A.A. Becker

Prediction of creep failure in aeroengine materials under multi-axial stress states

The creep and creep rupture behaviour of two, significantly different, aeroengine materials, namely a nickel-base superalloy at 700°C and a high temperature titanium alloy at 650°C, were studied. Experimental creep tests were conducted on uniaxial specimens and axisymmetric notched bars under constant tensile loads conditions. From the uniaxial creep test results, a creep continuum damage model was established for each of the materials. The skeletal point stress approach was used to obtain the approximate creep rupture stress criterion in the multi-axial generalization of the creep continuum damage models. This approximation was cross-checked using axisymmetric Finite Element (FE) analyses in a trial and error procedure. Multi-axial creep continuum damage models were then used in further FE creep analyses to predict the creep rupture times in specimens subjected to different tensile loads. The FE predictions of the rupture times in these notched specimens were found to be in good agreement with the experimental results for the nickel-base superalloy (Waspaloy) at 700°C and the titanium alloy (IMI834) at 650°C.

Interpretation of impression creep data using a reference stress approach

Abstract A sound, mechanics-based approach has been developed to allow conventional creep data to be obtained from impression creep test data; the reference stress method was used, to analyse the results of axisymmetric finite calculations, for this purpose. The equivalent uniaxial stress was found to be 0.296 times the mean indenter pressure and the effective gauge length was found to be about 0.755 times the indenter diameter. These values correspond closely with those which have been obtained by fitting indenter creep data to conventional uniaxial creep data.

A virtual reality surgery simulation of cutting and retraction in neurosurgery with force-feedback

A virtual-reality surgical simulator aimed at neurosurgery is presented. The simulator utilises boundary element (BE) technology to develop real-time realistic deformable models of the brain. The simulator incorporates the simulation of surgical prodding, pulling and cutting. Advanced features include the separation the cut surfaces by retractors and post-cutting deformations. The experience of virtual surgery is enhanced by implementing 3D stereo-vision and the use of two hand-held force-feedback devices.

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.

Analysis of the impression creep test method using a rectangular indenter for determining the creep properties in welds

The results of a theoretical and finite element investigation of an impression creep test method using a long rectangular indenter under plane strain conditions, rather than the conventional cylindrical indenter, are presented. The application of the technique for determining the creep properties of the various zones within welds is considered. The finite element method is used to obtain accurate (creep) or approximate (elasto-plastic limit load) reference stress solutions for the rectangular indenters placed at several positions in the base material, heat affected zone and weld metal. The effect of varying the geometric test parameters is reported. The possible advantages of the technique for determining some of the important creep properties in welded structures are identified.

Benchmarks for finite element analysis of creep continuum damage mechanics

Abstract Creep rupture life can be predicted using a continuum damage mechanics approach incorporated within a finite element (FE) formulation. The rate of change of a damage parameter, ω , ranging from ω =0 (no damage) to ω =1 (100% damage), is computed within each element, until failure occurs in the material cross-section. The main difficulty in the numerical formulation arises due to the very small time steps needed as the damage parameter increases to 1. This paper presents the results of a number of benchmark tests involving the FE analysis of creep continuum damage mechanics that can be used to verify the FE solutions. Two independent FE codes are used; an in-house code (FE-DAMAGE) and a commercial code (ABAQUS) in which a user-subroutine (UMAT) is incorporated. The results of a series of tests used to represent uniaxial, biaxial, triaxial and multi-material creep and damage behaviour are presented.

A Comparison Between Measured and Modeled Residual Stresses in a Circumferentially Butt-Welded P91 Steel Pipe

Residual macrostresses in a multipass circumferentially butt-welded P91 ferritic steel pipe have been determined numerically and experimentally. The welded joint in a pipe with an outer diameter of 290 mm and a wall thickness of 55 mm is typical of power generation plant components. An axisymmetric thermomechanical finite element model has been used to predict the resulting residual hoop and axial stresses in the welded pipe. The effects of the austenite to martensite phase transformation have been incorporated into the simulation. Residual stresses have been measured using the X-ray diffraction technique along the outer surface of the pipe and using the deep-hole drilling technique through the wall thickness at the center of the weld. Good correlation has been demonstrated between the residual hoop and the axial stresses obtained numerically and experimentally. The paper demonstrates the importance of using a mixed experimental and numerical approach to determine accurately the residual macrostress distribution in welded components.

Development of a representative test specimen for frictional contact in spline joint couplings

The frictional contact conditions in a helical spline joint coupling under torsional and axial loads are studied using finite element analyses. Comparisons of spline root torsional stresses are made with photoelastic measurements. Surface contact tractions, relative slip distributions and subsurface localized stress component and equivalent stress distributions are presented for both the external and the internal spline contact surfaces. The work is important as a basis for understanding and predicting the fatigue, contact fatigue, fretting fatigue, wear and fretting wear characteristics of the coupling. A representative test specimen concept is presented which seeks to capture the local contact variable distributions in the spline coupling, with a view to predicting the wear and the contact and fretting fatigue performance of the coupling.

Comparison of the creep and damage failure prediction of the new, service-aged and repaired thick-walled circumferential CrMoV pipe welds using material properties at 640°C

Finite element creep and damage analyses were performed for a series of new, service-aged, fully repaired and partially repaired circumferential welds in CrMoV main steam pipes, under an internal pressure and a uniform axial stress, using simplified axisymmetric models. The material properties used were those of a 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo weldment at 640°C. Failure lives and failure positions were obtained using damage modelling and these were compared with corresponding results obtained from steady-state analyses. The effect of the weld width, in the practical range, and the effect of the axial (system) loading, on the failure life and position, of the various weld situations were also investigated. Comparison of the various failure predictions allows the effects of the differences in the relative properties of the constituents of the weld, the geometry and system loading, to be identified. The information is useful for assessing the performance of practical service-aged/repaired welds in power plants pipework.

Failure prediction for multi-material creep test specimens using a steady-state creep rupture stress

The steady-state stress distributions in single- and multi-material notched and waisted specimens were investigated for practical creep test specimens using material properties obtained for materials from a service-aged CrMoV pipe weldment. The tri-axial stress conditions existing in notched and waisted specimens machined from welded pipes were identified. By using a steady-state effective failure stress, it has been shown that an approximate method, based on steady-state stress, for the prediction of rupture life and failure position can produce reasonably accurate results. The applicability of the approximate method was confirmed by comparing the results obtained using it with those obtained from corresponding creep continuum damage modelling. These results indicate that use of steady-state stress analysis, as an approximate technique, may be useful for assessing creep failure behaviour, for determining the effect of specimen size and for generating material properties for welded components.