A moving-load controlled-displacement fracture-toughness testing machine

A testing procedure using a moving-load controlled-displacement fracture-toughness testing machine is described. It provides simple means for studying fracture-toughness gradients in structural materials caused by varying thickness, heat-treatment variations, mechanical working gradients, welding, and different corrosive environments. The energy-release rate at the onset of crack propagation and the plane-strain fracture toughness can be measured directly without compliance calibration or stress-intensity evaluation. Specimens made of 7075-76 aluminum-alloy were used in the testing, both at constant and linearly varying net-section thickness. The large number of fracture-toughness points obtained allowed the probability distribution of toughness values to be studied with the curve showing a normal distribution. It is concluded that the strain-energy release-rate expression derived for this system is independent of crack length and the specimen geometry is simple. Furthermore, the technique of controlled crack propagation (discrete jump) provides a large number of data points along the specimen length.