The paper analyses some fretting fatigue results from the literature, reported by Nowell and by Szolwinski and Farris. The principal variables of contact size, peak pressure, remote specimen tension, ...and tangential force ratio are identified and these are used to construct an Artificial Neural Network (ANN), aimed at predicting total fretting fatigue life. The network is trained and validated using 90% of the data, and its success at predicting the results for the remaining 10% of unseen data is examined. The network is found to be very effective at separating the results into low life and ‘run-out’ groups. It is less successful at predicting lives for the low life specimens, but this is largely due to the difficulty of incorporating the runout and finite life tests together in the same dataset. The approach is seen to be potentially useful and identifies contact size as a key variable. However, the results highlight the need for significant numbers of experimental results if the method is to be used effectively in future. Nevertheless, the trained network comprises a useful tool for the prediction of future experimental results with this material.
•The paper presents a method for predicting fretting fatigue life by using machine learning.•The method highlights the importance of contact size in determining if the specimen lies in the long or short life regime.•Once trained the Artificial Neural Network is effective at classifying unseen data into long and short life regimes.•The approach highlights the need for comprehensive databases of fretting fatigue results.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UILJ, UPUK
•The effect of residual stresses on crack propagation under dents is evaluated.•A dislocation density approach is used to obtain the ΔK of cracks under dents.•Compressive residual stress increases ...predicted life and change the fatigue limit.
The effect of surface damage, such as dents caused due to the low velocity impact of hard blunt objects, on the fatigue life of mechanical components are investigated in this paper. A two-dimensional dislocation density approach is used to obtain the stress intensity factors of a crack propagating under dents. Both the contributions of the geometrical stress concentrator (notch), due to the presence of the dent, and the residual stress field, generated during the impact, on the stress intensity factor of the crack are obtained. A short crack growth model is then used to predict the fatigue life of nickel superalloys in the presence of two dent depths. The effect of the residual stress field has been shown to be the main contributor to the difference observed in predicted fatigue life between the two dent depths analysed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and ...hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R=0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB–DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK, ZRSKP
•Crack opening profiles, measured by DIC are used to capture elastic and plastic components of the crack tip field.•The Pommier and Hamam and the CJP model are shown to have a similar modification ...for crack tip plasticity.•We show that the crack tip field is similar to that reported in the literature.
Fatigue crack propagation occupies much of the life of engineering components, particularly in the short crack regime. It is important to understand the mechanisms of propagation in order to carry out damage tolerance assessment and to predict component service life. The paper describes experiments carried out at macro- and micro-scale using digital image correlation to measure near-tip displacements. From these, various key parameters governing crack growth are extracted, and different models of fatigue crack deformation are validated. In particular, it is concluded that the Pommier and Hamam and the CJP models for fatigue crack displacement and stress fields have rather similar approaches to capturing the effects of crack tip plasticity.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Measurements with digital image correlation of normal and tangential contact stiffness for ground Ti-6Al-4V interfaces suggest a linear relationship between normal contact stiffness and normal load ...and a linear relationship between tangential contact stiffness and tangential load. The normal contact stiffness is observed approximately to be inversely proportional to an equivalent surface roughness parameter, defined for two surfaces in contact. The ratio of the tangential contact stiffness to the normal contact stiffness at the start of tangential loading is seen to be given approximately by the Mindlin ratio. A simple empirical model is proposed to estimate both the normal and tangential contact stiffness at different loads for a ground Ti-6Al-4V interface, on the basis of the equivalent surface roughness and the coefficient of friction.
A fundamental problem that confronts peer-to-peer applications is the efficient location of the node that stores a desired data item. This paper presents Chord, a distributed lookup protocol that ...addresses this problem. Chord provides support for just one operation: given a key, it maps the key onto a node. Data location can be easily implemented on top of Chord by associating a key with each data item, and storing the key/data pair at the node to which the key maps. Chord adapts efficiently as nodes join and leave the system, and can answer queries even if the system is continuously changing. Results from theoretical analysis and simulations show that Chord is scalable: Communication cost and the state maintained by each node scale logarithmically with the number of Chord nodes.
The paper considers elastic contact of rough surfaces and develops a simple analytical expression for the stiffness of the contact under tangential loading, which predicts that the contact stiffness ...is proportional to normal load and independent of Young’s Modulus. The predictions of this model are compared to a full numerical analysis of a rough elastic contact of finite size. The two approaches are found to be in good agreement at low loads, when the asperity spacing is large, but the numerical approach predicts much lower stiffnesses at medium and high loads. It is shown that the overall stiffness cannot exceed that of the equivalent smooth contact, and a simple means of modifying the analytical approach is proposed and validated.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A finite-element model of the interaction of an elastic–plastic asperity junction based on cylindrical or spherical asperities is used to predict sliding friction coefficients. The modelling differs ...from previous work by permitting greater asperity overlaps, enforcing an interface adhesional shear strength, and allowing material failure. The results of the modelling were also used to predict friction coefficients for a stochastic rough surface. The asperities were based on the titanium alloy Ti-6Al-4V, and the magnitudes of the predicted friction coefficients were generally representative of experimental measurements of sliding friction. The results suggest that friction arises from both plasticity and tangential interface adhesion.
► Model of sliding friction based on elastic–plastic asperities using finite-element analysis. ► Higher asperity overlaps, tangential adhesion, and asperity failure allow realistic friction predictions. ► Model based on Ti6/4: tests reveal friction coefficients within the range of model predictions. ► Explanation of sliding friction based on plastic deformation and tangential interface adhesion. ► Some level of tangential interface adhesion seems necessary to generate realistic friction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
A biaxial fatigue experiment is described which is capable of simulating the loading experienced by a dovetail blade root in an aircraft gas turbine. A comprehensive stress analysis of the ...experimental configuration has been undertaken and a semi-analytical approach has been developed to provide accurate estimates of surface tractions and subsurface stress fields. The forces (normal and shear) and moment for the semi-analytical model are calculated from the global finite element analysis of the dovetail in which coarse elements were used. The bulk stress and the bending stress in the disk and blade were evaluated by calculating the membrane and bending stresses across the thickness of the disk and blade. Short crack arrest methods have been applied to the results of the stress analysis to predict the fatigue performance of the blade specimens. The results show a good agreement with the experimental observations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
•Eigenstrain approach works well to model the fatigue of LSP treated specimens.•Interactions between the LSP process and geometric features are important.•Not all instances of LSP applications ...provide improvement in fatigue life.•Eigenstrain method allows modelling the effects due to balancing tensile stress.
Finite element models, using the eigenstrain approach, are described that predict the residual stress fields associated with laser shock peening (LSP) applied to aerospace grade aluminium alloys. The model was used to explain the results of laboratory fatigue experiments, containing different LSP patch geometries, supplementary stress raising features and different specimen thickness. It is shown that interactions between the LSP process and geometric features are the key to understanding the subsequent fatigue strength. Particularly relevant for engineering application, is the fact that not all instances of LSP application provided an improvement in fatigue performance. Although relatively deep surface compressive residual stresses are generated which can resist fatigue crack initiation in these regions, a balancing tensile stress will always exist and its location must be carefully considered.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
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