The Committee on Rapid Inexpensive Tests for Determining Fracture Toughness of the National Materials Advisory Board recently recommended that action be taken by the American Society for Testing and Materials to standardize the slow bend precracked Charpy test for the purpose of providing an index of plane-strain fracture toughness based on the nominal strength of the Charpy specimen. This paper reports results of an investigation to explore the possibility of using nominal strength values for this purpose. Precracked Charpy specimens were cut from plate stock having well established KIc values. The alloys investigated were 4340 steel, 18Ni maraging steel, several high-strength aluminums, and a titanium alloy. For each alloy a range of toughness levels were tested. Nominal strength values from the Charpy specimens were calculated from their maximum loads, and the ratio between these strength values and the tensile ultimate strength σut was correlated with KIc2/σut2.
The results show that the range of plane-strain toughness over which a useful relation between Charpy strength ratios and KIc can be established is limited by excessive plasticity in the Charpy specimens at high-toughness levels. The effect of this plasticity is to cause a gradual loss of sensitivity of the Charpy strength to changes in KIc. The Green and Hundy limit developed for a nonstrain hardening rigid plastic material is useful in estimating the sensitivity limits. If the toughness range is suitably restricted to avoid this loss of sensitivity, it appears that the Charpy strength ratio is a useful index of KIc and can be used to estimate KIc with confidence equal to that as obtained by using W¯/A values derived from the same specimens.