applied fracture mechanics

180Applied Fracture Mechanics0.2% proofstressTensilestrengthTable 1. Mechanical properties of materialsElongation(%)Reduction ofareaVickershardnessSample A 610 745 26.3 65.5 238Sample B 842 1037 15.4 51.0 329Figure 4. Shapes of specimens and test apparatus: (a) specimen for LC tests, (b) specimen for PC tests,(c) contact pad, and (d) test apparatusPlain fatigue tests were also carried out without contact pads using run-out frettingspecimens at 2×10 7 cycles, and the size of fretting non-propagating cracks, which were thecause of plain fatigue fracture in most cases, were investigated. In addition to the cracklength, the plain fatigue tests were aimed at obtaining ΔKth from the fretting pre-crack underconstant R, –3, –1, 0, 0.5, by increasing the applied stress step by step until the specimensbroke. In these tests, the number of run-out cycles was defined as 10 7 and maximum andminimum applied nominal stresses were not to exceed 0.2% yield strength. The crack-profilepath from the initial point was also measured at the fracture surface by using a lasermicroscope to analyze the behavior of the crack propagation. When the specimen did notbreak from the fretting non-propagating crack, its depth was measured by polishing thecrack surface until it disappeared.3. Fretting fatigue test results3.1. Fretting fatigue strengthFigure 5 shows the stress amplitude σa against the number of cycles (S–N) diagrams for PCand LC conditions. Figure 6 shows the effect of the contact pressure on fretting fatigue