Fracture parameters for sintered steels

Measurements of the plane strain fracture toughness K_Ic of sintered steels have frequently been invalid because the requirement that P_max/P_Q<1.1 (where P_max = maximum load and P_Q=load used to calculate K_Ic) has not been met. We show that the reason for the criterion not being met is that sintered steels have a considerable crack growth resistance K_R. Values obtained in the past for K_Ic probably have been over-estimates of the initiation value of the crack growth resistance K_i and under-estimates of the maximum crack growth resistance K_∞. The important point is that the assessment of the toughness of sintered steels by a single parameter is not appropriate. Test methods to determine the crack growth resistance of sintered steels are discussed. Crack growth, which is difficult to detect by visual observation, can be determined by compliance techniques. Because of the porous nature of sintered steel, fatigue cracks are unnecessary at the tip of the notch and indeed are undesirable as they can easily cause errors in toughness measurements through inadvertent overloading. The thickness requirement for plane strain measurements can also be relaxed.