5 Da/R0 0.00 0.01 0.02 0.03 KId(MPam1/2) 4 5 6 7 8 T/s0=0.25 T/so=0.0 T/s0=-0.25 Fig. 1. Crack growth resistance curves for 1r =0.01, 2r =0.001 at three different T-stress Da/R0 0.00 0.01 0.02 0.03 KId(MPam1/2) 4 5 6 7 8 T/s0=0.25 T/so=-0.25 T/s0=0.0 Fig. 2. Crack growth resistance curves for 1r =0.002, 2r =0.001 and 0 ˆs =2.5 0s , at three different levels of T-stress and MPa m s KI / 10 7 = & . Da/R0 0.00 0.01 0.02 0.03 KId(MPam1/2) 2 3 T/s0=0.25 T/so=0.0 T/s0=-0.25 Fig. 3. Crack growth resistance curves for 1r =0.002, 2r =0.003 at three different T-stress Da/R0 0.00 0.01 0.02 0.03 0.04 0.05 KId(MPam1/2) 1 2 3 4 5 T/s0=0.25 T/so=0.0 T/s0=-0.25 Fig. 4. Crack growth resistance curves for 1r =0.002, 2r =0.001 and 0 ˆs =2.5 0s , at three different levels of T-stress and MPa m s KI / 10 6 = & . Effects of loading rates For comparison, two impact velocities I K& = 6 10 and MPa m s/ 107 are used with material constants 1r =0.002, 2r =0.001 and 0 ˆs =2.5 0s . The R-curves at the higher impact speed are shown in Fig. 2, while those R-curves at the lower speed are shown in Fig. 4. There is a reduction in the fracture toughness with the lower impact speed. This is because high strain-rate increases the stress ahead of the crack-tip. Similarly, the fracture stress is enhanced by the rate-dependent cohesive law. Further, plastic deformation is restricted at low loading rates. The crack speed at different loading rates are examined in Figs. 2 and 4. It is found that the crack speed is larger at low impact speeds than at high impact speeds. The steady-state crack speed at 0.25 / 0 = - s T is 518m/s at the higher impact speed and 648m/s at the lower impact speed. Effects of crack-tip constraints Although the loss of crack-tip constraint plays an important role in the toughness enhancement of
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