Strength characteristics of rock anchored by NPR bolt with different preloads

This study compares the strength characteristics of rocks anchored by NPR bolts and ordinary bolts with varied preloads,based on the mechanical properties of NPR bolts(with a negative Poisson’s ratio).The results show that the uniaxial compressive stress-strain curve of ordinary anchored rocks exhibits noticeable abrupt changes.After reaching peak strength,the bolt breaks,whereas the stress-strain curve of NPR-anchored rocks is smoother.The NPR bolt enters the stage of continuous resistance after reaching maximal strength and does not break.As the preload increases,the strength of the anchored rock grows linearly.A calculation equation for the strength of the anchored rock is proposed based on the preload.The theoretical equation fits the test results well,and the fitted parameters show that NPR bolts can better increase the strength of the rock.The concept of dynamic toughness UC of anchored rock is proposed to reflect the comprehensive mechanical properties of anchored rock,including strength and plasticity.As the preload increases,the UC of ordinary anchored rock first decreases and then increases,while the UC of the NPR anchored rock does not change significantly with the preload when the strain is small,and the UC increases with the increase of the preload when the strain is large.

Study on Electromyographic Characteristics of Neck Muscles in College Students with Chronic Neck Pain

Objective:To provide new insights for the evaluation and diagnosis of chronic neck pain(CNP).Methods:22 patients with CNP and 22 healthy individuals were recruited from South China Normal University,who were all college students.The subjects'neck extensor muscle strength in the head neutral position,the natural anteversion position,and the maximum forward flexion position were measured by an isometric muscle strength tester respectively.The neck extensor strength of CNP patients and healthy subjects were compared.Results:In the neutral position,the maximum isometric muscle strength of neck extensor muscles was 12.31 kg for CNP patients and 15.16 kg for healthy individuals,resulting in a ratio of 81%strength in patients compared to healthy subjects.This difference was highly significant(P<0.000).In the natural anteversion position,the respective values were 12.6 kg for CNP patients and 15.05 kg for healthy individuals,with a ratio of 83%,and a significant difference between groups(P<0.001).In the maximum forward flexion position of the head,the values were 13.36 kg for CNP patients and 16.15 kg for healthy individuals,with a ratio of 82%,and a highly significant difference(P<0.000).Conclusion:The neck extensor muscle strength levels in college students with CNP were significantly lower compared to healthy college students across all measured positions.

Construction controlling and strength increasing characteristics of locally developed epoxy asphalt mixture

The construction controlling elements,including construction performance at low ambient temperatures,reserved time ranges,and the strength increasing rule,for locally developed epoxy asphalt mixture（LDEAM） are studied through laboratory tests.Air void and Marshall stability are chosen as the performance measures for evaluating the construction quality.The wheel rolling test is used to simulate the actual construction situations,while the Marshall test and the Brookfield viscosity test are adopted to study the strength increasing rule.The experimental results reveal that the LDEAM can be constructed under a low ambient temperature condition of 10℃,and its longest reserved time is 70 min at a reserved temperature of 120℃.Test also shows that the strength of the LDEAM increases with time and temperature before cured.Finally,the theoretical reason for the strength increasing rule is analyzed on the basis of the organic chemistry theory.

Microstructures,Strength Characteristics and Wear Behavior of the Fe-based P/M Composites after Sintering or Infiltration with Cu–Sn Alloy

Some properties of the Fe-based P/M composites sintered and reinforced by infiltration with Cu-Sn alloy were described.It is shown that the hardness of the sintered material is 2.5 times lower,tensile strength is 1.7 times lower and the wear resistance is 2.5-3.3 times lower in comparison with those of the infiltrated material.The presence of pores on the friction surface of the sintered material affects the features of the wear process.Due to the specific morphology of copper in the infiltrated material,the phenomenon of selective mass transfer is observed and worn surfaces have a spongy-capillary texture.

Characteristic strength and acoustic emission properties of weakly cemented sandstone at different depths under uniaxial compression

As coal mining is extended from shallow to deep areas along the western coalfield,it is of great significance to study weakly cemented sandstone at different depths for underground mining engineering.Sandstones from depths of 101.5,203.2,317.3,406.9,509.9 and 589.8 m at the Buertai Coal Mine were collected.The characteristic strength,acoustic emission(AE),and energy evolution of sandstone during uniaxial compression tests were analyzed.The results show that the intermediate frequency(125-275 kHz)of shallow rock mainly occurs in the postpeak stage,while deep rock occurs in the prepeak stage.The initiation strength and damage strength of the sandstone at different depths range from 0.23 to 0.50 and 0.63 to 0.84 of peak strength(σ_(c)),respectively,decrease exponentially and are a power function with depth.The precursor strength ranges from 0.88σ_(c)to 0.99σ_(c),increases with depth before reaching a depth of 300 m,and tends to stabilize after 300 m.The ratio of the initiation strength to the damage strength(k)ranges from 0.25 to 0.62 and decreases exponentially with depth.The failure modes of sandstone at different depths are tension-dominated mixed tensile-shear failure.Shear failure mainly occurs at the unstable crack propagation stage.The count of the shear failure bands before the peak strength increases gradually,and increases first and then decreases after the peak strength with burial depth.The cumulative input energy,released elastic energy and dissipated energy increase with depth.The elastic release rate ranges from 0.46×10^(-3)to 198.57×10^(-3)J/(cm^(3)s)and increases exponentially with depth.

Fracture evolution characteristics of sandstone containing double fissures and a single circular hole under uniaxial compression

The uniaxial compression experiments on the sandstone samples containing double fissures and a single circular hole were carried out by using electro-hydraulic servo universal testing machine to investigate the effect of rock bridge angle β and fissure angle α on mechanical properties and evolution characteristics of cracks.The results show that the peak strength,peak strain and elastic modulus of defected specimens decrease comparing with those for intact sample,and show a decreased trend firstly and then increase with β changing from 0° to 90°.The peak strength and elastic modulus achieve the minimum value as the rock bridge angle is 60°,while the peak strain reaches the minimum value with the rock bridge angle of 45°.The crack initiation of tested rock samples occurs firstly in stress concentration areas at tips of prefabricated fissures under uniaxial compression,and then propagates constantly and coalescences with the prefabricated hole.Some secondary cracks initiate and propagate as well until buckling failure happens.The rock bridge angle has a great influence on crack initiation,coalescence,final failure mode,crack initiation stress and transfixion stress.The peak strength varies significantly,while the elastic modulus and peak strain change slightly,and the failure modes are also different due to the influence of fissure angle.

Microstructural Characteristics of Underwater Shock Consolidated Aluminum Composites

Metal matrix composites (MMCs) offer extra strength and high temperature capabilities in comparison with unrein-forced metals. Aluminum composites possess higher stiffness, strength, fatigue properties and low weight advantages. Carbon fiber reinforced Al composites (Al-Cf) and silicon carbide particulate reinforced Al composites (AI-SiCp) were shock densified using axisymmetric assemblies for underwater explosions. Unidirectional planar shock waves were applied to obtain uniform consolidation of the composites. The energy generator was a high explosive of 6.9 km/s detonation velocity. Irregular morphological powders of Al were the base material. The reinforcement ratio was 15 Vol. pet for Al-Cf composites and 30 Vol. pet for AI-SiCp composites. The microstructural and the strength characteristics of the shock consolidated Al composites are reported.

Strength failure and crack coalescence behavior of sandstone containing single pre-cut fissure under coupled stress, fluid flow and changing chemical environment

In order to study the strength failure and crack coalescence characteristics of cracked rocks, uniaxial compression experiments were conducted on cylindrical sandstone specimens, sampled from Longyou Grottoes of Zhejiang Province, China, with a single pre-cut crack soaking in different chemical solutions. Based on the results of uniaxial compressive test under different chemical solutions and velocities of flow, the effect of strength and deformation characteristics and main modes of crack coalescence for cracked rocks under chemical corrosion were analyzed. The results show that the pH value and velocity of the chemical solutions both have great influence on the sandstone sample's uniaxial compressive strength and deformation characteristics. Cracked sandstone samples are tension-destructed under uniaxial compression, and the crack propagation directions are consistent with the loading direction. The phenomena of crack initiation, propagation and coalescence of sandstone are well observed. Four different crack types are identified based on the crack propagation mechanism by analyzing the ultimate failure modes of sandstone containing a single pre-cut fissure. The failure process of specimen in air is similar with the specimen under chemical solutions, however, the initial time of crack occuring in specimen under chemical solutions is generally earlier than that in the natural specimen, and the crack propagation and coalescence process of specimen under chemical solutions are longer than those of the natural specimen due to softening of structure of rock caused by hydro-chemical action. Immersion velocity of flow and chemical solutions does not have influence on the ultimate modes of crack coalescence.

Mechanical properties and failure characteristics of fractured sandstone with grouting and anchorage

Based on uniaxial compression experimental results on fractured sandstone with grouting and anchorage, we studied the strength and deformation properties, the failure model, crack formation and evolution laws of fractured sandstone under different conditions of anchorage. The experimental results show that the strength and elastic modulus of fractured sandstone with different fracture angles are significantly lower than those of intact sandstone. Compared with the fractured samples without anchorage,the peak strength, residual strength, peak and ultimate axial strain of fractured sandstone under different anchorage increase by 64.5–320.0%, 62.8–493.0%, and 31.6–181.4%, respectively. The number of bolts and degree of pre-stress has certain effects on the peak strength and failure model of fractured sandstone. The peak strength of fractured sandstone under different anchorage increases to some extent, and the failure model of fractured sandstone also transforms from tensile failure to tensile–shear mixed failure with the number of bolts. The pre-stress can restrain the formation and evolution process of tensile cracks, delay the failure process of fractured sandstone under anchorage and impel the transformation of failure model from brittle failure to plastic failure.

Improved Breakdown Strength in(Ba_(0.6)Sr_(0.4))_(0.85)Bi_(0.1)TiO_3 Ceramics with Addition of CaZrO_3 for Energy Storage Application

（Ba（0.6） Sr（0.4））（0.85） Bi（0.1） TiO3 ceramics doped with x wt%CaZrO3（x= 0-10） were synthesized by solid-state reaction method. The effects of CaZrO3 amount on the dielectric properties and structure of（Ba（0.6）Sr（0.4））（0.85） Bi（0.1） TiO3 ceramics were investigated. X-ray diffraction results indicated a pure cubic perovskite structure for all samples and that the lattice parameter increased till x=5 and then slightly decreased. A homogenous microstructure was observed with the addition of CaZrO3. Dielectric measurements revealed a relaxor-like characteristic for all samples and that the diffusivity γ reached the maximum value of 1.78 at x=5. With the addition of CaZrO3, the dielectric constant dependence on electric field was weakened, insulation resistivity enhanced and dielectric breakdown strength improved obviously and reached 19.9 k V/mm at x=7.5. In virtue of low dielectric loss（tan d〈0.001 5）, moderate dielectric constant（er 〉1 500） and high breakdown strength（Eb 〉17.5 k V/mm）, the CaZrO3 doped（Ba（0.6）Sr（0.4））0.85 Bi（0.1） TiO3 ceramic is a potential candidate material for high power electric applications.

Mechanical properties of nerve roots and rami radiculares isolated from fresh pig spinal cords

No reports have described experiments designed to determine the strength characteristics of spinal nerve roots and rami radiculares for the purpose of explaining the complexity of symptoms of medullary cone lesions and cauda equina syndrome. In this study, to explain the pathogenesis of cauda equina syndrome, monoaxial tensile tests were performed to determine the strength characteristics of spinal nerve roots and rami radiculares, and analysis was conducted to evaluate the stress-strain relationship and strength characteristics. Using the same tensile test device, the nerve root and ramus radiculares isolated from the spinal cords of pigs were subjected to the tensile test and stress relaxation test at load strain rates of 0.1, 1, 10, and 100 s-1 under identical settings. The tensile strength of the nerve root was not rate dependent, while the ramus radiculares tensile strength tended to decrease as the strain rate increased. These findings provide important insights into cauda equina symptoms, radiculopathy, and clinical symptoms of the medullary cone.

Physical and mechanical properties of sandstone containing a single fissure after exposure to high temperatures

In order to investigate the physical and mechanical properties of sandstone containing fissures after exposure to high temperatures,fissures with different angles α were prefabricated in the plate sandstone samples,and the processed samples were then heated at 5 different temperatures.Indoor uniaxial compression was conducted to analyze the change rules of physical properties of sandstone after exposure to high temperature,and the deformation,strength and failure characteristics of sandstone containing fissures.The results show that,with increasing temperature,the volume of sandstone increases gradually while the quality and density decrease gradually,and the color of sandstone remains basically unchanged while the brightness increases markedly when the temperature is higher than 585 ℃;the peak strength of sandstone containing fissures first decreases then increases when the temperature is between 25℃and 400℃.The peak strain of sandstone containing fissures increases gradually while the average modulus decreases gradually with increasing temperature,and the mechanical properties of sandstone show obvious deterioration after 400 ℃.The peak strain of sandstone containing fissures increases gradually while the average modulus decreases gradually with increasing temperature;with increasing angle αof the fissure,the evolution characteristics of the macro-mechanical parameters of sandstone are closely related to the their own mechanical properties.When the temperature is 800 ℃,the correlation between the peak strength and average modulus of sandstone and the angle α of the fissure is obviously weakened.The failure modes of sandstone containing fissures after high temperature exposure are of three different kinds including:tensile crack failure,tensile and shear cracks mixed failure,and shear crack failure.Tensile and shear crack mixed failure occur mainly at low temperatures and small angles;tensile crack failure occurs at high temperatures and large angles.

The use of point load test for Dubai weak calcareous sandstones

Intact rock is typically described according to its uniaxial compressive strength （UCS）. The UCS is needed in the design of geotechnical engineering problems including stability of rock slopes and design of shallow and deep foundations resting on and/or in rocks. Accordingly, a correct measure-ment/evaluation of the UCS is essential to a safe and economic design. Typically, the UCS is measured using the unconfined compression tests performed on cylindrical intact specimens with a minimum length to width ratio of 2. In several cases, especially for weak and very weak rocks, it is not possible to extract intact specimens with the needed minimum dimensions. Thus, alternative tests （e.g. point load test, Schmidt hammer） are used to measure rock strength. The UCS is computed based on the results of these tests through empirical correlations. The literature includes a plethora of these correlations that vary widely in estimating rock strength. Thus, it is paramount to validate these correlations to check their suitability for estimating rock strength for a specific location and geology. A review of the available correlations used to estimate the UCS from the point load test results is performed and summarized herein. Results of UCS, point load strength index and Young＇s modulus are gathered for calcareous sandstone specimens extracted from the Dubai area. A correlation for estimating the UCS from the point load strength index is proposed. Furthermore, the Young＇s modulus is correlated to the UCS.

SIZE EFFECTS OF WINDOW GLASSES IN TALL BUILDINGS

A certain strength is required for the window glasses in tall buildings due to the high wind pressure. The strength for large size glasses is generally deduced from the testing results of specimens with samll sizes by the Weibull statistics theory. By a great deal of experiments the authors have studied the thickness , span and width effects of flat glasses of on bending strength under biaxial stresses. Compared with the Weibull theory, the accuracy of strength is determined and a modification is made for the volume effect proposed by Weibull. The size effect of Weibull modulus is investigated as well and the reasonable specification for flat glass strength is suggested.