Microstructural evaluation of cumulative fatigue damage below the fatigue limit

The objective of this work is to evaluate the microstructural changes induced near and below the fatigue limit in a pressure vessel steel plate, SA508. Dislocation cell to cell misorientation differences, θ, which increase with fatigue damage accumulation, are measured by the Selected Area Diffraction (SAD) method. The misorientation difference, θ, of the sample failed just above the fatigue limit is about 4.0 degrees on the average, which is about the same as that for the failure conditions of low cycle fatigue at higher stresses. The θ value increases even below the fatigue limit, but it does not increase at stresses which are lower than 50% of the fatigue limit