This paper investigated the effect of variable normal load on the fretting fatigue mechanism. A kinetics-based Q- P curve analysis method was proposed to assist testing system design and experiment result analysis. Based on this method, a biaxial fretting fatigue testing system was designed. Experimental and numerical investigation was carried out to discuss the effect of biaxial loading phase difference (the phase difference between bulk load and cyclic normal load) and stiffness ratio (the stiffness ratio between pad fixture and flat specimen) on the fretting fatigue mechanism. Based on the critical plane approach and proposed Q- P curves analysis method, it is found that the fretting status is partial slip regime under small stiffness ratio conditions. The stress/strain which is influenced by phase difference is the main factor of fretting fatigue damage in this condition. Furthermore, the mean stress of the normal load on the critical plane is compressive stress. It directly influences the fatigue damage under partial slip regime. With the increase of stiffness ratio and phase difference (from 0° to 90°), the fretting status changes from partial slip regime to gross slip regime, which means that the influence of wear increases. Wear inhibits the initiation of fatigue cracks, which has a positive influence on fretting fatigue life. As a result, the fretting failure mode gradually changes from fatigue to wear.
Many fretting-related engineering failure cases are under varying normal load and bulk load conditions. This complex loading condition makes fretting running regime very complicated. Since the fretting running regime significantly influences the fretting-induced fatigue and wear, it is worth discussing the fretting running regime under these loading conditions, which is barely studied at present. Based on the kinetics analysis of the commonly used fretting structure, a Q- P curves analysis method was derived. Different loading conditions (proportional and non-proportional) and fretting running regimes (partial slip and gross slip) were carefully discussed. It reveals that the coefficient of friction, stiffness of the specimen and pad support sheets, and loading parameters of variable normal load and bulk load are the main factors which influence the fretting running regime. Due to the variable normal load, the Q- δ curves and the tangential force history become extremely intricate. While the proposed Q- P curves analysis method shows a satisfactory result for the illustration of the fretting running regime under these complex loading conditions. It provides a novel methodology for the fretting mechanism analysis under variable normal load conditions.
This article is a review of models for predicting ultra-low cycle fatigue life. In the article, the life prediction models are divided into three types: (1) microscopic ductile fracture models based on cavity growth and cavity merger; (2) fracture models based on porous plasticity; and (3) ductile fracture models based on continuum damage mechanics. Furthermore, the article provides a critical assessment of the current state of research on ultra-low cycle fatigue life prediction models, highlighting the limitations and challenges faced by each model type. Ultimately, this review aims to provide a comprehensive overview of the different models available for predicting ultra-low cycle fatigue life and to guide future research in this important area of materials science and engineering.
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