Strain-controlled Tests Research Articles

Experimental investigation on electro-mechanically coupled cyclic deformation of laterally constrained dielectric elastomer

A series of electro-mechanically coupled cyclic tests at large deformation are carried out to characterize the cyclic deformation of a laterally constrained dielectric elastomer (DE) in this work. In the strain-controlled cyclic tests of the dielectric elastomer (e.g., VHB 4910 DE) with a constant or cyclic voltage, cyclic stress softening occurs and is influenced by the phase-angle difference between the applied cyclic strain and cyclic voltage. In the stress-controlled cyclic tests of VHB 4910 DE with a constant or cyclic voltage, ratchetting (a cyclic accumulation of inelastic strain) takes place; the ratchetting strain is considerably enhanced by applying higher voltage level, higher stress level and lower stress rate, and is also affected by the phase-angle difference between the applied cyclic stress and cyclic voltage. Moreover, the remarkable recovery of residual strain after the cyclic tests demonstrates that the cyclic stress softening and ratchetting of VHB 4910 DE mainly stem from the viscoelasticity. The comprehensive experimental observations are very useful to develop, calibrate and validate an electro-mechanically coupled constitutive model of dielectric elastomers.

Assessment of Dynamic Response of Cohesionless Soil Using Strain-Controlled and Stress-Controlled Cyclic Triaxial Tests

Soils subjected to earthquake motions undergo random variations in stress, strain and frequency during the entire period of shaking. The spatial and temporal complexity of the strong motions is commonly addressed through simplistic strain-controlled or stress-controlled cyclic loading tests as a part of laboratory investigations. Such a methodology is utilized and reported in the present study for assessing the dynamic response of cohesionless sand obtained from River Brahmaputra, India. In order to assess the dynamic response and liquefaction potential of the cohesionless Brahmaputra sand, both stress-controlled and strain-controlled cyclic triaxial tests were performed on reconstituted cylindrical specimens prepared at different relative densities (Dr) ranging from 30 to 90%. The specimens were subjected to varying effective confining pressures (σ′c: 50–200 kPa), shear strain amplitudes (γ: 0.015–4.5%) and cyclic stress ratios (CSR: 0.05–0.3). For all the tests, the frequency of the applied harmonic loading was maintained at 1 Hz. The magnitude of excess pore-water pressure (PWP) generated during successive loading cycles of the strain-controlled tests was found to be considerably lower than that generated in a stress-controlled test. The strain-controlled test reveals a reduction in the development of excess PWP with the increase in confining pressure and relative density, thereby indicating a decrease in liquefaction potential with increasing confining pressure. The stress-controlled test highlighted that based on a particular CSR, an increase in confining pressure results in the requirement of higher deviatoric stress and smaller numbers of cycles are required to initiate liquefaction. Although apparently misleading, it is imperative that during earthquake, same deviatoric stress is applied on the entire soil deposit. Thus, for specimens at larger depths having higher confining pressure, the CSR value is smaller, and thereby successive deeper layers require more number of stress cycles to liquefy. Hence, the study revealed that both stress-controlled and strain-controlled tests can be successfully used to assess the dynamic properties and liquefaction potential of cohesionless specimens. Based on the findings, the developed cyclic shear stress ratio (CSR) = 0.5 and cyclic shear strain amplitude (γ) = 0.5% are considered as the limiting conditions to achieve the onset of liquefaction through strain-controlled and stress-controlled approaches, respectively.

A test method for the determination of the cyclic material properties of electrical steel strip under strain-controlled loading

Abstract The rotors of electric machines are made of thin steel sheets, called electrical steel. Under service conditions, local strain may exceed the elastic limit i. e. for a lifetime assessment local strain might be of great significance. The main challenge for a reliable determination of the cyclic material properties of 0.3 mm thin electrical steel is to avoid buckling in the area of compression of fully reversed strain-controlled tests. Two approaches have been investigated. First, the test setup was improved and adapted to thin electrical steel specimens for which four different linear supported anti-buckling devices were developed and tested for fully reversed strain-controlled loading with the result that the best solution was a new approach using DLC coating instead of a polytetrafluoroethylene strip for reducing the friction between the specimen and the anti-buckling device. Secondly, the influence of increasing the specimen thickness by stacking single specimens through adhesive bonding is presented here.

Hysteresis energy based low cycle fatigue properties analysis in extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy at low temperature

The influences of low-temperature on low cycle fatigue (LCF) behaviors and fracture mechanisms of extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy with T6 state were investigated by strain-controlled LCF tests. The results show that the tensile strength increases, elongation and LCF life decreases with decreasing the temperature. The cyclic stress response behaviors of alloy are stable at various temperatures of room temperature (RT), 0 °C, −20 °C and −40 °C. And cyclic hysteresis energy densities are also stable after a short period of fluctuations at different conditions. In consideration of the different temperatures, the fatigue life evaluation model based on hysteresis energy is adopted. And the relationship between temperature and LCF life is derived from the parameters (Wo and β) of hysteresis energy model. The increase of hysteretic energy will lead to greater plastic damage, which is the main reason for the decrease of fatigue life, and the corresponding value of Wo significantly decreases. In addition, the fatigue crack initiates from the free surface of specimens and there exist a mass of fatigue striations in crack propagation region. The secondary cracks appear at low temperature, and the corresponding value of β slightly increases.

Deformation and Fatigue Behaviour of A356-T7 Cast Aluminium Alloys Used in High Specific Power IC Engines.

The continuous drive towards higher specific power and lower displacement engines in recent years place increasingly higher loads on the internal combustion engine materials. This necessitates a more robust collection of reliable material data for computational fatigue life prediction to develop reliable engines and reduce developmental costs. Monotonic tensile testing and cyclic stress and strain-controlled testing of A356-T7 + 0.5 wt.% Cu cast aluminium alloys have been performed. The uniaxial tests were performed on polished test bars extracted from highly loaded areas of cast cylinder heads. The monotonic deformation tests indicate that the material has an elastic-plastic monotonic response with plastic hardening. The strain controlled uniaxial low cycle fatigue tests were run at multiple load levels to capture the cyclic deformation behaviour and the corresponding fatigue lives. The equivalent stress-controlled fatigue tests were performed to study the influence of the loading mode on the cyclic deformation and fatigue lives. The two types of tests exhibit similar fatigue lives and stress-strain responses indicating minimal influence of the mode of loading in fatigue testing of A356 + T7 alloys. The material exhibits a non-linear deformation behaviour with a mixed isotropic and kinematic hardening behaviour that saturates after the initial few cycles. There exists significant scatter in the tested replicas for both monotonic and cyclic loading.

Effect of HIPing and degassing on the low cycle fatigue behavior of A319 cast alloy

Microstructure and low-cycle fatigue (LCF) behavior of A319 cast alloy were investigated in this work. Microstructure analysis indicated that both degassing and hot isostatic pressing (HIPing) process could help to reduce and eliminate porosity. The strain-controlled low-cycle fatigue tests were conducted at total strain amplitude ranging from 0.2% to 0.4%. The stress-strain hysteresis loops were acquired and the effect of HIPing and degassing on cyclic deformation behavior was analyzed combining the cyclic stress response curves. Moreover, the effect of HIPing and degassing on LCF life was analyzed, and a fatigue life model based on the area fraction of defects was proposed which was found to be suitable to predict life. Finally, the fatigue fractography observation showed that the presence of porosity could accelerate the initiation and propagation of cracks and the fatigue failure of materials.

The Role of Load Control Modes in Determination of Mechanical Properties of Granite

Circumferential strain-control tests were suggested to obtain the complete stress–strain curves (SSCs) of intact rock. However, compression test performed by the axial strain-control loading is mostly used to obtain the strength parameters and the SSCs. The influences of load control modes on the mechanical behavior of rock have not been fully investigated and understood. In this study, triaxial compression tests are conducted on granite specimens loaded by using both axial strain-control and circumferential strain-control modes to examine influences of load control modes on the determined mechanical properties. The occurrence mechanism and influencing factors of class II behavior are studied, and the potential application of the class II SSC in the evaluation of brittle failure of hard rock is also discussed. Results show that the peak strength, elastic modulus and fracture angle are generally higher in the axial strain-control test than in the circumferential strain-control test, and the failure in the former is also more violent than the latter under the same confining pressure. The much lower and constant circumferential deformation rate applied in the circumferential strain-control test is the dominant factor that favors the recovery of the complete post-peak SSC. The lower measured values of strength and elastic modulus are also associated with the lower deformation rate in the circumferential strain-control test, with a higher degree of damage and cohesion weakening occurring inside the rock. The development of the class II SSC is attributed to the axial strain recovery during the release of the stored elastic energy, which is influenced by the ratio of elastic energy to dissipated energy in the pre-peak stage, and is also associated with the consumed energy with respect to plastic deformation or initiation and propagation of new cracks in the post-peak stage. The class II SSC can aid the differentiation of the energy accumulation and consumption of different brittle rocks, which are usually characterized by a more abrupt and larger stress drop in the axial strain-control test. It can then be used to evaluate the strain rockburst potential of a particular rock type.

Effect of Auto-Tuning on Serrated Flow Behavior

The mechanical response of a servo-hydraulic testing system is affected by the stiffness of test specimens. An adaptive controller helps in auto-tuning the system by setting the optimal proportional-integral-derivative values for the subsequent test as the stiffness changes. This paper presents the effect of auto-tuning of various channels on the flow response of several commercial Al and Mg alloys and a mild steel. Strain-controlled monotonic tensile tests were performed at a given strain rate of 1 × 10−4 s−1 after auto-tuning of position, load, and strain channels in different combinations. Serrated flow or Portevin–Le Chatelier effect was observed in the Al alloys after auto-tuning of either position channel only or position and load channels. However, the serrations of Al alloys were shielded after further auto-tuning of strain channel. The stress-strain curves of Mg alloys and mild steel were observed to be basically free of serrations under any combinations of auto-tuning, which confirms that the serrated flow is a property of specific materials rather than a machine system noise.

Experimental study on pure-shear-like cyclic deformation of VHB 4910 dielectric elastomer

The cyclic deformation of VHB 4910 dielectric elastomer is experimentally investigated by performing a series of strain-controlled and stress-controlled pure-shear-like cyclic tests at room temperature. In the strain-controlled cyclic tests, obvious Mullins effect is observed in the first cycle, and continuous stress softening is characterized in the subsequent cycles; the Mullins effect and continuous stress softening are intensified by prescribing larger peak strains and higher strain rates. In the stress-controlled cyclic tests, remarkable ratchetting takes place, and its evolution becomes more significant if higher stress levels, lower stress rates and longer hold time at peak stress are prescribed. Moreover, a partial recovery of residual strain indicates that, besides obvious hyper-elasticity, the VHB 4910 dielectric elastomer also exhibits both recoverable viscoelasticity and measurable irrecoverable visco-plasticity during the cyclic tests.

Viscoelastic properties of the central region of porcine temporomandibular joint disc in shear stress-relaxation

In this study, shear relaxation properties of the porcine temporomandibular joint (TMJ) disc are investigated. Previous studies have shown that, in fatigue failure and damage of cartilage and fibrocartilage, shear loads could be one of the biggest contributors to the failure. The aim of the present study is to develop an evaluation method to study shear properties of the disc and to do a mathematical characterization of it. For the experiments, twelve porcine discs were used. Each disc was dissected from the TMJ and, then, static strain control tests were carried out to obtain the shear relaxation modulus for the central region of the discs. From the results, it was found that the disc presents a viscoelastic behavior under shear loads. Relaxation modulus decreased with time. Shear relaxation was 10% of the instantaneous stress, which implies that the viscous properties of the disc cannot be neglected. The present results lead to a better understanding of the discs mechanical behavior under realistic TMJ working conditions.

Asymmetric cyclic response of tensile pre-deformed Cu with highly oriented nanoscale twins

History-independent, stable and symmetric cyclic response has been detected in as-deposited bulk polycrystalline Cu with highly oriented nanotwins [Nature 551 (2017) 214–217]. In this study, to deepen the understanding of cyclic deformation in nanotwinned (NT) structures, small levels of tensile pre-strains were applied on NT-Cu, followed by strain-controlled symmetric tension-compression cyclic tests. Distinct from the symmetric cyclic response of as-deposited NT-Cu, the magnitude of the maximum stress in tension is much larger than that of the minimum stress in compression, indicating that the cyclic response of tensile pre-deformed NT-Cu is highly asymmetric. The degree of its cyclic asymmetry gradually decays as the number of cycles or the plastic strain amplitude is increased. The tensile pre-deformed NT-Cu recovers to its symmetric cyclic response after cyclic deformation at sufficiently large plastic strain amplitude, analogous to that detected in as-deposited NT counterparts. Molecular dynamics simulations and microstructural observations revealed that the observed asymmetric cyclic response is mainly related to the activation and movement of threading dislocations with extended misfit dislocation tails lying on the twin boundaries (TBs) during tensile pre-deformation. During cyclic deformation, threading dislocations in adjacent twin interiors tend to link their long tails with one another to form correlated necklace dislocations (CNDs) with a symmetric structure. The CNDs move back-and-forth along the twin boundaries without directional slip resistance, contributing to the transition from asymmetric to symmetric cyclic response of NT-Cu.

Finite-Element Crystal Plasticity on Phase-Field Microstructures: Predicting Mechanical Response Variations in Ni-Based Single-Crystal Superalloys

The mechanical response of Ni-based single-crystal superalloys is known to be sensitive to the microstructural state, i.e., the shape and size of the γ′ precipitates when exposed to high-temperature conditions. The magnitude and sign of the natural lattice misfit between the γ and γ′ phases play the most crucial role in establishing a controlled size, shape, and distribution of γ′ precipitates during heat treatments as well as in defining the direction of rafting, viz. the directional coalescence of the γ′ precipitates. In this study, a bottom-up scale bridging strategy of using phase-field informed finite-element (FE) crystal plasticity on realistic microstructures is followed to better understand the effect of the microstructural state on the macro-scale performance of a $$ \left\langle {001} \right\rangle $$ -oriented Ni-based single-crystal superalloy. Strain-controlled tensile tests using FE crystal plasticity were performed on a set of different microstructural states: cuboidal, rafted, and topologically inverted imported from 3D phase-field simulations. The study revealed that a cuboidal microstructure with a natural lattice misfit of − 0.004 is the most ductile. As observed experimentally, the microstructure with rafts perpendicular to the loading axis (N-type) is more ductile than the cuboidal one. The P-type microstructure, i.e., with rafts parallel to the loading axis, is found to have the lowest ductility, which was attributed to lesser dislocation mobility.

An Energy-Based Unified Approach to Predict the Low-Cycle Fatigue Life of Type 316L Stainless Steel under Various Temperatures and Strain-Rates.

An energy-based low-cycle fatigue model was proposed for applications at a range of temperatures. An existing model was extended to the integrated approach, incorporating the simultaneous effects of strain rate and temperature. A favored material at high temperature, type 316L stainless steel, was selected in this study and its material characteristics were investigated. Tensile tests and low-cycle fatigue tests were performed using several strain rates at a temperature ranging from room temperature to 650 °C. Material properties were obtained in terms of temperature using the displacement-controlled tensile tests and further material response were investigated using strain-controlled tensile tests. Consequently, no pronounced reduction in strengths occurred at temperatures between 300 and 550 °C, and a negative strain rate response was observed in the temperature range. Based on the low-cycle fatigue tests by varying strain rates and temperature, it was found that a normalized plastic strain energy density and a strain-rate modified cycle were successfully correlated. The accuracy of the model was discussed by comparing between predicted and experimental lives.

Dynamic behaviour of micaceous sand with varying mica content and its association with compactability, compressibility and monotonic shear response

ABSTRACT Cohesionless soils are highly susceptible to liquefaction under dynamic loading with high-strain amplitude simulating earthquake loading conditions. The presence of flaky mica particles among spherical sand influences the microscopic particle level mechanisms due to complex geometric arrangements caused by bridging, ordering and pore filling phenomena. Such variations substantially affect the compactability, compressibility and shear strength behaviour under monotonic and dynamic loading conditions. The current experimental investigations evaluated the effect of mica content on dynamic behaviour of micaceous sand through a series of strain-controlled cyclic triaxial tests at constant frequency and double axial strain amplitude of 0.5 Hz and 0.6% per cycle. The variation in dynamic parameters such as shear modulus and damping was determined at different mica contents along with cyclic stiffness degradation. Cumulative dissipated strain energy was also estimated to understand the contact-force networks of varying sand and mica matrix from sand-sand to mica-mica through sand-mica particle contact.

Mechanical behavior of an unsaturated clayey silt: an experimental and constitutive modelling study

This paper reports an experimental study and subsequent constitutive modelling focused on the stress-strain and volumetric responses during deviatoric stress application of a partially saturated clayey silt. The material was statically and isotropically compacted at constant water content towards a pre-defined pre-consolidation stress. A series of strain-controlled triaxial compression tests on a state of the art device and isotropic experiments are presented and discussed. The triaxial tests started at the same stress state (i.e., identical matric suction and mean net stress) and were conducted at the same constant suction. Several stress paths under isotropic conditions (i.e., drying–wetting, loading–unloading, and wetting–drying) were followed to induce different overconsolidated states before shearing the specimens. The test results are initially interpreted using the elastoplastic Barcelona Basic Model (BBM). Independent tests were selected to determine the model parameters associated with the volumetric behavior of the soil. The BBM was not able to capture the dilatant behavior observed during shearing. An enhancement of the BBM is proposed in this work, which includes both, a more general hardening law and sub-loading concepts. The main capabilities and limitations of the original BBM and the enhanced model are discussed and compared. The modified BBM was able to handle the dilatancy observed in the experiments and provided a more realistic description of the experimental stress-strain response.

단결정 니켈기 초내열합금 CMSX-4의 고온 저주기 피로특성

Isothermal low cycle fatigue (LCF) behavior of a crystal nickel-based superalloy CMSX-4, a material for high-pressure turbine first stage rotor blade, was investigated at elevated temperatures. Strain-controlled LCF tests were performed under various test conditions, such as mechanical strain amplitude. Stress response and cyclic deformation were investigated, and equations of LCF life prediction were derived through the Coffin-Manson method. In addition, fatigue-induced fracture mechanism and microstructural evolution were investigated, using scanning electron microscopy (SEM). Results revealed that cyclic behavior of the CMSX-4 superalloy, was characterized by cyclic softening with increasing number of cycles at 800oC and 900oC. LCF of the CMSX-4 superalloy at 800oC and 900oC could be affected mainly by elastic damage in fatigue processing. Fatigue cracks were initiated in the surface oxide layer of the specimen. The plane of fracture surface was tilted toward 001 direction. The fatigue fracture mechanism was quasi-cleavage fracture at 800oC and 900oC. In all broken specimens, the γˊ phase morphology maintained cuboidal shape.

Low cycle fatigue behavior of additively manufactured Ti-6Al-4V under non-proportional and proportional loading

Experimental tests were conducted on additive manufactured Ti-6Al-4V titanium alloy to investigate the mechanical and crack properties under multiaxial cyclic loading. Selective Laser Sintering technique (SLS) was employed to fabricate four types of cylindrical hollow specimens. The typology of each specimen is defined by the orientation of the layers and by the application of a stress-relieving heat treatment after the production process. Stress-strain cyclic curves of the materials were obtained to investigate the material cyclic plastic behavior, that resulted independent of specimen variety. Strain-controlled multiaxial low cycle fatigue tests under proportional and non-proportional loading paths were carried out on the specimens. Not heat-treated specimens exhibited a higher low cycle fatigue resistance both for proportional and non-proportional loading. Drastic initial softening was detected in the majority of the tests. Additional hardening was detected in part of non-proportional tests, which is atypical for this alloy. The mutual influence of applied load and microstructural characteristics on fatigue life are finally discussed.

Simultaneous enhancement of stress- and strain-controlled fatigue properties in 316L stainless steel with gradient nanostructure

A gradient nanostructured (GNS) surface layer with full austenitic phase was synthesized on AISI 316L stainless steel by surface mechanical rolling treatment at ∼280 °C. The mean grain size is ∼45 nm at the top surface and increases gradually with depth. Deformation-induced martensite (DIM) transformation was suppressed and the microstructural refinement was dominated by dislocation activities and twinning during the formation of the GNS layer. Axial tension-compression fatigue tests showed that fatigue strength and life are simultaneously enhanced in the GNS samples relative to the corresponding coarse-grained counterparts in both stress- and strain-controlled tests. This is very different from fatigue behavior of conventional nanostructured materials, i.e. an enhanced stress-controlled fatigue strength with a decreased strain-controlled fatigue life. Besides contributions from the enhanced mechanical properties and the suppressed surface defects formation, analyses of fatigue mechanisms demonstrated that the promoted formation of DIM during cyclic strain plays a crucial role in enhancing fatigue properties of the GNS samples in strain-controlled tests.

Implement the Laplace transform to convert viscoelastic functions of asphalt mixtures

The relaxation modulus of an asphalt mixture is adopted for evaluation of pavement structural soundness. However, it is difficult to perform a strain-controlled relaxation test on an asphalt mixture to obtain the relaxation modulus. The aim of this paper is to obtain the relaxation modulus expression in the Laplace transform domain and then perform a Laplace transform to derive the relaxation modulus from creep compliance. A uniaxial static creep test was conducted on four types of asphalt mixtures at 20 °C, 30 °C, and 40 °C and under a load of 0.3 MPa. The Prony series and power law were used to model creep compliance with good fitting of R2 > 0.95. Because the Laplace transform of the power law was a gamma function, the Prony series was used to simulate creep compliance and then applied to interconverting the viscoelastic functions. The Simpson and Gauss methods were used to convert the relaxation modulus from creep compliance based on a convolution formula and the calculation results from the two methods were shown to be equivalent. Based on the Laplace transform formula for creep compliance and the relaxation modulus, an equation for calculating the relaxation modulus in the Laplace domain was proposed. Numerical inversion of the Laplace transform, using the fixed Talbot method, was implemented to compute the relaxation modulus in the time domain and the results were compared with those derived by using the Gauss method. The average error is less than 10−4 and the computational speed of the fixed Talbot method is a factor of 25 faster than that of the Gauss method, demonstrating the high precision and high efficiency of the Laplace transform method for computing the relaxation modulus. The method developed in this study provides an effective tool for interconverting viscoelastic functions of asphalt mixtures.

Experimental Studies on Effect of Load Repetition on Dynamic Characteristics of Saturated Ahmedabad Cohesive Soil

The present study investigates the hysteresis response, stiffness degradation/cyclic softening and energy dissipation features of Ahmedabad cohesive soil. Saturated specimens of Ahmedabad cohesive soil were reconstituted using slurry consolidation technique at different void ratios to perform the following series of tests, strain-controlled cyclic triaxial tests, shear strength, and compressibility tests. Load repetition exhibited significant degradation in shear modulus of slurry consolidated cohesive soil specimens under cyclic loading conditions. A maximum stiffness degradation of 78% was observed for the specimen possessing minimum initial void ratio. Hysteresis response of Ahmedabad soil revealed alteration in the rotation angle of hysteresis loops and their sizes with varying initial void ratios of the Ahmedabad cohesive soil. A substantial influence of initial void ratio on the energy dissipation behavior of cohesive soil was depicted in the present study. The cumulative energy dissipation of the specimen prepared with lower initial void ratio was obtained to be 7.1 times higher than the specimen possessing maximum initial void ratio.