3.2 | FRACTURE TOUGHNESS UNDER INSTRUMENTED CHARPY IMPACT CONDITION
Fracture toughness tests under instrumented Charpy impact condition were conducted in accordance with GB/T 229-200719. The single edge-through cracked Charpy impact specimens were utilized with the thickness (\(B\)), width (\(W\)), length (\(L\)), span (\(S\)), and initial crack length (\(a_{0}\)) of 10 mm, 10 mm, 55 mm, 40 mm and 6 mm, respectively.
Three repetitive test results of the load-deflection curves are presented in Figure 5. For the load-deflection curve without yield stage, the crack initiation time (\(t_{f}\)) was determined as the time at which the load reached the peak value (\(P_{\max}\)) and the dynamic fracture toughness could be determined by linear elastic fracture mechanics20
\(K_{\text{Id}}=\frac{P_{\max}S}{BW^{1.5}}f\left(\frac{a_{0}}{W}\right)\)(5)
\(f\left(\frac{a_{0}}{W}\right)=\frac{3\left(\frac{a_{0}}{W}\right)^{0.5}\times\left[1.99-\left(\frac{a_{0}}{W}\right)\times\left(1-\frac{a_{0}}{W}\right)\right]\times\left(2.15-3.93\frac{a_{0}}{W}+2.7\frac{a_{0}^{2}}{W^{2}}\right)}{2\times\left(1+2\frac{a_{0}}{W}\right)\times\left(1-\frac{a_{0}}{W}\right)^{1.5}}\)(6)
The peak loads are 3.16 kN, 3.29 kN and 3.27 kN, with the corresponding\(t_{f}\) being 0.128 ms, 0.116 ms and 0.120 ms, respectively. According to Equation (5), the dynamic fracture toughnesses are 47.7 MPa.m0.5, 49.6 MPa.m0.5, 49.3 MPa.m0.5, and the loading rates (\({\dot{K}}_{\text{Id}}=K_{\text{Id}}/t_{f}\ \)) are 3.72×105 MPa.m0.5/s, 3.88×105 MPa.m0.5/s and 3.74×105 MPa.m0.5/s, respectively. The average dynamic fracture toughness and loading rate are 48.9 MPa.m0.5 and 3.78×105MPa.m0.5/s, respectively.