Advanced Materials Research Vols. 44-46

Abstract: Main progresses are reviewed and prospected from four elements on the fatigue reliability analysis research of China railway vehicles during recent decade. First, material fatigue database has been moderately developed and an emphasis was placed on the systemized fatigue properties. Second, a great of on-line inspections have been performed on the wheel-track contact forces and service stresses of key structures. A reliable vehicle dynamics simulation technique has been developed for simulating the contact forces and a multi-variable quadratic regression approach has been proposed to calculate the service stresses for recuing imperfect inspections. Third, systemized fatigue reliability researches have been made on improved fatigue limit measurement, probabilistic S-N curves, scale-deduced fatigue behavior, design Goodman-Smith diagram, and reliability analysis approach to address on the real super-long life of railway vehicle’s structures. Fourth, a critical safety state theory is established to address the time-dependent off-round wheel wear and the interactive fatigue damage between components. Fatigue cracking threshold measurement is improved. And a fatigue crack growth modeling is developed to cover entire physical process from the threshold to the fracture. The axle critical safety sizes are determined under considering the off-round wheel and the wheel critical safety wear sizes are measured to ensure the axle expected safety life. All researches are valuable to be further studied.

Abstract: Engineering design must be cost-effective over its complete life cycle and this necessitates a statistical approach to product and service reliability. A particular design therefore has a finite probability of failure during its lifetime and this has implications in terms of ensuring that design procedures, management of design, and operation and inspection are appropriate, effective and sufficient. Sophisticated expert system software packages have enabled wide access to rapid development to prototype and production stages. The expert knowledge encapsulated in such systems may be inadvertently used outside its intended application envelope, leading to unexpected and unwelcome failures. This paper highlights the role of failure analysis and fractography in the context of product reliability. It will present case studies illustrating typical structural reliability problems that highlight issues connected with the ‘reliability’ of experts in assessing the reasons for failure.

Abstract: The impact of remanufacturing on the conservation of energy and resources has been well recognized during the last decade. When the relevant technologies are applied for high temperature components in power and process industries, a redesign of the component life should be required due to the time-dependent feature of high temperature failure. In order to provide some fundamentals for redesign and remanufacture of high temperature components, mechanical behavior of a two-bar structure with one bar being remanufactured is analyzed. An optimal repairing time is given. From the viewpoint of creep damage, various high temperature structures are analyzed by using damage coupled finite element method. Suggestions for life extension remanufacture are proposed for typical high temperature components.

Abstract: Fatigue fracture of some high-strength steels occurs at small defect in the subsurface zone of a material at low stress amplitude level and in a high-cycle region of more than 106 cycles (gigacycle fatigue life), whereas surface fatigue crack initiation occurs at high-stress amplitude and low cycles. There is a definite stress range where the crack initiation site changes from a surface to a subsurface defect, giving a step-wise S-N curve or a duplex S-N curve. From the experimental results, fatigue fracture mode was classified into three types, such as, surface inclusion induced fracture mode, subsurface inclusion induced fracture mode without granular bright facet (GBF) area and that with the GBF, depending on stress amplitude level and stress ratio. The GBF area was observed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye in gigacycle fatigue regime. It was made clear from the discussion with fracture mechanics that the transition of fracture mode was affected by compressive residual stresses on the specimen surface. Fracture-mode transition diagram was proposed through the experimental and theoretical investigation. Also, from the evaluation of the fatigue life based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.

Abstract: Specimens of a directionally solidified superalloy with different shot peening pressure were annealed at 1220oC in vacuum condition to get recrystallized surface layers with different micro-structures. Low cycle fatigue tests of these specimens were performed at room temperature and 400oC by using an electrohydraulic load frame in the SEM system for real-time observation. The initiation and propagation of cracks were inspected and the influence of the micro-structure of the recrystallized layer on the material fatigue behavior was analyzed. The low cycle fatigue life of the specimens depends mainly on the characteristics of the recrystallized layer. When the shot peening pressure is lower, the recrystallized layer is thin and not integrated, and the fatigue life decreases obviously in comparison with that of the specimen without recrystallized surface layer. When the shot peening pressure increases, the recrystal grains are more integrated, and the fatigue life rises. A comparison of the recrystallized layers between the blade surface and the specimen surface has been done and it points that the incompact surface recrystal layer is very dangerous to gas turbine blades.

Abstract: By interpreting traditional stress-strength interference model as a statistical average of the probability that strength (a random variable) is greater than stress (another random variable) over its whole distribution range, the same model configuration, which was conventionally applied only to the case of same system-of-units parameters (e.g., stress and strength, both are measured in MPa), was applied to more general situation of different system-of-units parameters. That is to say, the traditional model was extended to more general situations of any two variables, as long as one of the variables can be expressed as a function of the other. Further more, the probabilistic fatigue life under random stress can be predicted, with known probabilistic fatigue lives under several deterministic cyclic stress amplitudes and known distribution of the random cyclic stress amplitude. The underlying principle is that the fatigue life under random stress is equal to the statistical average of the fatigue lives under cyclic stress of deterministic amplitudes which can be considered as the samples of the random stress.

Abstract: The structures of railway vehicles are required to service in super-long life regime. Determination of the probabilistic S-N curves including the regime should be a basic work to realize the real fatigue life prediction and reliability assessment. Based on the test results of Chinese railway LZ50 axle carbon steel, a statistical extrapolating method is proposed to determine the curves by applying the conventional test data in mid-long life regime. Some phenomena, i.e. the response of “fatigue limit”, a great of heat affecting using the high frequency ultrasonic fatigue test system, and the non-conservative test results using the conventional frequency tiny multi-specimen test system (machine C), are firstly mentioned in the existent super-long life researches. The too much conservative offers are also noted by the determinations of existent codes. To address the variable amplitude loads in production, the present method extrapolates continuously the curves from the mid- to super-long life regimes under the statistical controls of conventional fatigue limits. The practice for the present material indicates that the curves can well reflect the scattered data not only in mid-long life regime but also in super-long life regime. The non-conservative test results using machine C are also safely included by the curves.

Abstract: Transferring of fatigue behavior is investigated from material to structure for Chinese railway LZ50 axle carbon steel. S-N data in mid-fatigue life range and fatigue limit data of smooth small specimens are applied for material fatigue behavior. Real axles and similar smooth bigger specimens were tested with respect to fatigue limits. The data of real axles reveal the difference between material and a special structure. And the data of similar specimens play a role of bridge to connect material and material structures. Probabilistic transferring relations are deduced by the data from material, to similar specimens, and then to structures at the fatigue limit phase. Structural probabilistic S-N curves are obtained by the material fatigue behavior to extend the transferring relations to entire fatigue life range. Determinations of the fatigue behavior of the groove of LZ50 steel axle indicate availability of the present investigation.