ISSN: 0090-3973
Page Count: 8

Local Strain Behavior Prior to Fatigue Crack Nucleation
Quesnel, DJ
Graduate student and professor, Materials Research Center, The Technological Institute, Northwestern University, Ill.

Meshii, M
Graduate student and professor, Materials Research Center, The Technological Institute, Northwestern University, Ill.

Abstract

The local strain behavior of cold-rolled HSLA (niobium) steel was investigated in the vicinity of a circular hole for first loading, first unloading, and in the unloaded state after 50 000 and 82 000 cycles by determining ε^θ as a function of distance from an 0.02-in. (508-μm) circular notch by direct comparison of optical micrographs at approximately ×1000. This technique allows changes in length to be determined to ±0.15 μm which, over a 100-μm gage length, corresponds to an engineering strain resolution of ±0.15%. The resolution is sufficiently high to allow local strain distributions, as calculated from elasticity and plasticity theory, to be compared with the experimental data. A pseudo-plastic solution was developed from the plane stress solution by considering A. E. Green's three-dimensional solution and Neuber's relation between stress and strain concentrations. The monotonic deformation law obeyed by cold-rolled niobium steel was experimentally determined and combined with the pseudo-plasticity result to obtain a quantitative prediction of the local strain behavior on first loading. Experimentally obtained strain behaviors for first loading provide good agreement with the pseudo-plasticity result for each of the strain levels investigated. The strain distribution measured after the first unloading demonstrated that a compressive residual stress is generated during the first cycle. The strain distribution developed by repeated tension loading is also presented and discussed.

Keywords:
fatigue (materials), strain measurement, crack initiation, stress concentration

Paper ID: JTE10519J
DOI: 10.1520/JTE10519J
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