FATIGUE CRACK INITIATION AND EARLY GROWTH IN TENSILE-SHEAR SPOT WELDMENTS (SHEET STEEL, REPLICA METHOD, HIGH STRENGTH LOW ALLOY STEEL)

Fatigue crack development was observed in tensile-shear spot weldments made from a galvanized high-strength low-alloy steel. Fatigue cracks were measured by destructive examination of companion test specimens. A new technique was developed which allowed the direct observation of fatigue crack initiation and growth in spot-welded specimens which had been sectioned along the centerline of their nuggets. Measurements of fatigue cracks in these "presectioned specimens" were made from acetate replicas which had been made of the polished notch-root area of the exposed spot weld sections.;If life is defined as the number of cycles for a fatigue crack to grow through the thickness of the sheet (for this study, 1.4 mm), initiation of cracks 0.25 mm deep consumed up to 55% of the fatigue life of tensile-shear spot weldments. The portion of life devoted to initiation under a variable load history was found to be lower than that under constant amplitude loading. The improvement in total fatigue life resulting from the coining technique--a treatment in which the spot weld is compressed--was found to be shared between the initiation and propagation portions of life.;An initiation-propagation fatigue life model was modified using the data from this study and was found to correctly predict the fatigue lives of the specimens tested for the current study and for specimens tested by other researchers.;The presectioned-specimen method of monitoring fatigue cracks proved to be a reliable technique of measuring crack growth in an otherwise inaccessible area. The results of the presectioned specimens were comparable to those of the companion test series.