The thickness effect of side grooved CT specimens

Abstract

It is widely considered that the side grooved CT specimen gives a good approach to satisfying the plane strain condition. For a CT specimen 1 in (25·4mm) thick, a side groove of depth equal to 25% of the specimen size in the groove direction is recommended; this results in crack propagation occurring equally along the crack front. Recently some experimental studies have shown that a thinner CT specimen than is recommended by ASTM is enough to determine the fracture toughness value, J IC , by using the J-R curve. In the present study, the three-dimensional J- integral of the CT specimen is evaluated along the crack front by using the finite element method, and the thickness effects of the CT specimen are studied. First, elastic analyses are carried out for several types of CT specimens, by changing their thickness. It is shown that as the thickness decreases, the distribution of the J- integral value along the crack front becomes uniform. The CT specimens, with several depths of side grooves, are then also analyzed elastically. As the depth of the side groove increases, it is shown that the J values near the free surface become larger than those of the inner region. In the second stage, elasto-plastic analyses are carried out for three types of CT specimens. One is the standard type, the second is of 1 in (25·4mm) thickness with a 25% side groove, and the third is of 0·25 in (6·35mm) thickness with a 10% side groove. The distributions of the J- integral along the crack front in elasto-plastic states are obtained and compared. J M , the J value evaluated by using Merkle and Corten's conventional equation, is also obtained from the load versus displacement curve, and is compared with those obtained by the path integral method. To obtain the load versus displacement curves, two values are used as the plate thickness of the side grooved specimen. One is the net thickness, B n , and the other is the effective thickness, B e , proposed by Shih. It is shown that the effective thickness is useful in obtaining the J value similar to that obtained by the path integral method. The relations between crack tip opening displacement, CTOD, and the J value are studied and discussed.