Creep Strain Evolution Research Articles

Experimental study on the creep performance of Grade 8.8 and 10.9 high-strength bolts at elevated temperatures

High-strength bolts are widely used in beam-column connections which are typically subjected to significant pretension and experience high stress levels. Under fire conditions, high-strength bolts may exhibit noticeable creep deformation due to the combined effects of high stress and high temperature, which may result in significant deformation of the connection, thereby threatening the stability of the structure. This paper reports an experimental study on the creep behavior of Grade 8.8 and 10.9 high-strength bolts at elevated temperatures. A series of tensile tests are conducted on high-strength bolts at elevated temperatures, and varying stress ratios are set based on the yield strength measured form the tensile tests. Test results indicate that the temperature has significant influence on the mechanical properties of Grade 8.8 and 10.9 high-strength bolts. Through the entire temperature range, current specifications significantly overestimate the yield strength of high-strength bolts at high temperatures. At the same temperature, the development of creep strain over time is positively related to the stress ratio. An increase in stress ratio from 0.4 to 0.6 or 0.8 results in a maximum 7.8-time increase in creep strain. The temperature of 500℃ can be regarded as the critical temperature at which the creep effects of Grade 8.8 and Grade 10.9 high-strength bolts should be considered. The mechanical property prediction equations and a modified creep model for Grade 8.8 and 10.9 high-strength bolts were developed, which have high reliability in predicting the high-temperature mechanical properties and creep behavior of high-strength bolts.

Creep Phenomena, Mechanisms, and Modeling of Complex Engineering Alloys

Metal creep has been a subject of extensive study for more than 110 years because it affects the useful life of engineering components operating at high temperatures. This is even more true with ever-increasing operating temperatures of propulsion/power-generation systems and the environmental regulations to reduce greenhouse emissions. This review summarizes the recent development in creep modeling with regards to creep strain evolution, creep rate, creep ductility, creep life, and fracture mode, attempting to provide a comprehensive mechanism-based framework to address all the important creep phenomena and the long-standing issue of long-term creep life prediction with microstructural evolution and environmental effects.

Evolution and criteria for early creep damage

ABSTRACT The life-limiting fitness for service of high-temperature components is of interest in design, fabrication and later assessments of remaining creep life. Of the associated indicators, strain reflects creep in design and in service, while the discontinuities like creep cavities are targeted in the in-service inspections. Microstructure and hardness can provide supporting information on the material condition. Here, we assess such indicators for the creep-associated damage, particularly at the early stages. Improvements appear possible, e.g. in microscopy to support metallographic inspections and in utilising the widening inspection experience on newer materials. The present work successfully integrated the Wilshire/LCSP creep strain and rupture models with FE analysis for predicting creep strain evolution. Since the model applies for the whole creep curve, it can be used for early stages of creep down to the limit of negligible creep and to the lower limit of the window where creep cavitation damage can be observed.

Buckling of Stiffened Heterogeneous Shells Taking into Account Material Creep

The authors propose a mathematical model of shell structures taking into account the linear theory of hereditary material creep. This model is based on the total potential strain energy functional. Shells are reinforced with stiffeners, and the contact between the stiffener and the shell skin along a strip is taken into account. The Ritz method is applied to the functional, and a system of algebraic or integro-algebraic equations is found. The resulting system is first solved as a linear system (without account for the terms reflecting material creep), and the state of the structure under the specified load is determined. Then the iterative method is used to solve the creep problem for a given value of time. The paper presents the results of studying stiffened shallow shells of double curvature made of reinforced concrete under uniformly distributed load. The buckling of shells occurs over time as a result of the development of creep strains. Values of the buckling load as a result of material creep are found. It is shown that stresses are redistributed over the shell field over time and the maximum stress is observed near the contour of the structure.

Component test of nickel-based alloys and their welded joints under long-term creep loading

The aim of the paper is to investigate experimentally the creep behavior of improved materials and welded joints at component scale under near-to-service operating conditions, which are promising for the application in highly efficient and flexible future power plants.It focusses on the creep behavior under multiaxial loading of similar and dissimilar welded joints of high temperature resistant nickel-based alloys (A740H, A617B and HR6W) in the temperature range between 650 °C and 750 °C by means of thick-walled-component tests. In this case, the temperature, the external axial load and the internal pressure remain constant during testing.Numerical simulations are used for the estimation of the component axial load in order to induce a meaningful damage evolution without causing a total component failure. The metallographic investigation on the welds A740H/A740H and A617B/A740H indicated no test-related damage. Small cracks in circumferential direction were found on the welds HR6W/HR6W and HR6W/A617B after having reaching 2% local creep strain. The damage mechanism could be classified as a typical creep damage with carbides formation in the grain boundaries and thus grain boundary separations occur, which ultimately lead to the crack formation.In general, the FEM calculation underestimates the axial creep strain on the investigated welds, especially the initial creep strain evolution is not accurately predicted. However, the long-term creep strain evolution could be well predicted by FEM. In this case, the material parameters for the creep law should be further optimized in order to minimize the underestimation.

A robust life prediction model for a range of materials under creep-fatigue interaction loading conditions

This paper develops a robust creep-fatigue interaction (CFI) life prediction model which is superior to the existing methods. Specifically, the newly proposed creep damage incorporates the effect of creep strain, which introduces the critical creep strain energy density rate and creep strain evolution term. The predictions are carried out for various materials, including martensitic heat-resistant steel, austenitic stainless steel, nickel-based superalloy, and different loading conditions, including conventional CFI and hybrid-controlled CFI loadings. The results show that all of the tested data falls within ± 2.5 error band, demonstrating the broad applicability of the proposed model. Based on the proposed model, a continuous CFI failure envelope independent of material, temperature, and CFI loading type is proposed, with almost all the data points lying outside the envelope.

The Effect of Temperature on the Creep Ageing Characteristics of Al-Li Alloys

Abstract: In this paper, the evolution of creep strain and mechanical properties of 2195 Al-Li alloy is investigated at different temperatures (160/170/180°C), with the aim of discovering the mechanism of temperature influence on creep properties and providing a reference for improving the creep properties of the target material by adjusting the temperature in the future. This study demonstrates that the creep curve is strongly influenced by temperature, with a plateau and a transition period occurring at low temperatures. As the temperature increases, the creep curve gradually reverts to the typical two-stage creep characteristics due to the interaction of dislocation density proliferation and dislocation reversion within the material at high temperatures. In addition, by means of tests such as TEM, it was confirmed that the decrease in mechanical properties of the material caused by increasing the creep temperature is mainly due to the coarsening of precipitates and the appearance of precipitation-free zones at grain boundaries.

Larson Miller Parameter for the Prediction of the Creep Life of Unweld and Welded P91 Steel

For structural components that operate at elevated temperatures, martensitic P91 steel is preferable. It is widely used in steam generators in the fossil-fired thermal and nuclear power generation sectors due to its creep endurance and corrosion resistance. Several creep laws, such as Monkman-Grant, Theta project, Wilshire and Sinh model, Omega technique, and the Larson Miller Parameter (LMP), have been developed over time to predict and failure of materials susceptible to the creep phenomenon. However, only the Omega Law and Larson-Miller Parameter are the only two methods approved in API 579-1. In this work, the creep test for welded and unwelded P91were conducted at temperatures of 600°C under stresses of 165, 175 MPa, and 190 MPa. In comparison to the welded specimens, the unwelded specimens displayed a continuously more substantial development of creep strain with time, resulting in a higher steady-state creep rate and a shorter rupture life. The increased magnitude of creep-rupture data has been observed to impact the dependability of creep life. Because of more significant changes in service temperature and stress conditions, the dependability of P91 steel has deteriorated, as well as an increase in creep life. The rupture life has been predicted using the LMP method, which utilizes the constant C parameter. At the same stress, the predicted creep life for weld material shows a higher value than that of the parent material, which is consistent with the experimental result.

Generation of compressive residual stress at the root of tube-to-tubesheet welded joints in a heat exchanger

Stress corrosion cracking (SCC) is the main failure mode of tube-to-tubesheet welded joints in a heat exchanger, and weld residual stress can play an important role in the development of these cracks. Local post welding heat treatment (PWHT) is a feasible method to reduce weld residual stress for tube-to-tubesheet welded joints. In this paper, the residual stress evolution and relaxation mechanisms during local PWHT were discussed in depth. The effects of PWHT temperature and hold time on residual stress were clarified. A compressive stress manufacturing method based on local PWHT was proposed. The results show that the stress concentration at the root of tube-to-tubesheet welded joints is the main reason for the SCC of heat exchangers. The residual stresses in the weld and heated affected zone (HAZ) are significantly reduced during local PWHT. The residual stress relaxation of tube-to-tubesheet welded joints can be attributed to the creep strain development and deformation during local PWHT. The extrusion of the tube and tubesheet at cooling stage results in the generation of compressive stress at the root of tube-to-tubesheet welded joints. PWHT temperature has significant effect on the residual stress in the weld and HAZ in comparison with PWHT hold time. The application of the proposed compressive stress manufacturing method realizes the fabrication of compressive stress at the root of tube-to-tubesheet welded joints, and improves the economic benefit by reducing PWHT hold time. This work will provide a good guidance to reduce the SCC risk of heat exchangers.

Damage nucleation during transverse creep of a directionally solidified Ni-based superalloy

A directionally solidified Ni-based superalloy DS200 is investigated under low (750 °C) and high temperature (900 °C) transverse creep. In-situ creep tests with strain measurements are performed to capture creep strain evolution in the individual crystallographic grains. Moreover, the microstructure configurations that promote damage nucleation are identified as a function of the loading condition. High angle grain boundaries are the preferred sites for crack nucleation under all investigated loading conditions. However, grain boundary configurations promoting crack nucleation are observed to significantly differ as a function of the loading conditions. Such observations are discussed in view of creep behavior and damage processes.

The nature of change in the residual stress-strain state of dovetail lock joints, taking into account nonlinear strains

A general algorithm for calculating residual stresses based on the method of successive loadings is presented. The process of formation of residual stresses and strains in dovetail lock joints during its loading by centrifugal forces and subsequent unloading is simulated. The problem is solved in non-linear statement taking into account the elasticity, plasticity and creep strains. The fields of residual stresses and deformations in the grooves of the disk are obtained. The numerical solution of the problem was performed by the finite element method in the ANSYS software package. The influence of the opening angle on the nature of plastic strain zoning is analyzed. The opening angle was changed in the range from 45 to 70. It was established that with the development of creep strain, there is a change in the zone of nonlinear deformations and the nature of the distribution of residual stresses in the zone of the concentrator. The dependences of the value of maximum residual stresses on the opening angle , as well as their variation over time, are obtained.

Creep behaviour of structured clays in triaxial stress space: theory and experimental investigation

ABSTRACT This paper investigates the time-dependent behaviour of structured clays in triaxial stress space both theoretically and experimentally. A range of existing theoretical frameworks and phenomenological equations are reviewed and discussed in their ability to interpret the creep behaviour at different states in stress space. New experimental results are presented for two structured clays subjected to complex loading conditions. Each test consists of a defined stress path with intermediate stages of creep. Considerable creep deformations were observed at all anisotropic effective stress conditions. Measured viscous deformations were observed to be related to the stress state, approaching stress path and degree of structure present in the soil. The incremental strain ratio, , measured during constant stress was found to rotate counter-clockwise as a result of drained shearing towards failure for all tests. The experiments also show that for very small stress increments, phenomenological and empirical relations incorrectly predict the development of creep strains with time.

Characterization of heterogeneous creep deformation in vanadium-modified 2.25Cr1Mo steel weldments by digital image correlation

Low alloy ferritic steels are widely used to manufacture critical components that operating at elevated temperatures in the process industry, but a better understanding of the creep responses of their weldments are still needed to provide guidance for the design of high-temperature welded structures. In the present paper, the heterogeneous creep behavior of vanadium-modified 2.25Cr1Mo ferritic steel weldments was investigated by using the digital image correlation (DIC) technique. Three-dimensional plot of creep strain across the weld joint was constructed to intuitively represent the spatio-temporal evolution of creep strain for the base metal (BM), weld metal (WM) and the heat affected zone (HAZ). Combining the analysis of strain contour evolution during creep with the microstructural examination, it is found that creep strain was primarily concentrated in the HAZ, resulting in the creep cavitation at grain boundaries of the coarse-grain bainitic HAZ. In addition, BM exhibited a faster deformation rate than WM during creep. In order to quantitatively describe the heterogeneous creep deformation, a sub-region extensometer method based on the construction of virtual extensometers was proposed to obtain the representative creep strain data of individual regions of weldments. Furthermore, a parameter identification procedure combining a global optimization method (i.e. genetic algorithm) with an initial value determination scheme, was developed to determine the model parameters of BM, WM and HAZ for the hyperbolic-sine creep constitutive model. The results show that the heterogenous creep behavior of the V-mod 2.25Cr1Mo steel weldment can be well characterized by combining the sub-region extensometer method, the hyperbolic-sine creep constitutive model and its genetic algorithm-based parameter identification method.

Time-dependent behavior of rock-like specimen containing multiple discontinuous joints under uniaxial step-loading compression

Rock with multiple discontinuous joints widely exists in rock engineering, and its mechanical properties are complex, which greatly increases the difficulty of engineering design and construction. Time-dependent deformation characteristics and long-term strength evaluation of jointed rock masses remain poorly understood. In this work, the creep experiments of rock-like specimens with multiple discontinuous joints under uniaxial step-loading compression are carried out to explore the influence of joint geometry (rock bridge length, joint length, joint angle, and joint spacing) on creep strain, long-term strength, and failure pattern of specimens with multiple discontinuous joints. The following conclusions are drawn from the test results: 1) The deformation of jointed rock specimens has evident time-dependent effect, and the cumulative creep deformation increases as creep load increases; 2) The strength of jointed rock specimens under loading changes with time, and the ratio of long-term strength and creep peak strength ( σ∞/ σc) of the tested specimens ranges from 41% to 96%; 3) The distribution of initial joints affects the creep fracture modes of rock-like specimens. The rupture of rock-like specimens with different joints distribution is mainly caused by the growth of wing cracks and quasi-coplanar secondary cracks. Three different failure modes are observed from these specimens: i) tensile failure with cracks across the joint plane; ii) shear failure with cracks along the joint plane; and iii) tensile failure with cracks along the joint plane. Based on the principles of damage mechanics and fracture mechanics, a theoretical mechanistic model considering both the closure stage of pre-existing open joints and time-dependent propagation stage of new cracks is established. Considering the influence of joint length, joint angle and joint density, the evolution of creep strain of rock-like specimen with multiple discontinuous joints is analyzed. The theoretical model results agree well with the experimental results, which indicates that the established model can replicate the creep failure process of jointed rock mass. These theoretical and test results help us better understand the effect of multiple joints on the long-term behavior of rock mass.

Features of the kinetics of cyclic elastoplastic deformation diagrams at dwells in cycles and superimposition of variable stresses on them

The goal of the study is determination of the regularities of changes in cyclic strains and related deformation diagrams attributed to the existence of time dwells in the loading modes and imposition of additional variable stresses on them. Analysis of the obtained experimental data on the kinetics of cyclic elastoplastic deformation diagrams and their parameters revealed that in contrast to regular cyclic loading (equal in stresses), additional deformations of static and dynamic creep are developed. The results of the studys are especially relevant for assessing the cyclic strength of unique extremely loaded objects of technology, including nuclear power equipment, units of aviation and space systems, etc. The experiments were carried out on the samples of austenitic stainless steel under low-cycle loading and high temperatures of testing. Static and dynamic creep deformations arising under those loading conditions promote an increase in the range of cyclic plastic strain in each loading cycle and also stimulate an increase in the range of elastoplastic strain due to active cyclic deformation. At the same time the existence of dwells on extrema of stresses in cycles without imposition of additional variable stresses on them most strongly affects the growth of plastic strain ranges in cycles. Imposition of additional variable stresses on dwells also results in the development of creep strains, but their growth turns out to be somewhat less than in the presence of dwells without stresses imposed. The diagrams of cyclic deformation obtained in the experiments are approximated by power dependences, their kinetics being described in terms of the number of loading cycles using corresponding temperature-time functions. At the same time, it is shown that increase in the cyclic plastic deformation for cycles with dwells and imposition of additional variable stresses on them decreases low cycle fatigue life compared to regular loading without dwells at the same stress amplitudes, moreover, the higher the values of static and dynamic creep, the greater decrease in low-cycle fatigue life. This conclusion results from experimental data and analysis of conditions of damage accumulation for the considered forms of the loading cycle using the deformation criterion of reaching the limit state leading to fracture.

Experimental evaluation of localized creep deformation in grade 91 steel weldments

Spatially resolved measurement of localized creep deformation in heterogeneous creep resistant steel weldments is crucial but challenging for lifetime assessments of critical steam components in power plants. In this work, experimental approaches were established to quantitatively evaluate commonly observed localized creep deformation in multi-pass Grade 91 steel weldments. An in-situ digital image correlation (DIC) system was utilized with a creep testing frame to monitor and measure both full-field strain and localized strain accumulation across the weldments during long-term creep testing at elevated temperatures. The in situ DIC method measured not only the creep deformation behavior of the weld metal, heat affected zone (HAZ), and base metal, but also creep strain evolution for each sub-region within the HAZ itself, including coarse-grained HAZ, fine-grained HAZ (FGHAZ), and intercritical HAZ (ICHAZ). The DIC results revealed that local creep strain in the ICHAZ reached up to 90% strain before final rupture, whereas nominal creep strain measured by the standard extensometer of the tested cross-weld specimen was below 10%, indicative of Type IV cracking of the Grade 91 weld. Microstructural analyses revealed that the faster creep degradation/deformation in the HAZ was caused mainly by accelerated matrix grain recrystallization/growth and a reduced pinning effect from the segregated and coarsened precipitates in the FGHAZ and ICHAZ. The ultimate creep rupture occurred in the ICHAZ owing to its lowest creep resistance induced by the largest recrystallized grain size, the lowest fraction of coincidence site lattice, and the lowest local strain energies/dislocation densities.

Mechanism-based modeling of thermal and irradiation creep behavior: An application to ferritic/martensitic HT9 steel

In this work, the creep behavior of HT9 steel in both thermal and irradiation environments is predicted using an integrated modeling framework. Multiple physical mechanisms such as diffusional creep and dislocation climb are incorporated into crystal plasticity calculations using the Visco-Plastic Self-Consistent (VPSC) approach. Climb velocities are informed by mean field rate theory laws in place of empirical power law formulations. More interestingly, the climb velocities explicitly consider the contribution of irradiation-induced point defects, i.e., stress induced preferential absorption (SIPA) effect. The developed expressions are shown to apply under conventional thermal creep and to the more complex irradiation conditions as well. This physically-informed, mechanism-based model is used to simulate the creep strain evolution of HT9 pressurized tubes under various loading conditions. It is demonstrated that the experimental behavior of this material reported in the literature is well described by this theoretical framework. The role of each relevant mechanism is discussed.

Creep Failure and Damage Mechanism of Inconel 718 Alloy at 800–900 °C

The creep behavior and damage mechanisms of 718 alloys were investigated at 800–900 °C in air. The fracture morphology and microstructure evolution were observed by optical, scanning and transmission electron microscope. Besides, the creep damage tolerance (λ) and creep strain evolution curve were also calculated. The results showed that the creep curves of 718 alloys at 800 or 850 °C consisted of primary and tertiary stages, while the steady-state region became apparent at 900 °C. The apparent creep activation energy of 718 alloy was in the range from 446.3 to 491.8 kJ/mol. The alloy presented ductile fracture at 800 °C due to the nucleation, growth and linkage of creep voids. However, the failure of alloys at 850 or 900 °C presented necking to a point due to the microstructure degradation. Further investigations showed the softening of materials and the loss of mechanical performance could be mainly attributed to the coarsening or decrease of strengthening precipitates. Above 850 °C, it was found that γ′ phases would dissolve into matrix and stress promoted the re-dissolution of γ′ phases or led to the break of δ phases. Moreover, the creep strain evolution curves indicated that 718 alloys kept a relative stable state at 800–900 °C when the strain fraction was below 1.

Prediction of Creep Strain for Self-Compacting Concrete by Artificial Neural Networks

Artificial Neural Networks, ANN, technique is a computerized system that is built to simulate the neural networks in the human brain. Throughout the recent couple decades, ANNs had solved with a good degree of success many problems. In the present work, ANN model was developed by SPSS software for estimating creep strain development of self-compacting concrete mixes produced with different types of Portland cement, Type I and Type IL. The independent variables in this model were: age, compressive strength, modulus of elasticity, applied stress, initial strain, water to powder ratio, water to binder ratio, filler to cement ratio, clinker to cement ratio, aggregate size, and slump flow. The used data for model building were local, extracted from the present work. The predictions of the model have been compared to those of an international well-known model, ACI 209 Committee. The comparison revealed the good reliability of the present models in predictions (r = 0.998).

Creep deflection of Wood Polymer Composite profiles at demanding conditions

Durability and low maintenance make Wood Polymer Composite (WPC) profiles popular in decking applications. EN 15534-4 specifies minimum performance levels to guarantee WPC quality. However, despite such quality specifications, occasionally high temperature creep issues are reported. This paper evaluates the creep performance of three commercial WPC decking profile grades. All three WPC grades meet the requirements specified in EN 15534-4. Nevertheless, at slightly more demanding load conditions, some WPC samples fail around the creep deflection limits specified in the standard, and which are supposed safe. Reference outdoor testing in the moderate climate of the Netherlands shows creep deflection rates which are expected to lead to fatal failure of these WPC samples in a couple of years. It is concluded that predictive testing requires insight in progressive creep strain development relative to fatal failure strain level.