Influence of pores on the fatigue life of welded joints and investigation of acceptance criteria defined in standard documents

Abstract

Inner defects in welded joints constitute concurrent sites for fatigue failure in welds, especially when high fatigue resistance is enforced at weld toe and root. Existing standards limit the size of pores and other inner defects in proportion to the sheet or weld thickness, but for larger dimensions cutoff values are in place. This leads to overly high acceptance criteria when it comes to thicker sheets, as for example those used for wind turbine towers. To analyze the influence of pores on the fatigue life, specimens with targeted pore sizes larger than the usually accepted were tested under fatigue loading. In the present study, a numerical model for fatigue assessment was developed based on the experimental results, which allowed further variance of pore sizes and locations. It is shown, that for larger weldments the acceptable pore size can be increased by up to 87.5 % without the necessity to reduce the applicable design curve. But, the location of the pore with regard to the specimens surface has a large influence, with pores in a distance to the surface smaller than 13 % of the sheet thickness leading to a reduction in the applicable design curve by up to 4 FAT classes — an effect that is currently not considered in common weld quality documents.