NONDESTRUCTIVE CHARACTERIZATION OF FATIGUE PROCESSES IN CYCLICALLY LOADED WELDED JOINTS BY THE BARKHAUSEN NOISE METHOD

The fatigue behaviour of welded joints is dependent on the weld geometry, its microstructure, the cyclic material properties of the different weld zones and residual stresses in the weldment. In this investigation, a new nondestructive testing method is presented for a in-situ characterisation of fatigue damage in cyclically loaded welded joints. This micromagnetic method is based on the Barkhausen-Noise effect and with a new multi-parameter testing system, microstructure- and stress-sensitive parameters can be measured. The system is mobile, computer-controlled, has a high spatial resolution and needs no shielding, what makes it especially suited for diagnostics at existing steel constructions, e.g. buildings, steel bridges, cranes, steel parts of airplanes, etc.. Fatigue tests were performed on GTA-welds as single-step tests for a evaluation of the feasibility of the new method for characterising fatigue processes. Local plastic deformations in softened zones of the weld causing material damage were identified by a significant change of the micromagnetic parameters. Additionally, the static and cyclic mechanical deformation behaviour for the welded joints was studied by local strain measurements and FEM-calculations. The results of the present investigations indicate that material damage caused by microstructural changes and microcrack growth is strongly connected with the micromagnetic parameters and can be correlated to the mechanical behaviour and the fatigue life of welded joints. Furthermore, the micromagnetic testing method can be used for the identification of the different stages of the fatigue process in the material before failure of the weldment.