Fatigue strength assessment of welded mild steel joints containing bulk imperfections

This work investigates the effect of gas pores, as bulk imperfections, on the fatigue strength of welded mild steel joints. Two test series containing different butt joint geometries and weld process parameters are included in order to achieve two variable types of pore sizes. Based on the SRarea-parameter by Murakami, the test series can be grouped into imperfections exhibiting SRarea < 1000 mm and SRarea > 1000 mm. Fatigue tests at a load stress ratio of R = 0.1 are performed, which act as comparison for the subsequent fatigue estimation. To assess the fatigue resistance, the approaches by Murakami, De Kazinczy, and Mitchell are utilized, which highlight certain differences in the applicability depending on the imperfection size. It is found that, on one hand, Mur akamis approach is well suitable for both small and large gas pores depending on the applied model parameters. On the other hand, the fatigue concepts by De Kazinczy and Mitchell are preferably practicable for large defects with SRarea > 1000 mm. In addition, the method by Mitchell incorporates the stress concentration factor of the imperfection, which can be numerically computed considering the size, shape, and location of the gas pore, as presented in this paper.