FATIGUE-CRACK BENDING AND PROPAGATION FROM A PRECRACK UNDER MIXED-MODE CONDITIONS (BASED ON DISCONTINUOUS DISPLACEMENT ALONG CRACKS WITH A CENTER-HOLE NOTCH)

Fatigue crack bending and propagation behaviors were studied under mix ed-mode conditions, using annealed and fatigue slant precraks with a c enter-hole notch. The fatigue crack propagation directions were estima ted from the maximum tangential stress criterion using the stress inte nsity factors {(K-I)est, (K-II)est} evaluated from the discontinuous d isplacements (opening for Mode I, sliding for Mode II) along the precr ack, and they were in good agreement with the measured ones. The fatig ue crack propagation rates during bending were evaluated from the effe ctive stress intensity factor K-eff using {(K-I)est, (K-II)est}, in wh ich a total critical displacement ahead of the crack tip under mixed-m ode conditions was adopted as the fracture criterion based on the BCS crack theory. After the crack was bent, the subsequent fatigue crack p ropagated under the mode I behavior at the crack tip; therefore the (K -I)est evaluated from the discontinuous displacements measured along a bent crack could be regarded as the K-eff controlling the fatigue cra ck propagation rate of the crack bent from a slant precrack.