Rock mechanical characteristics and landscape evolutionary mechanism of the slit-type Danxia landform on the Chinese Loess Plateau

Since 2015,the newly discovered slit-type Danxia landform on the Chinese Loess Plateau has become a hot topic in the field of geomorphology worldwide.However,the relationships among its formation,evolutionary mechanism,and mechanical characteristics of its strata and rocks are not clear.In this paper,the Ganquan canyon group is used as the research object.Basic physical and mechanical indices of sandstone in the Ganquan canyon group were measured through field investigation and indoor experiments,and the deterioration trends for the mechanical parameters of sandstone in this area under the action of infiltration,acid dry-wet cycles,and freeze-thaw cycles were revealed.Lastly,the formation and evolutionary mechanism of the slit-type Danxia landform were discussed.The results showed that:(1)The sandstone in the canyon group had a low cementation degree and weak cohesive force,which was easily weakened under the action of water,resulting in a decrease in compressive strength and elastic modulus.(2)Acidic dry-wet cycles caused the mineral composition of the sandstone to be dissolved,and the micropores continued to grow and develop until new cracks were produced.Macroscopically,the compressive strength and elastic modulus of sandstone were greatly reduced,and this damage was cumulative and staged.The greater the acidity,the greater the damage.(3)As the number of freeze-thaw cycles increased,the uniaxial compressive strength and elastic modulus of the sandstone decreased continuously.During the freeze-thaw cycle process,the growth and development of cracks were primarily in fracture mode and usually developed along parallel bedding positions.(4)The interaction of tectonic activity and lithology with different weathering processes was a key factor in the formation and evolution of the slit-type Danxia landform.In conclusion,the intricate process of weathering influenced by historical climatic fluctuations has been pivotal in shaping the topography of Danxia landform.

Anchoring mechanical characteristics of Ductile-Expansion bolt

The application of ductile rock bolts has been a crucial method for solving the problems of large deformations,energy absorption and stability control issues in deep rock masses.To study the anchoring mechanism of the key expansive structure,this paper proposes a novel type of bolt—the Ductile-Expansion bolt,and conducts research on anchoring mechanics,energy absorption characteristics,and failure modes of the bolt.In addition,this paper defines the concept of load-volume ratio of metal rock bolts and proves the Ductile-Expansion bolt is capable of better improving the unit volume bearing capacity of the bolt material.Furthermore,laboratory and field tests verify the Ductile-Expansion bolt had better anchoring effect than the traditional rebar bolt,with the expansion structure favorably enhancing the ductility and energy absorption performance of the bolt.Finally,this paper microscopically analyzes the crack propagation and distribution morphology of the bolts by establishing a 3D coupled numerical model based on FDM-DEM.Numerical results illustrate the interface at the variable diameter of the Ductile-Expansion bolt serves as the transition zone between high and low stress levels.The expansion structure can impose radial compression on the medium around the bolt,which can improve the bolt anchorage performance.

Dynamic mechanical characteristics of NdFeB in electromagnetic brake

With the continuous development of artillery,the disadvantages of hydraulic recoil brakes gradually appear.At the same time,the appearance of high-performance Nd Fe B permanent magnet makes it possible to apply electromagnetic braking technology to recoil mechanism.In this paper,prototype tests of a certain artillery were carried out to verify the feasibility of the electromagnetic brake(EMB)and obtain the electromagnetic braking force.Due to the brittleness of Nd Fe B,in order to eliminate the worry about the safety of EMB,SHPB experiments of Nd Fe B were carried out.Then,based on the assumption of uniform crack distribution,the law of crack propagation and damage accumulation was described theoretically,and the damage constitutive model suitable for brittle materials was proposed by combining the Zhu-Wang-Tang(ZWT)equation.Finally,the numerical simulation model of the artillery prototype was established and through calculation,the dynamic mechanical characteristics of Nd Fe B in the prototype were analyzed.The calculation results show that the strength of Nd Fe B can meet the requirements of the use in the working process.From the perspective of damage factor,the damage value of the permanent magnet on the far right is larger,and the damage value of the inner ring gradually decreases to the outer ring.

Dynamic mechanical characteristics and application of constant resistance energy-absorbing supporting material

In deep underground engineering,rock burst and other dynamic disasters are prone to occur due to stress concentration and energy accumulation in surrounding rock.The control of dynamic disasters requires bolts and cables with high strength,high elongation,and high energy-absorbing capacity.Therefore,a constant resistance energy-absorbing(CREA)material is developed.In this study,the dynamic characteristics of the new material are obtained via the drop hammer tests and the Split Hopkinson Pressure Bar(SHPB)tests of the new material and two common bolt(CB)materials widely used in the field.The test results of drop hammer test and SHPB test show that the percentage elongation of CREA material is more than 2.64 and 3.22 times those of the CB material,and the total impact energy acting on CREA material is more than 18.50 and 21.84 times,respectively,indicating that the new material has high elongation and high energy-absorbing capacity.Subsequently,the CREA bolts and cables using the new material are developed,which are applied in roadways with high stress and strong dynamic disturbance.The field monitoring results show that CREA bolts and cables can effectively control the surrounding rock deformation and ensure engineering safety.

Microstructure and mechanical characteristics of AA6061-T6 joints produced by friction stir welding,friction stir vibration welding and tungsten inert gas welding:A comparative study

This study compared the microstructure and mechanical characteristics of AA6061-T6 joints produced using friction stir welding(FSW),friction stir vibration welding(FSVW),and tungsten inert gas welding(TIG).FSVW is a modified version of FSW wherein the joining specimens are vibrated normal to the welding line during FSW.The results indicated that the weld region grains for FSVW and FSW were equiaxed and were smaller than the grains for TIG.In addition,the weld region grains for FSVW were finer compared with those for FSW.Results also showed that the strength,hardness,and toughness values of the joints produced by FSVW were higher than those of the other joints produced by FSW and TIG.The vibration during FSW enhanced dynamic recrystallization,which led to the development of finer grains.The weld efficiency of FSVW was approximately 81%,whereas those of FSW and TIG were approximately 74%and 67%,respectively.

Mechanical characteristics and microcosmic mechanisms of granite under temperature loads

The relationships between mechanical characteristics of rock and microcosmic mechanism at high temperatures were investigated by MTS815, as well as the stress-strain behavior of granite under the action of temperatures ranging from room temperature to 1200 ℃. Based on a micropore structure analyzer and SEM, the changes in rock porosity and micro structural morphology of sample fractures and brittle-plastic characteristics under high temperatures were analyzed. The results are as follows： 1） Mechanical characteristics do not show obvious variations before 800 ℃; strength decreases suddenly after 800 ℃ and bearing capacity is almost lost at 1200 ℃. 2） Rock porosity increases with rising temperatures; the threshold temperature is about 800 ℃; at this temperature its effect is basically uniform with strength decreasing rapidly. 3） The failure type of granite is a brittle tensile fracture at temperatures below 800 ℃ which transforms into plasticity at temperatures higher than 800 ℃ and crystal formation takes place at this time. Chemical reactions take place at 1200 ℃. Failure of granite under high temperature is a common result of thermal stress as indicated by an increase in the thermal expansion coefficient, transformation to crystal formation of minerals and structural chemical reactions.

Mechanical characteristic variation of ballastless track in highspeed railway:effect of train–track interaction and environment loads

Due to the fact that ballastless tracks in highspeed railways are not only subjected to repeated train–track dynamic interaction loads,but also suffer from complex environmental loads,the fundamental understanding of mechanical performance of ballastless tracks under sophisticated service conditions is an increasingly demanding and challenging issue in high-speed railway networks.This work aims to reveal the effect of train–track interaction and environment loads on the mechanical characteristic variation of ballastless tracks in high-speed railways,particularly focusing on the typical interface damage evolution between track layers.To this end,a finite element model of a double-block ballastless track involving the cohesive zone model for the track interface is first established to analyze the mechanical properties of the track interface under the loading–unloading processes of the negative temperature gradient load(TGL)followed by the same cycle of the positive TGL.Subsequently,the effect of wheel–rail longitudinal interactions on the nonlinear dynamic characteristics of the track interface is investigated by using a vehicle-slab track vertical-longitudinal coupled dynamics model.Finally,the influence of dynamic water pressure induced by vehicle dynamic load on the mechanical characteristics and damage evolution of the track interface is elucidated using a fluid–solid coupling method.Results show that the loading history of the positive and negative TGLs has a great impact on the nonlinear development and distribution of the track interface stress and damage;the interface damage could be induced by the wheel–rail longitudinal vibrations at a high vehicle running speed owing to the dynamic amplification effect caused by short wave irregularities;the vehicle dynamic load could produce considerable water pressure that presents nonlinear spatial–temporal characteristics at the track interface,which would lead to the interface failure under a certain condition due to the coupled dynamic effect of vehicle load and water pressure.

Mechanical characteristics of soil-rock mixtures containing macropore structure based on 3D modeling technology

Soil-rock mixtures containing macropore(SRMCM)is a kind of geological material with special mechanical properties.Located in the project area of Lenggu hydropower station on the Yalong River,Sichuan Province,China,there is an extremely unstable Mahe talus slide with a total volume of nearly160 million cubic meters,which is mainly composed of SRMCM.The study on the mechanical properties of SRMCM is of great significance for the engineering construction and safe operation.In this paper,laboratory tests and discrete element numerical tests based on three-dimensional scanning technology were conducted to study the influence of stone content,stone size,and the angle of the macropore structure on shear characteristics of SRMCM.The failure mechanism of SRMCM was discussed from a microscopic perspective.This work explains the internal mechanism of the influence of stone content,stone size,and the angle of the macropore structure on the strength of SRMCM through the microscopic level of stone rotation,force chain distribution,and crack propagation.As the macropore structure that intersects with the preset shear plane at a large angle could act as a skeleton-like support to resist the shear force,the fracture of the weak cemented surface of soil and stone in the macropore structure is an important cause of SRMCM destruction.

Field tests on mechanical characteristics and strength parameters of red-sandstone

Large-scale field shear tests on ten specimens of the red-sandstone embankment at a highway in Hunan,China,were performed to examine mechanical characteristics and parameters of red-sandstone.The curves of thrust-displacement,failure mode,and shear strength parameters for red-sandstone with different water contents,different compactions,and different grain size distributions were obtained from the tests.A practical procedure of in-situ test for red-sandstone embankment was proposed to normalize the test equipment and test steps.Based on three-dimensional thrust-sliding limit equilibrium method,the formulas for calculating strength parameters of red-sandstone considering three-dimensional sliding surface were inferred.The results show that red-sandstone has typical complete curves of stress-strain,strain softening,which are caused by the special structure of red-sandstone;water content and compaction are important factors for strength and failure mode of red-sandstone;The average value of cohesion and internal friction angle of the specimens calculated by three-dimensional technique are 21.56 kPa and 29.29°,respectively,and those by traditional two-dimensional method are 25.52 kPa and 33.76°,respectively.

Analysis of the Stability and Mechanical Characteristics of the Jointed Surrounding Rock and Lining Structure of a Deeply Buried Hydraulic Tunnel

On-site monitoring and numerical simulation have been combined to analyze the stability of the jointed surrounding rock and the stress inside the lining structure of a sample deeply buried hydraulic tunnel.We show that the deformation around the tunnel was mainly concentrated in the range 51.37 mm∼66.73 mm,the tunnel circumference was dominated by shear failure,and the maximum plastic zone was about 3.90 m.When the shotcrete treatment was performed immediately after the excavation,the deformation of the surrounding rock was reduced by 58.94%∼76.31%,and the extension of the plastic zone was relatively limited,thereby leading to improvements in terms of the stability of surrounding rock.When the support was provided at different time points,the stress of the surrounding rock in the shallow part of the tunnel was improved everywhere.In the tunnel section with high ground stress and joint development,when 10 cm steel fiber concrete spray layer and 40 cm C25 concrete secondary lining were used,the maximum tensile stress on the lining structure was 0.89 MPa,i.e.,it was less than the tensile strength of concrete,which indicates that the internal force of the lining can meet the overall requirements.

Analysis of Mechanical Characteristics of Fly Ash Cenospheres Reinforced Epoxy Composites

Fly ash cenospheres（FACs） as a recycling material of industrial waste has become a competitor for other inorganic particle fillers. Epoxy resin（EP） composites reinforced with different content of FACs as well as different size grading ratio were prepared. The surface modification of FACs particles was conducted before incorporating into EP matrix. The impact and flexural strengths and the flexural modulus were investigated, and the fracture surfaces of the testing samples were analyzed using SEM. Results showed that FACs had an obvious effect on the mechanical characteristics of the FACs/EP composites. With increasing weight fraction of FACs, the impact and flexural strengths and the flexural modulus of EP composite samples increased, and reached the highest values when the weight fraction of FACs reached 15 wt%. The mechanical characteristics of the FACs/EP composites however deteriorated with further increasing of FACs content. For the EP composites reinforced with different size grading ratio of FACs, the larger proportion of small FACs particles, the better mechanical properties of the EP composites. The results were analyzed from the aspect of the plastic deformation, new surface formation and fracture absorption energy. The synergistic effect of the size grading ratio of FACs was not obvious, which would be further investigated.

Mechanical Characteristics of Radial Magnetic Bearing

This paper is based on the example of a radial magnetic bearing possessed of eight-pole, and derives the calculation formulas of static and dynamic mechanical characteristics of the bearing, in which the shape and curvature of surface, eccentricity and tilt of the journal are taken into account. Variations of the static and dynamic characteristics of the radial magnetic bearing versus static tilt parameters of journal are discussed. The outcomes show that the static tilt of the journal has influence on the mechanical characteristics of radial magnetic bearing, and change the static load capacity between two radial magnetic bearings and exert coupling effect between them. To study the dynamics of a practical rotor-magnetic bearing system, at least six stiffness coefficients in each radial magnetic bearing must be considered in ideal case, and twelve stiffness coefficients must be considered in general case of tilting journal. Such a find can be used for the coupled electromechanical dynamics analysis of rotor system equipped with magnetic bearings.

A Measuring Solution for Mechanical Characteristics of MO Disks

Mechanical characteristics of MO disk are vital for the designer of the drives and the manufacturers who provide the mass-product MO disks. So measuring mechanical characteristics is very significant. We compares the existing measuring methods and gives some novel measuring methods we adopted in details. The measuring system based upon these methods was introduced too. Some typical measuring results are also shown in this paper.

Structure,mechanical characteristics,biodegradation,and in vitro cytotoxicity of magnesium alloy ZX11 processed by rotary swaging

Rotary swaging(RS)of alloy Mg-1.03Zn-0.66Ca(ZX11)was shown to refine the average grain size to 4.5±1.2μm in a longitudinal section and 4.8±0.9μm in a transverse section.In addition,a small amount of Mg2Ca particles about 300nm in size and Mg6Zn3Ca2 particles with a size of about lOOnm was detected.This resulted in pronounced strengthening:the yield strength and the ultimate tensile strength rose to 210±8 MPa and 276±6 MPa,respectively,while the elongation hardly decreased(22.0±1.8% and 18.3±2.9% before and after RS).Furthermore,RS led to an increase in the fatigue limit of the alloy from 120 MPa to 135 MPa and did not impair its resistance to chemical corrosion.The studies in vitro showed that ZX11 induces hemolysis without inhibiting the viability of peripheral blood mononuclear cells and has a more pronounced cytotoxic effect on tumor cells in comparison with non-transformed cells.No significant difference of the latter effect between the initial and the deformed states was observed.

Research on low-frequency mechanical characteristics of the MR dampers in ship isolators

A new isolator composed of a steel rope spring and a magneto-rheological(MR) damper was designed and a study on low-frequency mechanical characteristics of MR dampers in isolators was carried out.It used the characteristics of the MR damper,such as fast response,controllable damping,small energy consumption,wide dynamic scope,and great adaptation.The relationships between MR damping forces and influencing factors were analyzed based on experimental data.The results show that damping force is not only related to structural dimensions,but also closely related to controllable current and vibration frequency.Finally,the empirical formula for damping forces was corrected,and the relationship between correction coefficients and factors analyzed.

Mechanical characteristics of composite honeycomb sandwichstructures under oblique impact

Carbon fiber reinforced polymer(CFRP)and CFRP-based composite honeycomb sandwich structures are particularly sensitive to impact.The mechanical characteristics of composite honeycomb sandwich structures under oblique impact are studied by numerical simulation and experiment.The oblique impact model is established,and the reliability of the model is verified by the oblique impact test.To further analyze the influence of structural parameters on energy absorption under oblique impact,the influence of impact angle,face sheet thickness and wall thickness of the honeycomb is numerically studied.The results show that the impact angle has an important effect on energy distribution.The structural parameters also have an effect on the peak contact force,contact time,and energy absorption,and the effect is different from normal impact due to the presence of frictional dissipation energy.Compared with normal impact,the debonding of oblique impact will be reduced,but the buckling range of the honeycomb core will be expanded.

Study on mechanical characteristics and safety evaluation method of steel frame structure after fire

Mechanical characterization of steel frame structure after fire are ana- lyzed based on fire dynamics, heat transfer theory, structural mechanics, and finite element theory. We study the temperature characteristics and mechanical prop- erties of steel flame structure under different fire locations and propose a safety evaluation method. We also analyze damage level of main flame components, maximum temperature of fire, thermal characteristics of flame components, fir- ing duration, etc. to provide useful information for fire resistance design of the steel frame structure and post-disaster safety evaluation.

Mechanical Characteristics of a Thrust Magnetic Bearing

Static and dynamic mechanical characteristics of a thrust magnetic bearing are studied owing to the inclination of the runner disk. The application refers to a thrust magnetic bearing for a turbo expander/compressor. The static tilt of the runner disk has remarkable influence on the mechanical characteristics of thrust magnetic bearing, it can change the static load distribution between two radial magnetic bearings and will exert violent coupling effect among a thrust magnetic bearing and two radial magnetic bearings. Such a finding can be used for the coupled electromechanical dynamics analysis of rotor system equipped with magnetic bearings.

Dynamic evolution in mechanical characteristics of complex supporting structures during large section tunnel construction

The shallow tunnelling method(STM)often uses temporary supports to divide large section tunnels into several closed or semiclosed sections so as to share the upper load.The complex support system composed of primary and temporary supports can ensure safety during tunnel construction.Based on the large section tunnel of Beijing Subway Line 12,the mechanical characteristics of support system by the double-side-drift method(DSDM)during excavation and demolition were analyzed through numerical simulation and monitoring.The study showed that the middle cave excavation was the most critical stage of the DSDM,during which the load on the supporting structure increased significantly.The temporary vertical support bore most of the new load during middle cave excavation.During the demolition stage,the load was redistributed,which caused arch settlement and section convergence.The removal of the temporary vertical support exerted the greatest impact in this process.The lateral temporary inverted arch changed from axial compression to axial tension after the middle and lower caves were excavated.Based on the mechanical characteristics of the support system,some engineering suggestions were proposed for large section tunnel construction.These research results can provide reference for the design and construction of similar large section tunnels.

Modelling and mechanical characteristics of PDC cutter-rock interaction by combining mixed fragmentation modes with dynamic rock strength

Polycrystalline diamond compact(PDC)bit is one of the most widely used drill bits for improving the rate of penetration in deep oil and gas well and geothermal well.However,the dynamic rock fragmentation mechanics characteristics of PDC bits are still unclearly.A coupled fragmentation mechanics model of PDC cutter-rock interaction is established by combining the mixed fragmentation modes with dynamic strength.The coupling influence laws of cutter angle,cutting depth,dynamic strength ratio,breaking modes on the horizontal force coefficient(HFC),vertical force coefficient(VFC)and specific energy are analyzed.The model of this paper can optimize cutter inclination angle,cutting depth and minimum specific energy.With the increase of the cutter inclination angle,the dynamic VFC changes into two modes.The definition of the dynamic modes depends on the dynamic strength ratio.As the cutting angle increases,the cutting force increases.The cutting force increases nonlinearly with increasing cutting depth.The specific energy of rock fragmentation increases nonlinearly with increasing cutting depth.With the increase of dynamic strength,the specific energy of rock fragmentation increases nonlinearly.When the input-energy increases,the rate of penetration response is divided into three stages.The results have important guiding significance for the PDC bit design and drilling parameters optimization to increase the rate of penetration and the efficiency of exploration and development.