applied fracture mechanics

184Applied Fracture Mechanics3.3. Profile path of crack propagationFigure 10 shows profile paths of crack propagation from the initial crack measured using alaser-microscope, where the stress amplitude is shown in parentheses for each case. Theangle of crack inclination against the normal direction was about 50-70° in stage I and about20° at the mixed mode region in stage II, as also shown in Fig. 10.Non-propagating cracks under PC conditions were almost all located in stage II except onecase (sample B at 400 MPa-mean stress) when no profile data were obtained because therun-out specimen was not fractured from the fretting pre-crack. On the other hand, underLC conditions, all non-propagating cracks were located near the boundary between stages Iand II. The boundary crack depth between stages I and II, d1, depended on the mean stress,contact pressure, and material strength. The following explains why this occurred.The values of d1 of sample B (higher static strength) were smaller than those of sampleA under the same test conditions.The values of d1 under PC condition were smaller than those under LC conditions at thesame mean stress.400 MPa-mean stress led to lower d1 than 0 MPa-mean stress under LC conditions.When mean stress was –100 MPa in the PC condition, d1 was extremely small (less than5 μm).Figure 10. Non-propagating and propagating crack profiles obtained from fretting fatigue tests: (a) PC(sample A), (b) LC (sample A), (c) PC (sample B), (d) LC (sample B), and (e) schematic view of fretting crack