Loading-unloading Tests Research Articles

Time-temperature superposition principle for the shear fracture behaviour of soft adhesive layers: From bulk to interface

Soft adhesive layers have attracted great attention as a commonly used adhesive technique, particularly because of their shear fracture behaviour at extreme loading rates and temperatures. Therefore, the present study examined the shear fracture properties of a soft adhesive layer through systematic rheological experiments, loading-unloading shear tests and shear fracture tests. Due to a similar viscoelastic characteristic in rheological properties and fracture properties, the time-temperature superposition principle (TTSP) was extended from the bulk to the interface to investigate its shear cohesive failure. A shear fracture toughness master curve was defined using the time-temperature equivalence shift factors derived from the rheological properties. The shear fracture behaviour of the soft adhesive layer could be predicted using data from a narrow range of loading rates/temperatures. The results demonstrated the potential of extending the TTSP approach from the bulk to a soft adhesive layer having viscoelastic character, which might be utilised to predict the shear fracture toughness of soft adhesive materials under extreme conditions.

Study on the evolution law of rock joint shear stiffness during shearing process through loading-unloading tests

Tunnels are usually located in highly jointed rock masses and most of them are in the area with frequent seismic activities. The joints have a great impact on the strength and failure mode of the rock mass. The jointed rock mass is subjected to frequent loading and unloading under complex geological and engineering conditions. Shear stiffness is a key parameter to reflect the shear deformation properties of the discontinuities. It is usually considered a constant value in the traditional calculation, but this assumption may not conform to engineering practice in practical engineering. Loading-unloading shear tests were conducted to study the evolution law of rock joint shear stiffness during the shearing process. The rock-like specimens with different joint roughness coefficients were used to simulate the natural rocks. The results show that the shear stiffness will gradually decrease with the increase of shear displacement. The shear stiffness of rock joints increases with the increase of joint roughness coefficient. According to the test results, a shear stiffness prediction model related to shear strain was derived. The prediction results were in close agreement with the laboratory test results, and the error range was within 12%. Which indicated that the established prediction model can better reproduce the shear stiffness change behavior of rock joints in different shear stages. In tunnel engineering design, the evolution of the shear stiffness of rock joints needs to be considered. More accurate results can be obtained by inserting the prediction model into the numerical simulation programs.

CNTs based capacitive stretchable pressure sensor with stable performance

In recent years, with the increasing demand for artificial robots and smart devices, flexible pressure sensors have attracted much attention. In this paper, to solve the problem of insufficient stretchability and complicated microstructure fabrication process, a capacitive pressure sensor based on flexible electrodes with wrinkle structure and composite dielectric layer was designed and prepared. The as-prepared sensor exhibits a sensitivity of 2.13 kPa−1 and the response time is about 0.1 s. During the loading-unloading test, the capacitive flexible sensor showed stable sensing performance and good repeatability with minor hysteresis effect. The research is of great significance for the application of flexible sensors in fields such as soft robots, E-skin and medical monitoring in the future.

Comparison of Effects of Different Sacrificial Hydrogen Bonds on Performance of Polyurethane/Graphene Oxide Membrane.

Processing robust mechanical properties is important for elastomeric materials. In this work, different molecular weights of polyethylene glycols (PEG) were used to modify graphene oxide (GO) in order to study the relationship between the number of hydrogen bonds and the properties of the polyurethane/graphene oxide membrane. The fact of PEG was successfully grafted onto the surface of GO was certified by Fourier transform infrared spectra, Raman spectra, X-ray photoelectron spectroscopy. The graft ratio was indicated by thermogravimetric analysis. The presence of hydrogen bonds in PUR/MGO composites membrane was proved by the cyclic loading-unloading test and stress relaxation test. The thermal stability and low-temperature resistance performance of PUR/MGO had been improved compared with PUR/GO. When the molecular weight of PEG grafted on the surface of GO was 600, the tensile strength and elongation at break of the composite membrane were optimal. The reason for the improvement of physical and mechanical properties was that the dispersion of filler in the rubber matrix and the compatibility between filler and rubber had been improved.

Effect of Crack Angle and Length on Mechanical and Ultrasonic Properties for the Single Cracked Sandstone Under Triaxial Stress Loading-Unloading

The disturbance due to coal mining causes the surrounding rock to undergo a complex process of stress changes during which the axial pressure and the confining pressure usually change accordingly at the same time. Existing studies generally investigated this process from a static perspective, which was not rigorous. The mechanical characterization of rock is very important to understand the failure of rock mass and the safety of mining during mining disturbance. Based on theoretical analysis, we conducted axial loading and radial unloading tests on the cracked sandstone, which was combined with the ultrasonic testing technology to examine its failure rules and to characterize and analyze its failure process using longitudinal wave velocity. The results demonstrated that crack length and angle had a significant impact on the strength and mechanical properties of sandstone, and the former had a greater impact on the strength of sandstone than the latter. As the crack length increased, the strength, elastic modulus, and deformation modulus of sandstone decreased, and the strength of sandstone increased as the crack angle increased. Elastic and deformation moduli first decreased and then increased. Furthermore, Poisson’s ratio increased slowly, then decreased slowly, and finally increased rapidly as the lateral pressure coefficient diminished, and Poisson’s ratio was more sensitive to changes in the angle. In this study, the change of longitudinal wave velocity reflected the whole process of sandstone failure. When the wave velocity was stable, the rock was at the yield limit point. Moreover, when the wave velocity was unstable, the sandstone was in a progressive failure period, and as a result, the wave velocity decreased and the sandstone cracked.

Effect of on-Line Annealing Temperature on Mechanical Properties, Microstructure, Fracture Structure and Superelasticity of Ti49.1Ni50.9 Alloy

Ti-50.9Ni (at%) shape memory alloy (SMA) ingot was prepared by the vacuum induction furnace, and was produced into Φ2.0 mm wire successively by forging, rolling and hot drawing. After complete annealing of 800 °C×5 min and cold drawing with 30% deformation, on-line annealing with different temperatures (500 °C×5 min, 600 °C×5 min, 700 °C×5 min) were employed to TiNi wires. Both tensile tests and fivecycles loading-unloading tensile tests with 7% strain were applied to study the mechanical properties, and superelasticity (SE) of TiNi wires. In addition, microstructure and fracture characteristics were investigated by OM, SEM and EDS. The results showed that the tensile strength of 500 °C×5 min annealing reached 1431 MPa, the elongation of 700 °C×5 min annealing reached 129%. The superelasticity behavior of 500 °C×5 min showed excellent performance in five-cycles loading-unloading tensile tests, and the transformation platform peak stress with >7% strain showed a small attenuation (about 4 MPa ), and cumulative residual strain was only 0.2%. In addition, fracture characteristics for all TiNi wires were all plastic fractures, there were a large number of dimples in fractures.

Characterization methods for strain‐induced damage in polypropylene

AbstractVarious methods are used to characterize the deterioration of mechanical properties in polymers. The focus is set on distinguishing between time‐dependent and irreversible damage in two different grades of polypropylene. First, digital image correlation is utilized to capture the stress–strain behavior during monotonic tensile tests. Changes in specimen volume are recorded throughout the experiment and serve as an indicator for crazes and voids. However, the elastic modulus, E, cannot be monitored throughout the entire experiment. Further analysis is performed in the form of cyclic load–unload tests. E and the residual strain, εres, as a function of the applied strain, εappl, are obtained for each cycle. Results show that E primarily suffers from the time‐dependent behavior of the tested polymers in this case. Subsequently, an alternative technique is applied, where specimens are prestrained and then allowed to relax. In the following dynamic mechanical analysis, viscoelastic effects can be avoided. Considerations on the onset and evolution of damage are made. Ultimately, these results are confirmed through microcomputed tomography, where the shapes and densities of defects are captured in high resolution.

High superelasticity NiTi fabricated by cold metal transfer based wire arc additive manufacturing

NiTi alloy exhibits superior superelastic properties but its wide application is limited by its poor workability. In this study, Ni-rich NiTi wire was used as the feedstock and a wall-shaped NiTi component with height of 130 mm was deposited on Ti substrate by cold metal transfer (CMT) based wire arc additive manufacturing (WAAM) process. The microstructure and mechanical properties of the as-deposited NiTi alloy specimens in the horizontal and vertical directions, as well as their correlations were discussed in detail. In particular, the superelastic properties of the deposited materials were evaluated by cyclic loading-unloading tensile test. It was found that a very high superelastic recovery rate of more than 90% can be reached after 25 cycles for samples tested along both the horizontal and vertical directions. The mechanism that governs the high superelasticity in the alloy was explored in terms of microstructure, chemical composition and phase transformation analyses.

The supporting capacity of ferrofluids bearing: From the liquid ring to droplet

In this paper, a simple ferrofluids (FFs) bearing, which consists of a rotor, a stator embedded with a cylinder magnet, and FFs is proposed. By controlling the volume of FFs, two types of liquid structures of the ring and droplet shapes are formed on the stator surface. Models of the bearing for the static supporting force are established and validated by experiments. The multicycle loading-unloading of the FFs bearings as well as the dynamic supporting force are performed. Results show that the static supporting force of the bearings can be predicted approximately by the theoretical model. Multicycle loading-unloading tests indicate that force values with excellent repeated accuracy can be achieved only within a limited range of oscillation in gap height. Compared with the FFs droplet bearing, the rotation speed manifests more significant impact on the dynamic supporting force of the ring structured bearing. Such design presents a new thought for micro-electro-mechanicalsystem (MEMS) that requires precisionsupporting or frictionless bearing.

Self-healable nanocellulose composite hydrogels combining multiple dynamic bonds for drug delivery

Herein, we developed two nanocomposite polysaccharide hydrogels TPP-CNC and TPP-CNF via simple mixing method, which were constructed with multiple dynamic bonds. The microstructural features, mechanical properties, rheological properties, healable ability and biocompatibility of the complex hydrogels were evaluated. The TPP-CNC and TPP-CNF complex hydrogels exhibited higher tensile strength than pure polysaccharide hydrogel, from ~259 KPa to ~890 KPa and ~910 KPa, respectively, that was attributed to the contribution of ionic crosslinked network and hydrogen bonds. In addition, the hydrogels indicated superior fatigue resistance and high energy dissipation ratio during loading-unloading tests because of the physical sacrifice bonds, which also decreased the self-healing time at room temperature (~15 min). More importantly, the drug loaded nanocomposite hydrogels showed sustained release, reduction burst release, increased release under acidic environment, and the drug release kinetics belonged to Fickian diffusion mechanism. Therefore, the nanocellulose polysaccharide hydrogels have the highly promising to explore as biomaterials for drug delivery.

Bilinear damage evolution in AA2011 wire drawing processes

This paper presents an experimental and numerical analysis of damage evolution in AA2011 aluminum alloy wires drawn under different scenarios. To this end, load-unload tensile tests were firstly carried out in order to characterize the degradation of the mechanical response in every cycle where the experimental results show a bilinear damage relationship in terms of the effective plastic strain. Therefore, a modification of the classical Lemaitre model is proposed in this work in order to reproduce bilinear paths of damage with the addition of only two parameters that can be directly obtained from the material characterization. Then, the damage predictive capability of this new experimental-based model is assessed in numerical simulations of the drawing process in one and two passes (considering for this last case the sequential and tandem configurations) where the computed predictions are compared with the corresponding experimental data showing a good agreement between them.

Effect of machining processes on the damage response and surface quality of open hole hybrid carbon/flax composites: An experimental study

The influence of conventional drilling (CD) and abrasive water jet (AWJ) machining on the mechanical behavior of hybrid carbon/flax composites was investigated. The specimens were manufactured using woven carbon fiber (C) and unidirectional flax fiber (F) in three configurations: unidirectional [0-90C2/0F6]S, cross-ply [0-90C2/(0/90)F6]S, and angle-ply [0-90C2/(±45)F6]S laminates. The examination of the hole surface revealed severe damage in the form of peel-up, push-out, and secondary delamination of AWJ machined holes, and only push-out delamination at the exit side of the CD drilled hole. A delamination factor was developed to quantify the delamination at the hole surface. The specimens were subjected to quasi-static load-unload tensile tests to assess their damage response and surface fracture. Unidirectional and angle-ply specimens drilled with AWJ accumulated a higher damage compared to CD specimens. This correlates with the delamination factor, indicating that the delamination at the entry and exit of the hole and secondary delamination greatly affect the damage evolution of the laminates.

Experimental study on energy and failure characteristics of rubber-cement composite short-column under cyclic loading

The recycling of waste tire rubber resources and its application in the field of civil engineering materials have the importance of the economy, environmental protection, and promoting the development of civil engineering materials. In this work, rubber cement mortar short-column was used as the research object, and the cyclic loading-unloading tests with finite cycles were carried out under three different loading modes (I, II, and III). A series of basic characteristics of rubber cement mortar under three different cyclic loading-unloading modes were further discussed and analysed, such as energy, deformation, damping ratio, failure mode mechanism, and crack development mechanism. The results showed that when the rubber content was 30%, the energy damage was reduced by 13.7 ~ 45.7%, and the cumulative damping ratio was increased by 57.0%. The evolution characteristics of the hysteresis loops in the loading-unloading curves in the form of "non-closed crescent" were explained. The development mechanism of cracks based on the principle of minimum energy was revealed, and the evolution characteristics of cracks were confirmed. The crack resistance effect of rubber cement mortar was explained by the property of damping energy dissipation of rubber particles. The purpose is to better apply rubber-cement composites to seismic structures, underground structures, and road engineering.

Research on the establishment of gas channeling barrier for preventing SCP caused by cyclic loading-unloading in shale gas horizontal wells

Sustained casing pressure (SCP) was an urgent problem to be solved during shale gas exploration and development. To cope with the issue of SCP, a method of establishing gas channeling barrier was proposed, and the location of the barrier was optimized, considering engineering and geological factors. A series of tests, containing uniaxial/triaxial compression tests and cyclic loading-unloading tests, were carried out to evaluate the accumulated plastic strain. Also, a full-scale cement sheath sealing integrity evaluation device was employed to verify the appearance of the micro-annulus. Computed tomography scan and nuclear magnetic resonance tests were performed to measure and analyze the distribution of microcracks and pores in the cement sheath after cyclic loading-unloadings. Numerical models of the wellbore assembly were established, which considered wellbore structures of different intervals of an actual deep shale gas well. Research findings indicated that after loading and unloading a certain number of times, the accumulation of plastic strain showed an increasing trend with the increase of the measured depth in the vertical section, but decreased with the rise of the measured depth in the horizontal section. The method of setting gas channeling barrier was proposed and could be used to avoid SCP by using the cement slurry with low elastic modulus, which could significantly reduce the cost of cementing operation. Many factors, including the non-uniform in-situ stress, wellbore structure, fracturing stage number, casing internal pressure, and formation mechanical properties were considered to establish the optimization method of barrier locations, and this method was verified by using the logging data of an actual well as well as the engineering and geological data. • An optimization method of establishing gas channeling barrier was proposed. • Loading-unloading of inner pressure leads to porosity increase of cement sheath. • First barrier should be established far away from the heel. Fig. 8 displays the gas leakage measurement results. As observed, gas leakage was detectable in the 22nd cycle at the injected gas pressure of 5 MPa. The gas channeling rate also increased as the injection pressure elevated. In order to observe which interface failed, the annulus was photographed, as presented in Fig. 9 . Obviously, micro-annulus appears at the inner surface of the cement sheath without internal cracks following repeated loading/unloading cycles. The obtained results show that the micro annulus mainly occurs in the inner interface of cement sheath.

The Soft-Strain Effect Enabled High-Performance Flexible Pressure Sensor and Its Application in Monitoring Pulse Waves.

Flexible and wearable pressure sensors attached to human skin are effective and convenient in accurate and real-time tracking of various physiological signals for disease diagnosis and health assessment. Conventional flexible pressure sensors are constructed using compressible dielectric or conductive layers, which are electrically sensitive to external mechanical stimulation. However, saturated deformation under large compression significantly restrains the detection range and sensitivity of such sensors. Here, we report a novel type of flexible pressure sensor to overcome the compression saturation of the sensing layer by soft-strain effect, enabling an ultra-high sensitivity of ~636 kPa-1 and a wide detection range from 0.1 kPa to 56 kPa. In addition, the cyclic loading-unloading test reveals the excellent stability of the sensor, which maintains its signal detection after 10,000 cycles of 10 kPa compression. The sensor is capable of monitoring arterial pulse waves from both deep tissue and distal parts, such as digital arteries and dorsal pedal arteries, which can be used for blood pressure estimation by pulse transit time at the same artery branch.

CONCRETE DAMAGE PLASTICITY MATERIAL MODEL PARAMETERS IDENTIFICATION

The procedure for identifying concrete damage plasticity material model parameters is presented in this paper. Concrete damage plasticity material model represents a constitutive model which is based on a combination of theory of plasticity and theory of damage mechanics. This material model is often used in solving geotechnical problems due to its realistic description of mechanical behavior of concrete material. Theoretical basis of concrete damage plasticity material model and material parameters identification procedure are presented in this paper. Proposed identification procedure is applied on experimental data from uniaxial compression and tension load-unload tests taken from literature. By applying experimental data, stress-strain curve is created. Based on stress-strain load-unload curve, stress-plastic strain and stress-degradation dependences are created which are necessary for material parameters identification. Using these dependences material parameters are determined. Verification of estimated parameters is performed in PAK software package using concrete damage plasticity material model. Finite element model is created for numerical simulations of uniaxial compression and tension tests. Numerical simulation results are compared with experimental data. By comparing numerical simulation results and experimental data it can be concluded that this procedure is effective for determining concrete damage plasticity model parameters.

Loading-unloading behavior of a clayey rock under thermo-hydro-mechanical conditions

Various investigations have been carried out on the hydro-mechanical behavior of geomaterials, but those of clayey rock are still limited. Among these existing studies, very few data are available related to the thermal effects, particularly the loading-unloading behavior of clayey rock at high temperatures. In this paper, undrained loading-unloading triaxial tests on undisturbed specimens of Boom Clay extracted from Mol, Belgium, were carried out with the aim to study (1) the influence of temperature and load path on the thermo-hydro-mechanical behavior of clayey rock, and (2) the formation mechanism of loading-unloading stress-strain hysteresis loops. The test results first revealed the stress paths–dependent stress-strain behavior of Boom Clay. The increase of initial mean effective stress p0′ has a positive effect on the mechanical behavior of Boom Clay. Secondly, temperature elevation induced the strength weakening of Boom Clay even under loading-unloading conditions. Thirdly, the p0′ and temperature had a bearing on the loading-unloading elastic modulus (defined as the peak-to-peak secant modulus of a hysteresis loop). Strain energy dissipation during the loading-unloading is then discussed from the perspective of thermodynamics. Finally, the study concludes that the stress-strain hysteresis loops for Boom Clay may be closely related to its high content of adsorbed water, and it may be driven by the combined effect of “friction resistance” and “viscous resistance.”

Anisotropic mechanical behaviour of columnar jointed rock masses subjected to cyclic loading: An experimental investigation

In order to investigate the anisotropic mechanical behaviour of the columnar jointed rock masses (CJRM) under cyclic loading, triaxial cyclic loading-unloading tests are conducted on artificial CJRM specimens with a geological structure similar to the actual CJRM under the confining pressures of 5 MPa. In this work, particular attention is paid on the influence of joint dip angles on the anisotropic deformation behaviour , strength, mechanical properties, and failure modes of the CJRM. The experimental results show that the relationship between characteristic stress thresholds and joint dip angles is exhibited as a typical U-shape. Crack unstable growth ( σ cu ) and crack damage ( σ cd ) stress thresholds are more anisotropic than the crack stable growth stress threshold ( σ cs ), peak stress ( σ p ) and residual stress ( σ r ). The CJRM anisotropy mechanical parameters induced by stress gradually increases. Three main failure modes are observed throughout the loading process with respect to the joint dip angles. The results presented in this paper are significant for understanding the mechanical behavior of anisotropy in rock engineering.

The microstructure and martensitic transformation of Ti-13 V-3Al light weight shape memory alloy deformed by high-pressure torsion

Ti-13 V-3Al (at%) high temperature shape memory alloy was considered attractive material due to its low density. The demand for the better shape memory performance is proposed to widen the scope of application. In the present study, Ti-13 V-3Al is processed by high-pressure torsion and annealing. The microstructure, martensitic transformation and mechanical properties were investigated. The severe plastic deformation introduced high density defects and decreased the stability of the α″ phase. The phase constitution is the sole β phase after high-pressure torsion, which was related to the Gibbs-Thomson effect. After the annealing treatment at 700 °C, numerous small parallel α″ martensite variants were observed, which was affected by the residual defects. More nucleation sites were provided, resulting that the reverse martensitic transformation was accomplished in a narrow temperature range. Moreover, the formation of (111) type I twinning become easier due to the assistance of the (111) stacking faults. The α″ martensite variants became larger and evolved to the V-shaped and the triangular self-accommodated morphologies as the annealing temperature increased to 800 °C. The twinning relationship in the different annealing temperature both were the (111) type I twinning. The properties were characterized by the tensile loading-unloading tests and microhardness tests. The shape memory effect was optimized and 4% full recoverable strain was obtained in the alloy annealed at 700 °C. The apparent hardness decreased from 323HV and 263HV as the annealing temperature raised from 700 °C to 800 °C.

Conductive and room-temperature self-healable polydimethylsiloxane-based elastomer film with ridge-like microstructure for piezoresistive pressure sensor

The fabrication of piezoresistive pressure sensor with self-healing function has attracted worldwide attention for its great prospects in intelligent robots, remote health monitoring and electronic skin. In this work, a conductive and self-healable elastomer (CSE) film with ridge-like microstructure was designed for flexible piezoresistive pressure sensor. Firstly, self-healable polydimethylsiloxane (PDMS)-based elastomer with ureido and imine groups was synthesized via a simple one-pot two-step polycondensation reaction with aminopropyl-terminated PDMS, isophorone diisocyanate and 1, 3, 5-triformylbenzene as monomers. Secondly, the synthesized PDMS-based elastomer solution was casted onto a sandpaper pre-sprayed with ureido pyrimidinone grafted carbon nanotubes/polyurea mixed solution, and the CSE film was obtained after drying and peeling off from the sandpaper. The piezoresistive pressure sensor fabricated with two single-electrode CSE films exhibited high sensitivity of 8.7 kPa−1 (0–6.1 kPa), low detection limit (50 Pa), fast response capability (response/relaxation time of 40/117 ms) and repeatability for 10,000 loading–unloading tests. Moreover, the sensor was able to fully recover to normal work after healing for 36 h at room temperature or even 200 s under the irradiation of near infrared (NIR) light. Importantly, the sensor was successfully applied for not only detecting human motions including radial pulse, speech recognitions and joint movements but also monitoring health status wirelessly through Bluetooth transmission. Our findings put forward a novel method to fabricate a high-performance self-healable piezoresistive sensor for wearable electronics, healthcare monitoring and electronic skin.