Fracture following plastic deformation

Alternatively, if detachment is associated with a brittle failure, then one must first determine if the fracture followed an elastic loading where an elastic model such as the JKR theory is appropriate or if it follows a plastic or elastic-plastic loading. In this latter case, the force needed to detach the particle from the substrate depends on the specific properties of the materials and the details of the deformations [63]. 

One can analyze the data from this type of work to classify materials with respect to their brittle fracture or plastic deformation tendencies or behavior [1,10]. Examples of Heckel plots are shown in Figure 3. This technique has also been used to follow bead compaction with modifications of the Heckel equation to account for rheological behavior [8]. 

Hiestand Tableting Indices Likelihood of failure during decompression depends on the ability of the material to relieve elastic stress by plastic deformation without undergoing brittle fracture, and this is time-dependent. Those which relieve stress rapidly are less likely to cap or delaminate. Hiestand and Smith [Powder Technol., 38, 145 (1984)] developed three pharmaceutical tableting indices, which are applicable for gener characterization of powder com-pactiability. The strain index (SI) is a measure of the elastic recovery following plastic deformation, the bonding index (BI) is a measure of plastic deFormation at contacts and bond survival, and the brittle fracture index (BFI) is a measure of compact brittleness. 

The fracture behaviour of some tough plastics can be inconsistent. For example, polycarbonate sometimes fails by yielding, but, on other occasions, a crack initiates and brittle fracture follows (Fig. 9.5). The explanation lies in the analysis of localised yielding. Section 8.2.4 showed that the indentation pressure could be three times the uniaxial compression yield stress. The yield stress can change by a similar factor during localised tensile deformation. 

These ductile features of these glasses may be observed microscopically in Fig. 3.18a and b, respectively. However, the stress—strain curves of these Zr-based amorphous glasses did not display appreciable macroscopic plastic deformation prior to catastrophic fracture, rather they mainly deformed elastically, followed by catastrophic failure along their shear bands. Examination of the fracture regions... 

That fraction of the applied work which is not consumed in the elastic-plastic deformation remains to create the new crack surface, i.e., the crack driving force. Therefore, a nonlinear fracture toughness, G, may be defined as follows ... 

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