International Journal of Fracture 53: R45-R52, 1992. R4 5 © 1992 Kluwer Academic Publishers. Printed in the Netherlands. EFFECTS OF PRESTRAIN ON THE CRACK ROTATIONAL FACTOR FOR PRESSURE VESSEL STEELS Zhu Hanxing, Hideo Kitagawa*, Li Changchun and Li Guangxia Department of Mechanics Huazhong University of Science and Technology Wuhan 430074, People's Republic of China *Nihon University, Japan Since DD19 was introduced in England, the fracture criterion COD has been widely used. Although the formula for COD has been amended, the crack rotational factor 7 always exists in the formula. In the design and defect-assess- ment of boilers and pressure vessels, a fracture mechanics standard is widely used. During the manufacture and use of this equipment, different plastic deformations may occur at the adapter zones. At present, various fracture mechanics parameters used in product design and defect safety-assessment are usually those of the undeformed materials and actually, many defects arise in the deformed materials. Therefore, it is necessary to combine experiment with FEM calculation for studying the effects of prestrain on the crack rotation factor 7 of pressure vessel steels. The tested materials were 20g and 16 MnR steels. Their major chemical composition and mechanical properties at room temperature are shown in Tables 1 and 2. The tension curves of the two steels were measured on an Instron testing machine, as shown in Fig. 1. The formula of the crack tip opening displacement 5 generally depends on the "crack hinge model", as shown in Fig. 2. It is recognized that the two sides of the crack rotate around the rotational center R as the applied load is increased. 7, the ratio of the distance between the rotational center and the crack tip to the ligament-size, is called the crack rotational factor. In Fig. 2, the two edges on the specimen's surface are not drawn out and the effects of elastic deformation on 8 and y are not considered for convenience. According to the definition of the rotational center and the geometrical relations among the various parameters, we have ~w -a)~58 5- 7(w - a ) + a From the above equation 7 can be obtained as (1) Int Journ of Fracture 53 (1992)