Ultra-low Cycle Fatigue Fracture Research Articles

Prediction of fracture behavior of beam-to-column welded joints using micromechanics damage model

Moment-resisting steel frames often fail by fracture failure of beam-to-column welded joints during a strong earthquake. This paper provides a numerical methodology based on micromechanics damage model instead of traditional fracture analytical methods to investigate the crack initiation and propagation of welded beam-to-column joints subjected to monotonic loading and ultra low cycle loading conditions. Firstly, parameters used in the micromechanics damage model for steel base metal, heat affected zone and weld metal were calibrated, respectively, against uniaxial tension test results and cycle test results of notched specimens. The evolution of void growth in the notched specimens under different loading conditions was compared. Secondly, fracture process of the welded joints subjected to tensile loading was simulated based on the micromechanics damage model. The predicted load displacement response agrees well with the other researcher's test results. Finally, the micromechanics damage model was applied numerically to investigate the ultra low cycle fatigue fracture behavior of the welded joints under constant amplitude as well as variable amplitude inelastic cyclic loading. According to the distribution and evolution of void in the welded joints obtained from finite element analysis, crack initiation and propagation were presented and the number of cycles to fracture was predicted. It is shown that the fatigue life predicted from finite element analysis based on micromechanics damage model agrees well with the other's test results.

A simplified approach to predict the failure of steel members under interaction effect of fracture and fatigue

This paper presents a simple approach to predict the failure of steel structures under the interaction effect of fracture and fatigue which is termed as ultra-low cycle fatigue (ULCF) failure. The approach mainly consists of simple fracture criterion, which is obtained from further simplification of available cyclic void growth model (CVGM). As for alternative component of ULCF analysis, a simple non-linear hardening model is proposed to employ in elasto-plastic analysis for situations where parameters of available mix hardening models (combined isotropic and kinematic hardening models) are difficult to determine. Initially the simplified ULCF fracture criterion is clearly presented and associated ULCF fracture prediction methodology is also indicated. Then the elasto-plastic hardening model is proposed and model behavior is compared with experimental behavior of few materials. The procedure to determine hardening constants and finite element (FE) formulation are also clearly mentioned. The simplified approach is then employed to determine ULCF fracture of some structural models. Hence verification of the simplified approach is confirmed by comparing the results with previous approaches-based estimations. Finally, study tends to conclude that the simplified approach produces reasonable accurate prediction to ULCF fracture of steel structures where magnitude of triaxiality remains relatively constant. Further, it was able to prove that commonly available elasto-plastic FEM packages can be reasonably employed to estimate ULCF fracture of steel structure when simplified ULCF fracture criterion governs the failure.

Creep-fatigue damage assessment by subsequent fatigue straining : Nakamura, T., Yaguchi, M., Ishikawa, A. and Asada, Y. Nucl. Eng. Design (1996) 162 (1), 97–106

The ductile crack initiation of structural steels due to ultra-low cycle fatigue (ULCF) loading often governs the limit state in steel structures. This paper proposes a modified Coffin-Manson model for ULCF fracture of structural steels considering the effect of stress triaxiality. To this end, tests on coupon specimens and circular notched specimens made of Chinese Q345qC steel were conducted to form different stress triaxiality states. Based on test results and corresponding finite element analysis, the parameters of modified Coffin-Manson model were determined, and empirical formulas were established between model parameters and stress triaxiality. Finally, detailed finite element analysis was conducted on circular notched specimen tests to validate the proposed model. The results show that the proposed model can be used to predict ULCF fracture of structural steels at different stress triaxialities with high accuracy.