Ortsaufgelöste Erkennung von Ermüdungsschäden an Schweißnähten mit Hilfe aktiver Infrarot-Thermografie

The paper presents two thermographical approaches for the detection of fatigue damages in welded joints of mild carbon steel S355. First, a recently developed extension of the thermoelastic stress analysis is utilised for early crack detection on butt welds. At low frequency fatigue loading the method separates the linear and nonlinear temperature response related to the harmonic loading. Significant changes during the test reveal the progressive fatigue damage evolution. On the tested components localised damage processes could be observed as early as 10 to 30 % of the total fatigue lifetime. The method is especially suited for laboratory testing since a defined and damage relevant loading is necessary. As a second approach the ultrasound excited thermography is applied using a high intensity ultrasonic transducer working at 20 kHz. Induced elastic waves lead to oscillations of the tested component. Dry friction at the crack faces provides a localised heat generation which allows for crack detection. It was found that the crack can be detected after 35 % of the fatigue lifetime. Although the crack detection on metal components is strongly dependent on the ultrasonic frequency a high potential for a future mobile application can be awarded to this method.