Influence of confining stress on fracture characteristics and cutting efficiency of TBM cutters conducted on soft and hard rock

Combined with numerical simulation, the influence of confining stress on cutting process, fracture conditions and cutting efficiencies of soft and hard rock has been conducted on the triaxial testing machine(TRW-3000) designed and manufactured in Central South University(China). Results are obtained by performing analysis on the fracture scopes of cement and granite plates,the characteristics of cutting force in cutting processes and the cutting efficiency. Firstly, the increase of latitude fracture scope and the decrease of longitude fracture scope are both more notable in the tests conducted on cement plates subjected to the increasing confining stresses; secondly, the increase tendency of peak penetration forces obtained from tests conducted on granite plates is more obvious, however, the increase tendencies of average penetration forces achieved from cement and granite plates are close to each other; thirdly, the cutting efficiency could be improved by increasing the spacing between cutters when the confining stress which acts on soft and hard rock increases in a certain degree, and the cutting efficiency of soft rock is more sensitive to the varying confining stresses.

APPLICATION OF INTEGER CODING ACCELERATING GENETIC ALGORITHM IN RECTANGULAR CUTTING STOCK PROBLEM

An improved genetic algorithm and its application to resolve cutting stock problem arc presented. It is common to apply simple genetic algorithm （SGA） to cutting stock problem, but the huge amount of computing of SGA is a serious problem in practical application. Accelerating genetic algorithm （AGA） based on integer coding and AGA＇s detailed steps are developed to reduce the amount of computation, and a new kind of rectangular parts blank layout algorithm is designed for rectangular cutting stock problem. SGA is adopted to produce individuals within given evolution process, and the variation interval of these individuals is taken as initial domain of the next optimization process, thus shrinks searching range intensively and accelerates the evaluation process of SGA. To enhance the diversity of population and to avoid the algorithm stagnates at local optimization result, fixed number of individuals are produced randomly and replace the same number of parents in every evaluation process. According to the computational experiment, it is observed that this improved GA converges much sooner than SGA, and is able to get the balance of good result and high efficiency in the process of optimization for rectangular cutting stock problem.

An Analysis of the Formation Mechanisms of Abrasive Particles and Their Effects on Cutting Efficiency

Magnetic induction-free abrasive wire sawing(MIFAWS)is a method that combines magnetic fields with traditional free abrasive wire sawing technologies.Magnetic abrasive particles(MAPs)are attracted on a magnetized wire,thus leading to an increase in their number into the cutting zone.The number of instantaneous-effective abrasive particles(IEAPs)adsorbed on the wire surface has a great influence on the cutting efficiency of the saw wire.In this study,a mathematic model of the movement of the MAP is presented,and the factors influencing the IEAPs number,including slurry-supply speed and slurry dynamic viscosity,are investigated both by means of simulation analysis and experiments.The results indicate that the number of IEAPs decreases with an increase in the slurry supply speed.The cutting efficiency increases gradually with the increase of slurry supply speed,but the growth rate of wire saw cutting efficiency slows down when the slurry supply speed exceeds a given threshold.The number of IEAPs adsorbed by saw wire increases with a decrease in the dynamic viscosity of the cutting fluid,while the cutting efficiency first increases and then decreases.The cutting efficiency attains its highest value when the dynamic viscosity of the cutting fluid is 0.0047 Pa⋅s.The experimental results agree with the simulation results,and provide some guidance for the practical application of the MIFAWS process.

Effects of discontinuities on penetration of TBM cutters

Based on the triaxial testing machine and discrete element method, the effects of embedded crack on rock fragmentation are investigated in laboratory tests and a series of numerical investigations are conducted on the effects of discontinuities on cutting characteristics and cutting efficiency. In laboratory tests, five propagation patterns of radial cracks are observed. And in the numerical tests, firstly, it is similar to laboratory tests that cracks ahead of cutters mainly initiate from the crushed zone, and some minor cracks will initiate from joints. The cracks initiating from crushed zones will run through the thinner joints while they will be held back by thick joints. Cracks tend to propagate towards the tips of embedded cracks, and minor cracks will initiate from the tips of embedded cracks, which may result in the decrease of specific area, and disturbing layers play as ‘screens', which will prevent cracks from developing greatly. The peak penetration forces, the consumed energy in the penetration process and the uniaxial compression strength will decrease with the increase of discontinuities. The existence of discontinuities will result in the decrease of the cutting efficiency when the spacing between cutters is 70 mm. Some modifications should be made to improve the efficiency when the rocks containing groups of discontinuities are encountered.

Disc-cutter induced rock breakage mechanism for TBM excavation in rock masses with different joint shear strengths

When tunnel boring machines(TBMs)excavate through jointed rock masses,the cutting efficiency is strongly affected by the shear strength of joints,the mechanism of which,however,remains poorly understood.In this study,a series of disc-cutter indentation tests were conducted on granite rock mass specimens with different joint shear strengths.During the indentation,the cracking process was recorded by a digital image correlation(DIC)system.The deformation and strength of specimens,cracking behavior,rock breakage mode and cutting efficiency were quantitatively investigated.In addition,to investigate the combined effects of joint shear strength,orientation and spacing on the rock breakage mechanism,numerical rock mass models were established based on a particle flow code PFC2D.Experimental results reveal that the cracking of primary and secondary cracks changes from the mixed shear-tensile to tensile mode in the initial stage,while the joint shear strength does not affect the cracking mode in the subsequent propagation process.The rock breakage mode is classified to an internal block breakage mode,a cross-joint breakage mode and a cutters-dependent breakage mode.The cross-joint breakage mode is optimal for improving the cutting efficiency.Numerical simulation results reveal that the increase in the joint shear strength changes the internal block breakage mode to cross-joint breakage mode for rock masses of particular ranges of joint orientation and spacing.These findings provide basis for improving the TBM cutting efficiency through jointed rock masses.

Analysis and comparison of laser cutting performance of solar float glass with different scanning modes

Cutting quality and efficiency have always been important indicators of glass laser cutting.Laser scanning modes have two kinds,namely,the spiral and concentric circle scanning modes.These modes can achieve high-performance hole cutting of thick solar float glass using a 532-nm nanosecond laser.The mechanism of the glass laser cutting under these two different scanning modes has been described.Several experiments are conducted to explore the effect of machining parameters on cutting efficiency and quality under these two scanning modes.Results indicate that compared with the spiral scanning mode,the minimum area of edge chipping(218340µm)and the minimum Ra(3.01µm)in the concentric circle scanning mode are reduced by 9.4%and 16.4%respectively.Moreover,the best cutting efficiency scanning mode is 14.2%faster than that in the spiral scanning mode.The best parameter combination for the concentric circle scanning mode is as follows:Scanning speed:2200 mm/s,number of inner circles:6,and circle spacing:0.05 mm.This parameter combination reduces the chipping area and sidewall surface roughness by 8.8%and 9.6%respectively at the same cutting efficiency compared with the best spiral processing parameters.The range of glass processing that can be achieved in the concentric circle scanning mode is wider than that in the spiral counterpart.The analyses of surface topography,white spots,microstructures,and sidewall surface element composition are also performed.The study concluded that the concentric circle scanning mode shows evident advantages in the performance of solar float glass hole cutting.

Coupling evaluation for material removal and thermal control on precision milling machine tools

Machine tools are one of the most representative machining systems in manufacturing.The energy consumption of machine tools has been a research hotspot and frontier for green low-carbon manufacturing.However,previous research merely regarded the material removal(MR)energy as useful energy consumption and ignored the useful energy consumed by thermal control(TC)for maintaining internal thermal stability and machining accuracy.In pursuit of energy-efficient,high-precision machining,more attention should be paid to the energy consumption of TC and the coupling relationship between MR and TC.Hence,the cutting energy efficiency model considering the coupling relationship is established based on the law of conservation of energy.An index of energy consumption ratio of TC is proposed to characterize its effect on total energy usage.Furthermore,the heat characteristics are analyzed,which can be adopted to represent machining accuracy.Experimental study indicates that TC is the main energy-consuming process of the precision milling machine tool,which overwhelms the energy consumption of MR.The forced cooling mode of TC results in a 7%reduction in cutting energy efficiency.Regression analysis shows that heat dissipation positively contributes 54.1%to machining accuracy,whereas heat generation negatively contributes 45.9%.This paper reveals the coupling effect of MR and TC on energy efficiency and machining accuracy.It can provide a foundation for energyefficient,high-precision machining of machine tools.