On an unified model for predicting notch strength and fracture toughness of metals

Abstract

In the present paper an unified model for predicting the notch strength and the fracture toughness, K IC , is given and checked on the basis of summarizing the recent research. The crack initiation at notch tip may be assumed to occur due to the fracture of the hypothetical material element located at notch root. Hence the crack initiation criterions are developed from the notch strain analysis and the well-known fracture criterions for various kinds of metals. If the fracture of the notched element occurs right after the crack initiation at notch tip without any subcritical crack propagation, the notch strength, i.e. the fracture stress of the notched element will be equal to the stress for crack initiation at notch tip. As a result, the formulae for predicting the notch strength from tensile properties are obtained for various kinds of metals. The formula for predicting the apparent fracture toughness, K 1A , can be also given in the same way. The predicted notch strength of metals agree well with the test results in literature. However, it should be pointed out that the predicted notch strength in plane stress state is the lower limit of the test results. More important may be that a new material constant, the so-called nonsensitivity-to-notch factor, is introduced to evaluate the notch sensitivity, or the notch toughness of metals. It has been pointed out that the crack must be bluntened during loading in order to keep the mechanical equilibrium at crack tip and the radius of the bluntened crack tip must have a critical value. Taking the bluntened crack as a sharp notch and ρ c as the critical radius of the bluntened crack tip, the formula for predicitng the fracture toughness, K IC , from tensile properties is obtained for the ductile metals, based the assumption mentioned above. For the steels with lath Martensite microstructure, ρ c will be equal to the strain hardening exponent in mm, and for steels with mixed Martensite structure ρ c is equal to the uniform elongation in mm, or the Austenite grain size, or 9 times of the dimple size. The difference between the predicted values of K IC and the test results are mostly less than 10. The values of K IC of aluminium alloys can be also predicted by a modified formula and the tensile properties.