Crack propagation and fracture in contacting bodies

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Crack propagation and fracture in contacting bodies

Orringer, O.

When a body subject to contact loads develops a crack, the origin, orientation, and tendency to propagate or not depend on fabrication details and maintenance history as well as the service environment. Research over the past twenty years has led to better characterization of crack behavior in some contacting bodies (railroad rail and wheels). Computational methods in solid mechanics provide the basis for quantitative results, but the true understanding also requires application of materials science principles. The two disciplines are being brought to bear in combination; quantitative assessments of potential manufacturing improvements and prediction of effects on performance in service are now within reach. Of the various kinds of cracking observed in rail, the most common on North American heavy freight lines is the so-called'shell', which orginates 6 to 10 mm below and propagates roughly parallel to the running surface. The average life to shell formation is O(2 to 4x10⁷ wheel contact stress cycles. Origin of the cracks is an area of tensile stresses in the subsurface region. Most of the wheels in freight service are monobloc castings. The average life to shell formation is O(5 to 6x10⁷ contact stress cycles. Discussed are thermal and fatigue cracks on wheels with 0,67 % to 0,77 % C.