Experimental research on the fatigue crack growth behaviour of Q420C

The lack of test data on the fatigue crack growth behaviour of Q420C steel affects its application in the field of wind power engineering. To fill this gap, this paper carried out an experimental study and theoretical analysis of the fatigue crack propagation behaviour of Q420C steel under constant amplitude. Six compact tension specimens were separately tested under six loading conditions comprising two maximum loads (Fmax = 8 kN or 10 kN) and three load ratios (R = 0.1, 0.3, or 0.5). The test results show that for the two crack growth rate calculation methods given by the ASTM E647-15, the secant method provides large discrete results, while the incremental polynomial method can obtain a relatively smooth rate curve. In addition, for the incremental polynomial method, the normalisation of the number of cycles in the ASTM E647-15 should not be used to achieve acceptable results. The rate curves of Q420C steel show a clear load ratio effect. Quantitative analysis of the effect of load ratios has shown that two-parameter models based on 'unified theory', such as Walker's model, can accurately describe the effect of load ratios. However, the theory of plasticity-induced crack closure cannot account for the effect of load ratios, as the crack opening load Fop is less than the minimum load Fmin for load ratios greater than 0.1. Finally, a comparison is carried out against the predictions by BS7910; results verify the applicability of BS7910 to Q420C steel material and demonstrate the different degrees of conservatism of all recommended design curves in BS7910.

Automated summary

This paper presents an experimental and theoretical study of the fatigue crack propagation behavior of Q420C steel. The results show that the incremental polynomial method is more suitable for calculating the crack growth rate compared to the secant method. The rate curves of Q420C steel exhibit a clear load ratio effect, which can be accurately described by models such as Walker's model. The theory of plasticity-induced crack closure cannot explain the effect of load ratios. In addition, the applicability of BS7910 to Q420C steel is verified, and the different degrees of conservatism of the recommended design curves in BS7910 are demonstrated.