Abrasive machining technology as a non-traditional machining technology has been extensively employed to polish a variety of materials, especially the hard-to-machine materials. Chen et al. [6] proposed a novel shear dilatancy polishing method which was employed to precision-polish tungsten surface, and it was found that not only had the material removal rate been significantly improved by about 33.4%, but a better surface finish could be obtained. By using this method, Chen et al. [7] also investigated the effect of the surface quality on hydrogen/helium irradiation behavior in tungsten, which indicated that improvement of the surface quality could essentially enhance the ability of the resistance to hydrogen/helium irradiation behavior in tungsten. Since abrasive machining technology is very useful in the precision machining area, the fundamentals of this technology should be investigated as well. Scratching technology with a cutter or single abrasive is widely used in exploring the fundamentals of material removal mechanisms in abrasive machining of hard brittle materials [8]. Shi et al. [9] investigated edge chipping in the scratching of soda-lime glass, and found that considering the high strain-rate hardening effect led to a more accurate prediction of edge chipping and subsurface damage, which could provide suggestions in the fabrication of high-value optical devices. Moreover, Huang et al. [10] explored the plastic deformation and crack propagation caused by the scratching of single crystal alumina, where the plastic deformation could be attributed to the coupling effects of lattice disorder, dislocation loops, stacking faults, dislocation glide and basal twin formation. For ceramics which are made of bonded crystal grains with some enhanced materials at given high temperatures, it was found that both intergranular and transgranular fractures occurred during the machining of aluminium nitride ceramics [11]. Liu et al. [12] explored the material removal mechanisms in the processing of alumina ceramics based on scratching tests, and found that transgranular fracture, consisting of ploughing and ductile cutting, dominated the material removal process. Although some fundamental material removal mechanisms could be concluded by analysing the morphology of eroded surfaces, the dynamic scratching process could not be observed experimentally due to the very short scratching period between the tool and target surface and very small cracks formation inside the target as illustrated in Figure 1.
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Comparison of the formation of cracks between a real target and the solid specimen constructed with the Bonded Particle Model: (a) physical crack between grains, (b) physical crack inside grains, and (c) breakage (crack) among spherical elements in a solid specimen [13].
Numerical simulation is alternatively used to investigate some dynamic and complicated problems that cannot be addressed experimentally [14,15,16,17]. DEM, treating the brittle materials as a solid specimen with arbitrarily sized spherical elements bonded together with the assistance of the BPM (Bonded Particle Model), can model mechanical behaviour subjected to external force or moment, which are mainly governed by the formation, propagation and eventual intersection of cracks [13]. Figure 1 shows a comparison of the formation of cracks between a solid specimen constructed with BPM and a real target material. Physical cracks between and inside grains in a real target material occur once a large enough force or moment is applied to the target material as shown in Figure 1a,b, respectively, while similar breakages (cracks) also occur among the spherical elements in a solid specimen, as shown in Figure 1c. Many such cracks would occur inside a solid specimen that contains thousands of bonded spherical elements to represent the cracking process in brittle materials, especially for RB-SiC ceramic which is made of bonded grains by the addition of enhanced materials at given temperatures. Jiang et al. [18,19,20] employed DEM to model SiC ceramics, and explored the crack propagation and failure mode by simulating uniaxial compression and three-point bending tests. Zhang et al. [21] modelled ultrasonic-assisted scratching of alumina ceramic using DEM, and found that the crack initiation was restricted by the addition of compressive stress on the target during the machining process. However, these studies did not consider the different grains in the ceramics, such as RB-SiC ceramic which is mainly made of bonded SiC and Si grains, and the propagation of cracks inside and along these grains, namely, the intergranular and transgranular fractures, during the machining process. Thus, a more accurate model considering the effect of the different grains inside RB-SiC ceramic on the material removal process has been developed in this paper to represent the scratching process more realistically.
Furthermore, it is noted from Figure 2 that RB-SiC ceramic is mainly made of bonded SiC and Si grains, and also includes cementitious materials, being carbon powders, bonding agents and free SiC and Si grains. In general, the cracks between these bonded SiC and Si grains are considered as intergranular fractures, while the cracks inside these bonded SiC and Si grains are regarded as transgranular fractures, as can be observed from Figure 1a,b, respectively. Due to the different properties of SiC, Si and cementitious materials, both intergranular and transgranular fractures could occur in the scratching of RB-SiC ceramic which, in turn, could affect the surface/subsurface quality; hereby a novel grain-based DEM model is needed to accurately simulate RB-SiC ceramic as well as to evaluate the scratching process, as detailed below.
Surface/subsurface morphology after the scratching process in brittle material removal mode (vs = 20 m/s, hs = 1 μm and R0 = 1 μm). (a) Representation of transgranular and intergranular cracks. (b) Representation of median and lateral cracks (colour spherical elements, except blue, represent material removal in terms of grain fragmentation or grain spalling).
Abstract Shale gas exploitation initiated in North America has rapidly extended worldwide. Hydraulic fracturing is an emerging field technique for stimulating the gas reservoir. The study of cracking processes, particularly crack coalescence, is vital for a successful hydraulic fracturing to enhance the gas exploitation. Experimental studies have observed that the size effects of the constituent particles are significant on the cracking behavior of the rock specimens. To further investigate the size effects, the bonded-particle model (BPM), which is based on the discrete element method (DEM), is adopted in the present research. In flaw-containing specimens, by varying the crack resolution (Ψ= a/2R), which is the ratio of half flaw length (a) to particle size (2R), the size effects on cracking behavior under compressive loading are studied. By keeping the flaw length constant, the particle size is varied independently in the BPM analysis. Decreasing the crack resolution increases the first crack initiation stress, but it has no obvious effects on the uniaxial compressive strength. The trajectories of the first cracks and secondary cracks hence generated have a higher resolution and are well-defined in those specimens possessing a higher crack resolution. On the contrary, in lower crack resolution specimens, the macroscopic first cracks appear to be wider and less continuous. These findings from numerical simulation clearly demonstrate particle size effects on cracking behavior. Special attention should be paid to these effects in future numerical study using the bonded particle model. Introduction Shale gas exploitation initiated in North America has rapidly extended worldwide. Hydraulic fracturing is an emerging field technique for stimulating the gas reservoir. The study of cracking processes, particularly crack coalescence, is vital for a successful hydraulic fracturing to enhance the gas exploitation. Different cracking processes are observed in marble and gypsum, which possess different grain sizes (Wong & Einstein, 2009a, 2009b; Wong, 2008). To further investigate such effect, the bonded-particle model (BPM) is adopted in the present research. The BPM, which is one of the DEM-based particle models, has been widely used for rock simulations (Cho, Martin, & Sego, 2007; Hazzard, Young, & Maxwell, 2000; Potyondy & Cundall, 2004) since the particle assembly approach was initially developed by Cundall (1971) and Cundall & Strack (1979). Recently, a time-dependent bond breakage model (Wu, Zhu, & Zhu, 2011), synthetic rock mass approaches (Bahaaddini, Sharrock, & Hebblewhite, 2011; Mas Ivars, Pierce, DeGagné, & Darcel, 2008; Thompson, Mas Ivars, Alassi, & Pradhan, 2011), as well as flat-jointed BPM (Potyondy, 2012) have been developed and used for engineering applications. However, some basic cracking phenomena are not yet fully understood for BPM, such as the effect of particle size on cracking processes. This paper will analyze and discuss this effect.
ABSTRACT Bonded Particle Method (BPM) has recently been extensively used to simulate brittle materials. In BPM, brittle material gets represented as a dense assemblage of particles (grains) connected together by contacts (cement). Only particle motion law and contact constitutive model are therefore needed to perform the simulation. BPM does not require complicated plasticity constitutive laws, but it seriously depends on the contact micro-properties. Therefore a calibration process is needed to establish a unique set of these micro-parameters. In this research, discrete element code of UDEC is employed to apply BPM. This code can create random-shaped and non-uniform-sized particles interacting in frictional and cohesive contacts. Using this approach, contact micro-parameters are calibrated to fit the conventional test results (uniaxial and triaxial compression, and Brazilian tension). Introduction In recent years, Discrete Element Method (DEM) is extensively used to model brittle materials [1,2]. Jing and Stephansson [3] have recently provided fundamentals of DEM and its application in rock mechanics. One use of DEM is to represent rock-type material as a dense packing of non-uniform-sized particles that are bonded together at their contact points. (Fig.1). This method is already known as Bonded Particle Method (BPM) [4,5]. The main advantage of this method is that a crack can be modelled as a real discontinuity between particles not just as a modification in material properties [3,4]. The major requisite of BPM is to calibrate the micro-contacts properties to fit the material macro-response [4,5]. So far most BPM models have been performed by Particle Flow Code (PFC) [8] in which the grains are simulated as rounded discs. Potyondy and Cundall [4] showed that the tensile strength obtained by PFC is approximately 0.25 of the uniaxial compressive strength. This is unrealistically high, where the ratio of tensile to compressive strength is typically reported around 0.05 to 0.10 [7]. This inconsistency is due to the fact that PFC simulates the particle as a circular disc. Therefore grain interlocking is basically neglected. Without interlocking, other contact parameters must be chosen disproportionately stronger to achieve the right material compressive strength, which itself leads to an exceedingly high tensile strength. Thus, they argued that cluster logic is required to have closer results to real behaviour. In a recent research, Cho et al. [5] have proposed a clumped logic to resolve this shortcoming. However in addition to its difficulty, this method is merely applicable to strong brittle rocks. They did not mention if weak rocks can be simulated in this way. In this paper, we are going to investigate the brittle materials by BPM and using the Universal Distinct Element Code (UDEC) [9]. We study the effects of UDEC micro-properties on the material macro-response. A series of experiments (Brazilian tension, uniaxial and triaxial compression) are used to calibrate UDEC 1078 micro-properties. It will be showed that UDEC does not show the defects and difficulties of the other methods. 2ff7e9595c
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