Papers by Keyword: Fatigue Mechanism

Abstract: Fatigue fracture surfaces and crack morphologies of 316LN stainless steel that test in a simulated AP1000 first-loop water and air environment were investigated by SEM, LSCM and EBSD. The results showed that, the fatigue crack initiated at persistent slip band, impurities and grain boundary, and then propagated in a trans-granular manner with typical fatigue striations. Characteristics of corrosion fatigue, such as brittle fatigue striation, rhomboid corrosion product and the trace of corrosions were found on the fracture surface of first-loop water environment specimen. The strain on first-loop water environment specimen is unevenly distributed surrounding the crack, and the gradient is not obvious, while that on air environment ones is evenly distributed , and the distribution gradient is associated with the distance of crack from . The fatigue crack propagation was accelerated in the first-loop water environment, and the EAC mechanism is most likely to be HIC.

Abstract: Fatigue strength characteristic of laser welded joint between austenitic stainless steel and ferritic stainless steel was studied at room temperature and 300 °C.The results showed that fatigue strength at 300°C was higher than that at room temperature in lower applied stress amplitude region. Fatigue crack mainly initiated at stress concentration part at room temperature. However, fatigue crack origin changed toward to the base material and the heat affected zone with increasing testing temperature. Increase in ductility of the base material and the weld metal induced by increasing testing temperature reduces stress concentration effect and resulted in change in fatigue mechanism. Appropriate weld position in a component with temperature gradient can be considered by minimizing the effect of stress concentration and taking into account degradation of strength due to increase in temperature.

Abstract: In recent years, the core engineering components of high-speed train, automobiles and aircrafts are required to endure fatigue loads up from 10⁸ to 10¹⁰ cycles. The present study results show that in the very high cycle fatigue (VHCF) regimes of more than 10⁷ cycles, the fatigue failure of high strength steel materials can occur below the traditional fatigue limit, hence the VHCF investigations of high strength steels not only help to further understand the fatigue essence and mechanism, but also do research on the fatigue design and life assessment method. This paper summarizes works of VHCF researches for high strength steels in recent years, such as the characteristics of S-N curve, the observations on fish-eye, which is one of the typical characteristics of fracture surface, crack initiation, crack propagation, etc. The present work also analyzes the fatigue mechanisms and briefly discusses several factors that affect VHCF properties, such as hydrogen effect, inclusion effect, frequency effect. Some possible and prospective aspects of future researches are also proposed.

Abstract: The multistage strength degradation theory, which has recently emerged from studies on the material and structural behaviour of concrete, provides a clear description of the mechanism of fatigue. According to this theory, fatigue is caused by the sporadic sudden change of cracking behaviour in a system under cyclic loading, leading to intermittent strength reduction of the system and its eventual failure. As metal is the main engineering material plagued most by fatigue failure, this newly-established theory needs to be experimentally verified on metal, which is the aim of this study. The obtained test results present strong experimental evidence for the existence of multistage strength degradation processes in metals under cyclic loading, and the strength degradation is clearly triggered by the abrupt change of cracking behaviour. These tests confirm the relevance of the multistage strength degradation theory on metal fatigue, and the engineering implications of the study are discussed.

Abstract: This paper presents the results of durability tests of AW7075 aluminium alloy, which is widely used in producing high-durable and particularly reliable constructions in aeronautical and automotive industries. The plain-strain fracture toughness, depending on the orientation of fracture direction with respect to the rolling direction of the material, and the fatigue crack growth rate were determined. Based on scanning electron microscope (SEM) studies the two stages of fatigue fracture are shown: the first is plastic striations and the second is phenomenon of connecting micropores that were formed around intermetallic second phase particles. As a results of research a model of fatigue crack for the second stage is presented.

Abstract: In order to reveal the underlying theoretical reasons for that the traffic accidents are caused by driver fatigue, this paper specifically represents the formation and mechanism of driving fatigue. From the point of Human Factors Engineering, the driver’s physical system of bearing the driving load is constructed based on fatigue theories. In addition, the driving behavior is divided into three stages. Then the fatigue generated by each stage is strictly divided into mental fatigue and physical fatigue. According to the extent of driving fatigue, the status of driver's fatigue is divided into four types, thus, a cumulative mode in driver fatigue is constructed.

Abstract: From the basic theory of fracture mechanics and combining the fatigue mechanism of rotor crack propagation, energy theory and power relationship, the stress analysis and deformation of the rotor are discussed, and the acoustic emission mathematical model of the rotor crack propagation is deduced, which will provide the theoretical basis for the further study of the acoustic emission in the rotor crack fault detection.

Abstract: As the ideal candidates for high temperature structural materials, carbon/carbon (C/C) composites are no doubt involved in fatigue loading. Therefore, the study on fatigue behavior is meaningful. In this paper, the research on fatigue behavior of C/C composites was reviewed and the characteristic of fatigue behavior was summarized. Some viewpoints for further investigations for the study on the fatigue behavior of C/C composites are also made in this paper.

Abstract: Porous structure is an important component in C/C composites, which directly affects the fatigue behavior of materials. Therefore, it is necessary to discuss the evolution of porous structure in C/C composites under the fatigue loading. In present work, the character of porous structure in original C/C composites was summarized and the evolution of porous structure after fatigue loading was analyzed. The positive effect of the porous structure evolution after fatigue loading on the reinforcing behavior of fatigue was proposed as well, which could provide a basis for further studies on the fatigue mechanisms of C/C composites.