Fracture toughness measurements by instrumented impact test

Abstract

Tests on an instrumented Charpy impact machine using fatigue cracked and side notched test pieces are described. The results, mainly on mild steel, are interpreted in terms of K Ic and are compared favorably with known results obtained by three other methods of testing. These other methods, already described elsewhere, are tensile tests on pre-compressed notched pieces, in which the borderline between crack arrest and crack propagation is determined, running crack tests on steel with birefringent coating and the correlation proposed by Krafft between the fracture toughness and strain hardening exponent. It is suggested that each testing method gives an estimate of the minimum K Ic value relating to the trough of the K ic -strain rate curve. Although the interpretation of each may be criticized for uncertainties of unknown magnitude, the all-round agreement found substantiates the general conclusion that the results are meaningful. A suitably modified version of the Charpy test is thus a convenient method of determining K Ic for low strength steels at temperatures up to about the 50 per cent shear fracture appearance transition (about 20°C for the mild steel reported here). An approximate correction for the effect of test piece inertia is proposed. For testing times to maximum load of 0.04 msec, or longer, the correction is less than 10 per cent but for shorter testing times becomes increasingly significant.