The test method to evaluate fracture toughness can be divided by the loading type to specimen. A static load is given to specimen for testing KIC, JIC, CTOD, etc. while a dynamic load is given to those for testing Charpy impact test, DWTT, etc. In case of fracture toughness tests with static loading, the data of load and displacement should be gathered during test because they were driven from fracture mechanics. In case of fracture toughness tests with dynamic loading, however, we don’t gather any data during specimen broken. We measure an absorbed energy for fracture and shear area from fracture surface after Charpy impact test and a shear area from fracture surface after DWTT. To analyze the results from these toughness tests of dynamic loading type with using fracture mechanics, recently, an instrumented system is installed to these dynamic loading test machines. An instrumented system can measure load and displacement during specimen broken. On the displacement-load curve made with data from an instrumented system, in general, the area in the region before peak-force is the energy for crack initiation while area in the region after peak-force is that for crack propagation. Meanwhile, it takes a long time and effort to evaluate the shear area from both fracture surfaces after Charpy impact test and DWTT test. For Charpy impact test, the method how to calculate the shear area with the information from a load-displacement curve has been studied for a long time. So the method for it is well established and known. For DWTT, however, the method how to calculate the shear area from a load-displacement curve was not known well. In this paper, a shear area could be calculated from an instrumented data without any more time or effort in addition to test. A shear area could be expressed as a function of total absorbed energy, fractured area, maximum force, time at 50kN and time at maximum force. Especially, the material with shear area more than 85% could be distinguished from that with shear area less than 85% because the transition curve of DWTT changes dramatically around 85% shear area.

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