Experimental investigation on low-cycle fatigue and lifetime estimation of high-strength lockbolt material ML40Cr steel

High-strength lockbolts have been widely applied in large-scale structures, such as wind turbine supporting structures, due to their excellent anti-lose and fatigue resistance performance. However, there has been limited research on the material behaviour of lockbolts. This paper investigates the fatigue properties of the lockbolt material ML40Cr steel, such as cyclic behaviour, peak stress, elastic modulus, effective stress and back stress under various strain amplitudes and ratios. The cyclic behaviour and peak stress of ML40Cr steel exhibited a three-stage softening process during low-cycle fatigue, namely initial rapid softening, smooth softening, and rapid drop stages. Based on experimental observations, a modified Chaboche combined hardening (MCCH) model is proposed to predict the hysteretic behaviour of high-strength lockbolt material under low-cycle fatigue. This model considers cyclic softening behaviour in the kinematic hardening rule and strain amplitude effects in the isotropic hardening rule, accurately capturing the cyclic behaviour of the material. The commonly used Smith-Watson-Topper (SWT) model can conservatively predict the fatigue life of ML40Cr steel, aiding in initial safety design considerations for structural engineering applications. The Darveaux model proves too complex for practical fatigue life predictions. To address these limitations, this study proposes a modified Darveaux model for fatigue life predictions. The modified Darveaux model employs the energy of the stable hysteresis loop to predict fatigue life of the ML40Cr steel.