13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- to the stress wave, dynamic stress approaches the static stress st σ . From the comparison between Case 3 and Case 4, it is seen that of the maximum dynamic stress oscillation ( ) max st σ σ− at the notch root A is always the same although the final displacement of Case 3 is 15 times larger than that in Case 2. It is may be concluded that the maximum dynamic stress oscillation ( ) max st σ σ− is controlled by the tensile speed. Figure 6 shows the relationship between the tensile speed u t and ( ) max st σ σ− for 0.03mm ρ= and 0.2mm. Here the results for 5 10 , u t = 6 10 mm s and step load u t =∞ are also indicated when the maximum displacement is 1.5mm. It is seen that( ) max st σ σ− is proportional to the tensile speed when 5000 u t mm s ≤ . However, ( ) max st σ σ− becomes constant when 5 10 u t mm s ≥ . 4. Strain rate concentration for high speed tensile test specimen Figure 7 shows the strain rate at the notch root A for Cases 1-5. The strain rate increases dramatically at the start of applying forced displacement, Then, after several oscillations, the strain 0.1 1 10 100 1000 10 4 100 1000 10 4 10 5 10 6 10 7 ・・・∞ Difference of maximum dynamic stress from static stress (σ max - σ st ) Tensile speed u/t [mm/s] σ max -σ st when ρ=0.03 σ max -σ st when ρ=0.2 A Figure 6. Difference between the static and dynamic maximum stress concentration ( max st σ σ− ) vs. tensile speed -400 -200 0 200 400 600 0 0.001 0.002 0.003 0.004 0.005 Strain rate ε yA at the notch root A [s -1] Time[s] max ε ① ① const ε ① A ⋅ (a) Case 1 Figure 7. Strain rate at notch root A for ρ=0.03mm (Continued on next page) ⋅
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