Experimental Estimation of/Integral from Load-Front Face Displacement Record for Compact Tension Specimens

摘要

Fracture toughness of ductile materials is often characterized by a J-R resistance curve. The compact tension specimen is one of the two commonly used standard geometries to characterize this mechanical property. When a compact tension specimen is used, it is normally required to use the load line displacement (LLD) to calculate the J integral according to the relevant literature and the ASTM test standard E1820. In this test approach, usually a displacement gage is attached to integral knife edges machined into the specimen along the load line, to measure the crack opening displacement while the crack is advancing. The crack opening load line displacement data are then used to calculate the J integral. Along with the load data, the displacement data are also used to evaluate the crack extension, usually applying a normalization data reduction technique. This method has been widely used in characterizing the J-R curve and J_(Ic) value in a wide variety of metallic materials. However, in some cases, for example in nuclear engineering, due to material size limitations, a standard test specimen size is not achievable, and attaching or mounting the displacement transducer to measure the load line displacement of a small compact tension specimen is very challenging. An alternative approach is required. The present study is aimed at evaluating the use of front face crack mouth opening displacement in a compact tension specimen to estimate the J integral. Based on the energy principle, the paper proposes a J equation based on front face displacement (FFD) similar to the ASTM E1820-11 formulation that is used for the compact tension specimen. The results are compared with those using load line displacement based J calculations for P92 steel. The test results show that the FFD based J resistance data are within 3% of the LLD based data.