Strain-controlled Tests Research Articles

Experimental Analysis of Mechanical Properties of High Organic Soft Clay at Different Depths

In this study, with regard to the differences of formation stage and physicochemical properties for different high organic soft clay layers, a series of laboratory tests have been carried out to evaluate the mechanical properties of high organic soft clay in Northeast China. The conventional high-pressure consolidation and strain-controlled triaxial shear tests have been carried out to measure the compression and shear strength of high organic soft clay which formed in different ages. Furthermore, the comparisons of stress-strain relations between undisturbed and remoulded high organic soft clay samples under the confining pressure of 300kPa reveal the significant differences in compression and shear strengths of high organic soft clay at different depths, which can be interpreted by the differences in the degree of decomposition of the soil. The test results show that the degree of decomposition of high organic soft clay greatly depend on its formation stage at different depths, which is deemed to determine the mechanical properties. This study will provide a good guide to civil engineers on the constructions of the foundation.

Investigation of the effect of rubber on rheological properties of asphalt binders using superpave DSR

The main objective of this study was to investigate the effect of the rubber additive on the rheological properties of asphalt binders including Superpave rutting parameter, fatigue parameter, storage modulus at six rubber percentages: 0.0, 2.5, 5.0, 7.25, 10, and 15%. The Superpave Dynamic Shear Rheometer (DSR) was used to measure the complex shear modules value and the phase angle for the fresh asphalt binder and the rubber-modified asphalt binders using DSR strain-controlled tests. The DSR tests were conducted at one loading frequency of 10 rad/sec (1.59 Hz) and five temperatures: 58, 64, 70, 76, and 82°C. A 12 percent constant strain was applied in these tests according to the Superpave specifications. The rubber additive improved the complex shear modulus value of asphalt binder at all temperatures. Findings of the study showed that the rubber additive resulted in an increase in each of: the complex shear modulus value, the rutting parameter, the fatigue parameter, and the storage modulus (|G*|, |G*|/sin δ, |G*|sin d, and |G*|cos δ, respectively) of the asphalt binder. In conclusion, the rubber additive improved the rutting resistance as well as the fatigue resistance of the asphalt binder by increasing the complex shear modulus value and the storage modulus, and decreasing the phase angle.

Temperature and Freeze-Thaw Effects on Dynamic Properties of Fine-Grained Soils

Consider that the Mable Creek silt in central Alaska liquefied during the November 2002 earthquake. Subsequently, frozen samples of these fine-grained soils were retrieved for laboratory study. This paper presents the influence on dynamic properties by temperature changes from below freezing to near freezing, and by freeze-thaw cycles. Sample temperature change was simulated by gradually applying the increasing temperatures of –0.2, 0.5, 1, 5, and 24°C. Sample seasonal climate change was simulated by applying 1, 2, and 4 freeze-thaw cycles. Tests on specimens conditioned at 0.5 and –0.2°C through different thermal conditioning paths were also performed. Determination of the soil’s dynamic properties was investigated using triaxial strain-controlled cyclic tests. The shear modulus was found to decrease when the temperature increased from near freezing to above freezing; however, the damping ratio reached a maximum value when temperature was at or near freezing. Applying 1, 2, and 4 freeze-thaw cycles resulted in an increase in both the dynamic shear modulus and the damping ratio. Thermal conditioning paths with the same target near-freezing temperature were found to impact dynamic properties.

Classification of metallic alloys for fatigue damage accumulation: A conservative model under strain control for 304 stainless steels

Fully reversed uniaxial tests performed under total strain and stress control on 304 stainless steels specimens show that, under strain control the fatigue damage for High–Low (H–L) cycling is more significant than that using Miner’s rule, but under stress control opposite results are obtained. This has been attributed to opposite effects of pre-hardening under strain and stress control. Classical non linear damage accumulation models are not able to take into account this difference in sequence effect. Smith–Watson–Topper (SWT) and Fatemi–Socie (FS) criterion combined to linear damage accumulation can take into account this difference in sequence effect through the presence of maximum stress. However these models require an elastic–plastic constitutive law which is difficult to propose due to the presence of high cycle secondary hardening observed on 304 stainless steel. A conservative model for damage accumulation under variable amplitude strain control loading is thus proposed, which does not require a constitutive law. Linear damage accumulation is used, while sequence effect is taken into account using the elastic–plastic memory effect through cyclic strain–stress curves (CSSC) with pre-hardening. This modeling classifies metallic alloys in two groups for damage accumulation, with a stable (independent to pre-hardening) CSSC as for aluminum alloys and with an unstable (dependent to pre-hardening) one as for austenitic stainless steels. For the former case the modeling is identical to Miner’s rule. The modeling is approved based on a large number of tests on 304 stainless steel and is compared with SWT and FS models. In presence of mean stress the modeling permits in a qualitative way to explain the fact that tensile mean stresses in constant amplitude strain control tests are more detrimental than for constant amplitude stress control tests. Moreover it is shown that the SWT model is not always able to predict accurately the fatigue life in presence of a mean stress. Finally, it is concluded that for a 304 stainless steel, in order to take into account the mean stress in fatigue life, the mean stress effect has to be decomposed into two parts: maximum and “intrinsic” mean stress effects.

Low cycle fatigue of an extruded Mg–3Nd–0.2Zn–0.5Zr magnesium alloy

The purpose of this study was to evaluate strain-controlled cyclic deformation behavior of an extruded Mg–3Nd–0.2Zn–0.5Zr (NZ30K) magnesium alloy. The microstructure of this alloy consisted of a bimodal microstructure with equiaxed recrystallized grains and unrecrystallized coarse grains along with a large number of smaller second-phase particles present inside the grains and larger particles along the grain boundaries alongside a characteristic precipitate free zone (PFZ). The average grain size was about approximately 5–7μm. It was observed that unlike the higher RE-containing Mg–10Gd–3Y–0.5Zr (GW103K) magnesium alloy, the NZ30K alloy exhibited asymmetrical hysteresis loops in tension and compression in the fully reversed strain-control tests at a strain ratio of Rε=−1. This was mainly due to the presence of relatively stronger crystallographic texture, PFZ, and the resultant twinning–detwinning activities during cyclic deformation. While this alloy exhibited cyclic softening at lower strain amplitudes and cyclic hardening at higher strain amplitudes, it had an equivalent fatigue life to that of other extruded Mg alloys. Fatigue crack was observed to initiate from the specimen surface with some isolated facets of the cleavage-like planes near the initiation site. Crack propagation was basically characterized by serrated fatigue striations.

Influence of cyclic loading history on small strain shear modulus of saturated clays

Small strain shear modulus Gmax is an essential parameter in soil dynamics, and it is usually estimated based on the Hardin and Richart equation. However, many previous researches on sands have indicated that the Hardin and Richart equation does not consider the influences of cyclic loading history on Gmax. In this paper, effects of cyclic loading history on Gmax of saturated clays under undrained conditions are studied using a combination device of piezoelectric-ceramic bender element system and cyclic triaxial apparatus. The dynamic pre-loading includes both relatively high amplitudes of cyclic stresses and cyclic strains. Gmax without cyclic loading history is also investigated for the comparison purpose. Test results show that, at the same effective stress, both cyclic strain history and cyclic stress history will induce reduction of Gmax compared to the corresponding Gmax values with non-cyclic loading effects. In strain-controlled tests, the reduction of Gmax is slight and relatively stable; while in stress-controlled tests, the reduction of Gmax increases suddenly and remarkably when the effective stresses degrade to a certain degree. The comparison between double amplitude axial strain and residual excess pore water pressure behaviors show that the remarkable reduction of Gmax can demonstrate the cyclic failure of saturated clays.

Intrinsic nonlinearity from LAOStrain—experiments on various strain- and stress-controlled rheometers: a quantitative comparison

Strain-controlled large amplitude oscillatory shear (LAOStrain) experiments on a polyisoprene melt and a polyisobutylene solution were conducted on four different rheometers. The results are compared using nonlinear quantities such as the normalized intensity of the third harmonic (I 3/1) and the intrinsic nonlinearity in order to assess the reproducibility of the experiments. Two of the investigated instruments were strain-controlled rheometers, another two, were advanced stress-controlled rheometers. Since the stress-controlled rheometers are able to conduct strain-controlled tests when employing an active deformation control loop, the two different rheometer types could be compared. Experimental details like the gain of the deformation control loop, and the method of temperature control have been shown to play crucial roles in achieving reasonable reproducibility across the different instruments. Furthermore, deviations from the quadratic scaling of I 3/1 with the strain amplitude and the influence of instrument inertia on nonlinear quantities were observed for one of the stress-controlled instruments. The standard deviation of the intrinsic nonlinearity Q 0(ω 0) at a specific angular frequency as determined by measurements on the same instrument was found to be 8 % or lower. The relative deviations of Q 0 across different instruments were instead up to 12 % in the investigated frequency range with an exception for a specific instrument and one of the samples, where the deviation was considerably larger.

On the role of grain shape in static liquefaction of sand–fines mixtures

This technical note presents new experimental evidence for the role of grain shape in the static liquefaction behaviour of sand–fines mixtures. The laboratory experiments comprise a series of strain-controlled triaxial tests on several types of sand–fines mixtures that are intended to clarify a concern remaining in a previous study by the same authors: whether the observed effect of grain shape was due to the differences in hardness of the added fines. The comparison of new test results with the previous ones confirms that it is grain shape rather than grain hardness that is responsible for the observed differences in overall behaviour. Adding fines of rounded shape into a clean sand indeed results in a marked increase of liquefaction potential compared with adding fines of more angular shape into the same sand. It is also shown that the concept of the combined roundness is effective and useful for quantifying in an integrated manner the influence of grain shape and fines content on the overall friction angle of sand–fines mixtures at critical state.

Low-cycle fatigue of a friction stir welded 2219-T62 aluminum alloy at different welding parameters and cooling conditions

Strain-controlled low-cycle fatigue tests and microstructural evaluation were performed on a friction stir welded 2219-T62 aluminum alloy with varying welding parameters and cooling conditions. Cyclic hardening of friction stir welded joints was appreciably stronger than that of the base material. The cyclic stress amplitude increased, and plastic strain amplitude and fatigue lifetime slightly decreased with increasing welding speed from 60 to 200 mm/min but were only weakly dependent of the rotational rate between 300 and 1,000 rpm with air cooling. Friction stir welded joints with water cooling had higher stress amplitude and fatigue life than that with air cooling. Fatigue failure of the joint occurred in the HAZ where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.

Multiaxial fatigue behavior of AISI 316L subjected to strain-controlled and ratcheting paths

To investigate the material behavior of a stainless steel grade AISI 316L, subjected to loading conditions typical for the primary cooling circuit of a light water nuclear reactor, a set of uniaxial, torsional and multiaxial low cycle fatigue and strain-controlled ratcheting tests has been carried out. The main goal of these experiments is to analyze how the cyclic deformation response is influenced by strain amplitudes, loading paths and ratcheting, and to estimate how fatigue life is affected by these elements. Finally, the performance of three advanced multiaxial fatigue criteria (i.e. Smith–Watson–Topper, Fatemi–Socie and Jiang) has been critically evaluated for all the considered loading cases.

Development of Stress-Induced Martensitic Transformation in TiNi Shape Memory Alloy

TiNi shape memory alloy (SMA) was subjected to tension at strain-controlled test on quasistatic testing machine. The nucleation, development, and saturation of the stress-induced martensitic transformation were investigated, taking into account the obtained dependency of mechanical parameters and the specimen temperature changes measured by an infrared camera (IR). Three kinds of data obtained by the IR system were analyzed: the temperature distribution on the SMA sample surface, the temperature changes derived as average from the chosen sample area, and the temperature profiles obtained along the sample length. The temperature distribution shows nucleation of the transformation process and a creation of the transformation bands. The average temperature reflects the effects of thermomechanical coupling, accompanying exothermic martensitic forward and endothermic reverse transformation. The temperature profiles revealed the temperature difference between the band and the rest of the sample. The experimental results were supported with finite element method numerical analysis (FEM). The FEM software components for structural and heat transfer problems, coupled in partitioned approach, were used for thermomechanical analysis.

Unifying Monotonic and Hysteresis Material Properties for Notch Plasticity Analysis under Variable Amplitude Loads

This study was performed in support of accurate notch plasticity analysis under variable amplitude loads. Monotonic and cyclic strain-controlled tests were performed on flat dog-bone coupons machined from 6.35mm thick 7075-T651 aluminium alloy plates. The tests with low-high-low strain amplitude transitions revealed both an instant softening following the high strain amplitude cycle and a gradual recovery during the subsequent low strain amplitude cycles. The transition from monotonic-like to hysteresis-like material behaviour was found to be consistent with an overload induced softening and recovery process. A conceptual consideration is thus proposed to unify the characterisation of monotonic and hysteresis material properties. The implications of the present finding on notch plasticity analysis are also discussed.

Influence of Cyclic Deformation Induced Phase Transformation on the Fatigue Behavior of the Austenitic Steel X6CrNiNb1810

The austenitic steel X6CrNiNb1810 (AISI 347) was investigated in isothermal total strain-controlled tests at ambient temperature and T = 300 °C in the LCF-and HCF-range. The phase transformation from paramagnetic austenite (fcc) into ferromagnetic α´-martensite ́(bcc) leads to cyclic hardening and to an increase in fatigue life. At 300 °C no α´-martensite formation was observed in the LCF-range and the cyclic deformation behavior depends basically on cyclic hardening processes due to an increase of the dislocation density, followed by cyclic saturation and softening due to changes in the dislocation structure. In the HCF-range an increase in fatigue life was observed due to ε- and α´-martensite formation. Measurements of the mechanical stress-strain-hysteresis as well as temperature and magnetic properties enable a characterization of the cyclic deformation behavior and phase transformation in detail. The changes in the physical data were interpreted via microstructural changes observed by scanning-and transmission-electron-microscopy as well as by x-ray investigations. Additionally electromagnetic acoustic transducers (EMATs) developed from the Fraunhofer Institute of Non-destructive Testing (IZFP) Saarbrücken were used for an in-situ characterization of the fatigue processes.

The non-proportional behaviour of a nickel-based superalloy at room temperature, and characterisation of the additional hardening response by a modified cyclic hardening curve

Multi-axial deformation strain-controlled testing was completed at Swansea University to characterise the non-proportional hardening response of a nickel-based superalloy. The data was used to assess the validity of a non-proportional cyclic hardening curve correction to represent the hardening response of this alloy. A square strain path, equivalent to a fully out-of-phase loading, was found to exhibit additional hardening at moderate peak applied strains compared to the proportional in-phase case. This additional hardening was not observed at low strain levels or at a test condition applying a lower level of non-proportionality in the applied strain path, and was dependent on strain range.

Static and cyclic properties of clay subgrade stabilised with rice husk ash and Portland slag cement

In the present study, clay soil collected from new Banda, Uttar Pradesh, India has been treated with rice husk ash (RHA) and Portland slag cement (PSC). Based on unconfined compressive strength test results, the optimum mix obtained is of 82.5%Soil+7.5%PSC+10%RHA. The increase in strength of the optimum mix is about 29.8%, 37.2% and 48.55% for a curing period of 7, 14 and 30 days, respectively. The soaked California bearing ratio (CBR) test gives about 91.75% higher values as compared to unsoaked CBR test for a curing period of 30 days. Strain-controlled cyclic triaxial tests were conducted to study the variation of degradation index, shear modulus and damping ratio of the optimum mix with number of cycles for strain amplitudes of 0.4%, 0.6%, 0.8% and 1% and for frequencies of 0.2 and 1 Hz at an effective confining pressure of 100 kPa. It is observed that the degradation index decreases at a fast rate for the first 25–50 cycles. From the study, it is concluded that the aforementioned mix may be suitable for pavement subgrade material.

An internal variable dependent constitutive cyclic plastic material description including ratcheting calibrated for AISI 316L

The purpose of this work is to provide a reduced complexity constitutive model capable of accurately reproducing the material response of stainless steels under conditions of thermal stress-induced deformation occurring in the primary cooling circuit of a light water nuclear reactor. The proposed approach consists of a Chaboche-type material description, modified to allow the kinematic hardening parameters to vary as a function of a set of internal variables. The methodology implemented to calibrate the model using low cycle fatigue and strain-controlled ratcheting experiments performed on a stainless steel of grade AISI 316L is presented in detail. The improvement provided by the internal variable constitutive model is evaluated by a comparison with simulations carried out by means of an unmodified Chaboche description and with experimental data for strain-controlled, stress-controlled and thermo-mechanical low cycle fatigue tests.

Effect of Ultraviolet Light Aging on Fatigue Properties of Asphalt

The ultraviolet aging effects on fatigue properties of asphalt were studied by using Dynamic Shear Rheometer (DSR). The low-temperature fatigue performance of asphalt has been tested in this paper with the strain-controlled mode of DSR. The samples were compacted under the temperature of 10°C and the constant strain of 2.0% in strain-controlled mode fatigue test. And the results showed that ultraviolet aging decreases the fatigue life-span of asphalt significantly.

Cyclic deformation of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys: Effect of strain ratio

Cyclic deformation characteristics of electron beam welded (EBWed) joints between Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys were evaluated via strain-controlled low-cycle fatigue tests at varying strain ratios at a constant strain amplitude. The welding led to a significant microstructural change across the dissimilar joint, with hexagonal close-packed (HCP) martensite α' and orthorhombic martensite αʺ in the fusion zone (FZ), α' in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A distinctive asymmetrical hardness profile across the joint was observed with the highest hardness in the FZ and a lower hardness in the HAZ of Ti17 side than in the Ti17 base metal (BM), indicating the presence of soft zone. The strength and ductility of the dissimilar joint lay in-between those of two base metals (BMs). Unlike wrought magnesium alloys, the Ti–6Al–4V BM, Ti17 BM, and joint basically exhibited symmetrical hysteresis loops in tension and compression in the fully reversed strain-controlled tests at a strain ratio of Rε=−1. At a strain ratio of Rε=0 and 0.5, a large amount of plastic deformation occurred in the ascending phase of the first cycle of hysteresis loops of Ti–6Al–4V BM, Ti17 BM, and joint due to the high positive mean strain values. Fatigue life of the joint was observed to be the longest at Rε=−1, and it decreased as the strain ratio deviated from Rε=−1. A certain degree of mean stress relaxation was observed in the non-fully reversed strain controlled tests (i.e., Rε≠−1). Fatigue failure of the dissimilar joints occurred in the Ti–6Al–4V BM, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by fatigue striations.

High Temperature, Low Cycle Fatigue Characterization of P91 Weld and Heat Affected Zone Material

The high temperature low cycle fatigue behavior of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 °C and 500 °C. The cyclic behavior of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests. The weld material is shown to give a significantly harder and stiffer stress–strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress–strain response, which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the cross-weld tests.

Low Cycle Fatigue Properties of Alloy 617 base Metal and Weld Joint at Room Temperature

Alloy 617 is the one of the leading candidate materials for intermediate heat exchangers (IHX) of a Very High Temperature Reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. As a series of the work to better understand the LCF properties of Alloy 617 weld joints at high temperature, firstly, in this work, strain-controlled LCF testing of Alloy 617 base metal (BM) and weld joint (WJ) by a gas tungsten arc weld process (GTAW) were carried out. Fully reserved total-strain controlled LCF tests have been conducted at room temperature with four total strain ranges of 1.5, 1.2, 0.9, and 0.6%. For the LCF tests triangular test waveforms with a frequency of 0.25Hz were applied. The present paper is to characterize the LCF properties for Alloy 617 base metal (BM) and weld joint (WJ) from the cyclic stress response behavior and fatigue fracture behavior, with the comparative method. The cyclic stress response behavior was influenced by the level of total strain ranges and the material properties. Though base metal (BM) had shown higher plastic strain accumulation, the observed fatigue life of the weld joint (WJ) is lower than the base metal (BM). Coffin-Manson relationship and energy-life models can be used to determine the fatigue life.