C. Fukuoka et al., CONDITIONING MONITORING BY MICROSTRUCTURAL EVALUATION OF CUMULATIVE FATIGUE DAMAGE, Metallurgical and materials transactions. A, Physical metallurgy andmaterials science, 27(12), 1996, pp. 3841-3851
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.