Cyclic Creep Behavior Research Articles

Cyclic creep behavior and life prediction of DZ125 alloy under different stress ratios

Abstract In order to study the creep behavior and mechanism of DZ125 alloy under cyclic creep under different stress ratios, cyclic creep experiments are performed under a temperature of 850°C and peak stress of 500 MPa, with stress ratios of 0.1, 0.3, 0.5, 0.7, 0.9, and 1, and the mechanical behavior and microstructure of creep samples under different stress ratios are compared and analyzed. The accuracy of several life prediction models is compared. The results show that cyclic hardening or cyclic softening will occur under different stress ratios due to grain boundary slip and dislocation plugging. According to the results of fracture analysis, as the stress ratio rises, the proportion of creep damage increases, and the fracture mode shifts from transgranular fracture to intergranular fracture. Several life prediction models are selected, and the results show that the FS frequency separation model and M-G model have better life prediction accuracy.

Analytical model for static and cyclic creep behavior of pressure-sensitive adhesives

The increasing interest in adhesives, such as pressure-sensitive adhesives (PSAs), is often held back by uncertainties regarding their long-term behavior, which includes creep behavior. However, little information exists in the literature regarding creep behavior of PSAs. In this paper, a unified phenomenological creep model for PSAs is applied in SLJs for static and cyclic creep, seeking to develop a tool to predict creep behavior of PSAs. The model was validated by comparing the predicted creep curves with experimental data. It captured the three creep phases and predicted the creep curves for different stresses and temperatures, which is uncommon in literature.

A novel fatigue life prediction methodology based on energy dissipation in viscoelastic materials, synergistic effects of stress level, stress ratio, and temperature

This work introduces a new methodology to predict the fatigue life of viscoelastic materials by considering the creep effect on fatigue behavior under the concurrent effects of stress level, stress ratio, and temperature. The model established based on the total amount of energy dissipated during fatigue loading. To estimate the amount of dissipated energy under varying stress ratios and temperatures, two shift factors, ψcyclic(R,T) and ψcreep(R,T), were derived, attributed to the cyclic and creep parts of fatigue loading. These shift factors were subsequently incorporated into defined equilibrium equations to create the relationship between estimated dissipated energy and fatigue life. The input data for the model consisted of the dissipated energy and cyclic creep values obtained from experiments conducted at a reference stress ratio of 0.5 and reference temperature of 20 °C, together with the storage and loss moduli measured from one dynamic mechanical analysis (DMA) experiment in the temperature range of 15 °C–60 °C on a fully-cured epoxy adhesive. To validate the accuracy of the results, the predicted fatigue life at three temperatures of 20 °C, 40 °C and 55 °C, each loaded under three stress ratios of 0.1, 0.5, and 0.9 were compared with the experiments conducted under the same conditions. Almost all predicted results were in good agreement with the experiments, nevertheless; at the stress ratio of 0.9 at 20 °C, due to the significant change in the cyclic creep behavior, the accuracy of the prediction was lower. The developed model was used as a new constant life diagram (CLD) formulation, which afterwards was further developed to plot three-dimensional constant life diagrams (3-D CLDs) in which the constant life surfaces were a synergistic function of stress ratio and temperature.

Influence of anelastic recovery on the cyclic creep behavior of AlCoCrFeNi2.1 eutectic high-entropy alloy

The AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) is regarded as a promising candidate for structural materials in high-temperature equipment, which may experience the cyclic creep and anelastic recovery caused by low-frequency peak-shaving loading. To investigate the influence of anelastic recovery on the microstructure and cyclic creep behavior of AlCoCrFeNi2.1 EHEA, the stress-varying (SV) tests at 800 °C are conducted to introduce the anelastic strain for comparison with the stress-constant (SC) test. The results show that the steady-state creep rate (SSCR) is accelerated with the increase in the cyclic-creep cycle and valley stress in SV tests. The anelastic recovery happened during the valley-stress holding (VSH) stage significantly alters the microstructure in the B2 phase of EHEA under SV conditions. This anelastic-recovery-related microstructural evolution is subsequently elucidated on the basis of back stress theory. Finally, the influence of microstructural evolution on the cyclic creep behaviour is revealed. The findings obtained will enhance the comprehension of high-temperature deformation mechanism of dual-phase EHEAs.

An improved life prediction strategy at elevated temperature based on pure creep and fatigue data: Classical strain controlled and hybrid stress–strain controlled creep-fatigue test

Creep-fatigue (CF) interaction is an important issue for designing the next generation power plants. Thus, strain-controlled CF tests (SCCFTs) and hybrid stress–strain controlled CF tests (HCCFTs) of 9Cr3Co3W1CuVNbB steel were conducted. The cyclic responses and creep behaviors in SCCFTs and HCCFTs were investigated. In addition, an improved CF life prediction method was proposed, and only pure creep and fatigue data were needed. Then, the relaxed stress in SCCFTs and the accumulated creep strain in HCCFTs were predicted. Moreover, the oxidation damage was incorporated into the non-linear interactions. The optimally predictive results of the studied material, Grade 91 steel, and 304 stainless steel were in ±3 error band with strain energy density exhaustion model.

Characterization of cyclic dynamic and creep responses of pure aluminum by instrumented indentation

<p indent="0mm">Dynamic behavior under cyclic loading conditions and creep behavior under quasi-static loading conditions were investigated by instrumented indentation of pure aluminum to reveal the influence of loading conditions (e.g., load, loading rate, and holding time) on indentation responses. The results of cyclic indentation show that, with the increase in the holding time, load, and loading rate of the pre-cyclic loading segment, the effect of creep on the subsequent cyclic indentation can be weakened, and the measured mechanical properties (e.g., contact depth, indentation hardness, and elastic modulus) can reach constant levels more rapidly with the increase in cycle number. The results of the creep tests show that the elastic modulus and indentation hardness decrease with the increase in holding time, the steady-state creep stress exponent obtained by instrumented indentation is independent of load, and a long holding time is not suggested for the creep tests. Indentation hardness and steady-state creep resistance measured within the inner grains are higher than those measured near the grain boundary regions.

A universal constitutive model for hybrid stress-strain controlled creep-fatigue deformation

On account of stress relaxation, the conventional strain-controlled creep-fatigue interaction (CCFI) tests make it difficult to obtain the creep-dominated data. To make up for this deficiency, a novel hybrid stress-strain controlled creep-fatigue interaction (HCFI) loading which is capable of controlling the ratio between fatigue damage and creep damage is developed. Besides, extensive comparisons of two CFI responses give better understanding of the newly proposed HCFI loading. Experimental results show that the cyclic creep behaviors under the two CFI loading conditions are obviously different. The creep strain for HCFI tests accelerates with cycle number and increases with test parameters significantly, while the creep strain decelerates with cycle number during CCFI loadings and it is not sensitive to dwell conditions. Besides, HCFI tests show a pronounced difference in failure life as the test parameters vary. These different phenomena challenge the current constitutive models, and thus, towards developing a simulation-based design methodology for high temperature components, a universal viscoplastic constitutive model based on Walker model is proposed, in which the self-adaptive hardening parameters and dynamic recovery factors are incorporated to capture the distinct cyclic deformation behaviors during HCFI and CCFI loadings. Good agreements between the experimental and simulated results verify the robustness of the proposed model in capturing various cyclic loadings, including strain-controlled cyclic loading (low cycle fatigue and CCFI), stress-controlled ratcheting-creep loading and HCFI loading, of P92 steel and Inconel 718 at different temperatures. Accordingly, apart from a loading profile able to generate wide ranges of CFI damage, this work also gives a universal constitutive model, which may provide a new solution for existing CFI design from loading waveforms to modeling methods.

A simplified method for studying cyclic creep behaviors of deep-sea manned submersible viewport windows

This paper predicts the cyclic creep behaviors of deep-sea manned submersible PMMA viewport windows that experience the cyclic dive, operation and rise stages. The three-element spring-dashpot viscoelasticity/damage model is used to predict the viscoelastic behaviors of PMMA viewport windows at 1–7 km water depths during the dive stage, the creep behaviors during the operation stage, and the creep recovery behaviors during the rise stage. This model has been validated by creep experiments of PMMA materials and viewport windows under 30 MPa stress levels. The fatigue damage behaviors due to variable hydraulic pressure during the dive and rise stages are equivalently represented by the cyclic creep damage behaviors under each approximate constant pressure, which is obtained by dividing these two stages into finite pressure increments. By considering a series of factors including the dive velocity, time, depth and sequence, the cyclic creep behaviors and lifetime of the conical and spherical viewport windows at 1–7 km water depths are explored using finite element analysis. The developed method and numerical algorithm are validated by using the pressure loading-pressure holding (89.5 MPa)-pressure unloading experiments of the PMMA conical viewport window. Results show: 1. for the same dive depth and sequence, the maximum strain during the creep stage, the residual strain and damage at the end of rise stage for the conical viewport window are about twice that for the spherical viewport window with the same thickness and cone angle, and the predicted cyclic creep lifetime for the spherical viewport window is about twice that for the conical viewport window. 2. the fatigue damage is validated to be comparable to the creep damage for two kinds of viewport windows, which produces large effects on the lifetime and load-bearing ability of structures. 3. the cyclic pressure diving to different water depths aggravates the creep and damage behaviors of shells, but the pressure loading sequence corresponding to different intermediate depths (for example, 1 km-3km–5km and 3 km-5km–1km) hardly affects the cyclic creep behaviors of viewport windows.

Experimental investigation of textile structure reinforced composite leaf spring for their cyclic flexural and creep behaviour

This work dealt with the investigation of cyclic flexural strength and time dependent creep behavior of E-glass/epoxy-based different textile structure reinforced (chopped fibers, unidirectional (UD), bidirectional plain woven (2D), and 3D woven solid structures (orthogonal and Interlock) composite leaf spring. All textile structures were developed using a rigid rapier mutibeam weaving machine. Four different 3D woven orthogonal structures with varying binder percentage (3, 7, 10, and 13%) were developed to optimize weaving construction parameters for leaf spring application. The 3D orthogonal reinforced composite leaf spring exhibited improved cyclic flexural and creep performance and 3S1B stuffer-binder combination was found with the highest initial flexural strength, lowest drop in cyclic flexural strength and improved creep resistance. UD based composite leaf spring exhibited comparable properties to 3S1B based composite leaf spring. Composite leaf springs were further analyzed for their surface damage (tensile side) due to cyclic flexural loading. Structural variation (binder percentage) of 3D structure reinforced leaf spring has a significant influence on their failure (surface damage) morphology. 3D woven composite leaf spring with minimum binder tow percentage was found to be a potential material for automotive leaf spring from cyclic flexural strength, creep resistance, stiffness retention and failure morphology point of view.

Mechanism-oriented characterization of the anisotropy of extruded profiles based on solid-state recycled EN AW-6060 aluminum chips

Because of the great potential to reduce the amount of energy, the direct recycling of scrap like aluminum chips by hot extrusion is a hopeful alternative to the usual remelting process. Previous investigations showed that the chips, which are encased by oxide layers, are elongated due to the extrusion process. Therefore, the aim of this study is to test to what extend anisotropic properties, in analogy to fiber-reinforced materials, can be determined. The mechanical properties of cast-based and chip-based specimens with orientations of 0°, 30° and 90° to extrusion direction were characterized by means of mechanical quasistatic and cyclic experiments. It could be shown that quasistatic properties of the 0° orientation are highest for chip-based specimens, whereby the differences to the other orientations are slight. On the other hand, large differences in cyclic creep behavior between the orientations as well as in damage behavior could be determined.

Cyclic Creep Behavior and Modified Life Prediction of Bainite 2.25Cr-1Mo Steel at 455 °C

Uniaxial static and cyclic creep tests were carried out on bainite 2.25Cr-1Mo steel at 455 °C. Effects of the unloading rate from 0.6 to 39 MPa/s and valley stress duration from 0 to 30 min on the cyclic creep deformation behavior were discussed. The results indicated that the fracture behavior under static and cyclic creep conditions showed a consistent ductile mode. The strain accumulation rate under cyclic creep was significantly retarded as compared with static creep due to the presence of anelastic recovery which was apparently influenced by the unloading conditions. For cyclic creep tests, the unrecoverable strain component determined by a systematic classification of the stress–strain curve was the true damage. A modified life prediction method proposed based on the unrecoverable strain component presented a good life prediction for cyclic creep.

The effect of stress state on rafting mechanism and cyclic creep behavior of Ni-base superalloy

A three-stepped specimen of Ni-base superalloy was designed in the paper to conduct cyclic creep tests under different stress levels. The experiment aimed at studying the effect of loading cyclic numbers and the stress state on the rafting mechanism and the cyclic creep behavior. The design of three-stepped specimen can help to simultaneously analyze the extent of microstructure evolution under different stress states, which included uniaxial states in gauge sections and multiaxial stress states in beveled sections. Metallographic observations revealed that both the rafting behavior and topological inversion behavior of γ/γ′ microstructure occurred during the cyclic creep tests. The specific connectivity number of the γ′ phase NA (γ′) was introduced to assess the extent of these microstructure evolutions, which was found to be related to the loading cyclic numbers and stress levels. Then a parameter of normalized cyclic number Tr was proposed to consider the contribution of these two factors. The extent of microstructure evolution was observed increasing with the normalized cyclic number. And the relationship between evaluation parameters NA (γ′) and Tr was proved to follow an asymptotic equation. The finite element model that established in the paper assisted to qualitatively analyze the rafting or topological inversion process. It successfully predicted the cyclic lives and acquired the normalized cyclic number Tr. The magnitude and distribution gradient of principal stress field in finite element model effectively account for rafting behavior under different stress states.

Experimental and numerical investigation of cyclic creep and recovery behavior of bovine cortical bone

An experimental and numerical study of the creep behavior of the bovine metatarsal cortical bone is presented. The experimental procedure included the multiple cycle tensile creep-recovery tests at eleven creep stress levels between 20 and 120 MPa. The threshold values of stress above which viscoplastic strain and damage start to accumulate are determined. Furthermore, based on the experimental data, a one-dimensional constitutive model has been developed. We demonstrate that Abdel-Tawab's viscoelasticity/damage model combined with the viscoplastic model proposed by Zapas and Crissman can predict the nonlinear behavior of cortical bone tissue subjected to creep-recovery loading conditions. Within the framework of numerical investigations, an efficient algorithm for the integration of the proposed constitutive model at the material point level is derived and implemented into the finite element software ABAQUS using user subroutines. The computational algorithm shows a good capability to describe the tensile behavior of bovine cortical bone for the specific mechanical conditions analyzed.

Cyclic creep behaviour of two-phase Ti-6Al-2Mo-2Cr alloy

One of the important criteria for selection titanium alloys for discs and blades of turbine engine compressor is their fatigue and creep strength at room and elevated temperature. Fatigue and creep properties of two-phase titanium alloys show strong dependence on microstructure, especially morphology of the α and β phases which can be controlled to certain extent by proper selection of hot working and heat treatment conditions. Quantitative description of two-phase titanium alloys behaviour under loading and environmental conditions leading to combined creep and fatigue processes has been always very challenging task due to large number of factors affecting deformation and fracture behaviour of the material. In the course of the research cyclic creep behaviour of Ti-6Al-2Mo-2Cr alloy (VT3-1) was investigated and compared to low-cycle fatigue and static creep properties at the temperature of 450°C. Microstructure of the alloy was varied by means of the heat treatment. Constant load tensile creep tests were carried out. Tension-tension cyclic loading was applied at the constant stress ratio with and without hold time at maximum load. The effect of test parameters on the creep-fatigue life at elevated temperature was estimated. Characteristic features of fracture surfaces were identified by scanning electron microscopy methods.

Experimental investigation of influence of alternating cyclic loadings on creep behaviors of sandstone

Cyclic stress variation induced by mining activities has a significant effect on the stability of surrounding rock and mining pillar in deep underground engineering. The influence of alternating cyclic loading and unloading tests on creep behavior of sandstone was investigated to explore the mechanism of the effect of the cyclic stress variation on the stability of rock. Firstly, a series of uniaxial constant strain rate and multi-step monotonic creep tests of sandstone were carried out to determine the maximum high loading stress and the minimum low loading stress in the alternating cyclic creep tests. Then the alternating cyclic loading and unloading creep tests were conducted to investigate the effects of loading paths and loading histories of cyclic high stress creep and low stress creep on the creep behavior of sandstone. Acoustic emission events, axial strain rate and lateral strain rate were also analyzed. The experimental results show that loading paths and loading histories of high stress creep and low stress creep have a significant influence on the alternating cyclic creep behavior of sandstone. The loading paths could accelerate the damage in the rock sample, which can be demonstrated by much larger plastic strain occurring in the damaged sample and larger viscous strain in the creep behavior of less damaged sample. The loading histories could increase viscous strain and plastic strain in the damaged sample. The maximum and the minimum loading stresses determined by the unstable crack threshold provide the basis of the unstable cracks growth in rock sample. The work presented in the present paper is helpful to understand the mechanical behaviors of rock mass under cyclic underground mining activities.

Mechanism-oriented characterization of the load direction-dependent cyclic creep behavior of the magnesium alloys Mg-4Al-2Ba-2Ca and AE42 at room temperature

Due to their low density and high strength-to-weight-ratio magnesium alloys are very attractive for lightweight construction. Especially creep-resistant magnesium alloys can also be applied in the engine area of automobiles and therefore offer a high potential for weight reduction. For a safe and efficient use in such applications at elevated temperatures, a fundamental knowledge of the materials properties under cyclic loading is necessary. The aim of this study is the comparative investigation of the cyclic creep (ratcheting) behavior of magnesium alloys Mg-4Al-2Ba-2Ca and AE42 under different load ratios. Fatigue tests in tensile and compressive load ratios have been performed at room temperature. Fatigue strengths were estimated in load increase tests based on structure-sensitive material reactions. The results were correlated with fatigue lifetimes determined in constant amplitude tests. Furthermore, characteristic deterioration mechanisms were evaluated using a dedicated custom sensor system as well as by concomitant microstructural investigations.

Two-time-scales and time-averaging approaches for the analysis of cyclic creep based on Armstrong–Frederick type constitutive model

The aim of this paper is the analysis of inelastic behavior under periodic cyclic loading regimes for materials showing recovery effects. Starting with the Armstrong–Frederick type model and applying the two-time-scale asymptotic technique, the constitutive equation for the mean inelastic strain rate and the evolution equation for the mean backstress variable are derived. The advantage of the presented technique is the closed analytical form of the solutions such that the influence of the loading profiles on the resulting slow cycle-by-cycle strain accumulation can be analyzed explicitly. To validate the derived equations, the original constitutive model is integrated numerically by applying the time-step procedure for various cyclic loading profiles. Numerical examples are presented to illustrate cyclic creep behavior of X20CrMoV12-1 heat resistant steel.

Accelerated material data generation for viscoplastic material models based on complex LCF and incremental creep tests

Viscoplastic material models are quite important for realistic characterization of deformation behaviour of high-temperature components. However, their material parameter adjustment is expensive and usually requires extensive material testing. The high-temperature deformation behaviour of cast iron GJS X SiMo 4.1 was determined by accelerated complex material testing methods in order to establish a cost-effective identification of viscoplastic material model parameters. Incremental creep tests with mixed-load and temperature steps, together with relaxation and stepped-tensile tests, were carried out in order to determine the stress-dependent creep strain rate at characteristic temperatures and stresses. The cyclic plastic deformation behaviour was studied with complex low-cycle fatigue (C-LCF) evaluations at varying strain rates, amplitudes and temperatures. Thereafter, the results were employed to identify material model parameters of a Chaboche-type viscoplastic material model, which was applied to represent the cyclic deformation and creep behaviour with a correct representation of strain-rate effects at low strain rates.

Insights into dynamic strain aging under cyclic creep with reference to strain burst: Some new observations and mechanisms part-II: Microstructural aspects

Cyclic creep behavior of 316LN austenitic stainless steel (SS) was investigated at 823K at different combinations of mean stress (σm), stress amplitude (σa) and stress rate. Characteristic strain bursts were observed being attributed to a pronounced influence of dynamic strain aging (DSA). Detailed microstructural investigation carried out through transmission electron microscope (TEM) revealed that dislocation substructure evolving under a process of strain burst during cyclic creep mainly consists of planar deformation bands. The number density of bands was found to be strongly sensitive to σm- σa-stress rate combination employed. An important substructural feature found in this study was the formation of microtwins. Either planar slip or twinning was found to dominate the substructure depending on the loading combination, which was demonstrated through a dislocation distribution map. Dislocation substructure was further correlated with evolution of surface relief studied through atomic force microscopy (AFM) and field emission gun-scanning electron microscopy (FEG-SEM), which depicts the formation of slip markings and nucleation of cracks from persistent slip markings during the course of a strain burst. Finally, well-known theoretical models explaining the mechanism of DSA during tensile deformation were suitably modified for load-controlled scenario and the origin of strain burst as a function of σm or stress rate was explained based on the same. Dislocation density measurements were carried out for specimens undergoing strain burst during cyclic creep, which was utilized for reconstituting the models.

Insights into dynamic strain aging under cyclic creep with reference to strain burst: Some new observations and mechanisms. Part-1: Mechanistic aspects

Investigations on the cyclic creep behavior of 316LN austenitic stainless steel (SS) are carried out at 823K as a function of mean stress (σm), stress amplitude (σa) and stress rate. Strain burst was found to be a common observation at 823K, which is attributed to the periodic locking and unlocking of dislocations in regular intervals under pronounced influence of dynamic strain aging (DSA). Three distinct stages are identified in the cyclic creep curve based on total accumulated strain or cyclic life. Strain accumulated or life spent in these stages is found to be a strong function of the loading condition (σm – σa – stress rate combinations). Critical strain and threshold cycles for the occurrence of a strain burst were found to have a negative mean stress dependence for fixed σa and stress rate and positive stress rate dependence for fixed σa and σm. Frequent appearance of strain bursts towards the end of life results in excessive geometric softening with clear implications on the failure mode which changes from fatigue failure to creep (necking) in specific cases. Dominant damage modes (cyclic hardening/mean stress dependent hardening or geometric softening/fatigue softening) observed under different loading conditions in cyclic creep in particular where strain burst is prominent is illustrated through a map. Characteristic steps in the form of “spider-web” patterns marking sudden increase in the rate of Stage-II crack propagation is noticed on the fracture surface which is a signature of strain bursts occurring during cyclic creep.