Comparison of the Modified CTOD Measurement Method with the Double Clip Gauge Method in a Compact Tension Specimen

For allowable defect analyses, the fracture toughness of materials needs to be accurately predicted. In this regard, a lower fluctuation of fracture toughness can lead to reduction in safety and economic risks. Crack tip opening displacement (CTOD), which is the representative parameter for fracture toughness, can be measured by various methods, such as the δ5, the J-conversion method, the single clip gauge method, and the double clip gauge method. When calculating CTOD from test results, the principle of similar triangles, which adopts the plastic hinge model, is influenced by the rotation factor, rp. Therefore, in order to reduce the fluctuation of CTOD, the exact value of rp must be defined. This study investigates various methods to predict fracture toughness in metallic materials, and assess the pros and cons of each method. Moreover, the equation of rp is modified by using a double clip gauge in compact tension (CT) to reduce the fluctuation of CTOD. The rp value is derived from 0.55 to 0.68, using the double clip gauge method. Finite element analysis is used to derive the rp values, which range from 0.50 to 0.66, in order to verify the validity of the derived rp values. This ensures the validity of the rp value derived from the experiment. In addition, the fluctuation of CTOD, based on the modified equation of rp, is lower than that using the single clip gauge method, according to BS 7448.