EVALUATION OF THE FRACTURE-TOUGHNESS OF THE DEEP-CARBURIZED LAYER

The composite nature of case-hardened materials, meaning the existence of the gradients of microstructure, toughness, and residual stresses, ensures that the fracture in the case layer is quite different from t hat of homogeneous materials. In this paper a new method to evaluate t he fracture toughness of the deep-carburized layer is discussed. This method uses the ''pop-in'' phenomenon that happens when the crack prop agates through the carburized layer. The results show that the ''pop-i n'' phenomenon is caused by the gradient distribution of the fracture toughness, the microstructure, and the residual stresses in the carbur ized layer. The K-IC-X (distance from notch) distribution curve calcul ated from the load and the crack length at the ''popin'' point is the basis for the evaluation of the fracture toughness.