Cyclic Loading-unloading Tests Research Articles

Fatigue-thermal damage characteristics of red sandstone with a hole under high-temperature cyclic loading coupling and microstructural degradation

In underground coal gasification (UCG) projects, deeply buried subterranean projects are significantly influenced by high temperatures, alongside disruptive loads such as excavation, blasting, and drilling. Therefore, it is essential to study the fatigue behaviors of rocks under the coupled effects of thermal elevation and cyclic loads. This study performed uniaxial compression and cyclic loading-unloading tests on red sandstone specimens with cavities. The high-temperature fatigue behaviors of specimens were explored from various aspects, encompassing stress, deformation, energy, damage, and the microstructural variations of the specimens after high-temperature procedures were assessed by utilizing scanning electron microscopy (SEM). At temperatures below 600 °C, SEM images indicate a contraction of microcracks in the specimens, whereas temperatures above this threshold lead to their expansion. Notably, the total, elastic, and dissipated energy density of the specimens exhibit a curve of second degree with the upper limit stress. Additionally, elastic and dissipated energy densities linearly correlate with input energy density. Furthermore, the coupled damage and axial strain of the specimens positively correlate with the temperature. The damage mode of thermal fatigue is attributed to the accumulation and penetration of transgranular fractures within the red sandstone.

The effect of insufficient creep of tectonic coal under hydrostatic pressure on deformation energy measurements

To investigate the implications of insufficient creep on deformation energy of tectonic coal under hydrostatic pressure, hydrostatic single and graded cyclic loading-unloading tests and graded creep experiments under varying stress gradients were conducted, deriving the following conclusions. Coal specimens under constant hydrostatic pressure exhibit a lab-unattainable ultimate creep strain corresponding to sufficient creep. The generalized Kelvin model can roughly describe coal creep behaviour in lab. Insufficient or absent creep results in a bending-down phenomenon in the deformation energy curve when beyond 76.41% ∼ 82.45% of the maximum stress. With increasing creep, deformation energy for coal samples increases by 0.29% ∼ 2.79%. A correction term incorporating stress and creep strain is introduced and exhibits a slow-then-fast growth trend with increasing creep strain, stabilizing at 1 after coal specimens experience full creep or reach critical stress. Original nonlinear model approximately characterizes the deformation energy of coal in actual geological formations, providing guidance for on-site production.

Characteristics of Pre-peak Mechanical Damage and Energy Evolution of Typical Hard-rock in Diversion Tunnel under Cyclic Loading-Unloading

An in-depth understanding of the pre-peak mechanical damage and energy-evolution characteristics of typical hard-rock in a diversion tunnel under cyclic load is of great significance to promote the safe and efficient construction of the diversion tunnel and the stability of surrounding rock. To study the pre-peak mechanical characteristics and the competition mechanism between energy storage and energy dissipation of typical hard-rock in a diversion tunnel under cyclic loading-unloading, combined with the internal drilling and blasting excavation of the actual engineering rock mass and the external vehicle cyclic load environment of the diversion tunnel, the cyclic loadingunloading tests of typical granite and tuff in diversion tunnel were carried out. Based on the analysis principle of mechanics and energy, the strain variables, modulus variables, energy variables and damage variables of granite and tuff under cyclic loading-unloading test were defined. The cyclic mechanical properties and energyevolution characteristics of granite and tuff under pre-peak load were analyzed. The competition mechanism between pre-peak energy storage and pre-peak energy dissipation of granite and tuff and the evolution law of strain damage variable and energy damage variable were revealed. The selection principle of rock sample size and the limitation of the test scheme were further discussed. The study of the damage evolution of rocks close to failure (pre-peak stage) under cyclic load is helpful to better understand the damage and failure mechanism of rocks in practical engineering problems. Keywords: Diversion tunnel, Cyclic loading-unloading, Granite/Tuff, Blasting cyclic load, Energy evolution.

A model for the temperature-dependent creep/recovery behavior of dry fiber fabric considering in-plane tension

In the manufacturing process of fiber-reinforced composite materials, the preforming of dry fiber fabrics is typically carried out under specific out-of-plane pressure, in-plane tension, and at designated temperatures. The fabric exhibits significant time and temperature-dependent permanent deformation as well as creep/recovery behavior. Gaining a better understanding of these phenomena and harnessing predictive capabilities will be instrumental in achieving more precise control over fiber volume fraction and material composition. In this paper, creep/recovery experiments and cyclic loading-unloading tests were conducted on CF3031 dry fiber preforms under various in-plane tensions and temperatures, measuring in-plane strains using Fiber Bragg Grating (FBG) sensors. Based on this data, a modified Burgers viscoelastic-plastic model has been proposed to describe the time-dependent creep/recovery behavior of dry fiber fabric preforming under the coupled effects of in-plane tension, out-of-plane pressure, and temperature variations. This model employs a single set of equations and parameters to represent the complete creep/recovery process of the material. The experimental results align well with the predicted outcomes across different compression scenarios.

An improved method to calculate the rock brittleness index PEECR based on linear energy storage law

The peak elastic strain energy consumption ratio (PEECR) is a rock brittleness index proposed by Gong and Wang. In the present study, based on the linear energy storage law of rock under triaxial compression, a new method was proposed to calculate the PEECR. The PEECR uses a simplified method to calculate the peak elastic strain energy. To solve this problem accurately, triaxial cyclic loading-unloading compression tests were carried out on shale. Strain energy parameters were calculated from the test curves. The results show that there is a linear relationship between the elastic strain energy and input strain energy, indicating that the linear energy storage law in rock is applicable to triaxial compression state. The universality of the linear energy storage law of rock under triaxial compression is also verified by the data in the published literature. Then, the peak elastic strain energy can be accurately determined using the linear energy storage law, and the PEECR is improved based on this. Finally, the PEECR and the improved PEECR were compared using the triaxial cyclic loading-unloading compression tests on three rocks (shale, red sandstone and granite), and the improved PEECR was compared with 11 existing energy-based brittleness indexes. The results show that the improved PEECR can further reflect the rock brittleness more accurately.

Experimental study on mechanical properties of Beishan granite in the mild temperature range

The response of mechanical property to temperature, especially in the mild temperature range, is important for the high-level waste (HLW) geological disposal engineering. To study the temperature effect on the deformation properties and the mobilization of strength of Beishan granite, the incremental cyclic loading-unloading compressive tests under various temperatures (T) and confining pressure (σ3) conditions (T = 25 °C, 60 °C and 90 °C, σ3 = 0 MPa, 5 MPa and 20 MPa) were carried out on Beishan granite. Results showed: (1) During the entire compression process, the cohesive strength is weakening while the frictional strength is strengthening. The CWFS model was developed considering the temperature effect. (2) For the peak strength, the cohesion decreased while the friction increased with increasing temperature in the range of 25 °C–90 °C. (3) The mechanism of cohesion weakening and friction strengthening with increasing temperature was discussed based on the micro-structure measurement. Clear thermal induced crack/pore closure was observed in granite after mild temperature treated, which contributes to friction strengthening and stiffness improvement. (4) The strength response to temperature (cohesion weakening and friction strengthening with increasing temperature) will result in ‘transition pressure (10.1 MPa for crack damage stress, and 14.9 MPa for peak strength)’ for Beishan granite under mild temperature that the surrounding granite would be strengthened by the elevated temperature under higher confining pressure. The transition pressure is important for high-level waste geological disposal engineering in repository layout design optimization.

Effects of 3d Transition Metal Substitutions on the Phase Stability and Mechanical Properties of Ti-5.5Al-11.8[Mo]eq Alloys.

The phase stability, mechanical properties, and functional properties of Ti-5.5Al-11.8[Mo]eq alloys are focused on in this study by substituting 3d transition metal elements (V, Cr, Co, and Ni) for Mo as β-stabilizers to achieve similar β phase stability and room temperature (RT) superelasticity. The ternary alloy systems with the equivalent chemical compositions of Ti-5.5Al-17.7V, Ti-5.5Al-9.5Cr, Ti-5.5Al-7.0Co, and Ti-5.5Al-9.5Ni (mass%) alloys were selected as the target materials based on the Mo equivalent formula, which has been applied for the Ti-5.5Al-11.8Mo alloy in the literature. The fundamental mechanical properties and functionalities of the selected alloys were examined. The β phase was stabilized at RT in all alloys except for the Ti-Al-V alloy. Among all alloys, the Ti-Al-Ni alloy exhibited superelasticity in the cyclic loading-unloading tensile tests at RT. As a result, similar to the Ti-5.5Al-11.8Mo mother alloy, by utilizing the Mo equivalent formula to substitute 3d transition metal elements for Mo, a RT superelasticity was successfully imposed.

Microstructure characteristics and superelastic properties of novel Ti–Cr–Sn superelastic alloys

Microstructures, texture, lattice deformation strain, and superelastic properties of four kinds of near β type Ti–Cr–Sn novel superelastic alloys were systematically investigated, i.e., Ti-2.5Cr-8.5Sn, Ti-3.0Cr-7.5Sn, Ti-3.5Cr-7.0Sn, and Ti-4.0Cr-6.5Sn (at.%). All the alloys after the solution-treatment at 1273 K followed by water quenching consisted of equiaxed β single phase grains at room temperature in addition to athermal ω phase particles. Mechanical tests revealed the largest superplastic recovery strains (excluding elastic strain) of 4.0% in Ti-4.0Cr-6.5Sn alloy. This is partially due to the large lattice deformation strain ∼6.32% and its well-developed 112β⟨110⟩β texture component when tensile test exqmined along the rolling direction (RD). Moreover, up to 100 cyclic loading-unloading tensile tests revealed that both Ti-4.0Cr-6.0Sn and Ti-2.5Cr-8.5 Sn alloys maintained almost perfect and large shape recovery strain (including elastic strain) over 4.5% without any additional thermo-mechanical treatment. Besides, the starting stress for inducing martensitic transformation in Ti-4.0Cr-6.5Sn alloy kept higher than 200 MPa during the 100 cycles. Based on these superior superelastic properties, it is concluded that these Ti–Cr–Sn superelastic alloys become practical alternatives to replace Ti–Ni not only for biomedical applications but industrial applications.

C2C12 HÜCRELERİ KULLANILARAK MİYOKARDİYAL DOKU MÜHENDİSLİĞİ İÇİN ÖRME HAVLU KUMAŞ İSKELENİN IN VITRO DEĞERLENDİRİLMESİ

Pile loop knitted scaffold has attracted attention for being used as a three dimensional (3D) biocompatible material particularly for the application of myocardial tissue engineering. In this research, nonintermingled, textured polyethylene terephthalate (PET) multifilament yarn with a filament count of 48 was used for producing a pile loop knit structure to investigate if structure could provide an appropriate microenvironment for the seeded murine C2C12 myoblasts in case of increasing in filament diameter (20.7 micron versus 9.1 micron from the previous study). The resulting structure had a pore network mainly consisting of macropores of 100-200 µm that were well interconnected with micropores (<90 µm). Furthermore, the mean values of the Young’s modulus of the structures at 20% strain were 240.67 kPa in the warp direction, and 85.73 kPa in the weft direction, which can be claimed not to be far from those declared in the literature for the human heart muscle. In addition, the cyclic loading-unloading test revealed no creep in the fabric after 100 cycles. According to the results of the WST-1 assay, the 48 filament-based scaffold was non-cytotoxic for C2C12 cells that were able to proliferate properly on the scaffold, which was visualized by SEM and confocal images. In this respect, pile loop knit structures keep promising results for being used as a cardiac patch.

Experimental Study on Mechanical Properties and Failure Mechanism of Damaged Sandstone

Solid materials such as rocks can contain primary defects, and internal defects are activated in the event of mining disturbance, which causes rock damage and destruction. Therefore, it is of great significance for rock engineering to study the mechanical properties and failure mechanism of damaged rock. In this study, damaged prefabricated crack sandstone specimens were prepared with the cyclic loading-unloading test, and the uniaxial loading test was carried out with damaged specimens. The evolution law of peak strength, elastic modulus, and peak strain of specimens with different damage degrees was studied, the quantitative relationship between the P-wave velocity and the damage degree was obtained, and the acoustic emission (AE) count and energy evolution characteristics of specimens with different damage degrees were analyzed. The energy evolution law of damaged specimens was revealed, and with the increase in damage degree, the elastic energy stored in the specimens can be converted into crack propagation more quickly, and the dissipated energy density increases rapidly, resulting in complete rock failure. The research results can provide theoretical support for the stability analysis and control of underground engineering rock mass in the event of multiple disturbances.

Experimental study on thermo-mechanical properties of Beishan granite under mild temperature

Temperature is believed to have an important influence on the mechanics behavior of granite. To investigate thermo-mechanical properties of Beishan granite, the host rock of China's preselected underground laboratory of high-level radioactive waste (HLW) geological repository, mechanical tests including uniaxial compression test and incremental cyclic loading-unloading test under mild temperature range (room temperature to 300 °C) were carried out with Acoustic emission (AE) activity monitoring. Results show: (1) The temperature threshold for Beishan granite is 150 °C. The UCS (uniaxial compressive strength) is increasing and pore is healing with increasing temperature in the range of room temperature to 150 °C, while the UCS is decreasing and pore is enlarging with increasing temperature in the range of 150 °C–300 °C. (2) The cracking activity in the compression process is significantly influenced by the ambient temperature. The AE activity of Beishan granite under different temperature exhibits three patterns (inactive-active for granite under room temperature, active-inactive-active under 60 °C–150 °C, active-inactive for granite under 200 °C and 300 °C). (3) Results of incremental cyclic loading-unloading compression tests show that the peak stress, peak strain and AE energy of granite increase with the elevated temperature. The strength and ductility are enhanced with increasing temperature in the mild range. (4) AE activity during the incremental cyclic loading-unloading test is used for loading-unloading response ratio (LURR) and felicity ratio (FR) analysis. The LURR value keeps increasing before pre-failure stress under lower temperature, but shows decrease trend before failure under higher temperature. The stress corresponding to the sharp drop on FR is identified as the stress threshold for rock damage, which are 0.57σc to 0.66σc for Beishan granite under the mild temperature conditions. This work will provide support for the long-term safety assessment for the HLW geological disposal engineering.

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.

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.

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.

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.

Experimental study on the damage process of marble under true triaxial pre-peak unloading conditions

Stress-induced instability is associated with rock damage. Here, the progressive brittle fracturing process in Jinping marble is studied by introducing two types of true triaxial pre-peak unloading tests, namely, the incrementally cyclic loading-unloading minimum principal stress test (ICM test) and the incrementally cyclic loading-unloading maximum and minimum principal stress test (ICMM test). By comprehensively analysing the irreversible strains, dissipated energy, acoustic emission (AE) characteristics and scanning electron microscopy (SEM) results, the rock damage evolution can be quantified and divided into two distinctive damage stages. At the boundary point, the irreversible strain increments reach their minimum values. In the gentle damage stage, the normalized irreversible strains increase linearly, and this process is associated with a small number of AE hits with low amplitude. The rapid damage stage is characterized by a nonlinear increase in the normalized irreversible strains, and this process is associated with a large number of AE hits with high amplitude. The dissipated energy mainly increases in the rapid damage stage. In addition, the rapid damage stage in the ICMM test mainly occurs in the last five cycles, due to the differences in the deviatoric stresses in each cycle. In both of these tests, the failure mode is principally characterized by tensile failure. Moreover, the precursory signals of rock fracturing and the influence of the loading paths on the strength are discussed.

Damage Characteristics of Rocks by Uniaxial Compression and Cyclic Loading-Unloading Test

Damage Characteristics of Rocks by Uniaxial Compression and Cyclic Loading-Unloading Test

Prediction of bending springback of the medium-Mn steel considering elastic modulus attenuation

The improvement of lightweight and safety is the development requirement of automobile parts. Therefore, the demand of thin-walled parts with ultra-high strength is more urgent. However, with the increase of strength, the springback phenomenon of parts is particularly serious, which greatly affects the forming quality of parts. Generally, the springback of automobile parts should be controlled within 5%. Thus, the accurate simulation and prediction of springback can provide important support for the manufacture of ultra-high strength automobile thin-walled parts. In this paper, the bending springback of the medium-Mn steel, as one of the third-generation automobile steels, was studied based on experiment and simulation under different conditions. Tensile tests were carried out to evaluate comprehensive mechanical properties with high strength and high plasticity. Cyclic loading-unloading tests were conducted to analyze the changing rules of unloading elastic modulus and inelastic recovery with strain. The correlation model between unloading elastic modulus and true plastic strain was further established and the model parameters were obtained. Furthermore, V-shaped bending tests were carried out under different conditions to analyze the effects of rolling direction, bending angle, and punch fillet radius on springback. The effect of rolling direction on springback angle is negligible. The bending angle has a positive effect on springback angle, while the punch fillet radius has a negative effect. Finally, the material model with constant and variable elastic moduli were developed by using user-defined material subroutines for the medium-Mn steel. V-shaped bending tests under constant and variable elastic moduli were simulated and compared with the experimental result. The simulated result with variable elastic modulus was closer to the experimental one, which proves the importance of considering the change of unloading elastic modulus in the numerical simulation of medium-Mn steel. The research result of this paper is helpful to enhance the simulation accuracy of springback of medium-Mn steel part and promote its industrial application.