PSI - Issue 6

Tonsho Fumiaki et al. / Procedia Structural Integrity 6 (2017) 269–275 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

5

273

d = 2

(1)

5. Consideration of the criterion for crack branching Critical condition for crack branching in steel remainsunclear until now so we consider the phenomenon from several point of view. 5-1 Energy flux density theory Gao[5] pointed out the difference between apparent crack speed and local speed, and then attributed that to the bifurcation of the crack path. He proposed energy flux density criterion, as shown in eq. (2) = ∗ 1 − (2) where :apparent crack propagation velocity, ∗ :static energy release rate ∗ 1 − approximately means the energy release rate in dynamic crack propagation(eq. (3)) derived in model calculation by Freund(1972), so when energy release rate can be calculated by numerical analysis, eq.(2) can be rewritten to eq. (4). = 2 ∗ (3) = (4) where : energy release rate at dynamic crack tip Eq. (2) shows maximum value at = 0.5 , which is in accordance with experimentally observable maximum speed. Also, Nishioka[6] showed energy flux density can be the critical condition for crack branching, though elastic resin material was used in these studies, so we need investigate whether energy flux density can be the critical condition in steel.