Cyclic Loading-unloading Tests Research Articles

Freeze-thaw resistance of eco-material stabilized loess

In the Loess Plateau in Northern China, repeated freeze-thaw (FT) cycles deteriorate the strength and structure of loess as a foundation soil, resulting in the instability or failure of supporting structure. Lignosulfonate is an eco-material, utilized as an effective and nontraditional stabilizer to improve the engineering properties of metastable soils. A series of laboratory tests, including unconfined compression tests, cyclic loading-unloading tests and scanning electron microscopy, on calcium lignosulfonate (CL)- and sodium lignosulfonate (SL)-stabilized loess were performed to investigate the stabilization effect, deterioration mechanisms of the FT cycles, and the resistance to FT cycles. Two traditional stabilizers, quicklime (QL) and sodium silicate (SS), were selected, and the engineering properties of QL- and SS-stabilized loess were compared with those of CL- and SL-stabilized loess. The results showed that the strength values of CL- and SL-stabilized loess specimens decreased by 34.2% and 50% respectively, after 20 FT cycles, whereas those of the traditionally SS- and QL- stabilized specimens decreased by 85.3% and 82.87%, respectively. The elastic moduli of SL- and QL-stabilized loess specimens decreased by 22.1% and 92.0%, respectively. The mean energy dissipations of nontraditionally treated specimens also decreased significantly less than those of traditionally treated specimens. Overall, the results showed CL and SL had better stabilization effects on engineering properties of loess than QL and SS, and their stabilized loess specimens exhibited stronger resistance to FT cycles. The study findings demonstrated the significant potential of lignosulfonate for extensive application in cold loess areas.

The effect of high-pressure torsion on the microstructure and outstanding pseudoelasticity of a ternary Fe–Ni–Mn shape memory alloy

Abstract Experiments were conducted to examine the effect of high-pressure torsion (HPT) processing on the microstructure and pseudoelastic behavior of a ternary Fe–10Ni–7Mn (wt.%) shape memory alloy in both the solution-annealed and intercritically-annealed conditions. X-ray diffraction (XRD) patterns and electron backscatter diffraction (EBSD) analyses showed that the initial microstructure of the alloy in the solution-annealed condition was a fully lath α′-martensite which partially transformed to a strain-induced austenite (α′→γ) by HPT processing. Also, the austenite formed in the dual phase (α′+γ) specimens after intercritical annealing treatment at 600 °C for 7.2 ks underwent a γ→e→α′ transformation during subsequent HPT processing such that a multi-phase microstructure was formed consisting of α′-martensite, austenite and e-martensite. The HPT processing led to a significant increase in the microhardness value to ~690 Hv due to a high density of dislocations and the associated grain refinement of the microstructure. Cyclic loading-unloading tensile tests at room temperature revealed a strain hysteresis and pseudoelastic behavior in the HPT-processed specimens with different initial microstructures. Outstanding pseudoelasticity values of about 67% and 75% were obtained at the fourteenth loading-unloading cycle after 20 HPT turns in the solution-annealed and intercritically-annealed specimens, respectively.

Evaluation of Deformation and Permeability Behavior of Amygdaloidal Basalt under a Triaxial Cyclic Loading Creep Test

Amygdaloidal basalt is a typical rock mass in the dam foundation of the Baihetan hydropower project in southwest China. With rising and drawdown of the reservoir water level, the permeability and creep deformation characteristics of the amygdaloidal basalt are much complicated in the long-term cyclic loading processes. A cyclic loading-unloading creep test on the amygdaloidal basalt was performed to evaluate its deformation and permeability behavior. The results showed that Poisson’s ratio and elastic modulus of the rock specimen varied significantly under different loading processes with a relatively large irreversible deformation. The permeability and strain rates of rock changed in two phases under lower deviatoric stresses, while there are three typical stages of strain growth with the final stress level of 121.8 MPa. For axial stress of 128 MPa, the creep deformation and creep rate in the axial direction are smaller than these in the lateral direction. Before the sample failure, the lateral deformation accelerates earlier than the axial deformation. The results also suggested that the permeability of the rock specimens decreases considerably during each loading process and then tends to be constant with time. No apparent change in steady permeability is observed with variation of stress. For 128 MPa axial stress, the permeability first decreases, then tends to be in a stable value, and at last increases during the sample failure.

Effect of working temperature on the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites

In the present study, the interfacial behavior of overmolded hybrid fiber reinforced polypropylene composites (hybrid composites) in the working temperature range from 23 °C to 90 °C was studied by experimental and constitutive methods. Monotonic and cycle loading-unloading single-lap-shear tests were employed to determine the interfacial properties of hybrid composites. The experimental results show that both interfacial shear strength and shear stiffness decrease with increasing working temperature. A regression function was adopted to evaluate the decaying degree of interfacial properties with increasing working temperature. The shear stress-displacement relationship under monotonic loading exhibits nonlinear behavior after an initial elastic region. The envelope lines of shear stress-displacement of hybrid composites under cyclic loading indicate that the nonlinearity in the curve is caused by the plastic deformation of polypropylene in the interphase region. A constitutive model was built to describe the nonlinear shear stress-displacement relation of hybrid composites at different working temperatures. A full suite of temperature-dependent plastic parameters in the model was obtained from cyclic loading-unloading tensile tests. The predicted shear stress–displacement curves agreed well with experimental results from different working temperatures. In addition, the failure mode of hybrid composites varied with working temperature.

Effect of stress induced damage on gas permeability of Beishan granite

Permeability is the key factor to control long-term stability of surrounding rock of high level radioactive waste (HLW) geological repository. In view of the low permeability of the intact granite in Beishan area, the preferred site for HLW repository in Northwest China, argon was used as the fluid medium to measure the gas permeability of Beishan granite under hydrostatic pressures and cyclic loading-unloading triaxial tests. It is found that the gas permeability decreases with the increase of confining pressure and their relationship approximately conforms to the power function. Due to irreversible plastic deformation, the permeability during unloading is less than that in loading process under the same confining pressure level. In the triaxial loading-unloading test, the acoustic emission technique applied well reflects the micro-crack propagation process and quantifies the cumulative damage in granite samples. It shows that the dilatancy stress is the threshold stress of the micro cracks in rock turn from stable growth to accelerated growth. This is the physical mechanism of significant increase of permeability after reaching the dilatancy stress.

Evolution of crystallographic orientation, precipitation, phase transformation and mechanical properties realized by enhancing deposition current for dual-wire arc additive manufactured Ni-rich NiTi alloy

Ni-rich NiTi alloys were deposited using the in-situ alloying wire arc additive manufacturing (WAAM) method, with varying deposition currents from 80 A to 120 A. The effects of deposition current on the crystal orientation, precipitation, phase transformation and mechanical properties of the WAAM-deposited NiTi alloys were investigated. The results show that increasing the deposition current during the WAAM process would result in noticeable coarsening of B2 grain and an increased volume fraction of high angle grain boundaries (HAGBs). Also, the texture intensity gradually decreased with increasing deposition current. The fabricated components are dominated by the B2 phase with quantities of Ni4Ti3 precipitates in all samples. When increasing the deposition current during the WAAM process, the size of Ni4Ti3 precipitates generally increased and gradually decomposed into a stable Ni3Ti phase which could be detected in the sample produced at 120 A. Furthermore, all of the characteristic phase transformation temperatures increased with the deposition current while the ultimate tensile strength dropped from 927.9 MPa to 613.8 MPa and elongation reduced from 8.7 % to 5.6 %. The cyclic loading-unloading tests revealed that similar trends for the evolution of irreversible strain (εir), recoverable strain (εre), recovery ratio, and elastic energy storage efficiency (η) during cycling were obtained in all samples processed with different deposition currents. The highest εre of 3.2 % and the highest recovery ratio of 53.9 % were obtained in the sample processed with 80 A at an applied stress of 700 MPa for ten cycles. The change of mechanical properties with varying deposition current is due to a combination of factors including precipitation hardening effect, grain refinement effect, and crystal orientation. These results can be useful for optimizing WAAM process parameters to fabricate NiTi components with acceptable structural properties.

Calcium phosphate bioceramics with polyvinyl alcohol hydrogels for biomedical applications

Polyvinyl alcohol (PVA) hydrogels show desirable characteristics to use as a biomaterial especially for soft tissue replacement. However, their bio inertness restricts their application in vivo. In this study, polyvinyl alcohol was blended with bi-phasic calcium phosphate and to develop a composite hydrogel by a physical freeze-thawing method, followed by annealing treatment. The synthesized bi-phasic calcium phosphate (BCP) and composite hydrogels were characterized by SEM, XRD and FTIR. The concentration of BCP was optimized and it was found that BCP modifies the hydrogel network by developing the secondary electrostatic bonding between matrix and reinforcement. The highest tensile and compressive strength could reach 5.2 ± 0.6 MPa and 14.9 ± 0.3 MPa respectively for PVA/2.5BCP and they exhibit time-dependent, rapid self-recoverable and fatigue resistant behavior based on the cyclic loading-unloading compression test. Similar observations were found for viscoelastic properties which are relevant for the tissue engineering application. Friction study showed the composite hydrogel had a cartilage-like frictional response, dominated by the interstitial fluid support. Besides composite hydrogel showed excellent antimicrobial activity against bacterial species, Escherichia coli, Staphylococcus aureus and Candida albicans fungi, and the cytocompatibility towards L929 fibroblast cells provides a potential pathway to develop a hydrogel as a promising substitute for tissue engineering scaffold material.

Nanohydroxyapatite Reinforced Chitosan Composite Hydrogel with Tunable Mechanical and Biological Properties for Cartilage Regeneration

With the continuous quest of developing hydrogel for cartilage regeneration with superior mechanobiological properties are still becoming a challenge. Chitosan (CS) hydrogels are the promising implant materials due to an analogous character of the soft tissue; however, their low mechanical strength and durability together with its lack of integrity with surrounding tissues hinder the load-bearing application. This can be solved by developing a composite chitosan hydrogel reinforced with Hydroxyapatite Nanorods (HANr). The objective of this work is to develop and characterize (physically, chemically, mechanically and biologically) the composite hydrogels loaded with different concentration of hydroxyapatite nanorod. The concentration of hydroxyapatite in the composite hydrogel was optimized and it was found that, reinforcement modifies the hydrogel network by promoting the secondary crosslinking. The compression strength could reach 1.62 ± 0.02 MPa with a significant deformation of 32% and exhibits time-dependent, rapid self-recoverable and fatigue resistant behavior based on the cyclic loading-unloading compression test. The storage modulus value can reach nearly 10 kPa which is needed for the proposed application. Besides, composite hydrogels show an excellent antimicrobial activity against Escherichia coli, Staphylococcus aureus bacteria’s and Candida albicans fungi and their cytocompatibility towards L929 mouse fibroblasts provide a potential pathway to developing a composite hydrogel for cartilage regeneration.

Mechanical properties and energy development characteristics of impact-prone coal specimens under uniaxial cyclic loading

Energy is the intrinsic driving factor of rock dynamic failure. In this paper, the mechanical properties and energy development characteristics of impact-prone coal are studied. The stress and strain data and the acoustic emission (AE) signals of the impact-prone coal specimens are obtained by conducting cyclic loading-unloading tests. The elastic modulus deterioration rate is defined to describe the deterioration of the mechanical properties of the specimens during the loading process. The calculation results show that the elastic modulus deterioration rate of the impact-prone coal specimens during the cyclic loading process is as low as 3%. The AE monitoring results show that the AE signal is rare in the loading-unloading cycle before failure, and a large number of AE signals are generated at the failure moment. The stress-strain data are used to analyze the energy storage and distribution law of the loading process of impact-prone coal specimens. The results show that the input energy density increases nonlinearly with the increase of the load, and the input energy is mostly stored in the rock as elastic strain energy. The dissipative energy density is linear with the axial load. The impact-prone coal specimens have a strong ability to store elastic energy, and the precursor of damage and failure is not obvious. The study in this paper reveals the cause of the impact-prone coal easily induced to coal bursts from the energy point of view.

Superelastic, fatigue resistant and heat insulated carbon nanofiber aerogels for piezoresistive stress sensors

Ultralight flexible carbon aerogels have been applied in many fields but are always hampered due to their weak mechanical stability, large energy dissipation, and complex preparation methods. Here, a novel polyimide-derived carbon nanofiber aerogel with high fatigue resistance and excellent flexibility is prepared by using the designed “fiber gluing” to construct the elastic continuous fibrous network. The as-prepared carbon nanofiber aerogels exhibit ultralow density of 6.6 mg cm−3, excellent fire-resistant performance when exposing to alcohol flames (650 °C), and robust elastic resilience even under 55% compressive strain. In particular, the carbon nanofiber aerogels show ultralow plastic deformation (only 1.3%) and low energy dissipation (<0.22) after 1000 cyclic loading-unloading tests at 55% compressive strain. Benefiting from their excellent flexibility and robust mechanical stability, the as-prepared carbon nanofiber aerogel are a promising candidate in the application of wearable devices and piezoresistive stress sensors.

Stimuli Responsive, Self-Sustainable, and Self-Healable Functionalized Hydrogel with Dual Gelation, Load-Bearing, and Dye-Absorbing Properties.

The motivation for designing low-molecular-weight gelators with self-healing characteristics originates from elegant examples in biology such as vines of the genus Aristolochia whose internal secondary growth exhibits rapid self-healing in their stems. In the present work, we had explored the stimuli-responsive dual gelation characteristics for the ester-functionalized surfactant (4-(2-(hexadecyloxy)-2-oxoethyl)-4-methylmorpholin-4-ium bromide, C16EMorphBr) in aqueous medium at 7.20% (w/v) critical gel concentration and pH 7.4. The hydrogel provides an excellent platform to study dynamic phase behavior within a supramolecular network as it exhibits transformation from a fibrillar opaque hydrogel to a transparent hydrogel upon heating. Molecular interactions, arrangement within the supramolecular framework, and mechanical properties of the hydrogels were characterized using Fourier transform infrared, small-angle neutron scattering, rheological analysis, and tensile strength and cyclic loading-unloading tests. The fibrillar opaque gel has been characterized for its morphology using scanning electron microscopy, field emission scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The self-sustained, self-healable porous fibrillar opaque xerogel was further explored for selectively absorbing anionic dyes and for its load-bearing characteristics. We conclude a perspective on designing a new-age gelator that can open entirely new avenues in environmental protection and wearable "smart" devices.

Nanocomposite interpenetrating hydrogels with high toughness and good self-recovery

It is particularly desirable to fabricate highly tough hydrogels with excellent self-recoverable properties for applications where high stress is required. In this work, we prepared a tough, fast self-recoverable nanocomposite hydrogel by chemical cross-linking of acrylamide (AM) monomers with vinyl-modified silica nanoparticles (VSNPs), combined with physical cross-linking of polyvinyl alcohol (PVA). The uniaxial tensile test showed that the nanocomposite hydrogel has excellent mechanical properties. The maximum elongation at break was as high as 666%, and the tensile strength was as high as 1.68 MPa. Cyclic loading-unloading tests revealed the excellent self-healing properties of the nanocomposite hydrogel. It is worth noting that the nanocomposite hydrogel exhibited higher strength after two loading-unloading cycles, due to the orientation of the PVA when stretched. In addition, the effects of PVA, VSNPs, and AM concentrations, and the number of PVA freeze-thaw cycles and freezing duration on the mechanical properties of the hydrogels were investigated in detail.

The mechanical behavior of epoxy asphalt mixture under cyclic loading

In order to investigate the mechanical behavior of epoxy asphalt mixture under cyclic traffic loading, cylindrical samples of the epoxy asphalt mixture were subjected to cyclic loading-unloading tests under varying confining pressures. The accumulation and dissipation of energy were observed in the energy evolutions. It turns out that the epoxy asphalt mixture exhibits elastic-plastic property in the first few cycles, and then the elastic property would be strengthened gradually in the cyclic loading tests. Applying a moderate confining pressure would significantly reduce the dissipated energy. The proportion of dissipated energy is about 25% from the unconfined test, and less than 15% from the confined test. As a result, more energy would be required if to produce equal permanent deformation under higher confining pressures. Therefore, a moderate increase in the confining pressure would be helpful in restraining permanent deformation, such as rutting.

Mechanical Characteristics and Energy Evolution Laws for Red Bed Rock of Badong Formation under Different Stress Paths

Exploring mechanical properties of red bed rock of Badong Formation Section two (b2) from the Three Gorges Reservoir is crucial to determine the instability mode of reservoir slopes. In order to reveal the energy evolution laws for b2 rocks under different stress paths, loading tests, unloading tests, and cyclic loading‐unloading tests were conducted, respectively. The results show that stress paths have significant influences on the mechanical properties and energy evolution characteristics of b2 rocks and the failure mechanism under three stress paths is revealed. Relative to the loading tests, the mechanical parameters of b2 rocks are deteriorated under the unloading conditions. In addition, the increasing trend of cumulative dissipation energy (CDE) is similar with that of total absorption energy (TAE) and the ESE‐strain curves are characterized by multistep rise. For cyclic loading‐unloading tests, the areas of hysteretic loop gradually increase as the cycle times increase, and the TAEs and the coefficient of the cumulative dissipation energy (CCDE) also increase gradually with approximately linear characteristics, while the CDEs gradually increase in an abrupt rate. This work can contribute to provide the failure mechanism of b2 rocks for evaluating the stability of reservoir slopes from the energy perspective.

Atypical Defect Motions in Brittle Layered Sodium Titanate Nanowires.

In situ tensile tests show atypical defect motions in the brittle Na2Ti3O7 (NTO) nanowire (NW) within the elastic deformation range. After brittle fracture, elastic recovery of the NTO NW is followed by reversible motion of the defects in a time-dependent manner. An in situ cyclic loading-unloading test shows that these mobile defects shift back and forth along the NW in accordance with the loading-unloading cycles and eventually restore their initial positions after the load is completely removed. The existence of the defects within the NTO NWs and their motions does not lead to plastic deformation of the NW. The atypical defect motion is speculated to be the result of the glidibility of the TiO6 layers, where weakly bonded cation layers are in between. Exploration of the above novel observation can establish new understandings of the deformation behavior of superlattice nanostructures.

Progressive Failure Analysis of Composite Laminates Including Shear Nonlinearity

A progressive damage model including the non-linear shear behavior was developed to investigate the failure mechanisms of composite laminates. The non-linear shear response of the T300/Cycom970 lamina was derived by cyclic loading-unloading tensile tests on [+45/-45]2S laminate coupons. The strain-based Hashin type failure criteria and ply discount method were employed to predict damage initiation and propagation. The model was implemented in commercial finite element software ABAQUS by a UMAT subroutine. The tensile behavior of open-hole tension specimen was predicted with this model. A reasonable good agreement was achieved between numerical predictions and experimental results.

Tensile Creep Behavior of Quasi-Unidirectional E-Glass Fabric Reinforced Polypropylene Composite.

The present work addressed the creep behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading. A series of creep tests were performed on the composite at three different loading stress levels. The creep response of off-axis samples of quasi-unidirectional composites under a constant loading level can be clearly observed. A phenomenological viscoplasticity model was built for describing the creep behavior of the composite. To improve the accuracy of prediction, cyclic loading-unloading tests were adopted to determine the material constants in the model. The predicted results in terms of the strains after a load over a period of time were found to be satisfactory, compared with the experimental results. In addition, same failure mechanism was found in off-axis samples under quasi-static and creep loading cases.

Strain rate-dependent mechanical behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

To make a better use of quasi-unidirectional fabrics in structural application, the strain rate-dependent tensile behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites was studied. Monotonic tensile tests were firstly performed on 10°, 20° and 90° off-axis samples of composites. The experimental results show that both mechanical properties and nonlinear stress-strain behavior of composites are dependent of strain rate. A regression function is adopted to describe the variation in overall mechanical properties with the imposed strain rate. A strain rate-dependent coupled damage-plasticity model is built for quasi-unidirectional composites. The damage variables and plastic parameters in the proposed model are determined through cyclic loading-unloading tensile tests and can be represented as a function of the imposed strain rate. In addition, the damage mechanism of composites is observed to evolve with strain rate. Increasing strain rate can hinder the initiation and evolution of damage in composites.

Temperature-dependent response of quasi-unidirectional E-glass fabric reinforced polypropylene composites under off-axis tensile loading

Abstract The temperature-dependent off-axis tensile behavior of quasi-unidirectional E-glass fabric reinforced polypropylene composites has been investigated both experimentally and constitutively in this paper. The nonlinearity in composites under off-axis loading is shown to be a combination of damage and plasticity regardless of temperature. A nonlinear constitutive model is established to predict the temperature-dependent nonlinear stress-strain curves of composites, in which the accumulation of plastic strain and evolution of damage in composites are described by the plasticity and damage theories, respectively. A full suite of temperature-dependent plastic and damage parameters in the model are determined through cyclic loading-unloading tests.

Highly Stretchable and Wearable Strain Sensor Based on Printable Carbon Nanotube Layers/Polydimethylsiloxane Composites with Adjustable Sensitivity.

Strain sensors that are capable of monitoring complex human motions with high accuracy are highly desirable for developing wearable electronics. This paper reports the fabrication of highly stretchable and sensitive multidirectional strain sensors with tunable strain gauge factors by employing a digitally controlled printer to incorporate carbon nanotube (CNT) layers into polydimethylsiloxane (PDMS) substrates. The fabricated sensors exhibit a high stretchability (up to 45%) and sensitivity with a gauge factor of 35.75. The gauge factors could be easily modulated by tuning the number of CNT printing cycles to accommodate diverse requirements. The cyclic loading-unloading test results revealed that the composite strain sensors exhibited excellent long-term durability. Particularly, in this work, for the first time, human-motion-induced strain was measured by a motion capture system and compared with the strain data obtained from the fabricated strain sensors. The deviation of strains measured by composite sensors is less than 20%, indicating the great accuracy of CNT/PDMS sensors to quantify the amount of motion-induced strain. Of significant importance is that due to the flexibility of the printing technique used, rosette-type sensors were fabricated to simultaneously measure strains along multiple axes. These superior sensing capabilities of the fabricated CNT/PDMS strain sensors give them great application potential in motion-detecting systems.