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The presence of inclusions is a known source of crack initiation and component failure in cast materials. In this work, the role of inclusions is investigated via a combined program of high‐temperature low‐cycle fatigue testing and computational modelling of a tempered martensitic steel, MarBN. Microstructural analysis has shown that manufacturing‐induced oxide inclusions are a key source of fatigue crack initiation. A fully coupled, critical plane life prediction and damage model is implemented in a unified cyclic viscoplastic user‐material subroutine and applied to predict damage and micro‐crack initiation for inclusions. It is deduced that more careful control of the development of inclusions in the manufacturing process will provide enhanced material and component performance for highly flexible and ultrasupercritical plant conditions. [ABSTRACT FROM AUTHOR]

Aeroengine spline couplings experience a wide range of loading conditions leading to contrasting service life limiting phenomena, including fatigue, fretting fatigue and fretting wear. Highly loaded couplings may employ incomplete contact axial profiles, while the contact geometry transverse to the spline axis is nominally complete with theoretical stress singularities at the contact edges. Life assessment of such components is consequently complex. The effect of torque overload conditions on the fatigue life of a barrelled, aeroengine type spline coupling is investigated experimentally. The experimental results are interpreted using three-dimensional finite element analyses, incorporating frictional contact and elastic–plastic material behaviour and the results of simple tension–tension fatigue tests. Torque-life and finite element predicted stress-life relationships are generated for spline life prediction purposes. Good correlation is obtained between the spline coupling and simple tension–tension fatigue test results, interpreted via the finite element predicted stress ranges. [ABSTRACT FROM AUTHOR]

Abstract: An improved unified cyclic viscoplastic material model for high temperature fatigue of P91 steel is presented. The primary enhancement over existing models is in relation to strain-rate independence of parameters, for accurate interpolation and extrapolation across a range of strain-rates and stress regimes, as relevant to flexible operation of high temperature power generation plant. The model combines a hyperbolic sine constitutive equation with anisothermal cyclic evolution of isotropic and kinematic hardening variables. The material model is developed from a thermodynamic framework and is implemented in multi-axial form within a user material subroutine. The user material subroutine is calibrated and validated for P91 steel across a range of cyclic (isothermal fatigue and thermo-mechanical fatigue) and non-cyclic high temperature loading conditions. A novel method for the identification of the cyclic viscoplastic material parameters is also presented. [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: 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]

This paper presents a combined experimental and computational methodology for fretting wear-fatigue prediction of pressure armour wire in flexible marine risers. Fretting wear, friction and fatigue parameters of pressure armour material have been characterised experimentally. A combined fretting wear-fatigue finite element model has been developed using an adaptive meshing technique and the effect of bending-induced tangential slip has been characterised. It has been shown that a surface damage parameter combined with a multiaxial fatigue parameter can accurately predict the beneficial effect of fretting wear on fatigue predictions. This provides a computationally efficient design tool for fretting in the pressure armour layer of flexible marine risers. [ABSTRACT FROM AUTHOR]

This paper presents a computational methodology for frictional contact mechanics of the pressure armour layer in flexible risers. This will allow, for the first time, quantification of key fretting variables, such as contact pressure, relative slip and sub-surface stresses in this complex geometry, under representative loading conditions. Fatigue lives are calculated using the 3-dimensional critical plane Smith–Watson–Topper multiaxial fatigue parameter. It is shown that COF has a significant effect on predicted trailing-edge tensile stresses in the pressure armour layer and, hence on fretting crack initiation in risers. It is also shown that operating pressure and bending-induced axial displacement significantly affect predicted crack initiation. These results will facilitate representative fretting wear and fretting fatigue testing of pressure armour layer material. [ABSTRACT FROM AUTHOR]

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: This paper presents two empirical parameters for the prediction of fretting. The first is a simple parameter, Dfret 1, which is shown to correlate test data against fretting fatigue life in terms of the so-called running conditions of contact displacement and normal load. The second, more complex, parameter is shown to have the ability to capture: (i) the effect of relative slip on fretting fatigue life and (ii) a well-known fretting fatigue contact size effect in a more rational framework. This parameter combines a multiaxial fatigue parameter, the constants for which can be obtained from plain-fatigue testing, and a surface damage parameter Dfret 2, which represents the surface damaging effect of fretting, the tribological constants for which can be obtained from fretting wear and fretting fatigue test data. A key benefit of Dfret 2 is that it circumvents time-consuming incremental wear simulations. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a combined experimental and computational methodology for fretting fatigue life prediction of aeroengine splined couplings under combined loading cycles involving cyclic torque and axial load, as well as rotating bending and fluctuating torque. The experimental method is based on the concept of a simplified representative test, which mimics the multiaxial fretting conditions between spline teeth via biaxial loading of specially-designed bridge pads and a fatigue specimen. The numerical method is based on a three-dimensional finite element model of the test rig assembly, including frictional contact effects, along with a multiaxial, critical-plane fatigue parameter for crack nucleation followed by crack growth prediction in the Paris regime using El Haddad small crack correction. The prediction methodology is shown to successfully capture the effect of the key fretting fatigue stress, which mimics the spline rotating bending moment, on total fatigue life. [Copyright &y& Elsevier]

Abstract: The fretting fatigue behaviour of a representative high-performance aero-engine spline coupling is studied under complex non-symmetrical and variable amplitude loads, consisting of major cycle torque and axial load, combined with minor cycle rotating bending moment and fluctuating torque. The objective of the study is to evaluate the effectiveness of finite element based predictions of: (i) a widely-used fretting fatigue damage parameter, namely a critical-plane Smith–Watson–Topper (SWT) parameter, and (ii) the fretting-fatigue-specific Ruiz criterion, for complex coupling geometries. The approach employs a high-resolution finite element model with localised mesh refinement to achieve detailed fretting fatigue results while maintaining an acceptable node count. The predictions are compared with corresponding laboratory test results and observations. Methods for overcoming the shortcomings of the SWT parameter, in the context of fretting fatigue, are discussed and a modified SWT parameter, which includes the effect of slip, is presented. [Copyright &y& Elsevier]