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71.
The inherently nonlinear phenomenon of fatigue crack propagation is modeled as a linear random process. To a first approximation, simple, nonstationary time series models are introduced and standard techniques for determining the parameters of autoregressive integrated moving-average processes are applied. Multiplicative time series models are next utilised for the representation of a group of crack history curves. Implementation of the models on the Virkler experimental data set yields satisfactory results. Reliable Gaussian approximations to the distribution of the time required by a crack to reach a specified critical length are obtained, and the usefulness of the approach is demonstrated when updating lifetime predictions after periodic inspections. 相似文献
72.
The isothermal strain‐life approach is the most commonly used approach for determining fatigue damage, particularly when yielding occurs. Computationally it is extremely fast and generally requires elastic finite element analyses only. Therefore, it has been adapted for variable temperatures. Local temperature—stress–strain behaviour is modelled with an operator of the Prandtl type. The hysteresis loops are supposed to be stabilized and no creep is considered. The consequences of reversal point filtering are analysed. The approach is finally compared to several thermo‐mechanical fatigue tests and the Skelton model. 相似文献
73.
The environmental and socio‐economic impacts of biomass utilization by co‐firing with brown coal in an existing thermoelectric unit in Greece or through its pure combustion in a new plant were studied and evaluated in this work. The 125 MWe lignite‐fired power plant in Ptolemais Power Station (Western Macedonia) was used as reference system. The environmental benefits of the alternative biomass exploitation options were quantified based on the life cycle assessment methodology, as established by SETAC, while the BIOSEM technique was used to carry out socio‐economic calculations. The obtained results showed clear environmental benefits of both biomass utilization alternatives in comparison with the reference system. In addition, co‐firing biomass with lignite in an existing unit outperforms the combustion of biomass exclusively in a new plant, since it exhibits a better environmental performance and it is a low risk investment with immediate benefits. A biomass combustion unit requires a considerably higher capital investment and its benefits are more evident in the long run. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
74.
卢金生 《理化检验(物理分册)》1995,31(6):27-30
通过对“软-软”、“中-软”、“中-中”、“中-硬”四组不同硬度匹配的滚子接触疲劳试验得出四条P-N曲线。并发现中硬度材料的接触疲劳寿命高于软材料,而当主试样为中硬时,与之相匹配的陪试件硬度越高,主试样的寿命越短,通过分析讨论解释了产生上述现象的原因及其失效机理。 相似文献
75.
Comparison of different absorption heat cycles is not always made on the correct manner. This also includes comparison of an ideal absorption cycle with a mechanical analogy. A new Carnot model operating with two heat engines and two mechanical heat pumps is defined to be the correct and logical way to describe the mechanical analogy for an absorption heat pump and an absorption heat transformer. General equations for the Carnot coefficient of performance, COPr, are exemplified and simulated for an absorption heat pump and an absorption heat transformer, and an entropy flow fraction diagram is introduced. The important fact that the absorption heat cycles must operate under the same conditions when they are compared is discussed. 相似文献
76.
T. MANN B. W. TVEITEN G. HÄRKEGÅRD 《Fatigue & Fracture of Engineering Materials & Structures》2006,29(2):113-122
The fatigue life of a welded aluminium T‐joint made from beams with rectangular hollow section (RHS) has been predicted using a crack propagation analysis and compared with experimental results from joints with different residual stress levels. To include the effect of the residual stresses, the stress ratio was calculated at the weld toe and, via Walker's equation, introduced into the analysis. How to obtain the Walker exponent has been discussed in detail. The introduction of a stress ratio at the weld toe provides good agreement between the experimentally and analytically found S–N curves. The effect of the residual stress was successfully included in the analysis. 相似文献
77.
Y. FARES M. CHAUSSUMIER A. DAIDIE J. GUILLOT 《Fatigue & Fracture of Engineering Materials & Structures》2006,29(8):588-596
The fatigue behaviour of bolts under axial load has always been considered from the component point of view for which fatigue limit is usually taken equal to 50 MPa, and few results are available to designers for limited lifetimes. Here, we take up this problem from a material point of view using a local approach. For each case of fatigue testing, using finite‐element (FE) model of the bolt, we determine the stabilized local stress at the root of the first thread in contact with the nut. To characterize bolt behaviour with these numerical results, we use Dang Van multiaxial fatigue criterion for which we extend application to the medium fatigue life. These results can be correlated with the experimental numbers of cycles to failure to determine material parameters of the generalized criterion. Using statistical Gauss method, we can make lifetime predictions for any level of risk of failure. In addition, we propose an analytical model to rapidly determine the local stress condition from nominal loading data (mean stress and alternating stress). This model dispenses us from a new modelling if the bolt is stressed in the same manner as the bolts used for behaviour characterization. Using this model and the generalized criterion, it is extremely easy to make lifetime predictions whatever the risk considered. 相似文献
78.
对转炉炼钢实现低铁耗和高碳低磷工艺进行了分析,介绍了南钢公司转炉炼钢实践低铁耗条件下高碳低磷工艺的具体应用措施。 相似文献
79.
van der Ploeg Eleonore; Dorresteijn Sasja M.; Kleber Rolf J. 《Canadian Metallurgical Quarterly》2003,8(2):157
Workers in medium- or high-risk professions are often confronted with critical incidents at the workplace. The impact of these acute stressors may be serious and enduring. Many workers also experience chronic job stressors, such as work overload or role conflicts. This study examined the frequently neglected relationship of acute and chronic stressors with self-reported health symptoms, such as posttraumatic responses, fatigue, and burnout. This association was investigated in a sample of forensic doctors in the Netherlands (N = 84). It was found that the more traumatic events the respondents experienced, the more problems they reported in coping with the traumatic events. Chronic job stressors were associated with posttraumatic responses (intrusions and avoidances) and with burnout and fatigue. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
80.
M. M. I. HAMMOUDA I. G. EL-BATANONY H. E. M. SALLAM 《Fatigue & Fracture of Engineering Materials & Structures》2003,26(7):627-639
An understanding into the macro kinetic and kinematic behaviour of fretted surfaces is provided. Making use of a modified version of a previously developed in‐house two‐dimensional elastic–plastic finite element analysis numerically simulates flat contact pad fretting fatigue tests. Basic macro mechanics concepts are adopted to idealise two bodies with rough contact surfaces and loaded at two different sites with arbitrary axial loading profiles. A time scale factor is devised to recognise the earliest candidate out of the events possibly accommodated at each loading increment. The present analysis utilises a relevant experimental set up developed in the Structural Integrity Research Institute of the University of Sheffield as an application. Computational results accurate to within 1.2% and corresponding to one contact pad span and six constant normal loads acting individually with four amplitudes of two sinusoidal axial load cycles are presented. The present computations include (1) the development of the global and local normal and tangential reactions and relative sliding displacement acting along the fretting surfaces and (2) contact pad deformation, generated stress fields and plasticity development within the neighbouring region of the fretted area. 相似文献