The effect of thermal cycling on the high-temperature creep behaviour of a single crystal nickel-based superalloy

Isothermal and thermal cycling creep behaviours of a single crystal nickel-based superalloy have been studied by means of tensile tests at 1150 °C and 80 MPa. We have demonstrated that thermal cycling creep rates are faster than isothermal creep rates and that lifetimes at high temperatures are shorter for creep tests under thermal cycling conditions. Furthermore, it is shown that thermal cycling creep lifetime increases as the thermal cycle frequency decreases

A unified constitutive model for asymmetric tension and compression creep-ageing behaviour of naturally aged Al-Cu-Li alloy

A set of unified constitutive equations is presented that predict the asymmetric tension and compression creep behaviour and recently observed double primary creep of pre-stretched/naturally aged aluminium-cooper-lithium alloy AA2050-T34. The evolution of the primary micro- and macro-variables related to the precipitation hardening and creep deformation of the alloy during creep age forming (CAF) are analysed and modelled. Equations for the yield strength evolution of the alloy, including an initial reversion and subsequent strengthening, are proposed based on a theory of concurrent dissolution, re-nucleation and growth of precipitates during artificial ageing. We present new observations of so-called double primary creep during the CAF process. This phenomenon is then predicted by introducing effects of interacting microstructures, including evolving precipitates, diffusing solutes and dislocations, into the sinh-law creep model. In addition, concepts of threshold creep stress σth and a microstructure-dependant creep variable H, which behave differently under different external stress directions, are proposed and incorporated into the creep model. This enables prediction of the asymmetric tension and compression creep-ageing behaviour of the alloy. Quantitative transmission electron microscopy (TEM) and related small-angle X-ray scattering (SAXS) analysis have been carried out for selected creep-aged samples to assist the development and calibration of the constitutive model. A good agreement has been achieved between the experimental results and the model. The model has the potential to be applied to creep age forming of other heat-treatable aluminium alloys

Deformation and rupture of stainless steel under cyclic, torsional creep

Copyright 2008 @ Engineering Integrity Society.Recent results from a long-term, strain-limited, cyclic creep test program upon stainless steel tubes are given. The test conditions employed were: constant temperature 500 °C, shear stress Ƭ = ± 300 MPa and shear strain limits ƴ = ± 4%. It is believed that a cyclic creep behaviour for the material has been revealed that has not been reported before in the literature. That is, the creep curves for stainless steel under repeated, shear stress reversals shows two basic square root dependencies: one upon time and the other upon cycle number. Consequently, the combined effect is such that the shear creep strain depends upon the square root of the product of cycle number and the time elapsed within that cycle. Despite extended times of cycling, with the test running into a period of over a year, no secondary or tertiary creep stages were ever observed within individual creep curves. Thus both the forward and reversed creep curves were exclusively primary in nature, within which the only visible evidence of a slow degradation of the deforming material was that the creep interval reduced successively between the imposed strain limits. However, it was found that the creep curve, when plotted within axes of cumulative creep strain and time, did recover a "pseudo-tertiary" stage. This stage concords with earlier results that showed tertiary creep to be a dominant contributor to the creep curve for this material under a steady torque. Given either the tensile ductility of the material or, a tensile creep rupture time, it is shown how final failure is predicted from the phenomenological square root law and an equivalence criterion

Measurement of local creep properties in stainless steel welds

A high temperature measurement system for creep deformation based on the digital image correlation (DIC) technique is described. The new system is applied to study the behaviour of a multi-pass welded joint in a high temperature tensile test and a load controlled creep test at 545°C. Spatially resolved tensile properties and time dependent creep deformation properties across a thick section type 316 stainless steel multi-pass welded joint are presented and discussed. Significantly lower creep strain rates are observed in the HAZ than in the parent material which is attributed to the introduction of substantial plastic strain in the parent material on initial loading. The weld metal shows the fastest creep rates and a variation that appear to correlate with individual weld passes. The visual information provides not only the local creep strain distribution but also the reduction of area and true stress distribution based on strains measured in the transverse direction. The results demonstrate the capability of the DIC technique for full field measurement of displacement and strain at high temperature long term creep tests

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 – 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 “time hardening” 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

Magnetic relaxation and collective vortex creep in FeTe0.6_{0.6}Se0.4_{0.4} single crystal

We study the vortex dynamics in high-quality FeTe$_{0.6}$Se$_{0.4}$ single crystal by performing magnetization measurements of the screening current density \emph{J}$_s$ and flux creep rate \emph{S}. Temperature dependence of \emph{S} shows a plateau in the intermediate temperature region with a high creep rate $\sim$ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for flux creep are directly determined by extended Maley's method. \emph{J}$_s$ with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach critical current density without flux creep by means of the generalized inversion scheme, which proves that the $\delta$\emph{l} and $\delta$\emph{T}$_c$ pinning coexist in FeTe$_{0.6}$Se$_{0.4}$ single crystal.Comment: 6 pages, 5 figure

A new creep model directly using tabulated test data and implemented in ansys

Nowadays plastics are increasingly used in highly stressed structures in all kinds of constructions. The time dependency, the so-called viscosity, is a crucial part of the material behavior of plastics. A typical form of viscosity is creep. Creep is the increase of deformation under constant load. In the FE-simulation creep behavior is usually described by creep law functions. The commercial software provide many creep law functions depending on time, stress, strain, temperature and multiple material parameters. To run a creep simulation, the user must define all the parameters which requires a certain effort. Curve-fitting procedures might be of help, the results, however, often are not precise enough. For these reasons, we introduce our new creep model doing the similar job as the creep law functions but being able to directly use the tabulated data of the creep tests without curve-fitting procedures. In this paper, we use the model to create a 3D stress-creep strain-time surface based on the tabulated data like isochronous curves, which is represented by bicubically blended Coons patches to provide a good convergence due to their differentiability. This creep model supports strain hardening, which shows more realistic behavior when the load changes significantly during the simulated proces

Plastic pre-compression and creep damage effects on the fracture toughness behaviour of Type 316H stainless steel

The influence of inelastic damage in the form of plastic pre-strain and creep damage, on fracture toughness of Type 316H stainless steel has been examined. Creep damage has been introduced into the 8% pre-compressed material by interrupting creep crack growth tests. Comparisons have been made between the fracture toughness test results from the as-received, pre-compressed and creep damaged materials. Furthermore, the effects of creep crack discontinuities on the crack tip strain fields have been examined by digital image correlation measurements. Inelastic damage was found to reduce the fracture toughness of the material, with creep damage having more severe effects than pre-strain

Linear matching method on the evaluation of cyclic behaviour with creep effect

This paper describes a new Linear Matching Method (LMM) technique for the direct evaluation of cyclic behaviour with creep effects of structures subjected to a general load condition in the steady cyclic state. The creep strain and plastic strain range for use in creep damage and fatigue assessments, respectively, are obtained. A benchmark example of a Bree cylinder subjected to cyclic thermal load and constant mechanical load is analysed to verify the applicability of the new LMM to deal with the creep fatigue damage. The cyclic responses for different loading conditions and dwell time periods within the Bree boundary are obtained. To demonstrate the efficiency and effectiveness of the method for more complex structures, a 3D holed plate subjected to cyclic thermal loads and constant axial tension is analysed. The results of both examples show that with the presence of creep the cyclic responses change significantly. The new LMM procedure provides a general purpose technique for the evaluation of cyclic behaviour, the plastic strain range and creep strain for the creep fatigue damage assessment with creep fatigue interaction

Theory of plastic vortex creep

We develop a theory for plastic flux creep in a topologically disordered vortex solid phase in type-II superconductors. We propose a detailed description of the plastic vortex creep of the dislocated, amorphous vortex glass in terms of motion of dislocations driven by a transport current $j$. The {\em plastic barriers} $U_{pl}(j)\propto j^{-\mu}$ show power-law divergence at small drives with exponents $\mu=1$ for single dislocation creep and $\mu = 2/5$ for creep of dislocation bundles. The suppression of the creep rate is a hallmark of the transition from the topologically ordered vortex lattice to an amorphous vortex glass, reflecting a jump in $\mu$ from $\mu = 2/11$, characterizing creep in the topologically ordered vortex lattice near the transition, to its plastic values. The lower creep rates explain the observed increase in apparent critical currents in the dislocated vortex glass.Comment: 4 pages, 1 figur