vibration fatigue tests by tri axis shaker design of an innovative system for uncoupled bending torsion loading

Abstract An innovative system for bending-torsion fatigue tests by tri-axis shaker is designed and presented. The system mounts a cylindrical specimen with eccentric tip mass, excited by horizontal and vertical base accelerations. A lateral thin plate prevents specimen horizontal displacement and allows torsional and bending deformations to be controlled independently. A lumped-mass model is first used to verify if input accelerations and resultant dynamic forces, required in testing, comply with shaker specifications. A finite element model is then used to perform both modal and harmonic analyses, necessary to determine the system natural frequencies and the dynamic response under horizontal and vertical accelerations. Experimental measures on a prototype are finally used to gather preliminary information for validating the numerical model and to verify that the proposed testing system can control bending and torsion loadings independently

An efficient procedure to speed up critical plane search in multiaxial fatigue: Application to the Carpinteri-Spagnoli spectral criterion

A more efficient procedure is proposed to speed up the Carpinteri-Spagnoli (CS) algorithm in numerical computations. The goal is accomplished by deriving the exact solution for the spectral moments and expected maximum peak of normal/shear stress in any rotated plane orientation. The algorithm then avoid the use of “for/end” loops to identify the five rotations that locate the critical plane in CS method. The procedure is especially advantageous if applied to three-dimensional finite element analysis, in which the stress spectra in thousands of nodes need to be processed iteratively. The procedure is based on theoretical results that have, however, a more general validity, being applicable to any multiaxial criterion that makes use of angular rotations to identify the critical plane

A numerical approach for static and dynamic analysis of deformable journal bearings

This paper presents a numerical approach for the static and dynamic analysis of hydrodynamic radial journal bearings. In the first part, the effect of shaft and housing deformability on pressure distribution within oil film is investigated. An iterative algorithm that couples Reynolds equation with a finite elements (FE) structural model is solved. Viscosity-to-pressure dependency (Vogel-Barus equation) is also included. The deformed lubrication gap and the overall stress state are obtained. Numerical results are presented with reference to a typical journal bearing configuration at two different inlet oil temperatures. Obtained results show the great influence of bearing components structural deformation on oil pressure distribution, compared with results for ideally rigid components. In the second part, a numerical approach based on perturbation method is used to compute stiffness and damping matrices, which characterize the journal bearing dynamic behavior

The “Projection-by-Projection” (PbP) criterion for multiaxial random fatigue loadings: Guidelines to practical implementation

This work is motivated by the increasing interest towards the application of the “Projection-by-Projection” (PbP) spectral method in finite element (FE) analysis of components under multiaxial random loadings. To help users and engineers in developing their software routines, this paper presents a set of numerical case studies to be used as a guideline to implement the PbP method. The sequence of analysis steps in the method are first summarized and explained. A first numerical example is then illustrated, in which various types of biaxial random stress are applied to three materials with different tension/torsion fatigue properties. Results of each analysis step are displayed explicitly to allow a plain understanding of how the PbP method works. The examples are chosen with the purpose to show the capability of the method to take into account the effect of correlation degree among stress components, and the relationship between material and multiaxial stress in relation to the tension/torsion fatigue properties. A case study is finally discussed, in which the method is applied to a FE structural durability analysis of a simple structure subjected to random excitations. The example describes the flowchart and the program by which to implement the method through Ansys APDL software. This final example illustrates how the PbP method is an efficient tool to analyze multiaxial random stresses in complex structures

Cyclic Plasticity and Low Cycle Fatigue of an AISI 316L Stainless Steel: Experimental Evaluation of Material Parameters for Durability Design

AISI 316L stainless steels are widely employed in applications where durability is crucial. For this reason, an accurate prediction of its behaviour is of paramount importance. In this work, the spotlight is on the cyclic response and low-cycle fatigue performance of this material, at room temperature. Particularly, the first aim of this work is to experimentally test this material and use the results as input to calibrate the parameters involved in a kinematic and isotropic nonlinear plasticity model (Chaboche and Voce). This procedure is conducted through a newly developed calibration procedure to minimise the parameter estimates errors. Experimental data are eventually used also to estimate the strain–life curve, namely the Manson–Coffin curve representing the 50% failure probability and, afterwards, the design strain–life curves (at 5% failure probability) obtained by four statistical methods (i.e., deterministic, “Equivalent Prediction Interval”, univariate tolerance interval, Owen’s tolerance interval for regression). Besides the characterisation of the AISI 316L stainless steel, the statistical methodology presented in this work appears to be an efficient tool for engineers dealing with durability problems as it allows one to select fatigue strength curves at various failure probabilities depending on the sought safety level

fem strategies for large scale thermo mechanical simulations with material non linearity

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The role of uncertainty of power spectral density data in estimating the fatigue damage of random uniaxial loadings through frequency-domain methods

This work treats of the statistical uncertainty of fatigue damage evaluated in frequencydomain, coming from the sampling variability of power spectral density data estimated from finite length records. The article derives the expression for the moment generating function (mgf) of the sample spectral moment of order q. The mgf permits the probability distribution of the sample spectral moment to be approximated by the distribution of a scaled chi-square random variable. This introduces the concept of “equivalent degrees of freedom” for a power spectrum. The confidence interval for both the q-th order spectral moment and the fatigue damage estimated by the “single-moment spectral method” are then obtained in closed form. A Monte Carlo study is finally used to verify the correctness of the proposed expression

An analytical approach to measure the accuracy of various definitions of the "equivalent von Mises stress" in vibration multiaxial fatigue

This paper aims to review and compare various definitions of the "equivalent von Mises stress" (EMVS), which is used as a criterion to analyse multiaxial stationary Gaussian random stresses in the frequency-domain. Since the original definition of EVMS proposed in 1994 by Preumont and Piéfort, several modified formulations have been suggested by other authors. In particular, this paper focuses on the definitions proposed by Braccesi et al. (2008) and Niesłony (2008). Such modified formulations aimed to correct some incoherencies in the Preumont’s definition of EVMS, which can lead to inaccurate estimations for particular combinations of material fatigue properties. In this work, analytical expressions are derived to check the accuracy of the original and modified EVMS criterion for different random loading types (e.g. pure bending, pure torsion, combined bending plus torsion). Such analytical expressions will allow engineers to understand when the original EVMS criterion and its modifications can safely be applied in fatigue design

Fatigue analysis of random loadings. A frequency-domain approach

Service loadings in structures and mechanical components can be modelled as random processes. The durability assessment under such complex loadings is commonly approached in time-domain by using counting methods and damage accumulation rules. An alternative approach could be developed in frequency-domain, where the random loading is characterised by its power spectral density. This book aims to provide an overview on methods for fatigue analysis of random loadings, with particular focus on frequency-domain approach. Classical time-domain load characterisation, counting methods and linear damage rule are first reviewed. Then, frequency-domain spectral methods for analysis of stationary random loadings are discussed, with particular emphasis on Gaussian and non-Gaussian load analysis. Application examples are also developed, with both numerical simulations and experimental load measurements. A general comparison of spectral methods is finally presented. This book should help to shed some light on the frequency-domain fatigue analysis of random loadings and it should be especially useful for researcher working in the field of structural and durability assessment under service loadings