CONDITIONING MONITORING BY MICROSTRUCTURAL EVALUATION OF CUMULATIVE FATIGUE DAMAGE

The objective of this work is to evaluate the damage induced below and above the fatigue limit (Delta sigma(t) = 360 MPa) in pressure vessel steels, such as SA508. Fatigue damage was induced in samples taken fr om an SA508 steel plate by various loading histories in order to exami ne 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 re plication method. Small cracks were observed after precycling both bel ow and above the fatigue limit. It was, however, found that fatigue te st bars had a longer lifetime after precycling below the fatigue limit , while precycling above the fatigue limit caused other specimens to f ail even when subsequently cycled below the fatigue limit. Cell-to-cel l 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 subseq uent cycling above the fatigue limit. It should be noted that the miso rientation at failure was always about 4 to 5 deg, regardless of loadi ng histories. Misorientation showed good correlation with the fatigue lifetime of the samples.