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 (F max = 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 F op is less than the minimum load F min 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. • This paper presents a thorough fatigue crack growth study of Q420C steel. • No normalisation of cycle number is needed in the incremental polynomial method specified in ASTM E647–15. • The load ratio effect has been confirmed that a higher load ratio leads to higher fatigue crack growth rates in Q420. • The effect of load ratio can be well explained by the unified theory, but not by the crack closure theory. • The design curves in BS7910 also hold for Q420C, of which the simplified curve is preferable in engineering applications. [ABSTRACT FROM AUTHOR]