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.
To investigate mechanical properties and understand deformation mechanisms of nanocrystalline materials under high strain rate, the dynamic compression tests for nanocrystalline Ni bulk prepared by high-energy ball milling and hot-pressure sintering were carried out at different high strain rate on Hopkinson bar and a mechanical modeling based on deformation mechanism under high strain rate loading was developed. The experimental results indicate that the nanocrystalline Ni sample has higher strength and good ductility and the strength of the sample increased with the increasing strain rate. Meanwhile, The predictions by the mechanical modeling based on mechanism of dislocation gliding and grain boundary sliding at high strain rates show good agreements with the experimental data.
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