Constant Strain Rate Tests Research Articles

Isotache model for consolidation with a small incremental load

To examine the applicability of an isotache model, two types of consolidation test were carried out: constant rate of strain (CRS) by changing the strain rate during the test, and an incremental loading (IL) test with small incremental ratios. It is found from the CRS tests that the stress shift caused by the strain rate change is nearly the same for all tested soils, but the behaviour after the change is different: some soils do not follow the isotache model. After selecting two typical soils (one follows the isotache model and the other does not), IL tests with various increment ratios and different durations at the loading stage prior to the incremental load were carried out to investigate how these parameters affect the strain–time relation. It is revealed that the strain–time relation is strongly governed by the magnitude of stress increments; in cases of small increments, most of the settlement is generated after the end of the primary consolidation. It is found that most test results can be interpreted by the isotache model. However, when the load duration at the loading stage prior to the incremental load is longer, the time–strain behaviour differs from that predicted by the isotache model.

Creep behavior of Sn–Bi solder alloys at elevated temperatures studied by nanoindentation

The creep mechanisms of eutectic Sn–Bi alloy were evaluated with indentation constant strain rate (CSR) method at elevated temperatures. The activation energy (Q) and creep stress exponent (n) of eutectic Sn–Bi alloy and other alloy compositions were measured in the temperature range from 25 to 100 °C. Prior to this, the indentation CSR testing protocol for evaluation of Q and n was validated through evaluating the pure Sn (grain size >100 µm) at various temperatures. The creep mechanism of large grain-sized Sn was found to be dislocation climb controlled by core diffusion in bulk. Dislocation climb though core diffusion and power-law breakdown were suggested to be the deformation mechanism for pure Bi and Sn–3%Bi alloy, respectively. For the two-phased eutectic Sn–Bi alloy, the creep mechanism was found to be strain rate and temperature dependent. Individual constituent phases were found to take turns to dominate the creep rate at different strain rates.

ACMEG-TVP: A thermoviscoplastic constitutive model for geomaterials

In this paper, a thermoviscoplastic constitutive model for geomaterials is presented. The model is formulated based on two aspects: (i) the nonstationary flow surface theory, which is used to describe the viscous behaviour of geomaterials, and (ii) the thermoplasticity concept, which is adopted to introduce the effect of temperature on geomaterials. In the model, the yield surface evolves with the viscoplastic strain, viscoplastic strain rate and temperature. Thus, it allows the description of combined effects of strain rate and temperature on the mechanical behaviour of geomaterials. The model has been used to simulate various tests involving different thermomechanical loading paths, including non-isothermal constant rate of strain tests, cyclic heating and cooling tests, creep tests with temperature changes and stress relaxation tests. The simulation results agree well with measurements, demonstrating the capacity of the proposed model to reproduce the various thermoviscoplastic behaviour of geomaterials.

Effect of nickel on hydrogen embrittlement behavior of medium-carbon high strength steels

The present study was attempted to explore the influence of Ni up to 1% on hydrogen embrittlement (HE) behavior of medium-carbon high strength steel by using both constant load test (CLT) and slow strain rate test (SSRT). The results show that HE index (HEI) decreases notably with the addition of 0.55% Ni, and this decreasing tendency tends to slow down with the addition of 1.02% Ni. HEI of SSRT is comparatively higher than that of CLT. The faction of brittle crack initiation area decreases with increasing Ni content. There is a decreasing tendency of both absorbed hydrogen content in Walpole solution and pre-charged hydrogen content with the addition of Ni. Both corrosion rate and pit depth of specimens after immersed in Walpole solution tend to decrease with increasing Ni content. Further electrochemical polarization test reveals that adding Ni could enhance pitting corrosion resistance. It is concluded that the addition of Ni could further lower the HE susceptibility of the tested steels mainly owing to enhanced corrosion resistance and decreased hydrogen absorption.

Initial strain energy-based thermo-elastoviscoplastic two-parameter damage self-healing model for bituminous composites—Part II: Computational aspects

This paper describes the implementation of the initial strain energy-based thermo-elastoviscoplastic two-parameter damage self-healing models for bituminous materials to compare the model predictions with experimental measurements. Computational algorithms based on the two-step operator splitting methodology are proposed and employed for numerical results. The elastic two-parameter damage self-healing predictor and the viscoplastic corrector are coupled with the Arrhenius-type temperature term via the net stress concept in conjunction with the hypothesis of strain equivalence and then systematically used for numerical implementation of the models. Four numerical examples of three-dimensional problems are first demonstrated for the effect of temperature and rest period for healing phenomenon. Second, experimental validation of the proposed model against monotonic constant-strain test at different temperatures and controlled-strain cyclic tension test is presented. It is observed that numerical simulations by the two-parameter model agree well with experimental results. In particular, the softening responses of the bituminous composites are well predicted for monotonic constant-strain rate test and the viscous damage behavior can be reasonably predicted by the proposed two-parameter damage models. It is noted that the two-parameter model can better predict the thermo-mechanical behavior of asphalt and is more versatile than the one-parameter isotropic damage self-healing model.

Comparison of flow behavior of as-cast and hot rolled Al-B4C composites by constant and differential strain rate tests

High temperature tensile flow behavior of aluminum-boron carbide (Al-B4C) composites of 0, 5 and 15% B4C, hot rolled to ~88% with intermediate annealing at 350 °C, was investigated by constant initial strain rate (CIS) test technique at 500 °C and strain rate jump test technique over the temperature range of 400–500 °C. In the as-cast condition, the flow stresses obtained between CIS and strain rate jump test techniques were found to be significantly different at 500 °C. The strain rate sensitivity index (m) was found to be ~0.1 over for all the composites in both as-cast as well as hot rolled condition. Tensile elongations were found to be 0.36 in both as-cast and hot rolled aluminum, whereas the same reduced in Al-5% B4C composite to 0.35 and 0.27, respectively. The values of activation energy (Q) for deformation of rolled aluminum and Al-5% B4C composite were determined to be 194.2 and 73.4 kJ/mol, respectively. The microstructural examination, using SEM and EBSD techniques, revealed cavitation in aluminum upon differential strain rate test, and grain refinement upon rolling, which increased later during tensile test.

Characterisation of Ballina clay

This paper describes a comprehensive characterisation study carried out on clay from the National Soft Soil Testing Facility located at Ballina in northern New South Wales (NSW, Australia). Ballina clay represents the estuarine soft clays of high to extremely high plasticity from the Richmond river valley in NSW. They are structured and lightly overconsolidated with an average organic content around 3%. Index properties as well as mechanical parameters were estimated from laboratory tests performed on tube specimens retrieved using a fixed-piston sampler. Index characterisation tests were combined with constant rate of strain tests, incremental loading tests and stress-path triaxial testing to evaluate compressibility, stiffness, permeability and strength parameters. These deposits display very high compressibility and a low undrained shear strength which is larger in triaxial compression. Ballina clay shows a non-linear stress–strain response either in one-dimensional compression or undrained shearing. The consolidation coefficient, and consequently the water permeability, reduces dramatically with the stress level in the overconsolidated zone, mainly due to soil destructuration. A brittle response has been observed during shearing that reduces the undrained shear strength by around 50% after peak. Geotechnical profiles describing the variation of index and mechanical properties with depth are provided and compared against in situ test results. It is shown that the use of the fixed-piston sampler, in combination with non-destructive methods, to assess and select samples for laboratory testing provided good quality and reliable test results which are in agreement with data interpreted from in situ tests.

Comparison of SCC Behavior of 304L Stainless Steels With and Without Boron Addition in Acidic Chloride Environment

The stress corrosion cracking (SCC) behavior of 304L B4 grade borated stainless steel (SS) as well as 304L SS was investigated by constant load and slow strain rate testing (SSRT) techniques. The microstructure, pitting, and SCC behavior of borated SS in the as-received, sensitized, and solution-annealed conditions were analyzed. Potentiodynamic anodic polarization and double loop electrochemical potentiokinetic reactivation (DLEPR) experiments were carried out to find out pitting corrosion resistance and degree of sensitization (DOS). The number of boride particles (composed of Cr, Fe, and B) were highest for the specimen solution annealed at 1423 K/2 h. Solution-annealing treatment at 1423 K/4 h was found to be beneficial in improving the corrosion resistance of borated 304L SS. Although the borated 304L SS exhibited a higher DOS, it showed improved pitting corrosion resistance compared to 304L SS. Constant load experiments revealed the time to failure to be the highest for the specimen solution annealed at 1423 K/4 h. SCC susceptibility index (Iscc) values obtained from SSRT tests were lower for solution-annealed borated 304L SS compared to the as-received and sensitized conditions. The improved SCC resistance of borated 304L SS was attributed not only to the solution-annealing treatment but also the higher stacking fault energy (SFE) value compared to 304L SS.

Criteria of Acceptance for Constant Rate of Strain Consolidation Test for Tropical Cohesive Soil

In this study, the rapid consolidation equipment (RACE) was developed as an alternative device to the conventional consolidation test using Oedometer, consuming merely a few hours for the whole precedure to determine the consolidation characteristics of cohesive soil. RACE operates based on the constant rate of strain (CRS) consolidation theory, which is a continuous loading method of testing, requiring a good estimation of the loading rate such that it is ideal for the achievement of steady state condition during testing. The steady state condition is achieved when the cv values from drained and undrained face of CRS converged with the cv from Oedometer test. A slightly modification has been made on the normal constant rate of strain (CRS) test by proposing a direct back pressure system to the specimen using a tube to saturate the soil sample. This research has produced a set of criteria for determining the suitable rate for the rapid consolidation test based on the ratio of normalized strain rate, β, and proposed a new coefficient in terms of a ratio of β to clay fraction (CF), as a part of new criteria for testing a fine soil. Four types of sample were tested with different rates of strain using the RACE and their results were compared with those conducted using the Oedometer on the same soil type, from which fairly good agreements were evident in many specimens. It was found from the study that the minimum value of normalized strain rate, β, for the CRS test is 0.005 and for the ua/σv ratio is suggested as 0.01. Also, the maximum β/CF for soils with clay friction lower and higher than 50 % are 0.008 and 0.001, respectively. The minimum β/CF value for both conditions is 0.0001.

Long-term life prediction of woven CFRP laminates under three point flexural fatigue

Abstract This technical contribution reports details of an investigation of the long-term flexural fatigue life behavior of plain weave carbon 3K-70P/epoxy (CFRP) laminates using accelerated testing methodology (ATM) coupled with time temperature superposition principle (TTSP). Initially, dynamic mechanical analysis (DMA) tests were performed on the CFRP laminates to determine the time-temperature shift factors followed by constant strain rate (CSR) tests at different temperatures to construct the creep strength master curve. The time-temperature shift factors were computed using an automated curve shifting programme. Horizontal shift factors were found to follow Arrhenius linear relationship for both above and below glass transition temperatures with different gradients. The scatter in the strength data was considered using Weibull analysis. Flexural fatigue tests were then conducted at different temperatures and loads. Finally, the master curves of flexural fatigue strength were constructed to predict the life of the CFRP laminates at arbitrary frequencies and load cycle ratios. Predicted fatigue behavior at non-default test parameters was found to be in good agreement with experimental data.

Development of a high temperature flow stress model for AerMet 100 covering several orders of magnitude of strain rate

Constant strain rate, constant velocity and Hopkinson Pressure Bar compression tests were carried out on AerMet 100 martensitic steel between 1130°C and 1250°C spanning strain rates from 0.01s−1 to 4000s−1. The results were used to generate a predictive flow stress model over the entire range of test conditions. The effect of initial austenite grain size on flow stress was found to follow the Hall–Petch relationship. This dependency was then removed through innovative heat treatments. The morphology of the flow stress curves were also dependent on mechanisms of microstructural evolution which were controlled by the test method, strain rate and temperature. Friction and adiabatic heating also had a major contribution. A novel method was proposed in order to define the flow stress, which was then used to determine the work hardening exponent of the Zener–Hollomon equation. It was found that a deviation from the linear trend was observed in Hopkinson Pressure Bar tests and reasons were given. An artificial neural network approach was used to determine a more accurate predictive flow stress model which included the effects of test method, temperature, stain rate and initial austenite grain size. The method showed that it was possible to predict the flow stress between 50 and 2000s−1 where mechanical testing’s results were absent.

ガラス繊維強化PBTの力学特性に及ぼす繊維含有率の影響

Evaluating long-term strength and mechanical properties of composite materials is important in order to use them as construction materials. When the matrix is polymer materials, the composite materials exhibit viscoelasticity. Therefore long-term strength prediction should be based on viscoelasticity. In this study, constant strain rate tests of PBT and short fiber reinforced PBT are performed. As a result, it is confirmed that time-temperature superposition principle can be applied the strength and elastic modulus of each material. And those shift factors coincide regardless of the fiber volume fraction. It is shown that the prediction of time dependence of the elastic modulus by shear lag model cannot be evaluated long-term elastic modulus correctly.

Evaluation of overconsolidation ratios from laboratory and in situ tests on Busan clay

The overconsolidation ratio (OCR) was determined through three different series of consolidation tests on the samples, which were retrieved using an oil-operated fixed-piston sampler at two sites in a deltaic deposit. The constant rate of strain (CRS) consolidation test and the end-of-primary (ILEOP) consolidation tests proved to be superior tests with higher quality of samples and produced close OCR values. The CRS and ILEOP tests provided OCR values close to those of the field measurements, whereas the conventional 1D consolidation (IL24) test underestimates the values. The OCR values obtained from the CRS and ILEOP tests lie between the upper and lower bounds of existing empirical formulas, whereas those from the IL24 test belong to the lower bound. Empirical formulas that were suited for both sites were newly developed through the correlations between the results of the two consolidation tests and three field tests. The empirical formulas, which have been developed based on the correlation coefficients, are appropriate for predicting the OCR values for a site where the great majority of data are given. This trend is more pronounced in the predictions obtained from the piezocone and the field vane tests than the predictions from the flat dilatometer test.

New strain energy-based thermo-elastoviscoplastic isotropic damage–self-healing model for bituminous composites—Part II: Computational aspects

This paper is the second in a series implementing initial strain–energy-based thermo-elastoviscoplastic isotropic damage–self-healing formulations for bituminous composites to compare the model predictions with experimental measurements. Computational algorithms based on the two-step operator splitting methodology are systematically developed and employed for numerical simulations. The elastic damage self-healing predictor and the viscoplastic corrector coupled with the Arrhenius-type temperature term via the net stress concept in conjunction with the hypothesis of strain equivalence are entirely considered as numerical implementation of the models. Several numerical examples of one-dimensional driver problems are first presented to show the effect of temperature and rest period for healing. Experimental validation of the proposed formulations against monotonic constant–strain test under different temperatures and controlled-strain cyclic tension test is presented. Qualitative and quantitative agreement between experimental results and numerical simulations is also observed. In particular, the softening behavior of the bituminous materials is well predicted for monotonic constant–strain rate test. The viscous damage behavior can be reasonably captured by the proposed damage models with the Arrhenius-type temperature term and the step-by-step computational algorithms.

<i>In Situ</i> Testing and Heterogeneity of UFG Cu at Elevated Temperatures

Experiments were conducted to investigate deformation-induced processes during in-situ tensile test at elevated temperature. Consequently the microstructure after creep loading was examined by 3D Electron Back Scatter Diffraction (EBSD) technique. The billets of coarse-grained copper were processed by equal-channel angular pressing (ECAP) at room temperature using a die that had an internal angle of 90° between the two parts of the channel and an outer arc of curvature of ~ 20°, where these two parts intersect. The pressing speed was 10 mm/min. To obtain an ultrafine-grained (UFG) material, the billets were subsequently pressed by route Bc by 8 ECAP passes to give the mean grain size ~ 0.7 μm. The constant strain-rate test in tension was performed at 473 K using testing GATAN stage Microtest 2000EW with EH 2000 heated grips which is configured for in-situ electron back scatter diffraction (EBSD) observations. Microstructure was examined by FEG-SEM TESCAN MIRA 3 XM equipped by EBSD detector HKL NordlysMax from OXFORD INSTRUMENT. The tensile test was interrupted by fast stress reductions after different deformation step and observation of microstructure changes was performed. Despite of a considerable interest in ECAP processing method, there are not many works documenting microstructure evolution and changes during creep testing and determining creep mechanisms of ultrafine-grained materials processed by ECAP. It was found that creep resistance of UFG pure Al and Cu is considerably improved after one ECAP pass in comparison with coarse grained material, however, further repetitive pressing leads to a noticeable deterioration in creep properties of ECAP material. Recently it was observed the coarsening of the grains in microstructure of ECAP copper during creep at elevated temperature. It was suggested that creep behaviour is controlled by storage and dynamic recovery of dislocations at high-angle boundaries. In the present work was found that ultrafine-grained microstructure is instable and significant grain growth has already occurred during heating to the testing temperature. Static recrystallization during heating led to the formation of high fraction of special boundaries Σ3 and Σ9. The tensile deformation at 473 K led to the additional grain growth and formation of new grains. Microstructure was investigated also by 3D EBSD.

Effects of mixed strain rates on low cycle fatigue behaviors of austenitic stainless steels in a simulated PWR environment

Though strain rates are hardly fixed during the actual transients in nuclear power plants (NPP), most of the existing environmental fatigue ε–N data were obtained under simple loading histories with constant strain rates. In an effort to incorporate the actual loading conditions, strain rate was changed during the low cycle fatigue (LCF) test. The LCF tests were performed in a strain control mode in both simulated pressurized water reactor (PWR) environment and 310°C air to evaluate the effects of mixed strain rates on fatigue life. The test results indicated that the modified strain rate approach and average strain rate method could be applied to the fatigue evaluation considering environmental effects. The cyclic stress response showed that the trend of hardening with mixed strain rate was consistent with a negative strain rate sensitivity, which was observed in the constant strain rate test. In addition, the increase in striation spacing on the fracture surface is in good agreement with the increase of bulk dislocation density and reduction of fatigue life.

Fractional description of time-dependent mechanical property evolution in materials with strain softening behavior

On the basis of using the variable fractional order to characterize the time-dependent mechanical property, a variable order fractional viscoelastic model is presented to represent the time-dependent evolution of mechanical property including strain softening behaviors. The developed model is applied to analyze the constant strain rate tension and compression tests in ductile metals and soils. It is shown that the model can reasonably characterize the hardening and softening stages of constant strain rate tests and describe the evolution of mechanical property. The time-dependent mechanical property evolution can be split into three parts: in the first stage, the mechanical property is almost invariable, the second phase is a mutation stage, and in the third stage it changes slowly and linearly with strain. The simulated results reveal that the evolution from strain hardening to strain softening is a continuous linear change of mechanical property. Furthermore, it is known that the order obtained through fitting some test data can be greater than 1, which corresponds to strain softening response, while for a long time the order was always assumed to be between 0 and 1.

Influence of strain-rate on hydromechanical behavior of highly compacted GMZ01 bentonite

During the long-term operation of a deep geological repository in crystalline rock, saturation of buffer/backfill materials will last for a long period of time. Investigation on time-dependent property of buffer/backfill materials is of great importance for long-term safety assessment of the repository. In this work, GMZ01 bentonite powder was statically compacted to specimens with a dry density of 1.70g/cm3 and suction-equilibrated to their target values. Then, a series of suction-controlled constant rate of strain (CRS) tests (including single-stage and step-wise CRS tests) were conducted for the investigation of the strain rate-dependent compression behavior of highly compacted GMZ01 bentonite. Comparisons were made between the results of the single-stage and step-wise CRS tests under given suctions for the verification of the applicability of the isotache concept on the compacted GMZ01 bentonite. Based on that, the step-wise CRS compression tests were performed and results were presented in terms of vertical stress–vertical strain curves. Based on the test results, the virgin compression index λ and the yield stress p0 were determined. Analysis indicates that λ decreases, while p0 increases with increasing suction. Furthermore, p0 increases with increasing CRS, which is consistent with previous researches on saturated soils. Finally, a logarithm function proposed between α(s) and suction s is verified and several relevant parameters are fitted, which represents the coupling of suction, time and stress of GMZ01 bentonite.

Discrete element modelling of creep of asphalt mixtures

Creep tests on asphalt mixtures have been undertaken under four stress levels in the laboratory while the discrete element model (DEM) has been used to simulate the laboratory tests. A modified Burger’s model has been used to represent the time-dependent behaviour of an asphalt mixture by adding time-dependent moment and torsional resistance at contacts. Parameters were chosen to give the correct stress-strain response for constant strain rate tests in Cai et al. (2013). The stress-strain response for the laboratory creep tests and the simulations were recorded. The DEM results show reasonable agreement with the experiments. The creep simulation results proved to be dependent on both bond strength variability and positions of the particles. Bond breakage was recorded during the simulations and used to investigate the micro-mechanical deformation behaviour of the asphalt mixtures. An approach based on dimensional analysis is also presented in this paper to reduce the computational time during the creep simulation, and this analysis is also a new contribution.

Brittle creep and subcritical crack propagation in glass submitted to triaxial conditions

AbstractAn experimental work is presented that aimed at improving our understanding of the mechanical evolution of cracks under brittle creep conditions. Brittle creep may be an important slow deformation process in the Earth's crust. Synthetic glass samples have been used to observe and document brittle creep due to slow crack‐propagation. A crack density of 0.05 was introduced in intact synthetic glass samples by thermal shock. Creep tests were performed at constant confining pressure (15 MPa) for water saturated conditions. Data were obtained by maintaining the differential‐stress constant in steps of 24 h duration. A set of sensors allowed us to record strains and acoustic emissions during creep. The effect of temperature on creep was investigated from ambient temperature to 70°C. The activation energy for crack growth was found to be 32 kJ/mol. In secondary creep, a large dilatancy was observed that did not occur in constant strain rate tests. This is correlated to acoustic emission activity associated with crack growth. As a consequence, slow crack growth has been evidenced in glass. Beyond secondary creep, failure in tertiary creep was found to be a progressive process. The data are interpreted through a previously developed micromechanical damage model that describes crack propagation. This model allows one to predict the secondary brittle creep phase and also to give an analytical expression for the time to rupture. Comparison between glass and crystalline rock indicates that the brittle creep behavior is probably controlled by the same process even if stress sensitivity for glass is lower than for rocks.