Study on Simple Method to Measure the Dynamic Fracture Toughness of Ductile Material

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In determining the impact fracture toughness, the dynamic stress intensity factor is computed if the plastic zone is limited in size compared with crack and specimen dimensions. on the other hand, the J-integral which is the inertia enhanced version of Rice's J-integral has to be evaluated if the specimen exhibits considerable plasticity prior to fracture. The time variation of the dynamic fracture mechanics parameter K_I(t) or J-integral under impact loading may be computed by the finite element method. However, this method requires a large computer time and is not convenient to use in material testing, The purpose of the present project is to develop a simple method to determine the impact fracture toughness of ductile materials. The main results are summarized as follows:(1) The J-integral can be computed by the conventional finite element method with about 5% errors without considering the singularity of the area integral. The singular area integral can be evaluated analytically by fitting the near-tip field with a function which is transformable into a line integral.(2) Simple formulae of J-integral for the three-point and one-point bend specimens have been derived by applying a plastic zone correction to the formulae of the dynamic stress intensity factors. A reasonable accuracy is obtained by the present method as compared with the elastic/viscoplastic finite element computation and the CPU-time is within 1% of the finite element method.(3) The impact fracture toughness test has been conducted. Methods to detect the initiation of crack have been examined. For relatively brittle materials the electric potential method is useful. However, it is impossible to detect crack initiation by this method if the specimen was fractured with a considerable amount of plastic deformation. Further investigation is needed for such ductile materials.