52 research outputs found
A parametric study on creep-fatigue strength of welded joints using the linear matching method
This paper presents a parametric study on creep-fatigue strength of the steel AISI type 316N(L) weldments of types 1 and 2 according to R5 Vol. 2/3 Procedure classification at 550ā¦C. The study is implemented using the Linear Matching Method (LMM) and is based upon a latest developed creep-fatigue evaluation procedure considering time fraction rule for creep-damage assessment. Parametric models of geometry and FE-meshes for both types of weldments are developed in this way, which allows variation of parameters governing shape of the weld profile and loading conditions. Five configurations, characterised by individual sets of parameters, and presenting different fabrication cases, are proposed. For each configuration, the total number of cycles to failure Nā in creep-fatigue conditions is assessed numerically for different loading cases including normalised bending moment M and dwell period t. The obtained set of Nā is extrapolated by the analytic function, which is dependent on M, t and geometrical parameters (Ī± and Ī²). Proposed function for Nā shows good agreement with numerical results obtained by the LMM. Thus, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) intended for design purposes and dependent on t, Ī±, Ī²
Novel direct method on the life prediction of component under high temperature-creep fatigue conditions
This paper presents a novel direct method, within the Linear Matching Method (LMM) framework, for the direct evaluation of steady state cyclic behaviour of structures subjected to high temperature ā creep fatigue conditions. The LMM was originally developed for the evaluation of shakedown and ratchet limits. The latest extension of the LMM makes it capable of predicting the steady state stress strain solutions of component subjected to cyclic thermal and mechanical loads with creep effects. The proposed iterative method directly calculates the creep stress and cyclically enhanced creep strain during the dwell period for the assessment of the creep damage, and also creep enhanced total strain range for the assessment of fatigue damage of each load cycle. To demonstrate the efficiency and applicability of the method to assess the creep fatigue damage, two types of weldments subjected to reverse bending moment at elevated temperature of 550C are simulated by the proposed method considering a Ramberg-Osgood model for plastic strains under saturated cyclic conditions and a power-law model in ātime hardeningā form for creep strains during the dwell period. Further experimental validation shows that the proposed direct method provides a general purpose technique for the creep fatigue damage assessment with creep fatigue interaction
Creep-fatigue life assessment of cruciform weldments using the linear matching method
This paper presents a creep-fatigue life assessment of a cruciform weldment made of the steel AISI type 316N(L) and subjected to reversed bending and cyclic dwells at 550C using the Linear Matching Method (LMM) and considering different weld zones. The design limits are estimated by the shakedown analysis using the LMM and elastic-perfectly-plastic material model. The creep fatigue analysis is implemented using the following material models: 1) Ramberg-Osgood model for plastic strains under saturated cyclic conditions; 2) power-law model in ātime hardeningā form for creep strains during primary creep stage. The number of cycles to failure N? under creep-fatigue interaction is defined by: a) relation for cycles to fatigue failure N dependent on numerical total strain range "tot for the fatigue damage !f ; b) long-term strength relation for the time to creep rupture t dependent on numerical average stress ĀÆ during dwell t for the creep damage !cr; c) non-linear creep-fatigue interaction diagram for the total damage. Numerically estimated N? for different t and "tot shows good quantitative agreement with experiments. A parametric study of different dwell times t is used to formulate the functions for N? and residual life L? dependent on t and normalised bending moment ĖM , and the corresponding contour plot intended for design applications is created
Creep-fatigue life assessment of high-temperature weldments using the linear matching method
This poster discusses Creep-fatigue life assessment of high-temperature weldments using the linear matching method
Linear matching method for parametric studies of weldments creep-fatigue endurance
This paper presents parametric studies on creep-fatigue endurance of the steel AISI type 316N(L) weldments defined as types 1, 2 and 3 according to R5 Vol. 2/3 Procedure classification at 550C. The study is implemented using the Linear Matching Method (LMM) and based upon previously developed creep fatigue evaluation procedure considering time fraction rule. Several geometrical configurations of weldments with individual parameter sets, representing different fabrication cases, are developed. For each of configurations, the total number of cycles to failure N* in creep-fatigue conditions is assessed numerically for different loading cases. The obtained set of N* is extrapolated by the analytic function dependent on normalised bending momentĖM, dwell period Dt and geometrical parameters. Proposed function for N* shows good agreement with numerical results obtained by the LMM. Therefore, it is used for the identification of Fatigue Strength Reduction Factors (FSRFs) intended for design purposes and dependent on proposed variable parameters
On cyclic yield strength in definition of limits for characterisation of fatigue and creep behaviour
This study proposes cyclic yield strength as a potential characteristic of safe design for structures operating under fatigue and creep conditions. Cyclic yield strength is defined on a cyclic stress-strain curve, while monotonic yield strength is defined on a monotonic curve. Both values of strengths are identified using a two-step procedure of the experimental stress-strain curves fitting with application of Ramberg-Osgood and Chaboche material models. A typical S-N curve in stress-life approach for fatigue analysis has a distinctive minimum stress lower bound, the fatigue endurance limit. Comparison of cyclic strength and fatigue limit reveals that they are approximately equal. Thus, safe fatigue design is guaranteed in the purely elastic domain defined by the cyclic yielding. A typical long-term strength curve in time-to-failure approach for creep analysis has two inflections corresponding to the cyclic and monotonic strengths. These inflections separate three domains on the long-term strength curve, which are characterised by different creep fracture modes and creep deformation mechanisms. Therefore, safe creep design is guaranteed in the linear creep domain with brittle failure mode defined by the cyclic yielding. These assumptions are confirmed using three structural steels for normal and high-temperature applications. The advantage of using cyclic yield strength for characterisation of fatigue and creep strength is a relatively quick experimental identification. The total duration of cyclic tests for a cyclic stress-strain curve identification is much less than the typical durations of fatigue and creep rupture tests at the stress levels around the cyclic yield strength
A comparative study between conventional and elevated temperature creep autofrettage
This paper presents a comparative study between conventional hydraulic and elevated temperature autofrettage. For modelling of both methods advanced plasticity and creep material models are used. The main governing equations for the models are presented as well. A beneficial influence of compressive residual stresses induced by both methods is demonstrated on a benchmark problem of cross bored block. The effectiveness and applicability of the two methods are estimated by conduction of compressive residual stress analysis and crack arrest modeling. Numerical simulation of the cyclic plasticity and creep problems are carried out by means of FEM in ANSYS Workbench with FORTRAN user-programmable subroutines for material model incorporating custom equations
Design optimisation of swellable elastomeric seals using advanced material modelling and FEM simulations
Swellable elastomeric seal is a type of specifically engineered packer that swell upon contact with wellbore fluids. Such packers have been widely employed in various oil-&-gas and minerals applications including slimming of well design, zonal isolation, water shut-off, and multi-stage fracturing. Downhole conditions are difficult to be reproduced using physical testing environment, but feasible to be simulated in virtual environment using CAE software. A better understanding of packersā mechanical behaviour in downhole conditions would provide a higher confidence and improvement of existing engineering design practices for manufacturing of packers. The numerical simulation can be incorporated into optimisation procedure searching for an optimal shape of packers with the goals to minimise the time to seal the borehole and maximise the contact pressure between the seal and borehole. Such an optimisation would facilitate the development of a packer with various designs optimised for different downhole conditions. The objective of this research project is to develop a design tool integrated into a CAE to implement parametric numerical studies using FEM simulation. However, development of such a CAE plugin is associated with a number of technical challenges specific to this class of multiphysics problems, which will be addressed and discussed in a poster
CAE-based application for identification and verification of hyperelastic parameters
The main objective of this study is to develop a CAE-based application with a convenient GUI for identification and verification of material parameters for hyperelastic models available in the current release of the FE code ANSYS Mechanical APDL. This Windows application implements a two-step procedure: (1) fitting of experimental stressāstrain curves provided by the user; (2) verification of the obtained material parameters by the solution of a modified benchmark problem. The application, which was developed using the Visual Basic.NET language, implements a two-way interaction with ANSYS as a single loop using the APDL script as input and text, graphical and video files as output. With this application, nine isotropic incompressible hyperelastic material models are compared by fitting them to the conventional Treloarās experimental dataset (1944) for vulcanised rubber. A ranking of hyperelastic models is constructed according to model efficiency, which is estimated using fitting quality criteria. The model ranking is done based upon the complexity of their mathematical formulation and their ability to accurately reproduce the test data. Recent hyperelastic models (Extended Tube and Response Function) are found to be more efficient compared to conventional ones. The verification is done by the comparison of an analytical solution to an FEA result for the benchmark problem of a rubber cylinder under compression proposed by Lindley (1967). In the application, the classical formulation of the benchmark is improved mathematically to become valid for larger deformations. The wide applicability of the proposed two-step approach is confirmed using stressāstrains curves for seven different formulations of natural rubber and seven different grades of synthetic rubber
Comparison of single-solver FSI techniques for the Fe-prediction of a blow-off pressure for an elastomeric seal
Summary: Assessment of leakage performance is a fundamental aspect in the design of elastomeric fluid seals. The key characteristic of the leakage performance is the blow-off pressure, which when it is reach, the leakage rate through the seal is no longer acceptable and the seal no longer performs its function. Prediction of the blow-off pressure under different operating conditions such as the amount of preload, pressure build-up rate, etc., would facilitate improvement in the seal design methodology and would allow subsequent efficient optimisation of a seal design. For an accurate prediction of the blow-off pressure, any computational methodology should ideally include Fluid-Structure Interaction (FSI), since the geometrical disposition of the seal, its deformation and its contact conditions with respect to the main structure are adversely affected by the fluid pressure. Two FSI single-solver techniques, available in the commercial FE-code Abaqus, are investigated in this study with application to the simulation of leakage. The first technique is Pressure Penetration Interaction, available for general structural analysis using a static implicit solver, which propagates the applied fluid pressure into contact openings. The second, more advanced FSI technique, is the Coupled Eulerian-Lagrangian (CEL) approach which is available for coupled fluid-structural analysis in a dynamic explicit solver. The CEL approach is the most versatile and can be applied to a wide range of FSI problems, e.g. simulation of a seal blow-off and fluid leakage through the resulting contact gap [1]. When trying to develop the art and science involved in the simulation of leakage phenomenon, a robust FSI technique has the potential to be a critically important design tool which could be applied to a wide range of more complex sealing geometries. This paper addresses a typical hollow rubber seal and investigates how the amount of initial compression and initial stretching affect the leak tightness of the seal using two different FSI techniques. The complexity of problem setup, the efficiency of the solver and the robustness of obtained solution will be compared and discussed
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