Study of Influence Factors on the CTOA Toughness Values by Experiment/Simulation Method

Abstract

Although the crack-tip-opening angle (CTOA) has been widely recognized as an efficient fracture criterion for modeling stable crack growth and instability during the fracture process, the variation of its toughness value with many different kinds of factors changing for specific material during steady crack extension has been the focus of attention, such as specimen thickness, crack tunneling, notch shape, displacement rate, etc. At first, the present paper reviewed a variety of fracture parameters assessing the stable crack extension for ductile fracture toughness of pipelines steels or aluminum alloys. It was summarized that a few of test techniques and calculation methods available for determination of CTOA toughness, and the development of relative testing standards. Furthermore, the focus for this paper was to discuss a variety of influence factors of the CTOA toughness values in accordance with the results from the CTOA testing and finite element simulation of X70 and X80 steel specimens with different ligament thickness and the relative published articles. To compare the different CTOA values obtained by using direct surface methods and indirect methods from finite element analyses (FEA), CTOA toughness values of X70 and X80 pipeline steel were measured with a modified double-cantilever beam (MDCB) specimen in the servo-hydraulic uniaxial test machine. The gauge thicknesses included 4, 8 and 10 mm. The result of this study showed that critical CTOA values decreased with gauge thickness increasing. It was found that the computed surface CTOA in 3-D finite element analysis was generally lower than direct surface CTOA in the experimental measurement.