D. Dini

Stress gradient effects in fretting fatigue

This paper highlights the importance of high stress gradients in fretting fatigue. These pose a serious difficulty in attempting to employ plain fatigue data to predict fretting fatigue performance. Stress gradient effects may also be responsible for a size effect which is found to occur in many fretting situations. Here we review a number of possible approaches for dealing with high stress gradients. We then explore a notch analogy which might be used to generate equivalent stress gradients in plain fatigue. Some sample comparisons are given for typical dovetail geometries.

Prediction of the slip zone friction coefficient in flat and rounded contact

Fretting fatigue (FF) experiments are usually carried out under a partial slip regime and tangential load control. This essentially means that the contact area is characterised by a central stick zone bordered by two slip regions. Evaluation of the coefficient of friction (COF) under such peculiar conditions is not an easy task. Furthermore, a variation of the COF with the number of fretting cycles has been already reported in previous studies. The current paper will extend the analyses already carried out for Hertzian contact to the flat-rounded geometry, recently used to simulate dovetail joints in turbine engine. The method presented enables prediction of the COF in the slip zones as a function of the measured mean value. The approach is completely general and may also be employed for other contact geometries.

Stress analysis of V-notches with and without cracks, with application to foreign object damage:

Gas turbine engines can be subject to ingestion of small hard particles, leading to foreign object damage. This can take the form of sharp V-notches in the leading edge of blades and there is a need to predict the initiation and propagation behaviour of fatigue cracks growing from the base of the notch. The notch geometry is quite extreme and is not normally covered in standard references for notch stress concentration factors. Similarly, stress intensity factor solutions for this geometry are not widely available. This paper uses the dislocation density approach to solve the two-dimensional elastic problem of a V-notch with a radiused root. Stress concentration factors are found for the notch itself, and stress intensity factors are determined for cracks growing away from the notch for cases of applied and residual stress distributions. Comparisons are made with existing notch solutions from the literature.

A review of asymptotic procedures in stress analysis: known solutions and their applications

A comprehensive review is given of the origins of asymptotic procedures in stress analysis. Specifically, attention is focused on the use of fracture mechanics to characterize the elastic stress state ahead of a crack tip. Analogies are then drawn between this configuration and the stress state adjacent to the apex of a sharp V-notch. Extensions of these asymptotic procedures to bonded and slipping contacts are then considered and it is shown that although power order singularities may be obtained, the solutions are more complicated. Lastly, the use of nested asymptotic procedures are considered in order to account for a small but finite radius at the tip of cracks and notches or at the edge of slipping contacts.

Residual strain analysis in polycrystalline aggregates using diffraction measurement and finite element modelling

This paper reviews some of the modelling and experimental techniques recently employed by the authors for the analysis of the macro- and mesoscopic behaviour of polycrystalline aggregates. A joined-up approach is used for assessing the deformation behaviour of polycrystalline materials at the macroscopic and grain levels, based on combining modelling using the crystal plasticity finite element (CPFE) method with experimental characterization by in situ loading and continuous lattice strain measurement by diffraction. The complementarity of the two methodologies is emphasized, as it helps to improve understanding of the physics underlying inelastic deformation, damage mechanisms, and fatigue crack initiation. The proposed approach is part of the attempt to develop a general framework that will enable reliable and accurate determination of the inhomogeneous fields of plastic strain, the regions of localized plasticity, and intergranular residual stresses. Examples are given of microscopically calibrated CPFE simulations being used to match the experimentally observed evolution of lattice stresses and strains in advanced structural alloys of industrial interest.

Analysis of plastic deformation and residual elastic strain in a titanium alloy using synchrotron x-ray diffraction

This paper presents the results of a study of elastoplastic deformation of titanium alloy Ti-6Al-4V subjected to four point bending prior to residual elastic strain measurements by high energy synchrotron diffraction. Both white-beam and monochromatic x-rays were used at the SRS Station 16.3 in order to record diffraction patterns as a function of beam position across the sample. This allowed the comparison between the two techniques to be readily made. Residual (elastic) strain was calculated as a function of position across the sample, based on different reflections of hcp titanium. Inelastic bending analysis was used to extract the plastic strains. The results demonstrate that (i) the level of plastic deformation can be deduced from the x-ray diffraction profile, (ii) the asymmetry of the material response to plastic deformation in tension and compression can be identified and (iii) differences in the behaviour of different grain groups can be seen.

Comparison of X-ray diffraction measurement of residual elastic strains: Monochromatic beam and image plate versus white beam energy-dispersive analysis

This paper presents the results of synchrotron X-ray diffraction measurement of residual elastic strains in a bent magnesium alloy bar using two different techniques. In the first technique the sample was scanned transversely to a monochromatic X-ray beam. The scattered X-rays were recorded on a phosphoric image plate that was translated at a proportional speed but in the opposite direction to the sample. This enabled the creation of a two-dimensional recording of one-dimensional diffraction patterns. In the second technique a white X-ray beam is used in combination with an energy-discriminating detector mounted at a fixed angle to record similar patterns at each gauge volume position within the sample. The motivation for comparing the results of two measurement techniques is to establish whether the same accuracy in a diffraction strain evaluation can be achieved. It was found that the translating image plate arrangement requires a certain amount of care in calibration and interpretation, but provides a quick and efficient method that can be competitive with the white beam energy-dispersive mode which is known to deliver a fast diffraction capability.

Further consideration of closure at the root of a sharp notch

The true contact length and corresponding contact pressure distribution arising at the root of a semi-infinite sharp notch, subject to compression, is found.

The influence function for self-equilibrating forces on a semi-infinite wedge

A solution is derived for the surface displacements arising on the faces of a semi-infinite sharp notch subjected to loading by two equal normal forces, one on each wedge face, applied an equal distance from the notch root. This is achieved by a novel, but robust, procedure using a line of strain nuclei applied along an inclined line within a half-plane and permits good convergence of the solution both in the neighbourhood of the points of application of the forces and at the notch root.

Asymptotic Methods Applied to the Correlation of Fretting Fatigue Lives

Local or asymptotic states of stress relevant to the characterization of the state of stress adjacent to the edges of incomplete contacts are discussed. It is shown that the multipliers for the asymptotic solutions effectively characterize the nucleation propensity of the geometries. This idea is tested by applying the approach to reliable fretting fatigue data in the literature, and re-examining it in this light.Copyright