Critical strain-energy release data

Charpy impact testing can also be used to determine critical strain energy release rates, G/, and to estimate the values of fracture toughness in states of plane strain and plane stress, and G c, respectively (49,89). Hence, experimental data can be obtained from standardized Charpy notched impact test (CNIS) as to their relevance for material selection and design. 

Pendulum-type machines are inexpensive and simple to use and maintain. Data can be produced from which fracture toughness Gic (more correctly known as the critical strain-energy release rate in Mode I) can be calculated (see Fracture and ASTM D5045-99 and later in this article). However, these machines are limited in the range of impact speeds, with a maximum of 3.4 m/s. Furthermore, the common practice of reporting only the total energy does not allow easy interpretation of the data, which can be overcome by using the instrumentation described previously. The use of instrumentation as described previously is often helpful for this purpose. 

There are, in addition, a number of other more complicated models. Bazant and his co-workers [34-38] have developed a smeared crack model, in which fracture is modelled as a blunt smeared crack band. The fracture properties are characterized by three parameters fracture energy, uniaxial strength, and width of the crack band. This approach lends itself particularly to computer-based finite element modelling of the cracks. For very large structures, this theory becomes equivalent to the LEFM approach. For smaller structures, however, the theory predicts a lower critical strain energy release rate, because the fracture process zone cannot develop fully. This theory has been shown to provide a good fit to experimental data from the literature. 

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