Effect of heat treatment on the mixed-mode impact behaviour of a low carbon steel

Abstract

As fracture mechanics has developed as a discipline, many parameters have been developed to characterize the instability condition. However, a majority of this work has been confined to the investigation of mode I fracture. Thus, we have standardized methods for experimentally determining K~o, J~c and J-resistance curves for mode I crack propagation. However, cracks in real materials can be subjected not just to tensile stresses but to complex stresss states so that the development of suitable parameters to characterize mixed-mode crack initiation and propagation is important in the evolution of suitable design criteria. Further, observations indicate that initially flat cracks in some tough materials tend to reorient themselves to oblique planes during growth. For these materials, crack propagation can be said to occur under combined mode conditions.A considerable amount of work on mixed mode I/III fracture toughness of materials is available using proportional loading methods [1-13] and all the work using such a loading method has recently been summarized [14]. The superposition of mode In loading results in drastic reduction in fracture toughness in some materials whereas in other materials it has little effect or even results in an increase in the fracture toughness. Fracture mechanism maps delineating regions of susceptibility to tensile and shear loads have been proposed to explain such differences [ 11,14].In the mixed-mode fracture toughness tests outlined above, the use of a modified compact tension specimen has enabled the testing of materials under a variety of combinations of mode I and mode III loadings. By using appropriate defined mixed-mode versions of the stress intensity factor K and J integral, the susceptibility of these materials to mixed-mode fracture can be quantified. In addition to compact tension specimens, three point bend specimens with an inclined crack can also be used to determine the mixed-mode fracture behaviour of materials [13].The aim of the present study was to evaluate the feasibility of extending the mixed-mode fracture concept to impact testing using a Charpy type test specimen. The modified Charpy test specimen is shown in Fig. 1. The crack inclination angle can be varied to provide a range of mode I and mode In load combinations. Four angles 90°,75°,60 ° and 45 ° were used in the present study. This modified Charpy test specimen provides for a simple method to determine the mixed-mode impact resistance of materials. Apart from providing information on dynamic I n t J o u r n oF ~a~. t u r e 80 (199~)