Abstract: Rotation correction method of unloading compliance for a compact tensile (CT) specimen is investigated, and a new reasonable rotation correction method for the CT specimen is proposed after making clear the problem of current method. Based on the new method, a transforming formula between crack mouth open displacement (CMOD,V₀) and crack open displacement (COD) along loading line (V_LL) for CT specimen in consideration of rotation effect is also proposed. The elastic-plastic finite element analyses show that the rotation center of the specimen is closed to the crack tip and does not locate at the center of the remaining ligament. Rotation radius R is little affected by the material constitutive relationship but it is relevant to the ratio a/W where the crack length a and width W are dimensions of the CT specimen, therefore, a monotonic polynomial formula to describe the relation between R and a/W is given. The rotation effect on COD measurement needs to be considered when larger plastic deformation occurs around the crack tip, and the evaluation of J-integral for CT specimen must use the corrected COD. Furthermore, two types of steels Cr2Ni2MoV and 16MnR are employed to verify the validity of the new rotation correction method experimentally. It shows that, the transforming formula between V₀ and V_LL with the rotation correction originating from plastic deformation near the crack tip is more reasonable for determining the plastic work for front force compact tension (FFCT) specimen, and the prediction results from the transforming formula are in agreement with the experimental results. The rigid rotation of the FFCT specimen has bigger influence on the estimation of J_R curves.

Key words: fracture mechanics, test method, ductile fracture toughness, compliance method, compact tensile specimen, rotation correction, finite element analysis