Unloading compliance (UC) method and normalization method (NM) are two of the most commonly used methods for determining the fracture toughness of materials. However, considerable differences often exist in the fracture toughness determined by these two methods, which solicits a new method to determine the fracture toughness accurately. In this paper, the compliance of crack length differences as measured by the crack length difference ratio Si is discovered, analysed and verified by experiments. Based on this compliance, a new accurate method, known as AJR, is developed and verified by test results. Factors that exhibit the advantages of the developed new AJR method are also investigated. It is found that the J-R curves determined by the new AJR method are more accurate than those determined by UC and NM. The new AJR method should be the first choice for steels with a small strain hardening ratio and low effective yield strength, and thicker CT specimens with shallower initial crack length. This is because the disagreement between UC and NM is unacceptably large. The developed new AJR method and the results presented in this paper can assist engineers and researchers to determine J-R curves and fracture toughness of steels more accurately and can contribute to the body of knowledge of fracture mechanics.
Normalization method is a practical method for determining the J-R curves and fracture toughness of steels. There is some concern, however, about the performance of this method on steels with small strain hardening exponent and yield strength due mainly to the assumption of infinite strain hardening exponent (n). This paper intends to analytically modify the normalization method by removing this assumption and incorporating the strain hardening in calculating the blunting corrected crack length. This modification enables the normalization method to be applied to steels with small strain hardening exponent and yield strength. Experiments are undertaken to prove the underperformance of the normalization method for steels with small strain hardening exponent and yield strength and to verify the modified normalization method (CNM). A comparison of fracture toughness determined by CNM with that by the unloading compliance method and normalization method corroborates the improved accuracy of the developed CNM. It is found in the paper that the developed CNM performs very well for materials with small strain hardening exponent and yield strength and performs better for specimens with smaller thickness and in accordance with all standards. The paper concludes that the developed CNM overcomes the deficiency of the normalization method for steels with small strain hardening exponent and yield strength.