The Fracture Toughness ($J_{Ic}$) of S45C (JIS) medium carbon steel plate is measured for compact tension specimens with various crack ratios and thicknesses using unloading compliance method. The unloading compliance method is used here to calculate the amount of the stable crack growth, Δa.
Comparisons between the values of J calculated from the energy rate principle proposed by Begley & Langes and those calculated from the other estimation techniques proposed by Rice, Merkle & Corten, and McCabe for compact tension specimens are made.
The $J_{Ic}$ value at the initiation of slow stable crack growth is determined from J-Δ a curve and, is seen to almost constant in the range of crack to width ratio, a/w=0.6-0.7.
