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                       34                                    Fracture Mechanics: Fundamentals and Applications






























                                                             (a)













                                                                                                            (b)










                                                             (c)


                       FIGURE 2.6 Crack propagation in various types of materials, with the corresponding fracture energy.
                       (a) ideally brittle material, (b) quasi-brittle elastic-plastic material and, (c) brittle material with crack mean-
                       dering and branching.
                       2.4 THE ENERGY RELEASE RATE

                       In 1956, Irwin [6] proposed an energy approach for fracture that is essentially equivalent to the Griffith
                       model, except that Irwin’s approach is in a form that is more convenient for solving engineering
                       problems. Irwin defined an energy release rate G, which is a measure of the energy available for an
                       increment of crack extension:
                                                               d ∏
                                                          G =−                                   (2.23)
                                                                dA
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