New Approaches to Predict Fatigue Parameters of Steels from Monotonic Properties and Estimation of Elasto-Plastic Localized Stresses and Strains.

摘要

Fatigue strain - life prediction model depends on six material fatigue parameters, fatigue strength coefficient sigma'f, fatigue strength exponent b, fatigue ductility coefficient epsilon'f, fatigue ductility exponent c, cyclic strength coefficient K', and cyclic strain hardening exponent n'.;In this study, a new nonlinear correlation between the Brinell hardness HB and ultimate tensile strength is proposed. The prediction results obtained from this model were compared with the results obtained using Roessle-Fatemi's method and experimental data. The correlation factor in the proposed model is higher than that found in the current literature.;The ultimate tensile strength is replaced by an equivalent Brinell hardness HB expression in the Modified Universal Slopes strain-life prediction model. This change results in sigma'f and epsilon'f fatigue parameters these parameters predicted using Brinell hardness HB. The new fatigue life prediction model was compared with the original Modified Universal Slopes model, and experimental data in the literature.;This model is valid for steels with hardness that ranges from 150HB to 660HB. The model is compared qualitatively and quantitatively with the Modified Universal Slopes life fatigue prediction model and experimental data. Different types of steels were employed to validate this model. The results show that the proposed model provides better fatigue life prediction when compared to the Modified Universal Slopes model, and experimental data. An accurate prediction of elasto-plastic cyclic deformation becomes extremely important in design optimization by providing accurate fatigue life prediction and that results in weight savings. Notch root stress-strain prediction is controlled by the two material parameters K' and n'. In this study a two-stage notch root prediction method is proposed. This was implemented using a correction factor to Neuber's rule notch strain amplitude as the first stage, and a linear interpolation scheme, between the results obtained from the first stage and elastic finite element analysis, as the second stage. The accuracy of this method is assessed by comparing the predicted results with the results obtained from elasto-plastic finite element analysis and Neuber's rule results. Various steels with different yield strengths were used in this study. Notch deformation behavior under cyclic fully reversed as well as variable amplitude loading conditions was monitored for a double notched flat plate and a circumferentially notched round bar to cover plane stress and plane strain conditions. Elastic as well as elasto-plastic finite element analyses were performed. Notch strain amplitudes in addition to fatigue life predictions obtained using the proposed method are in good agreement with the elasto-plastic finite element analysis when compared to predictions obtained using Neuber's rule. ABAQUS 6.13 software was used for elastic and elasto-plastic finite element analysis. Analytical methods together with fe-safe 6.5 software were used to obtain fatigue life under each loading condition.