Advances in Fracture and Failure Prevention

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Authors: Keiichiro Tohgo, Masahide Oka
Abstract: In this paper, the influence of the coarsening treatment of eutectic Si particles on fatigue strength and fracture toughness of a peak-aged Al-7%Si-0.4%Mg alloy casting has been investigated. By the coarsening treatment at 808°K for 100 hours, the eutectic Si particles grew to be spherical and were uniformly distributed, but casting defects remained without any changes. On tensile tests, the 0.2% proof stress and tensile strength were almost the same in both as-received material and Si-coarsened material, but the ductility was much higher in the Si-coarsened material than in the as-received one. Fatigue fracture occurred by crack initiation from the casting defects and succeeding crack growth on both materials, and the fatigue strength of the materials was affected by the size of defects rather than by the coarsening treatment. On the other hand, the fracture toughness was distinctly improved by the coarsening treatment. It is suggested that the improvement of fracture toughness in the Si-coarsened material is attributed to the energy dissipation due to the damage of Si particles around a crack and the rough fracture path.
Authors: S.Y. Son, S. Nishida, Nobusuke Hattori, W.T. Jeong
Abstract: The fatigue strength have been investigated about the plastic deformed specimens by roller working in Ti-6Al-4V alloy with a notch by 4 kinds of plastic deformation ratio. The plastic deformation values (Δ t) are employed 0.05, 0.1, 0.25 and 0.4 mm on notched specimens of Ti-6Al-4V alloy by roller-working. The hardness test, fatigue test and SEM observation was carried out on non-roller worked specimens and roller-worked specimens at room temperature. Based on the results of these tests, Though the hardness number of roller worked materials are higher than those of non-deformed Ti-6Al-4V alloy, the fatigue limit of roller worked materials becomes deteriorated in spite of roller working. Fatigue fracture had occurred in the non- deformed area where tensile residual stress exists. From the results of the SEM observation, stepped defect could be notch effect for reducing the fatigue strength of Ti-6Al-4V alloy.
Authors: Seon Jin Kim, Young Suk Kim, Sang Woo Kwon
Abstract: This paper illustrates the characteristics of parameters for three-parameter Weibull probability distribution of fatigue crack growth lives under constant stress intensity factor control experiments. Applying the statistical properties of the previous experimental results, the fatigue crack curves were simulated by using the non-Gaussian random fields simulation method and analyzed for the different specimen thickness and stress intensity level to determine the probability distributions of the fatigue crack growth life.
Authors: Byeong Choon Goo, Jung Won Seo
Abstract: Bogie frames of electric cars play an important role as a structural member for the support of vehicle loading. To guarantee more than 25 years' durability, a lot of study has been carried out for the prediction of the fatigue life of the bogie frame in experimental and theoretical domains. One of the new methods being applied in recent years for rolling stock structures is a reliability-based approach. In this paper, using one of these methods, we estimate fatigue life of the bogie frame of an electric car, which was developed by the Korea Railroad Research Institute. First of all, we have carried out tensile fatigue tests with several types of specimens: as-welded, ground on weld toe, reinforcement-removed and post weld heat treated specimens for each type aforementioned. In addition, we perform the probabilistic distribution tests of the S-N curves for the ground specimens. Normal, lognormal and Weibull distributions are used to model the distributions. Along with the obtained S-N relations, we use two approaches to evaluate the fatigue life of the bogie frame. In the first approach probabilistic distribution of the S-N curve is used. The S-N curves are obtained in function of failure percent from the specimen tests. Then the fatigue life is estimated by use of Miner-Palmgren hypothesis. In the second approach, load spectra measured by strain gauges are approximated by a two variable Weibull distribution. And a limit state function for the criterion of failure is derived by use of Miner-Palmgren hypothesis. The fatigue life is evaluated by a reliability engineering method.
Authors: Qing Yuan Wang, Norio Kawagoishi, Nu Yan, Q. Chen
Abstract: The objective of this study is to determine very long life fatigue and near threshold fatigue crack growth behaviors of 7075/T6 and 6061/T6 Al-alloys using piezoelectric accelerated fatigue at 19.5KHz. The experimental results show the fatigue failure can occur beyond 107, even 109 cycles, and endurance limits could not be obtained in the Al-alloys until 109 cycles. Fatigue voids are noticed on fatigue fracture in both alloys. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviors have been examined. Fatigue crack growth rates of small cracks in the Al-alloys are found to be greater than those of large cracks at the same stress intensity factor range.
Authors: Il Seon Sohn, Dong Ho Bae, Won Seok Jung, S.J. Park
Abstract: The suspension system of vehicle is directly influenced to ride and handling. Therefore, suspension part should have enough endurance during its lifetime to protect passenger. Spring is one of major suspension part of vehicle. Thus, in this paper, a fatigue design method for leaf spring based on proving ground response was proposed. At first, stress and displacement of leaf spring are measured through the proving ground test. And next, the maximum load acting on leaf spring assembly under driving condition was defined from the road load response. On the base of these results, fatigue tests for leaf spring assembly and 3-point bending fatigue tests for material of leaf spring were carried out. From the above, the maximum load-fatigue life relation of leaf spring material and assembly was defined, and 3-point bending test result has good agreement with leaf spring assembly fatigue test result. Thus, it is expect that economical fatigue design criterion for leaf spring assembly can be determined from fatigue data of simple smooth specimen by 3 point bending fatigue tests.
Authors: S.C. Kwon, P.K. Dutta, Yun Hae Kim, Soo Hyun Eum, Dong Hyuk Shin, R. Lopez-Anido
Abstract: A summary of the experimental fatigue characterization of FRP composite bridge decks at two extreme temperatures [-30 ° C (-22 ° F) and 50 ° C (122 ° F)] is presented. Each deck was initially subjected to one million wheel load cycles at low temperature and another one million cycles at high temperature [1,2]. The results presented in this paper correspond to the fatigue response of each deck for four million load cycles at low temperature and another four million cycles at high temperature. Thus, the each deck was subjected to a total of ten million cycles. Progressive degradation in stiffness with cycling was noted for each deck. Comparisons of responses were made between the different FRP composite deck configurations and materials.
Authors: K. Fujishita, Noriko Miura, Y. Takahashi, Yukihiro Nakayama
Abstract: Central Research Institute of Electric Power Industry (CRIEPI) has been conducting a research project on the verification of evaluation method for structural integrity of high-temperature components. As a part of the framework, we developed a flaw evaluation system on PC to predict creep-fatigue crack propagation, which was considered to be an essential crack driving force at high temperature especially in case where stresses caused by thermal loading were significant. The system is featured by easy and dialogical operation with graphical user interface. Fatigue and creep crack propagation behavior can be predicted using fatigue J-integral range and creep J-integral, respectively, which are the variations of the non-linear fracture mechanics parameter, J-integral. Both fatigue J-integral range and creep J-integral are calculated by the reference stress method with revised solutions developed by detailed finite element analysis.
Authors: T. Kobayashi, O. Muragishi, K. Nihei
Abstract: A fatigue detecting sensor that enables easy and precise estimation of fatigue damage of machines and structures was developed, based on the crack growth characteristics of metal. The sensor is composed of a metal foil with a single edge notch and a base metal foil. The two metal foils are bonded at the both far ends. The sensor is attached with an adhesive to the surface of structural member which is subjected to cyclic loads. The fatigue damage of the member is detected as the fatigue crack initiation from the notch tip of the sensor. The fatigue crack growth rate is independent of the crack length. The metal foil of the sensor is given pre-tension, so the sensor has stable crack growth characteristics independent of the average strain. The crack growth length is measured in a certain period after the installation. The length is converted to the fatigue damage of the member during the sampling period of sensor installation. Since no measuring instrument and signal wiring is necessary, the diagnosis procedure becomes easy and the cost for structural health monitoring could be reduced. With such small size of the sensor as conventional strain gages, the pin-pointed application of the sensor to the hot spot of structural stress concentration, where is important for the fatigue strength estimation of welded structures, is possible. Therefore, precise fatigue remaining life assessment could be carried out. Sensors with several strain ranges were developed, so application to vast strain range is possible. Even such a small strain range as 1/10000 is able to be detected. The sensors have been applied to various products like steel bridges, rolling stocks, ships, etc.
Authors: C.S. Jeong, Byeong Soo Lim
Abstract: At high temperatures typical for service conditions in fossil power plants, the creep fracture is dominated by the formation, growth and coalescence of cavities. Using high temperature pipe materials, P92 and P122, the characteristics of creep crack growth were analyzed in this study according to the cavities. The characteristics of cavities play a critical role in creep crack propagation and load line displacement. The effect of the load line displacement rate(dv/dt) and crack growth rate(da/dt) on the da/dt–Ct relation of creep crack growth was evaluated at different temperatures and Ki(initial stress intensity factor) values. The number of cavities increased with increasing temperature and Ki. The crack growth rate and load line displacement rate increased with the increase in the cavity numbers. The kind and distribution of these internal flaws were investigated by an intelligent phased array ultrasonic method and they were utilized in deriving the relationship with the creep crack growth rate, which will predict the creep characteristics of these materials.

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