Conditioning monitoring by microstructural evaluation of cumulative fatigue damage |
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Authors: | C Fukuoka Y G Nakagawa J J Lance R N Pangborn |
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Affiliation: | (1) the Research Laboratories, Ishikawajima-Harima Heavy Industries Company, Ltd., 135 Tokyo, Japan;(2) O & M Cost Control Technology, Electric Power Research Institute, 28262 Charlotte, NC;(3) the College of Engineering, Pennsylvania State University, 16802 University Park, PA |
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Abstract: | The objective of this work is to evaluate the damage induced below and above the fatigue limit (Δσ
t
=360 MPa) in pressure vessel steels, such as SA508. Fatigue damage was induced in samples taken from an SA508 steel plate
by various loading histories in order to examine the influence of prior cyclic loading below the fatigue limit. Cell-to-cell
misorientation differences were measured by the selected area diffraction (SAD) method. Surface cracking was also studied
by the replication method. Small cracks were observed after precycling both below and above the fatigue limit. It was, however,
found that fatigue test bars had a longer lifetime after precycling below the fatigue limit, while precycling above the fatigue
limit caused other specimens to fail even when subsequently cycled below the fatigue limit. Cell-to-cell misorientation usually
increases with accumulation of fatigue damage, but it was found that the misorientations measured after precycling below the
fatigue limit decreased again at the beginning of the subsequent cycling above the fatigue limit. It should be noted that
the misorientation at failure was always about 4 to 5 deg, regardless of loading histories. Misorientation showed good correlation
with the fatigue lifetime of the samples. |
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Keywords: | |
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