The effect of inertial loading in instrumented impact tests has been studied, and it is shown that during such tests large errors can be made in calculating the bending stress of a specimen at fracture if inertial loading is not taken into account. By instrumenting the tup, anvil, and specimen it is shown that in the period over which inertial effects occur, there is no simple relationship between the load measured on the tup, anvil, and specimen. To assist in interpretation and evaluation of impact load-time curves a model has been used which relates the spring constants and masses of the tup and anvil of the test machine and specimen to the frequency, phase, and relative amplitude of the oscillations occurring in the impact test. The importance of inertial loading is emphasized, in particular, with regard to dynamic measurement of plane strain fracture toughness KIc.

This paper reviews the background to the formation of a Working Group in the United Kingdom concerned with fracture testing of high-strength metallic materials. The results are given of two collaborative test programs designed to familiarize participants with testing techniques based on linear elastic fracture mechanics and interpretation of load-displacement curves. Results also are presented of specific studies on the influence of fatigue precracking stress intensity and the specimen dimensions such as thickness and crack length on KIc measurement. The current objectives of the BISRA Fracture Toughness (High Strength steels) Committee are to produce a British standard for plane strain fracture toughness (KIc) testing and prove its workability by a further collaborative test program.

As far as the development of new classes of ultra high strength steels is concerned the past decade has seen little progress apart from a few notable exceptions such as the trip steels. However, the introduction of fracture mechanics concepts to the evaluation of material properties has been responsible for an enormous breakthrough in the understanding of the factors which influence the fracture processes in conventional high strength steels. This approach has concerned not only the resistance of these materials to unstable fracture under rising load but also the stable crack propagation which can occur in aggressive environments under static load and in both aggressive and inert environments under cyclic loading conditions. Utilization of fracture mechanics has demonstrated the improvement which can be obtained in the properties of high strength steels through the use of thermomechanical treatments, hardening by intermetallic compounds and control of inclusions. Though much still remains to be learned in each of these fields, this paper highlights some aspects of the progress which has been made in the understanding of the fracture, fatigue and stress corrosion cracking of high strength steels.