An Application of Stress Intensity Factor to Fatigue Strength Analysis of Welded Invar Sheet for Cryogenic Use

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Abstract
Fatigue strength of several types of welded Invar joints were investigated at room temperature and 111 K. Lap fillet welded joints, in which a 1.5 mm thick Invar sheet was welded to a 1.5 mm or 3.0 mm thick Invar sheet or 12 mm thick stainless steel, were made by tungsten-inert-gas (TIG) welding. A joint, in which a 0.5 mm thick Invar sheet was inserted between two overlapped 1.5 mm thick Invar sheets, was made by resistance seam welding. In these joints, the root of the weld is the region most susceptible to fatigue crack initiation. Since S-N relationship depends upon the joint type, specimen size, and loading method in a fatigue test, a fatigue strength parameter independent of these conditions is necessary for the fatigue evaluation of structures. In this study, equivalent stress intensity factor range ΔKeq was adopted and investigated as such a parameter. By simulating the measured cyclic strain behaviors by elastic-plastic FEM analyses, the cyclic ranges of tensile, bending, and shear stresses that arise in the vicinity of the weld can be estimated. Using these ranges, ΔKI, ΔKII and accordingly ΔKeq were calculated for each specimen. As a result, it was found that the relationships between ΔKeq and cycles-to-failure in the range above 104 cycles were well within a narrow scatter band, regardless of the type of joints. It was concluded that ΔKeq is a very effective parameter for prediction of fatigue strength when the sharp root of a weld is the problem.

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