氧化铝空心球及玄武岩纤维对混凝土动力特性的影响

以氧化铝空心球为轻集料,玄武岩纤维为纤维增强材料制备轻质混凝土,利用直径100 mm的分离式霍普金森压杆试验系统,研究了不同应变率下素混凝土、玄武岩纤维混凝土、氧化铝空心球混凝土以及玄武岩纤维-氧化铝空心球复合混凝土的动态力学性能。结果表明:随着应变率的提高,四种试件的动压强度、动压韧性、动压比韧性及破坏程度均不断增大,表现出显著的应变率相关性;掺入氧化铝空心球使得试件破坏程度增大,动压强度及动压韧性减小,动压比韧性提高;掺入玄武岩纤维可以有效降低试件的破坏程度,并对试件具有明显的强韧化效果;通过复掺氧化铝空心球及玄武岩纤维,可以制备得到具有良好吸能特性的轻质混凝土。

Dynamic compressive strength and failure mechanisms of microwave damaged sandstone subjected to intermediate loading rate

To investigate the influence of microwave heating on the dynamic behavior and failure mechanisms of rock,dynamic compression tests were conducted on microwave-irradiated sandstone specimens using a modified split Hopkinson pressure bar(SHPB)system.Experimental results show that microwave radiation can effectively weaken the compressive strength of sandstone.Rock specimens show three different failure modes under impact load:tensile failure,tensile−shear composite failure and compressive−shear failure.The dynamic Poisson’s ratio,calculated using the measured P-and S-wave velocities,is introduced to describe the deformation characteristics of sandstone.With the increase in microwave power and heating time,the Poisson’s ratio declines first and then increases slightly,and the turning point occurs at 244.6℃.Moreover,the microstructural characteristics reveal that microwave radiation produces dehydration,pore expansion,and cracking of the rock.The damage mechanisms caused by microwave radiation are discussed based on thermal stress and steam pressure inside the rock,which provides a reasonable explanation for the experimental results.

Controlled low-strength material incorporating recycled fine aggregate from urban red brick based construction waste

Sixteen controlled low-strength material( CLSM)mixtures with various cement-to-sand( C/Sa) ratios and water-to-solid( W/So) ratios were prepared using recycled fine aggregate from urban red brick based construction waste.The fluidity and bleeding of the fresh CLSM mixtures were measured via the modified test methods, and the hardened CLSM mixtures were then molded to evaluate their compressive strength and durability. The results showthat the fluidity of the fresh CLSM mixtures is 105 to 227 mm with the corresponding bleeding rate of 3. 7% to 15. 5%, which increases with the increase in fluidity. After aging for 28 d,the compressive strength of the hardened CLSM mixtures reaches 1. 15 to 13. 96 M Pa, and their strength can be further enhanced with longer curing ages. Additionally, the strength increases with the increase of the C/Sa ratio, and decreases with the increase of the W/So ratio under the same curing age. Based on the obtained compressive strength, a fitting model for accurately predicting the compressive strength of the CLSM mixtures was established, which takes into account the above two independent variables( C/Sa and W/So ratios).M oreover, the durability of the hardened CLSM mixtures is enhanced for samples with higher C/Sa ratios.

Strength weakening effect of high static pre-stressed granite subjected to low-frequency dynamic disturbance under uniaxial compression

This study aimed to elucidate the strength weakening effect of high static pre-stressed rocks subjected to low-frequency disturbances under uniaxial compression.Based on the uniaxial compressive strength(UCS)of granite under static loading,70%,80%,and 90%of UCS were selected as the initial high static pre-stress(σ_(p)),and then the pre-stressed rock specimens were disturbed by sinusoidal stress with amplitudes of 30%,20%,and 10%of UCS under low-frequency frequencies(f)of 1,2,5,and 10 Hz,respectively.The results show that the rockburst failure of pre-stressed granite is caused by low-frequency disturbance,and the failure strength is much lower than UCS.When theσp or f is constant,the specimen strength gradually decreases as the f or σ_(p) increases.The experimental study illustrates the influence mechanism of the strength weakening effect of high static pre-stress rocks under low-frequency dynamic disturbance,that is,high static pre-stress is the premise and leading factor of rock strength weakening,while low-frequency dynamic disturbance induces rock failure and affects the strength weakening degree.

Sutureless Intestinal Anastomosis with a Novel Device of Magnetic Compression Anastomosis

Objective To explore the feasibility and efficiency of a novel magnetic compression anastomats(MCAs) in intestinal anastomosis.Methods A total of 36 male mongrel canines underwent intestinal anastomosis using traditional hand-sewn(n=18) or a novel MCAs(n=18).We compared the anastomosis time,postoperative complications,bursting strength of anastomoses,gross appearance,and pathology between two groups at each time-point of follow-up.Results The mean anastomosis time with MCAs was significantly less than that with hand-sewn(8.50±1.95 vs.31.1±4.32 minutes,P<0.001).The blood stools and intussusceptions occurred in both groups during follow-up period.Only 1 mongrel canine receiving intestinal anastomosis by MCAs experienced anastomotic leakage.The average bursting pressure of anastomoses obtained from mongrel canines undergoing intestinal anastomosis by MCAs was significantly higher than that by traditional hand-sewn at 1 week's follow-up time(P<0.05).Gross appearance of the anastomoses constructed by MCAs was relatively smoother and flatter.Pathological evalution of anastomoses revealed that general inflammation was greater in hand-sewn anastomoses than magnetic anastomosis.Conclusion The magnetic compression anastomat is a safe and effective device of sutureless intestinal anastomosis in canine models.

Effect of polypropylene fiber and coarse aggregate on the ductility and fluidity of cemented tailings backfill

Adding polypropylene(PP)fibers and coarse aggregates has become a popular way to enhance the strength and stability of the cemented tailings backfilling(CTB)body.It is essential to explore the influence of tailings-aggregate ratio and fiber content on the mechanical properties of CTB samples.The comprehensive tests of the unconfined compressive strength(UCS),slump and microstructure were designed,and the regression models were established to characterize the effect of the strength,ductility and fluidity.The results indicate that the tailings-aggregate ratio of 5:5 and PP fiber content of 0.5 kg/m^(3) are the optimum point considering the UCS,cracking strain,peak strain and post-peak ductility.The tailings-aggregate ratio is consistent with the unary quadratic to the UCS and a linear model with a negative slope to the slump.Microstructural analysis indicates that PP fiber tends to bridge the cracks and rod-mill sand to serve as the skeleton of the paste matrix,which can enhance the compactness and improve the ductility of the CTB.The results presented here are of great significance to the understanding and application of coarse aggregates and fibers to improve the mechanical properties of CTB.

Effect of strain rate and water-to-cement ratio on compressive mechanical behavior of cement mortar

Effects of strain rate and water-to-cement ratio on the dynamic compressive mechanical behavior of cement mortar are investigated by split Hopkinson pressure bar(SHPB) tests. 124 specimens are subjected to dynamic uniaxial compressive loadings.Strain rate sensitivity of the materials is measured in terms of failure modes, stress-strain curves, compressive strength, dynamic increase factor(DIF) and critical strain at peak stress. A significant change in the stress-strain response of the materials with each order of magnitude increase in strain rate is clearly seen from test results. The slope of the stress-strain curve after peak value for low water-to-cement ratio is steeper than that of high water-to-cement ratio mortar. The compressive strength increases with increasing strain rate. With increase in strain rate, the dynamic increase factor(DIF) increases. However, this increase in DIF with increase in strain rate does not appear to be a function of the water-to-cement ratio. The critical compressive strain increases with the strain rate.

Application of a fuzzy analytical hierarchy process to the prediction of vibration during rock sawing

A new predictive model for evaluating the vibration of a sawing machine was developed using a new rock classification system. The predictors are machine parameters and a rock sawability index. The new rock classification system includes four major parameters of the rock： uniaxial compressive strength, abrasiv- ity index, mean MoWs hardness, and Young＇s modulus. The FAHP approach was used when determining the weights of these parameters by six decision makers. Two groups of carbonate rocks were sawn using a fully-instrumented laboratory sawing rig at different feed rates and depths of cut. During the sawing trials system vibration was monitored as a measure of saw performance. Then, a new statistical model was obtained by multiple regression on the machining parameters and the rock sawability index. The model is very useful for the evaluation of the system vibration, and for selecting suitable machining parameters, from a limited set of mechanical properties.

Mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions

To investigate the mechanical properties of cement mortar in sodium sulfate and sodium chloride solutions, uniaxial compression test and ultrasonic test were performed. Test results show that the relative dynamic elastic modulus, the mass variation,and the compressive strength of cement mortar increase first, and then decrease with increasing erosion time in sodium sulfate and sodium chloride solutions. The relative dynamic elastic moduli and the compressive strengths of cement mortars with water/cement ratios of 0.55 and 0.65 in sodium sulfate solution are lower than those in sodium chloride solution with the same concentration at the420 th day of immersion. The compressive strength of cement mortar with water/cement ratio of 0.65 is more sensitive to strain rate than that with water/cement ratio of 0.55. In addition, the strain-rate sensitivity of compressive strength of cement mortar will increase under attacks of sodium sulfate or sodium chloride solution.

Microstructure,mechanical and wear properties of aluminum borate whisker reinforced aluminum matrix composites

The microstructural features and the consequent mechanical properties were characterized in aluminium borate whisker(ABOw)(5, 10 and 15 wt.%) reinforced commercially-pure aluminium composites fabricated by conventional powder metallurgy technique. The aluminium powder and the whisker were effectively blended by a semi-powder metallurgy method. The blended powder mixtures were cold compacted and sintered at 600 ℃. The sintered composites were characterized for microstructural features by optical microscopy(OM), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), transmission electron microscopy(TEM) and X-ray diffraction(XRD) analysis. Porosity in the composites with variation in ABOw contents was determined. The effect of variation in content of ABOw on mechanical properties, viz. hardness, bending strength and compressive strength of the composites was evaluated. The dry sliding wear behaviour was evaluated at varying sliding distance at constant loads. Maximum flexural strength of 172 MPa and compressive strength of 324 MPa with improved hardness around HV 40.2 are obtained in composite with 10 wt.% ABOw. Further increase in ABOw content deteriorates the properties. A substantial increase in wear resistance is also observed with 10 wt.% ABOw. The excellent combination of mechanical properties of Al-10 wt.%ABOw composites is attributed to good interfacial bonds, less porosity and uniformity in the microstructure.

Effect of fissure water on mechanical characteristics of rock mass

In view of the effect of fissure water in fractured rock mass on the strength of rock mass in engineering projects, we pre-pared specimens of cement mortar to simulate saturated rock mass with continuous fractures of different slope angles. By exerting static and dynamic loads on the specimens, the mechanical characteristics of rock mass with fissure water under these loads can be analyzed. Our experimental results indicate that the static compressive strength of saturated fractured rock mass is related to the slope angle. The lowest compressive strength of fractured rock mass occurs when the slope angle is 45°, while the highest strength occurs when the specimen has no fractures. Fissure water can weaken the strength of rock mass. The softening coefficient does not vary with the slope angle and type of load. The hydrodynamic pressure of fractured rock mass gradually increases with an increase in dynamic load. For a 0° slope angle, the hydrodynamic pressure reaches its highest level. When the slope angle is 90°, the hydro-dynamic pressure is the lowest.

Active earth pressure for subgrade retaining walls in cohesive backfills with tensile strength cut-off subjected to seepage effects

The commonly used Mohr-Coulomb(M-C) failure condition has a limitation that it overestimates the tensile strength of cohesive soils. To overcome this limitation, the tensile strength cut-off was applied where the predicted tensile strength is reduced or eliminated. This work then presented a kinematical approach to evaluate the active earth pressure on subgrade retaining walls in cohesive backfills with saturated seepage effects. An effective rotational failure mechanism was constructed assuming an associative flow rule. The impact of seepage forces, whose distribution is described by a closed-form solution, was incorporated into the analysis. The thrust of active earth pressure was derived from the energy conservation equation, and an optimization program was then coded to obtain the most critical solution. Several sets of charts were produced to perform a parameter analysis. The results show that taking soil cohesion into account has a distinct beneficial influence on the stability of retaining walls, while seepage forces have an adverse effect. The active earth pressure increases when tensile strength cut-off is considered, and this increment is more noticeable under larger cohesion.

Effects of zinc on static and dynamic mechanical properties of copper-zinc alloy

The effects of adding alloy element zinc on the static and dynamic mechanical properties of copper-zinc alloy were investigated. Tensile and low cycle fatigue behaviors of the C11000 copper and H63 copper-zinc alloy were obtained by using a miniature tester that combined the functions of in situ tensile and fatigue testing. A piezoelectric actuator was adopted as the actuator for the fatigue testing, and the feasibility of the fatigue actuator was verified by the transient harmonic response analysis based on static tensile preload and dynamic sinusoidal load. The experimental results show that the yield strength and tensile strength of the C11000 copper are improved after adding 37%(mass fraction) zinc, and H63 copper-zinc alloy presents more obvious cyclic hardening behavior and more consumed irreversible plastic work during each stress cycle compared with C11000 copper for the same strain controlled cycling. Additionally, based on the Manson-Coffin theory, the strain-life equations of the two materials were also obtained. C11000 copper and H63 copper-zinc alloy show transition life of 16832 and 1788 cycles, respectively.

Damaging Formula of the Frost Resistant Concrete with Poor Quality of Coarse Limestone Aggregate

In this paper, concrete with limestone coarse aggregate was studied due to frost action in saline and nonsaline environments. The main focus is to explain the damaging formula of concrete with poor quality of limestone aggregate in frost actions. All investigated concretes fulfill the recommendations of the European standard EN 206, Concrete--Specification, Performance, Production and Conformity limiting values for composition and properties of concrete （maximum W/C （water/cement） ratio, minimal class of compressive strength, minimal mass of cement and minimal percentage of entrained air）. The damaging formula of the frost resistant concrete is studied through scaling test of concrete during freeze/thaw process, frost resistant test of coarse limestone aggregate and chemical analysis of limestone. Experiments results showed that there is a correlation between CaO/MgO ratio and Al2O3 of limestone and frost resistance of concrete, using chemical composition for determining potential ACR （alkali-carbonate reactivity） will indicate higher risk of damaging effect of concrete.

Design and biomechanical study of a modified pedicle screw

Objective： In pedicle screw fixation, the heads of monoaxial screws need to be directed in the same straight line to accommodate the rod placement by backing out during operation, which decreases the insertional torque and internal fixation strength. While polyaxial screws facilitate the assembly of the connecting rod, but its ball-in-cup locking mechanism reduces the static compressive bending yield strength as compared with monoaxial screws. Our study aimed to assess the mechanical performance of a modified pedicle screw. Methods： In this study, the tail of the screw body of the modified pedicle screw was designed to be a cylindershaped structure that well matched the inner wall of the screw head and the screw head only rotated around the cyclinder. Monoaxial screws, modified screws and polyaxial screws were respectively assembled into 3 groups ofvertebrectomy models simulated by ultra high molecular weight polyethylene （UHMWPE） blocks. This model was developed according to a standard for destructive mechanical testing published by the American Society for Testing Materials （ASTM F1717-04）. Each screw design had 6 subgroups, including 3 for static tension, load compression and torsion tests, and the rest for dynamic compression tests. In dynamic tests, the cyclic loads were 25%, 50%, and 75% of the compressive bending ultimate loads respectively. Yield load, yield ultimate load, yield stiffness, torsional stiffness, cycles to failure and modes of failure for the 3 types of screws were recorded. The results of modified screws were compared with those ofmonoaxial and polyaxial screws. Results： In static tests, results of bending stiffness, yield load, yield torque and torsional stiffness indicated no significant differences between the modified and monoaxial screws （P〉0.05）, but both differed significantly from those ofpolyaxial screws （P〈0.05）. In dynamic compression tests, both modified and monoaxial screws showed failures that occurred at the insertion point of screw body into the UHMWPE block, while the polyaxial screw group showed screw body swung up and down the screw head because of loosening of the ball-in-cup mechanism. Conclusions： The modified screw is well-designed and biomechanically improved. And it can provide sufficient stability for segment fixation as monoaxial screws.