Effect of the shape of T-δ cohesive zone curves on the fracture response

Cohesive Zone Models (CZMs) are increasingly being used to simulate fracture and fragmentation processes in metallic, polymeric, ceramic materials, and composites thereof. A key feature of this approach is to represent the micromechanics of the fracture processes through a unique load-displacement relation. Most researchers consider magnitude of the energy, in addition to one of the two parameters (cohesive strength or critical displacement), to define the cohesive zone characteristics, ignoring the actual form (shape) of the relationship. Some of our recent work [1-3] and the work of others [17] has clearly shown that the energetics of the fracture process not only depends on the inelastic constitutive equation of the bounding material, but also on the choice of the cohesive zone model. CZM represents the embodiment of different inelastic micromechanisms active in the fracture process zone (FPZ). Since the micromechanisms are fundamental material characteristics, the choice of the CZM should depend on the specific material. The form (shape) of CZM represents the net effect of the processes and, hence, depends on the material system. In general, the shape of CZM is comprised of a rising, peak, and falling segment, and each segment exhibits a different influence on the energy dissipation, not only in the FPZ, but also (indirectly) on the bounding material. The commonly used exponential, bilinear, and trapezoidal models are analyzed to establish the relationships between the form (rise, peak value, and fall) characteristics of the T-δ curve and thermomechanical energy dissipation, plastic zone size, crack initiation load, and local stiffness behavior. By doing so, we provide specific guidelines to the CZM developers and users as to the criteria for the selection of appropriate CZM for a range of material system.