Probabilistic framework for multiaxial LCF assessment under material variability

The influence of material variability upon the multiaxial LCF assessment of engineering components is missing for a comprehensive description. In this paper, a probabilistic framework is established for multiaxial LCF assessment of notched components by using the Chaboche plasticity model and Fatemi-Socie criterion. Simulations from experimental results of two steels reveal that the scatter in fatigue lives can be well described by quantifying the variability of four material parameters Ï\u83fâ\u80²,εfâ\u80²,b,c. A procedure for choosing the safety factor for fatigue design has been derived by using first order approximation

Multi-Axial Fatigue Analysis of Thermite Rail Welds

This dissertation aims to examine the high-cycle fatigue behavior of thermite-welded railroad rails. In particular, the occurrence of fatigue defects in web and base regions of the rail is addressed, as frequently reported by field surveys. Fatigue life estimates are made by means of a multi-axial critical-plane fatigue algorithm that computes the fatigue damage based on the time history of stress tensors. The finite element method is used to analyze a full-scale replica of the wheel-track system comprising axle, wheel, thermite-welded rail, tie plates, and ties. The time-dependent rolling of the wheel and thermal stresses caused by seasonal temperature variations are incorporated into the finite element simulation. In addition, the effects of the track foundation stiffness and thermite weld geometry are explored. Fatigue crack nucleation is studied at three critical locations of the rail, where most reported weld service failures occur: web-to-base fillet, base center, and base corners. Under wheel loads, the results of the fatigue analysis indicate that it can take a long period of time for a fatigue crack to nucleate in the aforementioned regions of the rail if the material is nominally “defect-free”. Fatigue cracks tend to initiate in a transverse plane, next to the thermite weld, perpendicular to flexural tensile stresses that form in the rail base. The implementation of small planar surface defects at critical locations has shown to significantly reduce the fatigue nucleation life of thermite rail welds. Also, axial tensile stresses resulting from cold winter weather are found to considerably accelerate the fatigue nucleation process, especially in the web-to-base fillet region

Fatigue Reliability Sensitivity Analysis of Complex Mechanical Components under Random Excitation

Fatigue failure is the typical failure mode of mechanical components subjected to random load-time history. It is important to ensure that the mechanical components have an expected life with a high reliability. However, it is difficult to reduce the influence of factors that affect the fatigue reliability and thus a reliability sensitivity analysis is necessary. An approach of fatigue reliability sensitivity analysis of complex mechanical components under random excitation is presented. Firstly, load spectra are derived using a theoretical method. A design of experiment (DOE) is performed to study the stresses of dangerous points according to the change of design parameters of the mechanical component. By utilizing a Back-Propagation (BP) algorithm, the explicit function relation between stresses and design parameters is formulated and thus solves the problem of implicit limit state function. Based on the damage accumulation (DA) approach, the probability perturbation method, the fourth-moment method, the Edgeworth expansion is adopted to calculate the fatigue reliability and reliability-based sensitivity. The fatigue reliability sensitivity analysis of a train wheel is performed as an example. The results of reliability are compared with that obtained using Monte Carlo simulation. The reliability sensitivity of design parameters in the train wheel is analyzed

Reliability Analysis of Components Life Based on Copula Model

It is the general character of most engineering systems that failure statistical correlation of elements exists in the fatigue happened process due to the twin loads．Based on randomized Basquin equation，the constitutive relations are established between element life and random variable including twin loads，initial strength,fatigue strength exponent．Product-moment correlation coefficients are derived and used to quantify the dependence of logarithm life of elements．Aim at fatigue life correlation of elements in structural systems, the computation model of the system reliability is preliminarily established by means of using copula function. The new model can express the dependence of elements fatigue life in structural systems, can be used probability prediction of structural systems under common stochastic cyclic load, which gives a new path for reliability-based design and probability assesment in equipment systems with multi-mode damage coupling

Estudo do contacto roda/carril : análise de tensões e fadiga

Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 200

Design of power-transmitting shifts

Power transmission shafting which is a vital element of all rotating machinery is discussed. Design methods, based on strength considerations for sizing shafts and axles to withstand both steady and fluctuating loads are summarized. The effects of combined bending, torsional, and axial loads are considered along with many application factors that are known to influence the fatigue strength of shafting materials. Methods are presented to account for variable amplitude loading histories and their influence on limited life designs. The influences of shaft rigidity, materials, and vibration on the design are discussed

Mechanical Analyses of Multi-piece Mining Vehicle Wheels to Enhance Safety

In this research, experimental and numerical methods were used to analyse the performance of multi-piece wheel structures and two proposed innovative designs to enhance safety were validated by computer simulations. Fatality report analyses revealed that the majority (90%) of the multi-piece wheel failures were caused by use of lock rings. Experimental tire and rim base tests were conducted to understand the deflection characteristics of off-the-road tires and to validate the finite element model of a five-piece wheel/tire (sized 29.5-29) assembly. A linear relationship was found between the vertical displacement of the wheel and the maximum lateral deflection of the tire for both static and quasi static loading tests. A robust tire model was validated with an average accumulative error of 9.7% and an average validation metric of 0.96 for tire deflections, compared to the experimental tests. The rim base model was validated with an average error of 7.6% and an average validation metric of 0.93 for wheel deformations, and an average accumulative error of 12.7% and an average validation metric of 0.88 for strains, compared to experimental tests. Based on validated FE model of the five-piece wheel/tire assembly, geometry degradation (material wear out at critical regions) and material degradation (fatigue and corrosion) were studied to estimate their effects on fatigue lives. Two design innovations were proposed to enhance safety and fatigue life of the five-piece wheel. The threaded-connection design reduced the possibility of failure due to the mismatched wheel components. The BS band pull-out simulation revealed that the threaded-connection design was twice as strong as the conventional five-piece design in holding wheel components and the tire together, and the wheel may fail in a safer mode. The fatigue lives of the rim base were two orders of magnitude higher than those of the conventional five-piece wheel. The two-piece wheel design completely removed the possibility of wheel failure due to mismatched wheel components; the fatigue lives were increased by over two orders of magnitude, compared to the conventional five-piece wheel