A new cyclic loading method for measuring sheet fracture toughness

This paper reports on a new method for the measurement of sheet fracture toughness. In this method, samples cut in DENT geometry are cyclically loaded. The maximum load for each cycle is incremented after each cycle until the sample fractures. The work in the final cycle is then the work required to fracture the sample. The fracture toughness determined using the new cyclic technique is compared with that determined using the Essential Work of Fracture (EWF) technique, for samples with a wide range of fracture toughness, and it is shown that cyclic technique gives a fracture toughness around 8% lower than the EWF technique. For 3 samples, cyclic fracture toughness values were measured for ligament lengths from 3.3 to 14.0 mm and compared with the EWF fracture toughness in each case. The cyclic fracture toughness was found to be independent of the ligament length, except for very weak samples, and to be 5–10% lower than the EWF fracture toughness. The differences between the EWF and cyclic fracture toughnesses were probably due to the development of damage zones around the crack tips before the fracture. The advantages of the new cyclic technique over the EWF technique are that it requires less sample area as only one ligament length is required, is quicker and can be readily automated.