The fracture toughness, $J_{IC}$ of AISI 4140 steel for compact tension specimen was measured by unloading compliance method. Stable crack growth, Δa was obtained from measurement of compliance during a sequence of loadings and unloadings, and $J_{IC}$ was determined from J vs. Δa curve.
Ultrasonic crack detector was simultaneously used to determine the onset of crack growth, hence to measure $J_{IC}$ independently. This procedure, however, sometimes fails to give reasonable value of $J_{IC}$.
The compact tension specimens with various values of thickness and crack ratio a/w, were used and the results show that $J_{IC}$ value decreases slightly as the crack ratio, a/w or the thickness increases.
