Cyclic Creep Research Articles

Damage Accumulation Kinetics in Heat-Resistant Steels at the Plane Stress State Under Cyclic Loading at Different Temperatures

Experimental results are presented for damage accumulation in 10GN2MFA and 15Kh2MFA steels studied with the LM-hardness method at different stress states under cyclic loading at room and elevated temperatures that are consistent with the service conditions of structure elements. Damage accumulation kinetics is examined at primary and secondary creep stages in view of the multiaxial stress state and creep strain effects on the homogeneity coefficient. A higher level of initial strain on cyclic creep gives rise to more intensive damage accumulation in the metal, leading to the earlier onset of fracture in comparison with the smaller initial strain. Experimental critical homogeneity coefficients are given that correspond to the plastic strain stability loss when the stable deformation of the metal with its further fracture is affected by the reduction in the cross-section area and hardening modulus. The rate of homogeneity change in examined steels at different ratios of principal cycle stresses on cyclic creep is evaluated. The significant effect of a stress state at equilibrium between strain hardening and softening of the structure (specimen) metal at the secondary creep stage, is demonstrated. The effect of a stress state and temperature on damage accumulation kinetics of examined materials, in particular on the useful life of the metal on cyclic creep is established.

The Method of Estimating the Durability of Structural Materials from the Effects of Cyclic Loads

The article deals with the technique of predicting the durability of structural materials under the influence of low-cycle loads at different loading trajectories in a two-dimensional stress space at a flat stress-strain state. The use of this technique significantly reduces the number of time-consuming and expensive experiments while maintaining the quality of the results. The technique is based on the method of processing acoustic emission signals when the acoustic signals are allocated with a fractal dimension of the attractor 1≤ D2att ≤6 on a steady-state cyclic creep that reflects the accumulation of hazardous defects in the structural material. To predict the durability at a certain type of stress-strain state, but at any form of a cycle of low-cycle loading by the proposed method, it is enough to have the results of experiments at the maximum intensity of stresses, the minimum intensity of stresses and the saw-tooth form of the cycle of stress intensity change.

Thermomechanical properties and fatigue life evaluation of SnAgCu solder joints for microelectronic power module application

In this paper, a new constitutive model of SnAgCu lead-free solder, which covered the low-temperature and elevated-temperature creep properties, was constructed according to the thermomechanical property test under a wide range of temperatures (−40 to 120°C). This model was then implemented to the finite element model for simulation of the cyclic creep deformation of Sn3Ag0.5Cu solder in a chip inside the microelectronic power module (MPM). The accumulated creep strain of the solder layer was calculated and used to predict the thermo-mechanical fatigue life of the MPM. The applicability of the life prediction method was evaluated through a cross-check of the present results with that obtained by a double power model in the literature, as well as the thermo-mechanical fatigue test results. It was found that the thermo-mechanical fatigue lives, estimated by the accumulated creep strain and creep strain energy density from the established hyperbolic sine power law, are closer to the test results than that estimated by the double power law. The presented model can be seen as an improvement on the existing constitutive models of SnAgCu lead-free solder.

Laboratory investigations on the mechanical properties of cement treated RAP-natural aggregate blends used in base/subbase layers of pavements

The use of reclaimed asphalt pavement (RAP) matrials in road construction, maintenance, and rehabilitation projects has a number of economic and environmental benefits. This study evaluates the performance of cement-treated blends prepared by mixing the RAP content with fresh granular (G) material in different percentages and for different curing periods. For this purpose a comprehensive laboratory testing campaign was executed considering the resilient modulus tests, unconfined compressive strength tests, long-term cyclic tri-axial load tests, creep loading tests, and drying shrinkage tests on the blended samples. Analysis of the high-quality test result data demonstrates that applying even a small cement dosage, in the range of 1.5–4.5%, to the blended samples can considerably improve their resilience and strength properties in addition to limiting the accumulative strains occurring under the applied cyclic and sustained creep loading. However, drying shrinkage of the blended sample may be an adverse effect due to the addition of cement content. Additionally, statistical analysis of experimental results shows that resilience and strength properties of the cement-treated blends are significantly dependent on the blend composition and curing period, in addition to cement dosage. A regression model is presented for the estimation of compressive strength of the blended samples, which simultaneously takes into account the blend composition, curing period, and cement dosage.

Cyclic plasticity and thermomechanical alleviation in titanium alloys

Cyclic behaviour of titanium alloy IMI834 has been investigated with a new thermo-mechanically coupled discrete dislocation plasticity formulation, integrated with experimental observation. The mechanistic basis of the cyclic loading dependence is shown to be the establishment of dislocation pileups and back stress development such that on partial unloading, reversed strain occurs by thermally-activated dislocation escape and reverse glide during a secondary stress hold. Anomalous cyclic strain accumulation in both isothermal and anisothermal stress-loaded alloy IMI834 is explained by the reversed straining and dislocation structure re-arrangement mechanisms. The mechanisms also provide the underpinning explanation for the beneficial effect of elevated temperature excursions in diminishing cyclic creep accumulation and hence reducing dwell fatigue sensitivity in titanium alloys by thermal alleviation.

The effect of Gd addition on the cyclic oxidation behavior and creep life of alumina-forming Ni-based superalloy

The oxidation resistance and creep property of alumina-forming Ni-based superalloys with varying amounts of Gd are studied at 1100 ℃ in ambient air. Regardless of the Gd content, from the outermost surface inwards, a typical oxide scale consists primarily of NiO, spinel, spinel with oxide particles containing refractory elements, and Al2O3. However, spallation and abnormal growth of the scale are observed in alloys without Gd, and Gd addition significantly improves oxidation resistance. The creep rupture life is almost identical for the alloys with 0.04 wt.% Gd and those without Gd. The addition of Gd improves alloy oxidation resistance without compromising creep property.

A novel insight into the primary creep regeneration behaviour of a polycrystalline material at high-temperature using in-situ neutron diffraction

Primary creep regeneration (PCR) is observed during cyclic creep deformation in many engineering alloys. PCR is a phenomenon in which reverse inelastic strain fully, or partially, resets the early creep strain hardening memory of the material. The current understanding regarding the origin of the PCR behaviour in engineering alloys is limited to the phenomenological observations from the changes in dislocation structures whereas a good mechanistic understanding of the PCR behaviour is crucial for developing robust plasticity-creep predictive models. In this study, we investigated the micromechanical origin of PCR behaviour in type 316H stainless steel at 650 °C using high-temperature mechanical testing and neutron diffraction. A cyclic creep experiment was conducted in-situ at a neutron diffraction beamline, during which various degrees of unloading and reverse loading were applied to the specimen, followed by creep deformation under a load above the material's yield strength. Partial PCR was observed after reverse plastic loading for all the creep dwells, which is contrary to current high-temperature lifetime assessment's procedures advice which is to account for full recovery of primary creep after any reverse plastic loading. The extent of PCR is observed to be proportional to the magnitude of reverse plastic strain up to a level of 0.5% reverse plastic strain. From the measured neutron diffraction data, a strong correlation was observed between the changes in magnitude of the accumulated intergranular residual lattice strains and the macroscopic primary creep strain. Moreover, the increases of lattice strain to saturation and transition from primary creep to secondary regime occur at the same time. Based on these correlations it can be postulated that the PCR behaviour observed due to cyclic loading in type 316H stainless steel at elevated temperature originates from the accumulation of intergranular residual lattice strains during the reverse plastic loading and those time-dependent changes during the creep dwells.

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.

A Modified Creep Model for Cyclic Characterization of Rock Salt Considering the Effects of the Mean Stress, Half-Amplitude and Cycle Period

The fluctuation of the operation pressure during injection and withdrawal is a key factor affecting the long-term stability of surrounding rock in a compressed air energy storage (CAES) cavern. To assess the mechanical behavior of rock salt under different pressure fluctuations and to optimize the operating parameters of CAES caverns, uniaxial cyclic loading tests under the sine condition were carried out by changing three cycle parameters, namely the mean stress, half-amplitude and cycle period. The test results show that the deformation behavior during cyclic loading tests can be divided into transient, steady, and accelerated stages, which is similar to that of creep tests. The strain and steady strain rates increase with increasing cycle parameters, showing a nonlinear deformation property of rock salt under cyclic loading. To describe the nonlinear response that cannot be captured by the Burgers model, a stiffness scaling factor is proposed based on the test results. A modified cyclic creep model of rock salt is established by introducing this stiffness scaling factor into the conventional Burgers creep model. The agreement with the experimental data indicates that the modified cyclic creep model can accurately reflect the influence of the three cyclic parameters on the deformation properties of rock salt.

Mechanical behavior of timber-concrete composite members under cyclic loading and creep

Composite structures have emerged over the past several years, and are being increasingly studied and applied in civil engineering projects. In this study, a composite structure was combined with timber under tension and concrete to resist compression. Although several connection systems to bind wood and concrete exist, an innovative system was applied in the present study based on a specific wood treatment and the use of adhesives. Eight hybrid timber-concrete composite (TCC) members were fabricated and tested under static and cyclic loading, and creep. The results indicate that the minimum and maximum loads (which represent dead and live loads, respectively) were constant during a cyclic bending test. The mid-span deflection evolved during the test. An analytical model based on the compatibility of the strains was developed to predict the evolution of the displacement at the mid-span and integrate the effect of creep phenomena. Although the TCC members were cyclically tested under the maximum nominal load (live load), the evolution of the mid-span deflection was governed by the creep phenomena. Finally, the composite TCC members were subjected to a residual bending test until failure. TCC members with ordinary concrete showed a progressive loss of bending stiffness during a four-point bending test. This was caused by progressive debonding of the concrete slab and a diminution of the load capacity compared with TCC members that were not cyclically loaded. Additionally, a TCC member with ultra-high performance fiber-reinforced concrete (UHPFRC) achieved similar mechanical behavior as the TCC member tested under a static load.

Effect of activated carbon obtained from vinasse and marc on the rheological and mechanical characteristics of the bitumen binders and hot mix asphalts

This study examined the effect of the activated carbon, obtained from marc and vinasse agricultural wastes and used in the bitumen modification, on the rheological characteristics of bitumen binders and mechanical properties of hot mix asphalt. For this purpose, the activated carbon, obtained from vinasse generated at distillation step in the alcohol production from molasses together with grape marc, was used. The activated carbon was added to neat bitumen (PG 58–16) in 3 different ratios (5%, 10%, and 15% by weight of neat binder). The following tests were applied to the neat and modified bitumen: BBR (bending beam rheometer), DSR (dynamic shear rheometer), rotational viscosity, softening point, and penetration. The HMA samples were prepared using the neat and modified binders. The following tests were applied to the HMA samples: Marshall stability and flow, resistance against the damage induced by moisture, indirect tensile stiffness modulus, cyclic creep, and indirect tensile fatigue. Considering the binder test results, it was determined that using the activated carbon in the bituminous modification increased the stiffness and high-temperature performance of the bituminous binders, while the low-temperature performance was not affected much. Based on the mixture tests, it was found that using the activated carbon modified bitumen increased the Marshall stability, stiffness, resistance against the fatigue cracking, and permanent deformation of mixes. On the other hand, the use of activated carbon in bitumen modification adversely affected the resistance against the moisture-induced damage.

Experimental Study and Damage Model Study of Rock Salt Subjected to Cyclic Loading and Cyclic Creep

Due to the gas injection and production of underground salt caves during the operational phase, rock salt is often subjected to a combined stress of cyclic pressure and constant pressure. In order to investigate the damage evolution of rock salt under different combined stresses, the uniaxial cyclic loading test and cyclic creep test were carried out. The stress‐strain curves, energy characteristics, energy dissipation, and damage of rock salt in the two experiments were analyzed and compared. The test results show that the stress‐strain curves of the two tests presented three stages of “sparse”‐“dense”‐“sparse.” As the maximum stress increases, the stage of “dense” will decrease and the rock salt cycle life will decrease. The relationship between cycle life and Δσ (difference between maximum and minimum stress in the tests) is an exponential function under cyclic loading and a linear relationship under cyclic creep. Based on the experimental data, the energy dissipation of rock salt is analyzed. The damage variables were defined from the perspective of energy dissipation, and the damage evolution of rock salt under two tests was obtained. There are three corresponding stages of energy dissipation and damage: initial, constant speed, and acceleration. The damage model is obtained by inverse functioning the s function, and then the correction coefficient is added to the model to obtain the modified damage model. The modified damage model is compared with the experimental data. The results show that the model can accurately describe the three stages of rock salt damage. The significance of parameters in the modifying damage model is also discussed.

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.

Cyclic indentation of polymers: Instantaneous elastic modulus from reloading, energy analysis, and cyclic creep

Abstract

Study on the creep behaviors of Tianjin soft clays under cyclic loading

A creep test of remolded Tianjin saturated silty clay under cyclic loading was carried out. The test results showed that the cyclic creep curve exhibited attenuation- stabilization characteristics. At the initial stage of cyclic loading, the creep curve was approximately sinusoidal, and the axial strain increased with creep time. At the end of cyclic loading, the axial strain remained basically the same, and the stress-strain hysteresis curves tended to coincide, indicating that the soil creep tended to be stable. The cyclic creep was composed of cumulative plastic strain and reversible strain. The reversible strain of soil was related to the dynamic stress level. The larger the magnitude of cyclic stress, the larger reversible strain was generated. At the initial stage of cyclic loading, the cumulative plastic strain increased rapidly, and as the number of cyclic loadings increased, the cumulative plastic strain gradually entered the attenuation stage and eventually reached the stable stage.

A micro-macro confined compressive fatigue creep failure model in brittle solids

Fatigue properties under cyclic static compressive loading and unloading (i.e., compressive fatigue/cyclic creep properties) have an important meaning for evaluation of lifetime in brittle solids containing numerous microcracks. The total deformation during cyclic static compression consists of time-independent elastic, time-dependent viscoelastic, time-dependent viscoplastic and time-independent plastic deformations. In this study, the viscoplastic and plastic deformations are collectively called as plastic deformation below. The plastic deformation dominates the lifetime of brittle solids. The rebound of elastic and viscoelastic deformations at static unloading is widely studied. However, the rebound of plastic deformation at static unloading is rarely studied under constant lateral confining pressure, and the total time-dependent deformation during cyclic static compressive failure also is rarely studied in theory. Furthermore, the rebound of plastic deformation has a great significance for cyclic static compressive failure, and microcrack variable influences seriously plastic mechanical properties of brittle solids. However, the theoretical relationship between plastic deformation rebound and microcrack variable in brittle solids during a cyclic static compressive failure is rarely established. In this study, an analytical solution is proposed by coupling the confined cyclic static loading and unloading path, the Hooke-Kelvin viscoelastic model and the formulated micro-macro model, which explains the total time-dependent visco-elastic-plastic deformation caused by microcracks variable during cyclic static compressive failure. Effects of residual static axial stress after unloading and confining pressure on cyclic creep failure are studied. Irreversible elastic and viscoelastic deformations caused by residual static stress after unloading are illustrated. An interesting rebound phenomenon of plastic deformation induced by microcrack recovery is found. A critical value of axial stress causing a rebound of plastic deformation is also found. Effects of parameters Δσ1a, Δσ1b, and Δt in cyclic static loading and unloading path on the mechanical properties of cyclic static compressive failure are discussed. The proposed theoretical model provides a certain help for evaluation and prediction of fatigue lifetime in brittle solid engineering.

Numerical Prediction of Long-Term Deformation for Prestressed Concrete Bridges under Random Heavy Traffic Loads

AbstractMany prestressed concrete bridges exhibit increasing long-term deflections under heavy traffic loads. Cyclic creep from increasing heavy traffic has received gradually increasing attention ...

Experimental characterization and constitutive modeling of polyurethanes for structural applications, accounting for damage, hysteresis, loading rate and long term effects

Polymers are highly customizable materials that present high potential for applications in the development of damage-resistant structural systems. Out of the various polymeric materials, polyurethanes combining high strength (2–3 times that of normal concrete) with high deformation capacity (~0.04 strain at yielding) have been recently considered in the development of damage resistant bridge columns. However, development of structural systems incorporating polyurethanes requires fundamental understanding of their mechanical properties under various loading conditions, as well as development of versatile constitutive models describing such responses. Compared to conventional structural materials, the response properties of polyurethanes include hysteresis, damage (softening), and rate dependence (viscosity) in the elastic and inelastic range. To address this challenge, this study: (i) conducts an experimental program to characterize the mechanical behavior of an amorphous thermoset polyurethane which has been (and is currently being) considered in structural applications, (ii) develops a softening viscoelastic viscoplastic uniaxial material model to simulate the observed response under various loading conditions, and (iii) develops a heuristic systematic procedure to estimate model parameters from test data. The loading conditions of the experimental program include: (a) monotonic/cyclic compression-only and tension-only loading at strain rates up to 0.1/s, (b) multi-step strain-controlled monotonic loading including constant deformation segments to quantify relaxation and capture the so-called equilibrium path, (c) cyclic combined tension-compression uniaxial loading, and (d) relaxation and creep compressive tests. The test results showed nonlinear rate dependence, asymmetric response in tension and compression, monotonic/cyclic damage, and hysteresis.

Laboratory evaluation of nano-silica modification on rutting resistance of asphalt Binder

Rutting is identified and used as the first failure mechanism in most important design conditions of flexible pavements. Bitumen can play a decisive role as one of the main materials used in asphalt mixtures, and modifying the properties of bitumen can greatly delay these failures and minimizing them in some cases. In this study, the PG 64-22 bitumen was modified using 2, 4 and 6% weight percentage of nano-silica and the effect of this modifier on reducing rutting failure was evaluated compared to the control sample. The rutting parameters including non-recoverable creep compliance (Jnr) and the recovered creep (R%) were also evaluated, and then the effect of nano-silica on the improvement of these parameters at high temperatures was evaluated by simulating dynamic loading using Dynamic Shear Rheometer (DSR) and performing the MSCR test at 3 temperatures of 58, 64 and 70 °C. After investigating the bitumen behavior in the non-linear viscoelastic state, a significant effect was observed in the rutting parameters. So that, as a result of nano-silica addition, the cumulative strain decreased compared to the control sample and this decrease became more by increasing the modifier percentage, resulting from increasing the creep recovery at the end of each creep cycle. It was also concluded that, the increase in temperature in high and low stresses, has a negative effect on decreasing deformation recovery percent (R%) and the non-recoverable creep compliance (Jnr), both in unmodified and modified bitumen samples, but importantly, the amount of nano-silica effect on the improvement of these parameters was negative, which was discussed in detail in this study. Also, at a certain temperature, increasing the percentage of nano-silica modifier significantly increased R%, and also significantly decreased the Jnr. In the following, the difference of creep recovery in high and low stresses of Rdiff and the difference of the non-recoverable creep compliance in high and low stresses of Jnrdiff significantly improved, indicating a reduction in the sensitivity of asphalt samples to the applied stress and also a significant increase in resistance of the samples against rutting.