Isochronous creep rupture loci on deviatoric plane and its application to P92 steel creep behaviour under multiaxial stress state

The transformation relationship of the coordinate variables between principal stress space and deviatoric stress plane has been deduced and the isochronous creep rupture loci of disparate criteria have been described on deviatoric stress plane so as to analyze the creep behaviour under multiaxial stress state. The creep experiments of P92 steel smooth and notched specimens subjected to various stresses at 650 °C have been conducted. A modified constitutive model for the creep of P92 steel has been proposed and used to simulate the creep of P92 steel notched specimens with FEA software. The FEA results were consistent with the experimental data and the fracture morphology observation. It was found that the Hayhurst criterion had the best correlation with the experimental results of P92 steel under multiaxial stress state than other criteria through the comparison of the isochronous creep rupture loci on deviatoric plane.     

Effect of multiaxial stress state on creep damage in ASME T92 welded joint

This study reports the type IV fracture process and the influence of multiaxial stress state in ASME T92 welded joints during creep. The type IV fracture occurs at the fine‐grained heat‐affected zone (ie, FGHAZ), involving void initiation, growth, and coalescence, microcrack occurrence, propagation and extension, and eventual macrocrack with consequent joint failure. The creep damage is not uniformly distributed along the thickness direction in the FGHAZ, and the central part of the welded joint is the most seriously damaged region. The equivalent creep strain is higher at the external surface, but the stress triaxiality is larger in the centre section. Large equivalent creep strain could promote creep void initiation, whereas high hydrostatic pressure and stress triaxiality factor accelerate void growth in the FGHAZ of T92 joints. Besides, reducing groove angle and HAZ width of the joints is recommended to delay the occurrence of type IV cracking because of lower equivalent creep strain and stress triaxiality factor.     

Research on hardness variation for P92 notched specimen creep experiment

In order to study the hardness variation of P92 steel during creep in multiaxial stress state, creep experiments of specimens with various notches were conducted under different stresses at 650°C. The hardness and microstructure changes were investigated after creep experiments. The factors related to the hardness of P92 steel notched specimens were discussed. The Kachanov-Robotnov constitutive model for the creep of P92 steel was used to calculate the stress state and damage of P92 steel notched specimens during creep. The results showed that the hardness of P92 steel notched specimens decreased with the decrease of stress level and the increase of multiaxiality. The relationship among hardness, secondary phase precipitates, multiaxiality and damage were discussed.     

Closed-form plastic collapse loads of pipe bends under combined pressure and in-plane bending

Based on three-dimensional (3-D) FE limit analyses, this paper provides plastic limit, collapse and instability load solutions for pipe bends under combined pressure and in-plane bending. The plastic limit loads are determined from FE limit analyses based on elastic-perfectly-plastic materials using the small geometry change option, and the FE limit analyses using the large geometry change option provide plastic collapse loads (using the twice-elastic-slope method) and instability loads. For the bending mode, both closing bending and opening bending are considered, and a wide range of parameters related to the bend geometry is considered. Based on the FE results, closed-form approximations of plastic limit and collapse load solutions for pipe bends under combined pressure and bending are proposed.     

Study on creep mechanical behaviour of P92 steel under multiaxial stress state

The creep mechanical behaviour of P92 steel at 650°C has been studied by experimental research and finite element analysis. During the creep of P92 steel, there existed the notched strengthening effect, which was influenced by the shapes of the notch and the nominal stress. Under the condition of the same notch depth, the creep life enhancement factor increased with decreasing notched radius or the increase of stress. The multiaxial stress caused by the notch effect had a significant influence on the evolution of the microstructure and resulted in a transforming tendency from ductile to brittle at the root of the notch. The fracture position varied with the shapes of the notch: the U shaped notch started to fracture at the root of the notch, while the C shaped notch in the centre of the specimen. The creep process of notched specimens was simulated by embedding Kachanov–Rabotnov creep damage constitutive model into the interface program of finite element software. The result showed that damage distribution of notched specimens varied during the process of creep. The maximum damage location at the end of creep depended on the notch shape: with larger notch radius the maximum damage location was in the centre, while smaller radius of notch specimens was near the notch root, which was consistent with the analysis of the fracture morphology.     

Research on elastic-plastic creep damage of notched P92 steel specimens

Creep tests were performed on P92 steel specimens with notches of three different sizes at 650 °C. The results showed that the specimens switched from exhibiting ductility to showing brittleness at their center and at the notch root under multiaxial stress, but to varying degrees. This transformation was accompanied by a decrease in the reduction in area as well as in the number of dimples in the sample cross-section. The multiaxiality had a marked impact on the precipitation of the secondary phase, with its value determining the extent of precipitation of the secondary phase at the center and the root of the notch during creep. Using finite element analysis, an elastic-plastic creep damage model is embedded into the interface program and the creep behavior of the notched specimens was simulated. The results showed that plastic deformation at the notch root can accelerate specimen damage.     

Finite element analysis of notched bar skeletal point stresses and dimension changes due to creep

In this investigation, finite element calculations have been performed to obtain the creep stress distributions generated in circumferentially notched bar test‐pieces. They have also been made to determine the relation between axial extension and notch throat diameter changes. It has been found that an approximate skeletal point can be identified where the stress state is insensitive to the power law stress dependence of creep. Consistent trends in skeletal point stress ratios to those given in an existing Code of Practice for notch bar creep testing have been obtained. Nevertheless updated values, particularly for sharp notches, are proposed and these have now been inserted into a new version of the Code of Practice. In contrast, the link between extension and notch throat diameter changes has been found to depend on the creep stress index as well as the notch geometry. It is anticipated that the analysis can be used to establish the multi‐axial creep stress deformation and rupture behaviour of materials.     

Finite element analysis and experimental research on notched strengthening effect of P92 steel

Abstract

The notched strengthening effect during creep of P92 steel has been studied by finite element analysis and experimental research. It was found that there was a transforming tendency from ductile to brittle at the root of the notch and the extent of the transforming intensified with the increment of the nominal stress. It was the transforming tendency that increased the value of creep life enhancement factor. With the help of finite element software, Kachanov–Rabotnov creep damage constitutive model was embedded into the interface program and the notched specimens creep was simulated. The result has shown the Kachanov–Rabotnov model can be used to simulate the notched strengthening effect of P92 steel accurately when the material constant α?=?0·73.     

Micro mechanics based on vacancy diffusion coupled with damage mechanics related to creep deformation and prediction of creep fracture life

Abstract

In situ observations of crack growth and damage progression were conducted under creep conditions for P92 and titanium aluminides inter-metallic compound. A proposed analysis of stress induced particle diffusion was applied to stress induced vacancy diffusion. Results obtained from this analysis were successfully correlated with the experimental behaviour of macroscopic damage progression and a theoretical characteristic of creep deformation was derived. It was found to be in good agreement with experimental characteristics of creep deformation. Furthermore, the experimental characteristics of creep damage progression which concern voids and micro crack formations at grain boundary were found to be well correlated with those of deformation. From these results, correlation between vacancy diffusion in nano-scale, creep damage in mezzo-scale and creep deformation in macro-scale were successfully realized.     

Influence of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel

Effect of temperature on multiaxial creep behaviour of 304HCu austenitic stainless steel has been investigated. The multiaxiality was introduced by incorporating notches in smooth specimens. Creep rupture life increased with notch acuity ratio having a saturation/decline tendency. Notch strengthening increased with temperature, stress and notch sharpness. Multiaxial ductility decreased rapidly with notch sharpness and tended towards saturation. Fracture mode was found to change from transgranular ductile to intergranular creep depending on the stress, temperature and notch sharpness. Finite element analysis of notched specimens along with orientation imaging microscopic study was carried out to assess the deformation and damage at different normalised stress ratio. A temperature independent unique master plot for multiaxial rupture life as a function of stress has been established.     

基于Norton-Bailey模型的P92钢初期蠕变过程分析

以Norton-Bailey时间硬化模型和Kachanov损伤模型为基础,提出一种能够全面描述蠕变全寿命期蠕变变形发展的本构模型。与Norton-Bailey模型相比,该模型可以更准确地反映蠕变第一阶段的变形累积,以Kachanov损伤演化方程描述蠕变第三阶段的变化;推导模型第一阶段和第三阶段的参数确定方法;根据实验数据,确定P92钢的模型参数。将该模型嵌入到在ANSYS接口程序中,并用于高温合金钢P92的蠕变寿命计算,结果表明模型与实验数据基本吻合。     

Damage modelling: the current state and the latest progress on the development of creep damage constitutive equations for high Cr steels

This paper reviews the fundamentals of the development of creep damage constitutive equations for high Cr steels including (1) a concise summary of the characteristics of creep deformation and creep damage evolution and their dependence on the stress level and the importance of cavitation for the final fracture; (2) a critical review of the state of art of creep damage equation for high Cr steels; (3) some discussion and comments on the various approaches; (4) consideration and suggestion for future work. It emphasises the need for better understanding the nucleation, cavity growth and coalesces and the theory for coupling method between creep cavity damage and brittle fracture and generalisation.     

New plastic collapse moment equations of defect-free and throughwall circumferentially cracked elbows subjected to combined internal pressure and in-plane bending moment

Closed-form plastic collapse moments (PCM) equations were earlier proposed for throughwall circumferentially cracked (TCC) elbow subjected to pure in-plane bending moment. However, an elbow is often subjected to combined internal pressure and bending moment in actual service condition. Therefore, the present study investigates the effect of internal pressure on the in-plane PCM of a TCC elbow. The PCM of a cracked elbow is usually expressed as a product of two parameters: PCM of a defect-free elbow multiplied by a weakening factor due to the crack. Therefore, the present study also includes analysis of defect-free elbows. Elastic-plastic finite element analysis is employed for the present analysis. A total of 396 cases of elbows with various sizes of circumferential cracks (2θ = 0-150°), different wall thickness (R/t = 5-20), different levels of normalized internal pressure (p = PR/(tσ_y) = 0-1), different elbow bend radii (R_b/R = 2,3) and two different bending modes, namely closing and opening are considered in the analysis. Elastic-perfectly plastic stress-strain response of material is assumed. The load in the elbows is split in two components: a constant internal pressure applied initially followed by in-plane bending moment monotonically increasing in definite steps. PCM are evaluated from moment—end rotation curves by twice-elastic slope method. From these results, closed-form equations are proposed to evaluate PCM of TCC and defect-free elbows subjected to combined internal pressure and in-plane closing/opening bending moment. Attempt has been made to compare the predictions of the proposed equations with the available experimental/numerical results and to rationally explain the behaviour where no experimental/numerical data is available for comparison.     

The use of multiaxial fatigue models to predict fretting fatigue life of components subjected to different contact stress fields

This work describes the application of multiaxial fatigue criteria based on critical plane and mesoscopic (Dang Van, 1973, Sciences et Techniques de lÁrmement, 47 , 647—722) approaches to predict the fatigue initiation life of fretted components. To validate the analysis, several tests under closely controlled laboratory conditions are carried out in a Ti‐6Al‐4V alloy. These classical Hertzian tests reveal a size effect where fretting fatigue lives vary with contact size. Experimentally available data for fretting fatigue of an Al‐4Cu alloy are also used to assess the models. Neither the critical plane models nor the mesoscopic criterion considered can account for the effects of different contact stress fields on the initiation life, if the calculation is based only on highly stressed points on the surface. It is shown, however, that satisfactory results can be achieved if high values of the fatigue parameters are sustained over a critical volume.     

Influence of precipitation on dislocation substructure and creep properties of P91 steel weld joints

Abstract

Two trial weld joints were prepared using the GTAW and SMAW methods and they underwent creep testing at temperatures between 525 and 625°C. The longest time to rupture was 45,811 h. Two main processes occurred during creep exposures: recovery and precipitation of secondary phases. Slight coarsenings of the M₂₃C₆ carbide, precipitation of Laves phase and Z-phase were observed after long tests at high temperatures. Some differences in microstructure and creep failure were found in the individual zones of weldments. After long exposure at temperatures up to 600°C, fractures occurred in the fine-grain heat-affected zone as a result of a low density of fine vanadium nitride and a high density of coarse particles at grain and subgrain boundaries. At 625°C, growth of Laves phase caused a softening of the ferritic matrix and crack propagation in the weld metal.     

Some issues on structural integrity analysis of P91 welds in power plants subjected to high temperature creep*

Welds are metallurgically complex, with heterogeneous structures within the weld metal and heat‐affected zones. In order to carry out a structural integrity assessment, it is important to have knowledge of the initial metallurgical features and the associated material property variations so that it is possible to accurately model all the possible failure modes of welds using, for example, numerical techniques. This paper describes some fundamental issues concerning a holistic process for high temperature performance and failure prediction assessment of power plant welds. This includes welding process simulation and residual stress determination, microstructural evolution and the formulation of creep damage mechanics constitutive equations including weld metal anisotropy. Typical examples, from specific case studies for P91 welds, are used to illustrate the application. Future requirements for development of high temperature assessment methods for welds are suggested.     

Influence of changes of the bending plane position on the fatigue life

The paper concerns influence of changes of the bending plane position on the fatigue life. The obtained results were analyzed and compared with the fatigue results for oscillatory bending. The applied specimens were smooth, they had round sections, and they were made of the leaded brass CuZn40Pb2 (MO58). The results obtained under cyclic bending with the plane position change were compared with the results obtained for the specimens with the same parameters under pure oscillatory bending. A change of the bending plane position occurred every 10% fatigue life determined under pure oscillatory bending at the given amplitude of the bending moment, according to the defined fatigue characteristics. Calculated values of nominal stress in a cross section were recalculated according with cyclic material properties and values of elasto‐plastic stress were obtained.     

Vibration of non-ideal simply supported laminated plate on an elastic foundation subjected to in-plane stresses

In this paper, the effect of non-ideal boundary conditions and initial stresses on the vibration of laminated plates on Pasternak foundation is studied. The plate has simply supported boundary conditions and is assumed that one of the edges of the plate allows a small non-zero deflection and moment. The initial stresses are due to in-plane loads. The vibration problem is solved analytically using the Lindstedt–Poincare perturbation technique. So the frequencies and mode shapes of the plate with non-ideal boundary condition is extracted by considering the Pasternak foundation and in-plane stresses. The results of finite element simulation, using ANSYS software, are presented and compared with the analytical solution. The effect of various parameters like stiffness of foundation, boundary conditions and in-plane stresses on the vibration of the plate is discussed. Dependency of non-ideal boundary conditions on the aspect ratio of the plate for changing the frequencies of vibrations is presented. The relation between the shear modulus of elastic foundation and the frequencies of the plate is investigated.     

Linear matching method on the evaluation of plastic and creep behaviours for bodies subjected to cyclic thermal and mechanical loading

On the basis of the previously proposed Linear Matching Method (LMM), a new LMM model and its corresponding numerical procedure are developed in this paper to allow for the evaluation of plastic, creep and ratchet strains of structures subjected to a general load condition in the steady cyclic state. The constant and varying residual stress fields associated with differing mechanisms are obtained as well as the steady cyclic stress state of the whole component for further structural design and assessment. The total strain range for use in fatigue assessment, including the effect of creep and plastic strains, is obtained. A typical example of 3D holed plate subjected to cyclic thermal load and constant mechanical load is analysed here in detail to verify the applicability of the proposed numerical technique. The LMM results in the paper are compared with those obtained by ABAQUS step‐by‐step inelastic analyses. This comparison demonstrates that the LMM has both the advantage of programming method and the capacity to be implemented easily within a commercial finite element code, in this case ABAQUS. The LMM provides a general‐purpose technique for the evaluation of creep/fatigue interaction in the steady cyclic state. Copyright © 2006 John Wiley & Sons, Ltd.     

The effect of the spew fillet on an adhesively bonded single-lap joint subjected to bending moment

The mechanical behaviors of an adhesively bonded single lap joint with a spew fillet under bending moment, where the widths of the upper-adherend and lower-adherend are not the same, were studied experimentally and numerically. Using AA2024-T3 aluminum alloy as adherend and DP460 as paste adhesive, four different types of single-lap joint samples (without spew fillet, with spew fillet at joint edges, with spew fillet at joint ends and with spew fillet at all edges) were produced for experimental studies. Stress analyses in the single-lap joint were performed with a three dimensional non-linear finite element method by considering the geometrical non-linearity and non-linear material behaviors of both adhesives (DP460) and adherend (AA2024-T3). The single lap joint with the spew fillet with different widths had a significant effect on the stress distribution. Additionally, the spew fillet increased the load carrying capacity of the joint and decreased the stress concentration of the joint.