Fatigue Crack Bending and Propagation from a Precrack under Mixed-Mode Conditions. (Based on Discontinuous Displacement along Cracks with a Center-Hole Notch).

Abstract

Fatigue crack bending and propagation behaviors were studied under mixed-mode conditions, using annealed and fatigue slant precracks with a center-hole notch. The fatigue crack propagation directions were estimated from the maximum tangential stress criterion using the stress intensity factors{(KI)est(KII)est}evaluated from the discontinuous displacements(opening for Mode I, sliding for Mode II)along the precrack, and they were in good agreement with the measured ones. The fatigue crack propagation rates during bending were evaluated from the effective stress intensity factor Keff using{(KI)est(KII)est}, in which a total critical displacement ahead of the crack tip under mixed-mode conditions was adopted as the fracture criterion based on the BCS crack theory. After the crack was bent, the subsequent fatigue crack propagated under the mode I behavior at the crack tip; therefore the(KI)est evaluated from the discontinuous displacements measured along a bent crack could be regarded as the Keff controlling the fatigue crack propagation rate of the crack bent from a slant precrack.