LOW AND HIGH-CYCLE FATIGUE - A CONTINUUM SUPPORTED BY AFM OBSERVATIONS

It is proposed that fatigue damage evolution is controlled by surface displacements and these can be accurately measured by atomic force mic roscopy (AFM). As these displacements can be followed throughout the h istory of a fatigued component, the fatigue process in general represe nts a continuum of behavior. In 10 and 200 mu m grain size titanium, A FM measurements demonstrate that the fraction of plasticity contributi ng 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, cycli c hardening parameters, and the threshold stress intensity, the propos ed model represents a microstructurally-sensitive Manson-Coffin law fo r fatigue initiation. Coupling this with a more standard fracture mech anics approach for the latter stage of life allows a simple expression for life prediction. Over the range of 10(3)-10(6) cycles; this expre ssion predicts fatigue life of titanium exposed to air and saline envi ronments to first order. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.