Advances in Fracture and Failure Prevention

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Authors: Keum Oh Lee, Sam Son Yoon, Soon Bok Lee, Bum Shin Kim
Abstract: In recent, ferritic stainless steels are widely used in high temperature structure because of their high resistance in thermal fatigue and low prices. Tensile and low cycle fatigue(LCF) tests on 429EM stainless steel were performed at several temperatures from room temperature to 600°C. Elastic modulus, yield stress and ultimate tensile strength(UTS) decreased with increasing temperature. Considerable cyclic hardening occurred at 200°C and 400°C. 475°C embrittlement observed could not explain this phenomenon but dynamic strain aging(DSA) observed from 200°C to 500°C could explain the hardening mechanism at 200°C and 400°C. And it was observed that plastic strain energy density(PSED) was useful to predict fatigue life when large cyclic hardening occurred. Fatigue life using PSED over elastic modulus could be well predicted within 2X scatter band at various temperatures.
1135
Authors: Jong Won Yoon, E.H. Kim, Hi Won Jeong, Yong Taek Hyun, Seung Eon Kim, Yont Tai Lee
Abstract: Effect of silicon content on the creep properties of Ti-6Al-4Fe-xSi was studied. Creep resistance of Ti-6Al-4Fe-xSi alloys was superior to that of Ti-6Al-4V. Ti-6Al-4Fe-0.5Si alloy exhibited the highest rupture strength and creep resistance among the Ti-6Al-4Fe-xSi alloys investigated. The minimum creep rate of the alloys decreased with increasing silicon content up to 0.5wt.% and then it increased again when the silicon content was higher than 0.5wt.%. TiFe precipitates were formed mainly at the β phase area of Ti-6Al-4Fe-xSi alloys by consuming titanium and iron in β phase, when the alloys were thermally exposed at 500 and 600°C during the creep test. During the creep test, microvoids were induced at the TiFe/α phase interfaces and the cracks were formed along the TiFe/α phase interfaces by the coalescence of the voids. Those cracks were finally connected each other through the α phase.
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Authors: You Tang Li, Chang Feng Yan, Yao Bing Wei
Abstract: The rotating bending and the inverse rotating bending are the two different forms of fatigue fracture. Some fracture problems for middle carbon steel in extra-low rotating bending fatigue loading are studied in this paper. The experiments of inverse rotating bending fatigue have been made in special equipment designed by us. Several problems, the relations of the cycle times of fracture and the strain amplitude near notch tip to the deflection, the relations of the depth of crack propagation to the cycle times of fracture, are discussed through the experiments. The effects of depth and tip radius of notch to the cycle times of fracture are studied also in this paper. The suitable parameters of inverse rotating bending fatigue fracture model for middle-carbon steel in extra-low cycle times has been proposed.
1147
Authors: You Tang Li, Ping Ma, Chang Feng Yan, Fu Yuan Lang
Abstract: Safety designs and fracture designs are the two important fields of crack theory. The fracture problems of middle carbon steel under extra-low cycle rotating bending fatigue loading are studied in this paper. The experiments have been made in special rotating bending fatigue fracture machine designed by us. Several problems, the relations of the cycle times of fracture and the strain amplitude near notch tip to the deflection, the effects of depth and tip radius of notch to the cycle times of fracture, are discussed through the experiments. The mathematical expression of fracture toughness to the cycle times of fracture is obtained. The suitable parameters of rotating bending fatigue fracture model for middle-carbon steel in extra-low cycle times has been proposed.
1153
Authors: Kyung Su Kim, Seok Cheol Kim, Chun S. Shim, J.Y. Park
Abstract: A growing fatigue crack is known to be retarded on application of an overload cycle. The retardation may be characterized by the total number of cycles involved during retardation and the retarded crack length. The overload ratio plays an important role to influence the retardation behavior. The objective of the present investigation is to study the effect of different overload ratio on the retardation behavior. For ESET (eccentrically-loaded single edge crack tension) specimens, fatigue crack growth tests were conducted under cyclic constant-amplitude loading including a single tensile overloading with different overload ratios. The proposed crack retardation model predicted crack growth retardation due to a single tensile overloading. The predicted results were compared with the experimental ones to confirm the reliability of this model.
1159
Authors: Ki Sung Kim, Kyung Su Kim, Chun S. Shim, Hyung M. Cho
Abstract: In the fatigue tests of specimens, sufficiently large tensile overload included in cyclic constant amplitude loading causes the retardation in fatigue crack growth. Crack retardation remains for some period of time after the overloading. The number of cycles in the retarded crack growth has been shown to be related to the plastic zone developed due to the overload. The magnitude of the elastic-plastic zone around the crack tip of DENT(Double Edge Notched Tension) specimen after a overloading was measured by ESPI(Electronic Speckle Pattern Interferometry) system. The fatigue crack growth rate also was measured by a traveling microscope. The relationship between the measured magnitude of plastic zone and the crack growth rate was compared with the equations proposed by Wheeler. Crack growth retardation model that was characterized by crack growth length and the size of plastic zone was proposed and compared with test result. From the research, the validity of proposed model is examined on crack growth retardation, and consequently fatigue life.
1169
Authors: Qin Zhi Fang
Abstract: An automatic fatigue crack growth measurement system was developed, in which a special four-channel A-D acquisition board that could collect data in phase was used. The data collecting frequency is in the range of 4×(2~25600)Hz. The system is suitable for fatigue tests with the frequencies not higher than 250Hz. Eddy current transducers and standard load cell were used to measure displacement and load, respectively. The system can instantly calculate fatigue crack lengths, stress intensity factors and fatigue crack growth rates. As an application of the system, fatigue crack growth rates (FCG) and the thresholds of steels 42CrV and IR3Mo were presented.
1179
Authors: Bum Joon Kim, Byeong Soo Lim
Abstract: Fatigue strength and life of weldment at high temperature is very important for high temperature materials used in power plants. In this study, fatigue crack growth tests were performed at high temperature to examine the effect of microstructure on the crack growth resistance in P92 steel weldment. Microhardness of the weldment was also investigated for better analysis. CT specimens were made from a welded plate with notches at different distances from the fusion line. High fatigue crack growth rates were found along the fusion line and fusion line+2mm line into the heat affected zone.
1185
Authors: Y. Iino
Abstract: Room temperature fatigue crack growth behaviour of un-exposed(as-received) and high temperature air-exposed(at 1100°C for 24 to 1000 h) CMSX-10 was investigated. A small compact tension specimen with [001] loading direction and [110] fatigue crack growth direction was used. The fatigue crack growth results were analyzed using a stress intensity parameter K . It is shown that the air-exposure has no deteriorative effect on the crack growth behaviour, even though the microstructure has changed by the exposure(coarsening of γ' ). The crystallographic facet fracture of (1-11) and (-111) planes appeared and the intersection of the planes forms sets of lines(ridge and valley) parallel to [110] direction. The fatigue crack tip front is schematically shown.
1191
Authors: Huang Kuang Kung, Hung Shyong Chen, Bo Wun Huang
Abstract: Extensive studies of thermal oxidation have mostly focused on the isothermal oxidation of high-temperature polyimide-matrix composites; very few have addresses thermal fatigue or nonisothermal oxidation and the associated weight loss kinetics in the literature. According to thermal activation theory and experimental results concerning isothermal oxidation, a method for predicting anisotropic thermal-fatigue oxidation is developed for the composites.
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