Page 324 - Handbook of Materials Failure Analysis
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6 Fatigue Wear Particles   321

















                   (a)                             (b)

                  FIGURE 13.11
                  (a) The stripped down race car gearbox; (b) the damaged gear.
                  particles in this case study. Due to the inadequate lubrication, the catastrophic wear
                  particles have various oxide films, coloring from straw-yellow to green-blue.




                  6 FATIGUE WEAR PARTICLES
                  Fatigue wear particles are generated from the surface contact fatigue. Surface contact
                  includes rolling contact, sliding contact and impact contact. For a rolling element
                  bearing, sometimes, it would subject to the three different contacts which depend
                  on the machine operations. It is believed that microcracks would initiate in the sub-
                  surface after certain cycles of repeat contact stress. When the microcracks propagate
                  to the metal surface, the fatigue wear particles can be generated.
                     Micropitting and macropitting (spalling) are the forms of surface fatigue failures.
                  Micropitting usually generates the fatigue wear particle up to over a hundred
                  microns. Whereas macropitting produces significant amount of big-size fatigue wear
                  particles, which are up to mm scale. However, such large-size particles are rarely
                  caught in oil samples. But they can be collected by the magnetic plate installed in
                  the oil sump.
                     Figure 13.13 shows the typical fatigue wear particles. Fatigue wear particles usu-
                  ally feature with smooth surface and random, irregular profiles [2]. In comparison
                  with laminar particles, fatigue wear particles are thick. Figure 13.13a was a fatigue
                  wear particle (L100 μm) from an oil sample taken from a rolling element bearing
                  lube in a Yankee machine. Figure 13.13b was a large fatigue wear particle
                  (L720 μm) from an oil sample taken from a gearbox in a paper mill.
                     Some fatigue wear particles remain the striations on the particle surface. These
                  striations could associate with the crack propagations during the particle generation.
                  Figure 13.14a was a fatigue wear particle (L160 μm) from the rolling element bear-
                  ing, on which some striations are distinctive. Figure 13.14b displays another fatigue
                  wear particle from the same bearing. The striations had almost disappeared, but
                  could still be distinguished.
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