The loss of relatively large pieces of wheel tread material has been described as shelling or spalling. For the purposes of this paper shelling will be defined as loss of material from the action of rolling contact fatigue. The mechanism and characteristics of spalling has been presented in an earlier interpretive literature review. Shell cracks initiate at the near surface or surface of the wheel and grow inward. The cracks grow at an approximate 45° angle to the tread surface by shear as shown in figure 1. It appears that the shell forms due to shear failure along flow lines from to normal metal flow due to repeated rolling contact.

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