracture of metallic materials. It is assumed that the unique correspondence between uniaxial tension/compression and ductile fracture can be established. As a result, the advantages of the cohesive crack model are kept in order to simulate the stress and strain concentrations. Most metals and alloys are not homogeneous. Thus, macroscopic level behaviours are essentially originated from a microscopic one, as well as taking into account local fluctuations due to disorder. Especially, fracture is quite sensitive to disorder, and sometimes mean field theory without consideration of disorder is not able to provide accurate predications. The fractal approaches to scale effects are adopted to introduce the inherent effects from material disorder at different scales. The scale-independent cohesive and overlapping laws with respect to the classical stress–strain relations can be presented. Consequently, fractal approach can avoid the limitations of a classical mean-field approach, where disorder is simply averaged in an elementary representative volume. In addition, energy brittleness number sE is proven to be a characteristic parameter to evaluate the fracture instability not only in quasi-brittle materials but also in metallic materials under certain conditions.
Settore ICAR/08 - Scienza delle Costruzioni
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