Failure analysis of pre-stressed high strength steel bars used in a wind turbine foundation: Experimental and FE simulation

Two screw-thread pre-stressed high strength steel bars used in a wind turbine foundation fractured suddenly and unexpectedly after being loaded by a pre-stress for a number of hours. In this paper, on the one hand, a series of tests were conducted to determine the failure cause of the bars. The results suggest that the failure was caused by hydrogen-assisted cracking (HAC). An amount of hydrogen had invaded into the bars, and a number of inclusions were found in the bars. The large-size inclusions caused very high stress concentration around them, consequently resulting in excessive accumulation of hydrogen as the stress can enhance hydrogen diffusion and accumulation. The coexistence of high stress concentration and hydrogen accumulation around inclusions induced HAC, finally causing the failure. On the other hand, based on hydrogen-influenced cohesive zone modeling (CZM), finite element calculations were performed to simulate the crack initiation in the bars. The results indicate that the CZM has a potential to predict such kind of crack initiation assisted by hydrogen, and can offer a numerical technique to determine whether HAC may occur in an actual inclusion- and hydrogen-containing pre-stressed high strength steel structure.