G. R. Ahmadzadeh

Fatigue damage and life of SS304, S45C, and SNCM630 steel samples tested at various irregular axial‐torsional loading spectra were evaluated by means of 7 Smith‐Watson‐Topper (SWT) type damage descriptions. The overall damage was calculated over peak‐valley events of counted cycles by means of the product of stress and strain corresponding to the areas within stress‐strain hysteresis loops. Fatigue lives of 304 steel samples predicted by the SWT, Lorenzo‐Laird, Szolwinski‐Farris, and Chen‐Xu‐Huang models were found noticeably larger than those of experimentally obtained at various loading spectra. Predicted lives by these descriptions were found moderately in agreement with experimental data of S45C and SNCM630 steel samples. The predicted lives of steel samples by Socie, Lv et al, and the one presented in this study were closely agreed with experimental data due to their different descriptions. The modified SWT model further included the product of maximum shear stress and shear strain amplitude and related the overall damage to fatigue life through Coffin‐Manson equation. The choice of model descriptions for damage assessment was discussed based on their terms, areas underneath of hysteresis loops, and material damage mechanism.

The present study intends to examine ratcheting response of SiC_P particle-reinforced Al 6061 matrix (Al 6061/SiC_P) composite samples over asymmetric load cycles based on the kinematic hardening rule of Ahmadzadeh–Varvani. The Ahmadzadeh–Varvani hardening rule offered a simple framework by taking into account of both material and stress level dependent coefficients to predict the quasi-shakedown of ratcheting of composite samples with various volume fractions under single- and multi-step loading conditions. The coefficients in the Ahmadzadeh–Varvani rule were estimated by means of mathematical expressions involving material properties, mean stress, and stress amplitude for any given stress levels. Ratcheting strain progressively increased as composite materials experienced low–high loading sequences when stress level increased over steps of loading histories. Ratcheting strain curves of low-high and high-low loading histories were successfully predicted. The predicted ratcheting curves with high–low loading sequences have shown change in ratcheting direction consistent with the experimental data.

The present study predicts ratcheting response of 1070 and 16MnR steel samples using nonlinear kinematic hardening rules of Ohno–Wang (O–W) and Ahmadzadeh–Varvani (A–V) under uniaxial stress cycles. The ratcheting values predicted based on the O–W model were noticeably influenced by the magnitude of exponents and the number of backstress components. Taking into account both material and cyclic stress level dependent coefficients, the A–V hardening rule offered a simple framework to predict ratcheting strain over loading cycles. A comparative study of these hardening rules to assess ratcheting of 1070 and 16MnR steel samples undergoing uniaxial loading conditions resulted in a close agreement of the A–V and O–W models. The choice of hardening rules in the assessment of materials ratcheting was further discussed based on the complexity of the hardening rule, number of constants/coefficients required to characterize ratcheting response, and central processing unit (CPU) time required to run the models.

The present study evaluates ratcheting response of materials by means of the Armstrong–Frederick (A–F) hardening rule, the modified A–F rule (Bower's model), and further modifications of the hardening rule based on new introduced coefficients. The implemented modifications on the A–F‐based hardening rule aims to address stages of ratcheting over stress cycles. The modified hardening rule predicts the ratcheting strain rate decay over stage I and the constant rate of strain accumulation during stage II. The modified hardening rule consisted of the coefficients of the hardening rule controlling stress–strain hysteresis loops generated over stress cycles during ratcheting process (Bower's modification on A–F rule) plus the coefficients controlling rates over stages of materials ratcheting deformation. Stress–strain‐dependent coefficients in the modified rule are responsible to compromise overprediction of ratcheting of A–F during stage I and the premature plastic shakedown beyond stage I induced by Bower's model. Ratcheting strain rate coefficients improved the hardening rule capability to calibrate and control the rate of ratcheting in stages I and II and enabled the modified hardening rule to predict ratcheting strain over a prolonged domain of stress cycles. The modified hardening rule was employed to assess ratcheting response of 304, 42CrMo, 316L steel and copper samples under uniaxial loading conditions. The predicted ratcheting values based on the modified hardening rule and the experimental ratcheting strains were found in good agreements.