ACOUSTIC EMISSION METHOD APPLICATION TO CONTROL MICROCRACKING IN THIN-SHEET SPOT-WELDED JOINTS

Abstract

Shell-type body structures are referred to as support framework structures of modern vehicles (e.g. cars). Methods of electric resistance spot welding are widely used for their joints in the process of manufacturing, particularly when welding dissimilar low-carbon steels of light gauge (up to 3 mm). The process of welded body structures manufacturing is accompanied by the formation of non-equilibrium structures at the junctions and residual stress fields including welding stresses. Under further weathering (before operational loads application) relaxation processes occur in both non-equilibrium structures and residual stress fields, resulting in the stabilization of heterogeneous structurally stress and strain condition of an object. To evaluate relaxation resistance of a spot welded connection we used the following basic numerical characteristics: residual stress, stress decrease in the weld spot, average relaxation rate and the time before first crack formation in the local microscopic volume. A technique of stress relaxation testing of spot-welded joints for 08 08Ju steel grades and the results of testing are presented in the article. To register a localized area of microcracking in spot-welled joints when testing stress relaxation we used acoustic emission method. A technique for microcracking formation control in spot welded joints of light-gauge sheet metals applying acoustic emission method has been developed. This technique is the basis for studying relaxation resistance of welded joints. It has been found that relaxation resistance of a spot welded joint depends on the parameters of welding conditions, the value of initial load on welding spot and relaxation rate.