Experimental investigation on anisotropic fatigue crack growth characteristics of Q420C steel

The hot-rolled steel sheets often exhibit anisotropy in their mechanical characteristics due to the grain's elongation along the rolling orientation. The anisotropy characteristic also appears in the fatigue crack growth behaviour. However, it has not been thoroughly studied for some types of steel materials although they have been widely used for wind turbine towers. In this study, the longitudinal fatigue crack growth behaviour of the hot-rolled steel sheet (Q420C) is investigated by an experimental programme. A total of six specimens were tested under constant amplitude loading cases with different load ratios. It was found that longitudinal cracks are approximately 15% faster than transverse cracks at the load ratio of 0.1; at the load ratio of 0.3, they are approximately 22% faster. The anisotropy of crack growth rates is attributed to differences in the microstructure of the material. A comparison of the crack growth rate is carried out for Q420C steel and some widely used American-made steel materials. Furthermore, the applicability of design curves in the British Standard BS 7910 is explored, the crack closure phenomenon is analysed and the fatigue crack growth mechanism is discussed.

Automated summary

Hot-rolled steel sheets often exhibit anisotropy in their mechanical characteristics due to grain elongation along the rolling orientation. This anisotropy also affects fatigue crack growth behavior. However, there is limited research on this for some types of steel materials, despite their wide use in wind turbine towers. In this study, the longitudinal fatigue crack growth behavior of Q420C hot-rolled steel sheet is investigated through experiments. Results show that longitudinal cracks propagate approximately 15% faster than transverse cracks at a load ratio of 0.1 and about 22% faster at a load ratio of 0.3. The anisotropy in crack growth rates is attributed to microstructural differences. A comparison with American-made steel materials is made, and the applicability of design curves in the British Standard BS 7910, crack closure phenomenon, and fatigue crack growth mechanism are discussed.