T.H. Hyde

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.

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.

An investigation of the fatigue and fretting performance of a representative aero-engine spline coupling

The fatigue behaviour of a representative high-performance aero-engine spline coupling is investigated under test conditions designed to simulate in-service conditions. The test load cycles consist of major cycle torque and axial load, simulating maximum thrust, combined with minor cycle rotating bending moment and fluctuating torque, simulating life-limiting conditions at take-off. The objective of the study is to develop understanding of the fatigue behaviour of the coupling over a range of loading conditions, including interaction between low-cycle fatigue, fretting fatigue and fretting wear. This information is necessary for the development of fatigue and fretting-fatigue life prediction techniques. The test results are interpreted with the help of three-dimensional finite element models, which include the frictional contact between the spline teeth.

Creep analysis of pressurized circumferential pipe weldments—a review

This paper reviews work related to the high-temperature creep analysis of pressurized circumferential pipe weldments. It is important to define the problem and thus metallurgical features correctly; the identification of material microstructural-property variations within the heat-affected zone (HAZ) and the failure modes of welds are briefly included, as well as in-service experience of pipe welds. Experimental methods, including model and full-size component testing, are summarized and examples of typical tests results are described. Material constitutive equations, which can be used in describing the creep deformation of and failure mechanisms in welds, are briefly described. Numerical modelling using finite element (FE) methods, covering a range of approaches and analyses, taking account of the effects of material properties, pipe geometry, weld dimensions and system loading, on the stresses and failure behaviour of pipe weldments, are summarized. Typical results are presented to illustrate the potential uses and limitations of the FE methods.

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.

On the interpretation of results from small punch creep tests

The small punch creep testing method is highly complex and involves interactions between a number of non-linear processes. The deformed shapes that are produced from such tests are related to the punch and specimen dimensions and to the elastic, plastic, and creep behaviour of the test material, under contact and large deformation conditions, at elevated temperature. Owing to its complex nature, it is difficult to interpret the small punch test creep data in relation to the corresponding uniaxial creep behaviour of the material. One of the aims of this paper is to identify the important characteristics of the creep deformation resulting from ‘localized’ deformations and from the ‘overall’ deformation of the specimen. Following this, the results of approximate analytical and detailed finite element analyses of small punch tests are investigated. It is shown that the regions of the uniaxial creep test curves dominated by primary, secondary, and tertiary creep are not those that are immediately apparent from the displacement versus time records produced during a small punch test. On the basis of the interpretation of the finite element results presented, a method based on a reference stress approach is proposed for interpreting the results of small punch test experimental data. Future work planned for the interpretation of small punch tests data is briefly addressed.

Mixed mode fatigue crack growth at 550°C under plane stress conditions in jethete M152

Abstract This paper describes the experimental and theoretical results obtained from mixed mode (I and II) fatigue crack growth tests in Jethete at 550°C. Mixed mode crack tip conditions were achieved using the Compact Mixed Mode (CMM) specimen, using initial crack tip conditions corresponding to K I K II = 1.6 (mixed mode) and K I K II = 0.0 (or pure mode II). Two tests have also been performed under pure mode I conditions using the standard Compact Tension (CT) specimen. The results indicate that the mixed mode crack growth data can be correlated with the pure mode I data using a stress intensity factor range, ΔKϵ obtained from ΔKI and ΔKII, based upon the angular position, θ, of maximum plastic tangential strain. Hence a Paris law obtained from the pure mode I data can be applied to mixed mode behaviour. The direction of fatigue crack growth is shown to be dependent upon the KI/KII ratio and can be predicted using either the proposed maximum tangential strain criterion or the maximum tangential stress theory.