Jan Papuga

Fatigue life prediction for broad-band multiaxial loading with various PSD curve shapes

The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behaviour at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto-visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension-compression tests performed at 20°C and 300°C. A good agreement was obtained between numerical stabilized tractioncompression cycle curves (with and without pre-straining) and experimental ones. This 3 dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly non proportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria.Abstract The fatigue calculation procedure analysed here applies the power spectral density (PSD) function of the equivalent stress together with the known spectral method for estimating the probability density function of stress amplitudes included in random loading. Here, the narrow-band approximation, Wirsching–Light, Benasciutti–Tovo and Dirlik models are used, together with the SWT parameter. The prediction capability of these four methods was verified on a set of 107 tests results obtained under random axial, torsion and combined axial and torsion loading applied to a tubular specimen with a one-sided hole. Several PSD shapes and combinations of loading were applied. It is shown that the results for fatigue life calculated using the Benasciutti–Tovo and Dirlik methods are well correlated with the results of experiments under this type of loading.

Systematic validation of experimental data usable for verifying the multiaxial fatigue prediction methods

The paper discusses some of the issues, the researchers interested in verifying various multiaxial fatigue limit estimation solutions are facing to. Even recently, newly proposed criteria have been or are tested on dozens of experimental inputs. Papuga in [1] pointed out, that applicability of the most often used test batch is limited and only half of these data items is worth using for such purposes. This paper extends that analysis by describing the weak points of various data sets used in this domain for validating new proposals on multiaxial fatigue limit estimates. The conclusion from the extensive analysis is that the researchers should adopt other test sets only if they very well know their background. KEYWORDS. Multiaxial fatigue; multiaxial fatigue limit; fatigue prediction; fatigue prediction validation; multiaxial fatigue limit experiment.

Efficient lifetime estimation techniques for general multiaxial loading

In this paper, we discuss and present our progress toward a project, which is focused on fatigue life prediction under multiaxial loading in the domain of low-cycle fatigue, i.e. cases, where the plasticity cannot be neglected. First, the elastic-plastic solution in the finite element analysis is enhanced and verified on own experiments. Second, the method by Jiang describing the instantaneous damage increase by analyses of load time by time, is in implementation phase. In addition, simplified routines for conversion of elastic stresses-strains to elastic-plastic ones as proposed by Firat and Ye et.al. are evaluated on the basis of data gathered from external sources. In order to produce high quality complex analyses, which could be feasible in an acceptable time, and allow the period for next analyses of results to be expanded; the core of PragTic fatigue solver used for all fatigue computations are being re-implemented to get the fully parallelized scalable solution.

DATABASE FINLIV - FOCUS ON STAIRCASE METHOD

This paper introduces FinLiv database focused on gathering, manipulating and providing experimental static and fatigue data. One of its new features is the ability to include also fatigue tests realized by the staircase method.

Wiles of Using Hollow Specimens for Fatigue Tests

The paper reports an issue found in using the hollow specimens claimed usually as unnotched. If the goal is to test them under multiaxial loading, the target is to reduce the shear stress gradient by making them thin-walled. Anyhow, the analyses described here show, that this attempt induces an issue unknown previously to authors – the maximum stress in tension loading is found on the inner surface of the tube. The individual effects concerning material and geometry are described.