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This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material. Micro-scale specimens, representative of stent struts, are manufactured via laser micro-machining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterisation of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments. [ABSTRACT FROM AUTHOR]

Abstract: A methodology for the simulation of thermal fatigue delamination in composites and prediction of delaminated crack opening displacement (DCOD) and composite laminate permeability is presented. This is critical for the safe design of composite cryogenic fuel tanks. The methodology, which is based on an extended finite element method for simulation of crack growth, does not require a priori definition of initial crack (delamination) length or crack propagation path. In contrast, previous work required estimation of delamination length and corresponding number of thermal cycles based on experimental measurements. The methodology is validated against measurements from standardised static and fatigue delamination test methods. Prediction of delamination crack growth in a quasi-isotropic laminate under cryogenic fatigue loading is used to establish the effect of initial interlaminar defect length on subsequent crack growth, as well as the effect of delamination length on DCOD and permeability. A key additional benefit is that the proposed method can simulate both inter- and intralaminar crack growth in two- and three-dimensional geometries. [Copyright &y& Elsevier]

Highlights: [•] FE analysis of fretting wear induced multiaxial-stresses in thin steel wires. [•] Implementation of the SWT critical-plane approach in a 3D FE fretting wear model. [•] Novel damage accumulation method for 3D FE fretting wear simulation models. [•] Analysis of different methods for the fatigue coefficients estimation of wires. [•] Comparison the predicted lives with those obtained in fretting wear tests. [ABSTRACT FROM AUTHOR]

Abstract: Tidal turbine blades are subjected to significant thrust and torsional loadings due to the high density of the seawater in which they operate. These thrust loadings lead to high bending moments at the blade root, which can prove to be a serious design constraint for these devices and can have implications with respect to cost-effectiveness and scalability. This work presents a combined hydrodynamic-structural design methodology for a commercial scale (1.5 MW) tidal turbine. A hydrodynamic analysis of the blade is carried out to determine force distributions along the blade span under normal and extreme operating conditions. Using output from the hydrodynamic model, a pre-processor for computing blade structural properties is used to determine the strain distribution along the blade spar caps. The strain distributions from this analysis are then compared with a finite element model of the blade which is then used to compare the structural performance of glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) as spar cap materials. [Copyright &y& Elsevier]

Abstract: The dimensions of coronary stent struts are similar to those of the metallic grains of their constituent alloys. This means that statistical size effects (SSEs), which are evident in polycrystals with few grains through their dimensions, can have detrimental effects on the mechanical performance of stent struts undergoing large plastic deformation. Current trends in coronary stent design are towards thinner struts, potentially increasing the influence of SSEs. In order to maintain adequate device performance with decreasing strut thickness, it is therefore important to assess the role of SSEs in the plastic deformation of stents. In this study, finite element modelling and crystal plasticity theory are used to investigate SSEs in the deformation of struts in tension and bending. The relationships between SSEs and microstructure morphology, alloy strain hardening behaviour and secondary phases are also investigated. It is predicted that reducing the number of grains through the strut cross section and increasing the number of grains along the strut length have detrimental effects on mechanical performance. The magnitudes of these effects are predicted to be independent of the uniformity of the studied microstructures, but dependent on alloy strain hardening behaviour. It is believed that model predictions will aid in identifying a lower bound on suitable strut thicknesses in coronary stents for a range of alloys and microstructures. [Copyright &y& Elsevier]

Abstract: In this study a numerical model is developed to predict the effects of corrosion on the mechanical integrity of bioabsorbable metallic stents. To calibrate the model, the effects of corrosion on the integrity of biodegradable metallic foils are assessed experimentally. In addition, the effects of mechanical loading on the corrosion behaviour of the foil samples are determined. A phenomenological corrosion model is developed and applied within a finite element framework, allowing for the analysis of complex three-dimensional structures. The model is used to predict the performance of a bioabsorbable stent in an idealized arterial geometry as it is subject to corrosion over time. The effects of homogeneous and heterogeneous corrosion processes on long-term stent scaffolding ability are contrasted based on model predictions. [Copyright &y& Elsevier]

Abstract: Coupled thermo-mechanical finite element modelling of both isothermal axisymmetric compression and compression testing using the Gleeble thermo-mechanical simulator has been carried out in order to compare the levels of relative stress error which can arise during each test. A Norton-Hoff material model has been used as a basis for evaluating the testing methods and the calculation of the relative error by providing reference stress–strain curves. The errors arise from effects such as interface friction causing non-uniform deformation fields and heat generation in both testing cases and from initial specimen temperature profiles in the case of the Gleeble testing. These errors are shown to be up to approximately 20% and vary throughout the test from positive to negative and are therefore important to consider when processing experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Fretting fatigue is a complex problem and therefore it has generally been necessary to determine fatigue lives experimentally. This paper describes a method that integrates wear modelling with fretting fatigue analysis to permit the prediction of the effects of material removal, due to fretting wear, on fretting fatigue life. A finite element based method is employed to predict the fretting wear-induced evolution of contact geometry, contact stresses and a multiaxial fatigue damage parameter with cumulative damage effects, as a function of slip amplitude, for a laboratory fretting fatigue test arrangement. The results show that the approach is capable of capturing the experimentally observed effect of slip amplitude on fatigue life. The conditions simulated span across a range of partial slip and gross sliding conditions, corresponding to fretting fatigue tests available from the literature, and giving rise to the well-known phenomenon of a critical range of slip amplitude for reduced fatigue life. The methodology demonstrates that this is primarily due to wear-induced pressure redistribution, the nature of which is highly dependent on the slip regime. [Copyright &y& Elsevier]

Abstract Structural optimisation of a wind turbine blade is presented in this work. The optimisation was performed using a multi-objective genetic algorithm and finite element modelling to determine the optimal structural design for a glass fibre-reinforced polypropylene composite blade. A candidate blade design from the Pareto efficient set was manufactured and tested for a range of structural characteristics, including: mass, centre of gravity, deflections, strains and natural frequencies. Static testing was carried out using a Whiffle tree test rig and a laser scanner was used to determine the deflection of the blade to a high degree of accuracy. The finite element model results for the custom-made design are compared to the measured blade response. The FE model predictions for strains, mass and natural frequencies are in relatively good agreement with the test results; however, notable deviations in the deflections predictions are attributed to modifications to the blade for manufacture and the shell-based modelling approach. The differences are discussed in detail and recommendations for future design work are outlined. The test results of the bespoke blade are also compared to two additional designs to determine the level of improvement afforded by the genetic algorithm approach. The bespoke glass fibre blade demonstrated an improvement in tip deflection of 16% relative to the original blade design, with a slight decrease in mass. [ABSTRACT FROM AUTHOR]

A novel computational methodology for predicting three-dimensional microcracking and permeability in composite laminates is presented. The methodology simulates microcrack initiation and propagation using the extended finite element method (XFEM) and delamination using a mixed-mode cohesive zone model. Random microcrack initiation is modelled using a random (Weibull) distribution of fracture strengths. The Weibull distribution is adjusted to account for specimen volume, allowing mesh independent crack density predictions. An alternate method is also investigated, based on an elemental representation of defects using measured void geometry. The predicted microcracking and damage distributions are shown to correlate closely with 3D X-ray CT (computed tomography) scans of cryogenically cycled specimens. Crack opening displacements are consistent with laminate test measurements. Permeabilities, based on the dimensions of the leak paths, were found to be within the measured range for various CF (carbon fibre)/PEEK materials. [ABSTRACT FROM AUTHOR]

Abstract: This paper addresses coating fracture in hard brittle coatings subjected to combined normal and tangential loads through a finite element based methodology. The coating is modelled as an elastic layer perfectly bonded to an elastic substrate with a pre-microcrack, assumed to initiate at the contact trailing edge due to high tensile stress. The predicted results are consistent with previously published coating fracture results. The model predicts a significant effect of coating thickness on crack propagation for coatings with large elastic mismatch and the final propagated crack profile is predicted to depend on friction coefficient, coating fracture toughness and sliding displacement. [Copyright &y& Elsevier]

Abstract: The three-dimensional finite element implementation of a continuum damage mechanics formulation for multiaxial fatigue is presented, incorporating elastic modulus reduction due to fatigue damage. The implementation is validated against theoretical and published experimental results for uniaxial and notched multiaxial fatigue under different combinations of mean and alternating stresses for Ti–6Al–4V. An automatic incrementation scheme is developed for efficient computation of damage accumulation and hence stress redistribution. The method is also implemented in two-dimensional, plane strain for fretting fatigue and is shown to successfully capture the effect of contact slip on fatigue life for a round-on-flat fretting geometry. Comparisons are also made with a critical-plane multiaxial fatigue approach for fretting. The work is a first step towards a more general fatigue damage approach to unify wear and fatigue prediction for fretting. [Copyright &y& Elsevier]

Abstract: Fretting damage, in the form of wear and fatigue-cracking, is a common cause for concern in a wide range of dynamically loaded mechanical joints, and consequently, control of the rates of such damage is commonly addressed either by design or by the choice of materials with enhanced performance. Such material enhancements may be made to the material bulk, or just to the surface regions through surface engineering. In this work, a thin PVD W-DLC coating with a CrN interlayer was deposited onto both a high-strength steel and a titanium alloy. A cylinder-on-flat fretting configuration was employed under gross slip fretting conditions. It was observed that whilst a low coefficient of friction (COF) was measured throughout the wear of the DLC layer, there were variations in the rates of wear which could be correlated with compositional variations within the film. As wear progressed through the carbon-rich and tungsten-rich upper sub-layers of the W-DLC layer, the wear coefficient decreased. With further wear into the CrN underlayer, the wear coefficient and the COF increases. An incremental finite element (FE) wear model based on the Archard wear equation has been used to simulate the coating wear process, with particular emphasis on evolution of the contact geometry for the W-DLC layer. The predicted coating wear life is then shown to correlate, across a range of load–displacement combinations, with a single predictive coating life equation. [Copyright &y& Elsevier]

Abstract: A key challenge in the design of engineering couplings and contacting components relates to the development of an understanding of the comparative performance of contrasting contact geometries for a given application, including loading, applied deformations and geometrical space envelope. Although fretting is observed in many mechanical assemblies such as keyway-shaft couplings, shrink-fitted couplings, one specific example which has motivated the present work is the pressure armour layer of a marine flexible riser, where the groove and nub experience fretting contact damage. A Hertzian cylinder-on-flat contact geometry is commonly assumed for this groove-nub contact due to the ready availability of the contact (normal and tangential) analytical solutions for this geometry. In reality the contact geometry is closer to a rounded punch-on-flat. The present work adopts a finite element methodology to compare the significance of the Hertzian assumption to that of a rounded punch-on-flat, in terms of fretting behaviour. [Copyright &y& Elsevier]

Abstract: This paper is concerned with finite element modelling of microstructure-sensitive plasticity and crack initiation in fretting. The approach adopted is based on an existing method for microstructure-sensitive (uniaxial) fatigue life prediction, which proposes the use of a unit cell crystal plasticity model to identify the critical value of accumulated plastic slip associated with crack initiation. This approach is successfully implemented here, using a FCC unit cell crystal plasticity model, to predict the plain low-cycle fatigue behaviour of a stainless steel. A crystal plasticity frictional contact model for stainless steel is developed for microstructure-sensitive fretting analyses. A methodology for microstructure-sensitive fretting crack initiation is presented, based on identification of the number of cycles in the fretting contact at which the identified critical value of accumulated plastic slip is achieved. Significant polycrystal plasticity effects in fretting are predicted, leading to significant effects on contact pressure, fatigue indicator parameters and microstructural accumulated slip. The crystal plasticity fretting predictions are compared with J 2 continuum plasticity predictions. It is argued that the microstructural accumulated plastic slip parameter has the potential to unify the prediction of wear and fatigue crack initiation, leading in some cases, e.g. gross slip, to wear, via a non-localised distribution of critical crystallographic slip, and in other cases, e.g. partial slip, to fatigue crack initiation, via a highly-localised distribution of critical crystallographic slip with preferred orientation (cracking locations and directions). [Copyright &y& Elsevier]

Abstract: Numerical analyses based on the finite element (FE) method and remeshing techniques have been employed in order to develop a damage tolerance approach to be used for the design of aeroengines shaft components. Preliminary experimental tests have permitted the calculation of fatigue crack growth parameters for the high strength alloy steel adopted in this research. Then, a robust numerical study have been carried out to understand the influence of various factors (such as: crack shape, crack closure) on non-planar crack evolution in solid and hollow shafts under mixed-mode loading. The FE analyses have displayed a satisfactory agreement compared to experimental data on compact specimens (CT) and solid shafts. [Copyright &y& Elsevier]

Abstract: The application of thick thermally sprayed coatings on metallic parts has been widely accepted as a solution to improve their corrosion and wear resistance. Key attributes of these coatings, such as adherence to the substrate, are strongly influenced by the residual stresses generated during the coating deposition process. In high-velocity oxy-fuel (HVOF) thermal spraying, due to the relatively low temperature of the particle, significant peening stresses are generated during the impact of molten and semi-molten particles on the substrate. Whilst models exist for residual stress generation in plasma-based thermal spray processes, finite element (FE) prediction of residual stress generation for the HVOF process has not been possible due to the increased complexities associated with modelling the particle impact. A hybrid non-linear explicit–implicit FE methodology is developed here to study the thermomechanical processes associated with particle impingement and layer deposition. Attention is focused on the prediction of residual stresses for an SS 316 HVOF sprayed coating on an SS 316 substrate. [Copyright &y& Elsevier]

Multi-strand, copper conductors in submarine power cables (SPCs) for offshore wind are susceptible to fretting wear and fatigue, due to multiplicity and complexity of contacts, subjected to potentially severe dynamic loading. This paper presents (i) a global-local methodology for coupled hydro-aero-elastic dynamic loading of a representative SPC for identification of local inter-wire fretting-related conditions, (ii) fretting wear characterisation of copper conductor material, and (iii) local fretting multiaxial wear-fatigue finite element models for fretting fatigue life assessment of SPC copper conductor contacts. Predicted fretting fatigue lives are shown to be consistent with previously published bending fatigue test data on SPC copper conductors. Fretting fatigue life is shown to be significantly affected by aero-hydrodynamic loading, wear, slip regime and wire diameter. [ABSTRACT FROM AUTHOR]

Abstract: Fretting of the stem-head joint in a prosthetic hip implant is investigated experimentally and computationally. An FE-based methodology for fretting wear-fatigue prediction in a prosthetic hip implant is developed. Tribological and profilometry tests are performed for two head/stem material combinations: Co–28Cr–6Mo/DMLS Ti–6Al–4V and Co–28Cr–6Mo/forged Ti–6Al–4V. The hardness and wear resistance of DMLS Ti–6Al–4V are shown to be superior to those of forged Ti–6Al–4V. The significance of wear in a hip joint for 10 years of service in a normal weight person for moderately intense exercise is predicted for both material combinations. Both material combination joints are shown to have excellent wear resistance which suggests that the wear debris emission will not be significant. [Copyright &y& Elsevier]

Abstract: A finite element methodology for thermo-mechanical fatigue analysis of a subcritical power plant outlet header under realistic loading conditions is presented. The methodology consists of (i) a transient heat transfer model, (ii) a sequential anisothermal cyclic viscoplastic model and (iii) a multiaxial, critical-plane implementation of the Ostergren fatigue indicator parameter. The methodology permits identification of the local thermo-mechanical stress–strain response at critical locations and prediction of fatigue life and cracking orientation for complex transient, anisothermal, cyclic elastic–plastic-creep material behaviour. Measured plant data, in the form of steam and pipe temperature transients and steam pressure data, are employed to identify heat transfer constants and validate the predicted thermal response, with particular attention given to plant start-up and attemperation effects. The predictions indicate out-of-phase temperature-strain response at the header inside surface and in-phase response on the outside surface. Cooling transients are predicted to control damage and crack initiation at the inner bore, whereas heating transients are predicted to have a more damaging effect at weld locations. A representative test cycle is presented, which is shown to capture the salient thermo-mechanical cyclic damage of the realistic cycle. The predicted results correlate well with industrial experience in terms of crack (initiation) orientation, location and life. [Copyright &y& Elsevier]

Abstract: The effect of fretting wear on stress evolution in a thin, hard diamond-like carbon coating deposited on a high strength steel is simulated using a finite element (FE) based method under gross sliding conditions. The effects of coating stiffness, thickness, and coefficient of friction are studied. The trailing edge tensile stress and the leading edge compressive stress are generally predicted to reduce with wear, while the shear stress at the interface is predicted to increase. An extrapolation procedure for predicting coating wear life is shown to provide accurate and slightly conservative estimates compared with explicit modelling to complete coating wear. [ABSTRACT FROM AUTHOR]

Abstract: The fretting wear behaviour of Ti–6Al–4V is experimentally investigated using a cylinder-on-flat specimen arrangement for different loads, strokes and numbers of fretting cycles, including partial slip and gross sliding conditions. An elastic–plastic, finite element (FE) based, incremental wear simulation approach is employed to study the evolution of the surface and sub-surface fretting variables, using a kinematic hardening (continuum) plasticity model to represent the cyclic plasticity behaviour. The predicted evolution of plastic deformation and fretting variables, such as contact pressure, relative slip, sub-surface stresses and critical-plane Smith–Watson–Topper (multiaxial) fatigue parameter, due to the simulated wear are compared with the observed wear scar characteristics and locations of crack initiation. It is shown that the characteristics of predicted plastic strain and damage accumulation are significantly different for the gross sliding and the partial slip cases and that the differences can be correlated with the observed differences in surface morphology and surface cracking. It is shown that the location of plasticity due to shear yielding at the surface promotes wear across the contact region for the gross sliding case and at the stick–slip interface for the partial slip case. [Copyright &y& Elsevier]

In this paper, a global-local fretting design methodology for the pressure armour layer of flexible marine risers is outlined. This includes global dynamic riser analysis, geometrical and analytical sub-models and local nub-groove contact finite element analysis. Furthermore, a fretting test rig is developed and utilised to quantify coefficient of friction and wear coefficient under representative nub-groove loading conditions. The combination of the global-local computational methodology and experimental characterisation of pressure armour wire material allows for the development of running condition fretting maps. This identifies design criteria for critical riser global curvatures that are associated with minimum number of cycles to failure. The design methodology presented in this paper is applied to a realistic riser design study, using extreme sea-state loading conditions. In this case study, the predicted pressure armour fretting fatigue lives are found to be in the same range as the plain fatigue lives of the tensile armour layer. • Global-local fretting design methodology for the pressure armour layer of flexible marine risers. • Global dynamic riser analysis, geometrical and analytical sub-models and local nub-groove contact finite element analysis. • Development and utilisation of a fretting rig to quantify friction and wear coefficients under representative nub-groove loading conditions. • Identification of design criteria for critical riser global curvatures associated with minimum number of cycles to failure. • Realistic riser design study, using extreme sea-state loading conditions. [ABSTRACT FROM AUTHOR]