An application of the differential thermographic technique for welded joints fatigue evaluation

摘要

The paper deals with the possibility of using the differential thermography (Thermoelastic Stress Analysis) to predict the fatigue resistance of welded joints on the basis of the local stress/strain field at the weld toe. The study is inspired by a local strain method, the WEL.FA.RE. method, based on the local amplitude of strain ε_a measured by 3mm grid-length strain gauges bonded with the axis at a 2.5mm distance from the real weld toe. The WEL.FA.RE. method suggests to determine the fatigue limit of welded joints simply by means of an experimental curve and the measurements of the local amplitude of strain ε_a to the weld toe directly on the structure under service conditions. In this work, both strain gauge and TSA techniques have been used to this purpose. In order to understand the development of the fatigue phenomena, the entire local strain field to the weld toe has been monitored by means of the thermoelastic stress analysis (TSA) technique and the results have been compared to those obtained with strain gauge. Structural steel specimens have been fatigue tested under alternate symmetric loads (tension-compression) and the local strain amplitude to the weld toe has been measured with two experimental technique (strain gauge and thermoelastic stress analysis) and compared in view of choose which one is more suitable for the WEL.FA.RE. method. The analysis of the thermoelastic data has showed that TSA is able to provide adequate spatial resolution to describe the complexity of the strain field along the cord. Furthermore the phase image has turned out to be an effective parameter to assess the crack initiating and growth. So, thermoelasticity has the capacity to be used as a non destructive technique for the evaluation of the structural integrity of the welded joints.