Naval welded joints local stress assessment and fatigue cracks monitoring with quantitative thermoelastic stress analysis

International audienceFatigue assessment of welded assemblies is a key challenge both for industrial and academic research. This paper assesses possibilities of thermoelastic coupling quantitative analysis for the comprehension of fatigue phenomena occurring with welded structures. A test protocol has been set up to measure temperature field on cruciform as-welded specimens under cyclic loading thanks to an infrared camera. Identification of temperature variations associated to thermoelastic coupling (order of magnitude of 0.1 K) allows to map temperature which are directly correlated to stress tensor first invariant. From this result, analysis of thermoelastic coupling gradient allows to determine the part of bending stress due to specimen distortion (up to 20% higher than nominal stress). Besides, thermal conduction influence when evaluating a local stress at weld toe is studied thanks to a numerical thermal model. In addition, infrared images post-treatment based on image segmentation is proposed for fatigue crack monitoring at weld toes (minimal crack size detection of 1 mm). Experimental validation is performed through analysis of marked fracture surface. Finally, fatigue crack growth rates are evaluated with this approach for different load amplitudes