Papers by Author: Yong Xiang Zhao

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Authors: Bing Yang, Yong Xiang Zhao
Abstract: A new method is proposed to estimate the parameters of probabilistic fatigue crack growth rate models, including the Paris equation and its’ improved type. To take the statistical characteristics of whole test data into account, the method inherits the idea from the general maximum likelihood method which is widely used in parameters estimation of fatigue S-N curves, ε-N curves, and da/dN-"K curves, and extends the conventional correlation coefficient optimization method into parameters evaluation not only for mean curve, but also for standard deviation curve and probabilistic curve. Analysis on the test data of 16MnR steel indicates that present method is available and feasible. Comparing to the general maximum likelihood method, present method has simpler algorithm, and can avoid constructing and solving the likelihood function, so it is speedier in calculation.
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Authors: Yong Xiang Zhao, Bing Yang, Wei Hua Zhang
Abstract: A series of uniaxial-compression tests were conducted on some representative brittle rock specimens, such as granite, marble and dolerite. A multi-channel, high-speed AE signal acquiring and analyzing system was employed to acquire and record the characteristics of AE events and demonstrate the temporal and spatial distribution of these events during the rupture-brewing process. The test result showed that in the primary stage, many low amplitude AE events were developed rapidly and distributed randomly throughout the entire specimens. In the second stage, the number of AE increased much slower than that in the first stage, while the amplitude of most AE events became greater. Contrarily to the primary stage, AE events clustered in the middle area of the specimen and distributed vertically conformed to the orientation of compression. The most distinct characteristic of this stage was a vacant gap formed approximately in the central part of the specimen. In the last stage, the number of AE events increased sharply and their magnitude increased accordingly. The final failure location coincidently inhabited the aforementioned gap. The main conclusion is that most macrocracks are developed from the surrounding microcracks existed earlier and their positions occupy the earlier formed gaps, and the AE activity usually becomes quite acute before the main rupture occurs.
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Authors: Yong Xiang Zhao, Guo Xiang Song
Abstract: Cyclic deformation behaviour is experimentally investigated using an incremental step method under a spectrum straining block loading mode for China grade B cast steel wheel. Results reveal that the material acts cyclically as non-Masing behaviour with a Bausinger effect. Different from monotonic tensile situation, material under fatigue loads was subjected to a plastic dissipation process without a yielding sign. Fatigued areas of specimen fracture surfaces are around 35%. Cyclic stress-strain (CSS) curve exhibits constant lower than monotonic tensile curve. Difference is enlarged as strain level increasing in elastic regime of the monotonic deformation. And this difference sustains a level of around 100 MPa as over yielding point. In addition, randomness is also observed for the present cyclic deformations. It indicates that random CSS relations should be comprehensively applied even if under stress based fatigue analysis.
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Authors: Yong Xiang Zhao, Bing Yang
Abstract: Cyclic deformation characters and description method are investigated to the grade B cast steel for Chinese railway rolling wagon bogie frames. Incremental step test method with 8 strain amplitude loading mode were employed. Results reveal that the material acts as non-Masing behaviour with a significant Bausinger’s effect and appears a slight cyclic softening. Great of dimples in the transient fracture district indicate that present material is ductile. Significant discrepancy between the monotonic and cyclic beaviour was revealed to indicate that there is different monotonic and cyclic deformation behaviour. The scattered cyclic stress-strain relations indicate that appropriate description for the cyclic deformation of present material should be a probabilistic modeling. And then, a probabilistic modeling with measurements of survival probability and confidence is developed to give a good description to the cyclic deformation behaviour.
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Authors: Yong Xiang Zhao, Bin Zhang
Abstract: Damage modes, causes, and effects of railway wheel set are investigated from wide production fields. And then, the reliability strategies are classified as service reliability and development reliability. The development one includes design, material and manufacturing, quality approval/authentication, which is a basis of the service one. The service one is the true product quality, which has not only the basis of development reliability but also the effects of non-destructive monitoring level, damage analysis and assessment capacity, and repairing and renewing policies. Reliability of wheel set is so important that a series of code-based regularities have been issued by professional societies or associations. Optimal design is a most effective way to improve wheel set reliability. And in service, non-destructive inspection and repair opportunities are valuable to be synthetically programmed. The critical damage defect/flaw sizes should be scientifically determined. Three advances in strength reliability field should be introduced. They are affordable and appropriate method for reliability assessment and fatigue life prediction including the super long life regime, no-Paris type crack growth models which include crack initiation, stable growth, and approach fracture, and fatigue S-N relation derived from short cracking growth behaviour.
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Authors: Yong Xiang Zhao, Bing Yang, Ming Fei Feng
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.
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Authors: Bing Yang, Yong Xiang Zhao
Abstract: , was experimentally investigated by a replica technique to seven smooth hourglass shaped specimens for railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in MSC stage. This behavior was corresponding to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. The banded structure appeared a barrier because each band was rich in hard layered pearlites for the crack to cross. In PSC stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. The two barriers are inherent in the material and the crack initiation and growth are subjected to an evolutionary process under competition between the inherent resistances from the barriers and the increasing driving force from the growing crack size. This provides a prehensive understanding of the crack initiation and growth.
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Authors: Lian You Yu, Yong Xiang Zhao
Abstract: Fatigue limit measurement is investigated experimentally on the grade B cast steel for Chinese railway freight car bogie frames. Small sample up-and-down test method was employed for the present study. Results reveal that fatigue cracks initiated mostly from the material cast shrinking cavities. Distinct river-like flowers and second cracks appeared on fracture surface in perpendicular to fatigue crack path. Lots of dimples are distributed in transient fracture district to indicate that present material is ductile. Maximum likelihood approach is applied for measuring the probabilistic fatigue limits, in which the limits are defined as the fatigue strengths following normal distribution at an expected fatigue life. Statistical parameters are then estimated by a likelihood function method. A comparison analysis to the existent conventional, Dixon-Mood and Zhang-Kececioglu approaches indicates that the maximum likelihood approach is the approach meeting the definition.
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Authors: Yong Xiang Zhao
Abstract: For an engineering structure with an actual fatigue life over that corresponding to a so-called fatigue limit, appropriate reliability assessment and fatigue life prediction are essential for developing the structure and sustaining its high quality in service. Basic clues are explored. A competition fatigue initial mechanism is shown to provide a requirement of material primary quality management. Affordable deduced material and structural probabilistic S-N curves are presented by fitting into material mid-and-long life S-N data and fatigue limits and, then, comparing to structural fatigue limits. Random cyclic stress-strain relations are depicted for constructing random stressing history of structures. Reliability assessment and fatigue life prediction are established to synthetically consider the interference of applied stresses deduced from the random cyclic stress-strain relations and capacity strengths derived from the structural S-N relations with an expected life. Affordable and appropriate method has been then developed to realize the reliability assessment and fatigue life prediction including the super long life regime. Availability of the present method has been indicated through a reliability analysis to the velocity related reliabilities and fatigue lives of a railway axle.
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Authors: H.Q. Liang, Yong Xiang Zhao, Y. Li
Abstract: Four grouped rectangle specimens with thirty four samples, plus eighteen samples for fatigue limits, are tested for constructing the fatigue reliability curves of China grade B cast steel wheel. It reveals that fatigue cracks commonly initiated from the surface weakest phases and grew with typical fatigue striations and sub-cracks perpendicular to the growth direction. Cyclic plastic seems to be strengthened as cyclic stressing level increasing. Fatigue reliability curves are constructed on a consideration of addressing the real super long life. They are evaluated by a concurrent probability extrapolation method. And the measured curves verified that appropriate description has been reached on the test data.
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