It is proposed that fatigue damage evolution is controlled by surface displacements and these can be accurately measured by atomic force microscopy (AFM). As these displacements can be followed throughout the history of a fatigued component, the fatigue process in general represents a continuum of behavior. In 10 and 200 μm grain size titanium, AFM measurements demonstrate that the fraction of plasticity contributing to surface damage can be expressed as a single function over nearly five decades of cycles. Regarding this function, the effect of grain size appears to be small. In terms of damage accumulation rates, cyclic hardening parameters, and the threshold stress intensity, the proposed model represents a microstructurally-sensitive Manson-Coffin law for fatigue initiation. Coupling this with a more standard fracture mechanics approach for the latter stage of life allows a simple expression for life prediction. Over the range of 103-106 cycles, this expression predicts fatigue life of titanium exposed to air and saline environments to first order.
Bibliographical noteFunding Information:
Discussions particularly relevant to strain-range partitioning, cumulative damage and small crack propagation rates are acknowledged with S. Antolovich, D. L. Davidson, Y. Katz, and S. Suresh. This research was supported by the Office of Naval Research under Grant N00014-91-J-1998 and for one of us, J. W. Hoehn, under the Basic Energy Sciences Division, Materials Science, DOE Grant No. DE-FG02-96ER45574.