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

In conventional fracture-toughness testing, the line of application of the loads remains fixed with respect to specimen geometry. In this testing machine, the load moves with the advancing crack front, and displacement is used as the controlled variable to propagate and arrest a crack. The energy-release rate at the onset of crack propagation and, hence, the plane-strain fracture toughnessKIc can be measured directly without compliance calibration or stress-intensity evaluation. The specimen is in the form of a flat plat 25 by 50 cm which is simple to machine and provides about 30 values ofKIc. The versatility of the machine is demonstrated by making a statistical analysis ofKIc for 7075-T6 Al by showing the effect of plate thickness on the fracture toughnessKc using a tapered specimen, and by evaluatingKc in 7075 Al as a function of aging temperature in a thermal-gradient-treated specimen.