Comprehensive investigation on unloading compliance and normalization methods for determining accurate fracture toughness of steels

Fracture toughness is an important mechanical property of steel used in engineering design and failure assessment of steel structures. Fracture toughness is usually determined from laboratory tests, an essential step of which is to establish the J-Resistance (J-R) curves. Unloading compliance method (UCM) and normalization method (NM) are two of the most commonly used methods for determining the J-R curves and fracture toughness of steels. However, a considerable difference exists in fracture toughness determined by these two methods, which can cause confusion in true values of the fracture toughness. Inaccurate fracture toughness not only greatly affects the design and assessment of structures, but also causes severe economic loss and poses safety risk to the public. The main objective of this research is to comprehensively investigate the UCM and NM and then to develop new methods to determine the fracture toughness of steels accurately. A relatively large number of fracture tests are conducted using specimens with different types of steel. The effects of affecting factors on the performance of UCM and NM are thoroughly examined. The results show that many factors affect the agreement between the UCM and the NM and influence the accuracy of the two methods. With the causes of the error of these two methods identified, a new modified normalization method is analytically developed to eliminate the effects of mechanical properties on the accuracy of NM and enable it to be applied to steels with small strain hardening exponent and yield strength. Furthermore, a new real J-R curve method is developed and verified with test results. It is found in this research that the J-R curves determined by the new real J-R curve method are much closer to the accurate or real J-R curve than those determined by either the UCM or the NM. This research work can contribute to the current knowledge of the J-R curves and can assist engineers and researchers to prevent structural failures due to inaccurate determination of fracture toughness.