Creep Stress Relaxation Research Articles

Lattice Discrete Particle Modeling of concrete coupled creep and shrinkage behavior: A comprehensive calibration and validation study

Abstract Concrete aging and deterioration are the result of a complex combination of multiple interacting time-dependent phenomena. Two major ones are shrinkage and creep, which affect and in turn are affected by both environmental conditions and any additional damage mechanisms like Alkali-Silica Reaction (ASR), corrosion, freeze/thaw and others. While many modeling approaches can represent the overall uncoupled contribution of these phenomena, they can not capture their coupled effects especially when large internal self-equilibrated stresses are produced like in the case of ASR free expansions. Macroscopic continuum models can not capture the induced meso-scale creep deformation and stress relaxation due to these conditions. Recently, the Lattice Discrete Particle Model (LDPM) was extended to account for coupled creep, shrinkage and ASR deformations and has shown noticeable success in capturing these meso-scale phenomena. The model uses a multi-physics formulation to evolve temperature, humidity, cement hydration, and alkali ion diffusion in both space and time at the meso-scale between large aggregate pieces. Creep and shrinkage deformations are formulated based on a discrete version of the Micro-prestress Solidification theory. It was shown that using this detailed multi-scale multi-physics framework is essential for the accurate prediction of both plain and reinforced concrete long-term behavior. The major challenge associated with using this comprehensive formulation is the appropriate calibration of its multiple parameters. This requires multiple experiments to be performed on the same material, which is usually not the case in many experimental campaigns. Till today, no established recommendation existed concerning the optimal calibration sequence dependent on the available data. This paper, presents a detailed procedure for the calibration of the hygro-thermo-chemical and creep/shrinkage parameters in an uncoupled manner. The procedure is thoroughly investigated using 9 different experimental datasets that vary in complexity and level of detail. Results show the effectiveness of the procedure and the capabilities of the modeling framework.

Experimental and modeling investigation of the creep-fatigue interactive deformation behavior of PM super alloy FGH96 at evaluated temperature

In this paper, the fatigue tests of powder metallurgy (PM) FGH96 were performed at 650 °C with four different types of dwell to study its cyclic mechanical behavior and creep-fatigue interactive deformation behavior. As shown in test results, PM FGH96 shows obvious feature of cyclic softening. Load level and dwell time are found to be important factors affecting the velocity and degree of cyclic softening. Some regular patterns are clearly displayed in the curves of creep deformation and stress relaxation, which is speculated to affect the characteristics of FGH96's sensitivity to dwell. Based on the test data and analysis, a practical cyclic plasticity-creep model was established by combining Chaboche model and simplified θ-projection model, and its capability of describing the deformation of creep-fatigue interaction behavior has been proved. The investigation of deformation and cyclic properties will be useful for instructing the establishment of creep-fatigue interactive life predicting model in the future.

Creep and stress relaxation behavior for natural cellulose crystal of wood cell wall under uniaxial tensile stress in the fiber direction

We investigated the temporal changes in creep and stress relaxation behavior in both microscopic crystalline cellulose and macroscopic strain of wood specimen using Japanese cypress (Chamaecyparis obtusa Endl.) to understand the viscoelastic properties of wood cell walls. Specimens 600 µm in thickness were observed by the X-ray diffraction and submitted to tensile load. The crystal lattice strain of (004) plane and macroscopic strain of specimen were continuously detected during creep and stress relaxation tests. It was found that the creep compliance based on macroscopic strain showed a gradual increase after instantaneous deformation due to loading and then the parts of creep deformation remained as permanent strain after unloading. On the other hand, crystal lattice strain showed a different behavior for macroscopic strain; it kept a constant value after instantaneous deformation due to loading and then increased gradually after a certain period of time. These differences between macroscopic and microscopic levels were never found in the stress relaxation tests in this study. Relaxation modulus at the macroscopic level only showed a decreasing trend throughout the relaxation process. However crystal lattice strain kept a constant value during the macroscopic relaxation process. In addition, the microfibril angle (MFA) of wood cell wall has a role of mechanical behavior at microscopic level; crystal lattice strains were smaller with increasing MFA at both creep and relaxation processes. Creep compliance and stress relaxation modulus at the macroscopic level decreased and increased with increasing MFA, respectively. Our results on the viscoelastic behavior at microscopic level evidenced its dependency on MFA.

Study of precipitation-assisted stress relaxation and creep behavior during the ageing of a nickel-iron superalloy

To study the stress relaxation and creep behavior of a nickel-iron base superalloy during the ageing process, solution annealed microstructures were subjected to loading experiments at various temperatures. Neutron diffraction (ND) measurements were carried out during the loading process to record the elastic lattice strain response of the γ, γ″ and γ′ phases. The results show that over time, increased length contraction occurred at all three temperatures and the sample tested at 720 °C experienced the most significant shrinkage. During the precipitation of the γ″ phase, the chemical composition of the γ matrix will change as the niobium atoms migrate to the form γ″ precipitates, which directly contributes to a change in the lattice parameter of the matrix and governs the samples' volume shrinkage. The creep strain at 720 °C was positive, while negative creep could be seen at 500 °C and 620 °C. The observed negative creep at 500 °C and 620 °C was caused by the samples' shrinkage, and the creep strain at 720 °C was positive because creep plastic strain is relatively large and thus dominates the measured creep deformation. The stress relaxation results exhibited a significant increase with the increase in test duration at 500 °C, 620 °C and experienced a normal decrease at 720 °C. Stress increase during the relaxation tests is a manifestation of the phenomenon of “negative creep”. The precipitation induced volume shrinkage was found to be the key reason for the observed negative creep and abnormal stress relaxation.

Influence of nanocellulose on mechanics and morphology of polyvinyl alcohol xerogels

Xerogels are porous networks of crosslinked polymers that are useful for biomedical applications such as drug delivery, scaffold engineering, tissue regeneration, cell culture and wound dressing. However, inferior mechanical properties curtail their applications to a considerable extent. Nanocellulose fibers and crystals are often added into the polymer matrix to improve their mechanical strength. Here, nanocellulose in the mass ratios of 7%, 13% and 18% are loaded into polyvinyl alcohol (PVA) matrix followed by thermo-morpho-mechanical characterization. With increase in nanocellulose content, thermal degradation occurs at a lower temperature. It is observed that addition of higher quantity of nanocellulose crystals leads to the formation of weak cellulose-rich regions causing xerogel rupture. This is predominantly observed for xerogel loaded with 18% nanocellulose crystals. Similarly, addition of higher quantity of nanocellulose fibers increase brittleness of the xerogels causing fracture. This is predominantly observed for xerogel loaded with 18% nanocellulose fibers. Creep strain and stress relaxation is observed to decrease with addition of nanocellulose loading owing to molecular chain restriction and polymer chain immobility.

Effect of cold creep on fatigue crack growth behavior for commercial pure titanium

In this paper, the effect of the cold creep on fatigue crack growth rate are investigated by experiment considering wide ranges of load amplitudes and load ratios with or without certain degrees of dwell time. To further illustrate the cold creep influence, a series of finite element results are carried out. Finally fracture morphology analyses are also made to explain related crack growth behavior. Results show that fatigue crack growth rate either increases or decreases with certain degrees of hold load and dwell time imposed, depending on the values of the load amplitude and load ratio. This finding is the same as that in the fatigue life. Discussion shows that the effect of creep on fatigue crack growth can be attributed to the competition of the creep stress relaxation and creep strain. A process of inhibiting and promoting crack growth is performed. The fatigue-creep interaction diagram based on the analyses is depicted, by which the interaction between cold creep and fatigue can be well understood. Finally further studies and discussion are carried out by fracture morphology analyses from the dislocation pile-ups, which is in accordance with the finite element results from local creep behavior. This research will provide support to the clear understanding of the cold creep effects on the fatigue crack growth behavior.

Effect of the hardening rules on the creep age forming prediction of 7050 aluminum alloy with experimental verification

The creep age forming (CAF) has been used in the aerospace sector due to its attractive characteristics that allows producing a component with low residual stress. The process has been studied from the finite-element simulations which are used mainly to predict the springback. However, to accomplish the simulation, it is necessary to set the CAF constitutive equations in the finite-element software. In addition, it is also necessary to define the hardening rule which is applied to determine the creep strain. This work aims to investigate CAF applying the finite-element analysis with the time-hardening rule and strain-hardening rule and thus predicting creep strain, stress relaxation, and springback. The finite-element simulations were accomplished in dies with single and double curvatures and the blank’s material was the alloy AA7050. Furthermore, the Marin–Pao model was implemented in the MSC.Marc software through a user subroutine. This model was fitted to the creep experimental curves and it generated good agreement with the experimental data. The results of the simulations that used the time-hardening rule were similar to the strain-hardening rule, and therefore, if it had been chosen a hardening rule, it would not have generated a significant impact in the CAF simulation results. At the end, the simulated springback was compared to the experimental springback from the literature and the percentage error ranged from 0.46% to 15.33% that indicate the proximity with the literature data. Moreover, other experimental validation was performed, and when compared to the results of this methodology, the calculated error in springback was 6.3%.

Effect of elastic follow-up and ageing on the creep of an austenitic stainless steel

Elastic follow-up is a mechanical boundary condition lying between constant load and constant strain control. It exists in many engineering components operating at high temperature and can result in dramatically different creep stress relaxation and strain accumulation rates in a localized region of a component. We have performed creep tests under constant load, constant strain and elastic follow-up control on an aged (additional 22,000 h) 316H austenitic stainless steel after service in a nuclear power station for 65,000 h. Primary and secondary forward creep models with parameters derived from the constant-load data were able to describe constant-load creep adequately, but not able to predict stress relaxation and elastic follow-up. We show that this is because ageing has increased the constant-load creep strain rate significantly but has no effect on stress relaxation creep strain rate. Ageing promotes the formation of ferrite/chi phase at grain boundaries which are preferential sites for creep cavitation under load control. However, creep cavitation is less likely under constant strain and elastic follow-up control because a high creep strain rate, large creep strain and stress cannot coexist under these boundary conditions.

The Dynamic Mechanical Analysis of Highly Filled Rice Husk Biochar/High-Density Polyethylene Composites.

In this study, rice husk biochar/high-density polyethylene (HDPE) composites were prepared via melt mixing followed by extrusion. Effects of biochar content and testing temperature on the dynamic mechanical analysis (DMA) of the composites were studied. Morphological analysis of the rice husk biochar and composites were evaluated by scanning electron microscopy (SEM). The results showed that biochar had a positive effect on dynamic viscoelasticity, creep resistance and stress relaxation properties of the composites, but the creep resistance and stress relaxation of the composites decreased with the increase of temperature. SEM analysis showed that HDPE components were embedded in the holes of the rice husk biochar, and it is believed that strong interaction was achieved.

Modeling of creep and stress relaxation of the nickel‐base alloy NiCr20TiAl at isothermal and non‐isothermal loading conditions

AbstractThe design and dimensioning of new as well as the assessment of operating high‐temperature components in service require a precise prediction of creep and stress relaxation. The increasing share of renewable energies forces fossil‐fired power plants for increasing numbers of start‐ups and shut‐downs. Consequently, transient loading conditions need to be taken into account. In order to meet this demand, non‐isothermal creep equations are necessary, which enables a consistent prediction of creep strain and stress relaxation in a wide range of temperatures and stresses. In this paper, an approach for the visco‐plastic modeling of creep and stress relaxation for non‐isothermal loading conditions is presented. The strain portions creep, “negative creep” and initial plasticity, occurring at elevated temperatures are described by temperature‐dependent phenomenological equations. Within this paper, the adjustment of the parameters is based on a wide database of hot tensile tests, creep and annealing experiments. The nickel‐base alloy NiCr20TiAl has been examined in a temperature range from 450 °C to 650 °C. The developed material models have been successfully validated with isothermal and non‐isothermal relaxation experiments. Further, the recalculation of a staged relaxation test demonstrates the capability of the defined material laws in a wide stress range under isothermal and non‐isothermal loading conditions.

Influence of thermal ageing on the creep behaviour of a P92 martensitic steel

It is well established that thermal ageing reduces the life of engineering components operating at elevated temperature. However, there is still not an accurate approach to estimate the change in the service life as a result of thermal ageing of the material. In this study, the effect of accelerated thermal ageing, for a thousand hours prior to creep testing at a temperature of 650℃ for a martensitic P92 steel has been compared to un-aged steels. The effect of thermal ageing on a primary-secondary stress dependent creep model has been explored, by conducting a set of uniaxial creep experiments on both un-aged and aged P92 steel. It was observed that thermal ageing enhanced the creep deformation for a narrow range of stresses, and systematically reduced the creep rupture life. These applied for conditions by up to a quarter of the original life. Results from creep crack growth tests for similarly aged material reduced incubation time and accelerated the crack growth rate. However, creep stress relaxation tests revealed no evidence of the influence of thermal ageing on rate of relaxation.

Comparative Study of Creep and Relaxation Characteristic of Serrate Structure Surface

The creep and stress relaxation tests of dentate discontinuity poured by cement mortar are carried out by using biaxial creep machine under the same normal stress and shear stress levels. According to the creep and stress relaxation test results: If don't consider the instantaneous deformation of creep curve, there are many similarities of the creep curve and stress relaxation curve, but the trend of the creep curve was opposite to that of the stress relaxation curve; Creep equation and relaxation equation of Burgers model is selected to fit the creep curve and stress relaxation curve, the fitted curves agree well with the experimental data and correlation coefficient is higher. The values of G1 of the creep equation and relaxation equation are close, but the values of G2, η1 and η2 of the creep equation is one magnitude order higher compared with that of the relaxation equation, so it is not simply considered that creep and relaxation are equivalent.

Preparation, optimization and property of PVA-HA/PAA composite hydrogel

PVA-HA/PAA composite hydrogel is prepared by freezing-thawing, PEG dehydration and annealing method. Orthogonal design method is used to choose the optimization combination. Results showed that HA and PVA have the maximum effect on water content. PVA and freezing-thawing cycles have the maximum effect on creep resistance and stress relaxation rate of hydrogel. Annealing temperature and freezing-thawing cycles have the maximum effect on compressive elastic modulus of hydrogel. Comparing with the water content and mechanical properties of 16 kinds of combination, PVA-HA/PAA composite hydrogel with freezing-thawing cycles of 3, annealing temperature of 120°C, PVA of 16%, HA of 2%, PAA of 4% has the optimization comprehensive properties. PVA-HA/PAA composite hydrogel has a porous network structure. There are some interactions between PVA, HA and PAA in hydrogel and the properties of hydrogel are strengthened. The annealing treatment improves the crystalline and crosslinking of hydrogel. Therefore, the annealing PVA-HA/PAA composite hydrogel has good thermostability, strength and mechanical properties. It also has good lubrication property and its friction coefficient is relative low.

Interface diffusion-induced creep in metal matrix particulate composites under triaxial loading

Analytical solutions for interfacial diffusion-induced creep rate and stress relaxation in metal matrix particulate composites are developed under triaxial loading. Based on the Eshelby inclusion theory, the driving forces for interface diffusion and interface slip are obtained, respectively, and the relationship between them is clarified. The effects of the triaxial loading, the volume fraction of the inclusion, and the modulus ratio of the inclusion and matrix on the creep rate and stress relaxation are analyzed. In addition, the error caused by the scale effect is corrected by fitting the finite element results with the nonlinear least square method. The present solutions provide an important reference for various engineering applications of metal matrix particulate composites.

근막이완요법(myofascial release)이 Ergometer 운동 후 심박변이도와 대퇴직근 경직도에 미치는 영향

The purpose of the study is to invastigate the effects of myofascaial release on heart rate variability and stiffness of rectus femoris after ergometer exercise. In a cross-over design, 9 young(22.75±1.89years) healthy males performed two incremental 70% HRR cycling bouts, and the neither 10 min of myofascial release or passive rest. The parameters of heart rate variability were obtained from both time-and frequency domain analysis of 5min ECG recording at rest, immediately after exercise and recovery 10min. Standard deviation of the node to node interval(SDNN) and root mean square of successive differences(RMSSD) and the low and high frequency components were calculated as normalized units. In order to measure the mechanical properties like muscle stiffness and elasticity, MyotonPro(Myoton AS ,Tallinn, Estonia) was applied to muscle belly of rectus femoris muscle at rest, immediately after exercise and recovery 10min. From the Myoton measurement, Five indexes (Frequency, Logarithmic Decrement, Dynamic Stiffness, Creep and Mechanical Stress Relaxation) were obtained. The results are as follows: 1. Analysis of HRV revealed a significant increase in sympathetic activity and a significant decrease in parasympathetic activity following ergometer exercise with no significant difference between both groups. 2. The muscle tension and stiffness increased significantly while the elasticity decreased immediately after exercise while no significant effect of myofascial release on mechanical properties indexes was found during recovery period. The results represent that these application of MR technique might influence personal feeling and none-specific recovery from muscle fatigue without any statistically significant change of muscle mechanical properties and heart rate variability.

Creep deformation and stress relaxation of a martensitic P92 steel at 650 °C

This paper develops methods to predict creep stress relaxation in the presence of combined boundary conditions and explores the influence of primary-secondary stress dependent creep properties on predictions for a martensitic P92 steel at temperature of 650°C. A series of forward creep and elastic follow-up experiments have been conducted. A summary is provided of empirical creep equations for forward creep and creep stress relaxation (elastic follow-up) tests, including the link to the experimental procedure. The creep stress relaxation tests have been performed with two rigs to give elastic follow-up factors of 1.17 and 1.7. Both time hardening and strain hardening approaches have been considered where the strain hardening model provided more accurate predictions compared to time hardening; except at relatively low stress levels. The difference between stress relaxations predicted using strain hardening and time hardening approaches are considerable. The model predicts the creep stress relaxation accurately in the early stage of relaxation, indicating that the majority of stress relaxation occurs where primary creep needs to be taken into account. This study highlights the importance of stress dependent creep model to predict stress relaxation, especially with high level of initial residual stresses.

Viscoelastic braided stent: Finite element modelling and validation of crimping behaviour

Braided stents are fabricated at their relaxed dimensions and need to be crimped in order to fit into the catheter for deployment. Once the covering sheath is removed, the braided stent self-expands at the diameter of the diseased lumen, the radial force restoring the luminal patency. The mechanical properties of the viscoelastic monofilament used in the fabrication of the braided stents are analysed. The monofilament is subjected to both uniaxial tensile and extensional creep testing, with the purpose of predicting the material's time dependent characteristics. An accurate finite element model is required to enable correct estimation of the mechanical behaviour of braided stents subjected to crimping. In this report, Abaqus (v6.14) was used to develop a linear viscoelastic constitutive material model, where the creep compliance function and stress relaxation function were derived from experimental data. The material model was validated by performing simulation of radial compression for a braided stent and a comparison between the experimentally evaluated radial forces of the braided stent and the finite element prediction is presented. The regression analysis performed for the viscoelastic material model led to an R-squared value of 94.4%, showing a very good agreement between the predicted values and the experimental test data.

Identification of the Hereditary Kernels of Isotropic Linear Viscoelastic Materials in Combined Stress State. 2. Proportional deviators

The relationships between the hereditary and creep kernels are established. The hereditary kernels define the scalar properties of isotropic linear viscoelastic materials in a combined stress state. The creep kernels are obtained in uniaxial-tension and pure-torsion tests. The constitutive equations are chosen so as to meet the hypothesis of proportional deviators. The problems of analyzing the creep deformation and stress relaxation of thin-walled tubular specimens under combined tension and torsion are solved and tested experimentally

IMPROVING THE RELIABILITY OF THE PISTON OF HIGH-SPEED DIESEL BOOSTED

The problem of a piston reliability ensuring in the high-power diesel engine service and specifically the tear in particular areas of the piston skirt is discussed in the paper. The creep process and stress relaxation simulation for aluminium silicon alloy Al-25 is performed. The alloy creep threshold in a short-time term is ascertained. The material creep area on the piston surface is determined. The approach for tear elimination in the investigated area is suggested.

Dependence of creep age formability on initial temper of an Al-Zn-Mg-Cu alloy

The initial temper of the material may directly affect the whole creep age forming (CAF) process. In terms of creep deformation and stress relaxation, using the constant-stress creep aging and constant-strain stress relaxation aging tests, the relationship between initial temper and CAF formability is investigated for an Al-Zn-Mg-Cu alloy at 165°C for 18h. Three tempers are selected as the initial tempers in CAF, viz., solution, retrogression and re-solution. The CAF formability of this alloy with initial temper of retrogression is the best, and the creep strain of the retrogression tempered specimen after creep aging of 18h is about 1.21 and 1.34 times than that of the solution and the re-solution tempered specimens, respectively. The calculated stress exponents of this alloy with three initial tempers range from 7.3 to 9.5, indicating that the CAF of this alloy is mainly controlled by the dislocation creep. The various formability for three initial tempers are attributed to different inhibitions of the transgranular precipitates on the dislocation movement. For the retrogression temper, the initial fine and uniformly distributed precipitates are seriously coarsened after 6h of CAF, which minimally inhibit the dislocation movement. While, for the re-solution temper, the fine precipitates are re-precipitated in the matrix of the alloy, which observably hinder the dislocation movement and lead to the worst formability.