Crack detection via strain measurements in fatigue testing

Fatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure\u27s operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements

Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment

High-Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives

A probabilistic study of welding residual stresses distribution and their contribution to the fatigue life

Welding is a joining process that is associated with heating cycles which leads to considerable change in local material microstructure and the formation of high welding Residual stresses (RS) in the welded joint. Residual stresses can have a detrimental effect on the fatigue strength of welded joints. In this paper, previously published data from measurements of residual stresses in various types of welded joints are compiled. In total, more than 100 test results are studied covering steels with yield strengths between 307 MPa and 1050 MPa in different welded details (butt joints, longitudinal and transverse attachments, cruciform joints, as well as K-joints) with varying thicknesses. The collected data is used to study the distribution of welding residual stresses (regardless of the welding parameters) at weld toe and through the thickness of the welded plate. Probabilistic analysis is then used to arrive at a model that represents the value and distribution of residual stresses in welded joints. This model is used to predict and explain the scatter in fatigue test data from recent fatigue testing of welded samples