Characterization of grinding-induced cracks in ceramics

A method of determining the size distribution of grinding-induced cracks using a simulation of the grinding process is presented. The simulation uses the grinding wheel profile, grinding conditions and the normal grinding force as input. The grinding operation is considered to be a continuous indentation process arising from a series of abrasive edges. The grinding-induced cracks are estimated based on the indentation fracture mechanics model. Applying the simulation procedure to creep feed grinding of alumina with resinoid diamond wheels resulted in estimates of crack sizes ranging from 3 to 37 μm for the four different grinding conditions that were used in the study.     

Abrasive wear of a single CBN grain in ultrasonic-assisted high-speed grinding

Research into the single grain cutting mechanism is important for understanding complex grinding mechanisms. Based on the characteristics of ultrasonic vibration, the motion equation of the grain is established, and the generated trajectory is theoretically analyzed. By adopting the method of combining high-speed grinding technology with ultrasonic vibration, abrasive wear forms of single cubic boron nitride (CBN) grains under common and ultrasonic conditions are studied. Further studies are conducted on the influence of the grain itself and the main grinding parameters on abrasive wear. Research shows that the main forms of abrasive wear during ultrasonic-assisted grinding are shearing wear and removing wear. However, common grinding leads to micro-crushing wear and a small amount of abrasion wear; the different forms of wear correspond to different grinding force signals. The greater the initial grain protrusion height, the greater is the abrasion of the protrusion; for the same grain protrusion height, the abrasive wear due to ultrasonic-assisted grinding is larger than that due to common grinding. As the grinding depth increases, the abrasive wear of both processing modes increases; however, in the case of ultrasonic machining, the abrasive wear increases slowly and is larger than that under common grinding. This study provides a certain decision basis for real-time monitoring of the ultrasonic-assisted high-speed grinding process. Additionally, it provides guidance and reference for the manufacture and selection of the grinding wheel and for the selection of reasonable processing parameters.     

Feature extraction method of abrasive belt wear state for rail grinding

Abstract

Abrasive belt has the advantages of high removal efficiency and high waste collection; furthermore, it does not easily damage the rail when applied in rail grinding. However, the poor wear consistency and short service life restrict its development in this field. It is important to explore the theory and technology of monitoring the abrasive belt wear condition to evaluate the means of improving the grinding performance of the belt. This article discusses the contact stress state between the belt and the rail surface. In addition, the study establishes the belt rail grinding force model during operation using the abrasive grain distribution function. The analysis of the force model proposes that the belt abrasion condition be monitored by using the force ratio parameter of the abrasive belt in the grinding process. The extraction method of the force ratio information is proposed in accordance with the grinding environment. The results of experiments successfully verified the validity of the method.     

Edge burr development when using a cemented carbide micro-drill: Formation and control mechanisms

To explore the mechanism of formation and methods of control of edge burr in the grinding and forming of cemented carbide micro-drill, the morphology, material composition, and the edge structure of the generated area of the edge burrs are studied. Through indentation and scratch experiments, the critical grinding depth (h_c) of grinding machining is calculated to be 0.793–1.052 μm. The grinding experiments have verified the effects of the actual grinding depth h₀ of different single abrasive particles on the edge burr and the effectiveness of the method of controlling the burr by increasing the cutting angle of the abrasive particles. The experimental results show that edge burrs are mainly concentrated on the cutting edge close to the outer cylinder of the micro-drill. When the actual grinding depth h₀ of a single abrasive particle is less than the critical grinding depth h_c, workpiece material is mainly subjected to plastic deformation removal, the length L of the edge burr along the edge direction is 10–20 μm, the width W of the burr perpendicular to the edge direction is 1–3 μm. The formation of edge burrs is mainly related to the actual grinding depth and the abrasive grain cutting angle γ of abrasive grains. With the increase of h₀, the length L of the edge burr decreases, and the width W thereto first increases, then decreases. Increasing the cutting angle of abrasive particles can control the edge burr. By changing the grinding direction of the grinding wheel, the cutting angle of the abrasive particles can be changed from an acute angle to an obtuse angle, thereby eliminating the edge burr without affecting the performance of micro-drilling.     

The effect of anisotropicity of Norway spruce (Picea abies) during two-body abrasion

The effect of anisotropicity of Norway spruce (Picea abies) during two-body abrasive wear was investigated by rubbing the wood with five different orientations while using constant surface pressure and a sanding belt with very fine abrasive grits. The anisotropic nature was found to affect the microstructure of the worn surface and the breakage mechanism of the surface. The properties of the particles that were released from the surface during abrasion were dependent on the grinding orientation and if the particle originated from early- or latewood. The wear process was influenced by the anisotropic nature of wood.     

Effect of composition and microstructure on slurry abrasion response of hardfaced martensitic stainless steel

Hardfaced martensitic stainless steel alloy was deposited on mild steel substrate by flux cored arc welding method. The slurry abrasion studies of weld-deposited hardfaced steel were performed using slurry abrasion test rig with 250–300 μm silica sand particles. The effect of weld compositional gradation on the abrasive wear resistance of hardfaced stainless steel at a distance of 0.6, 1.2, 2.4, 3.0 and 3.6 mm from the top surface was studied. The observed abrasion rates were rationalized in terms of mass loss, hardness and distance from the top surface i.e. diluted surfaces beneath the top surface. The abrasive wear mass loss increased with increasing distance beneath the top surface, which was attributed to the coarsening and morphology change in martensite phase. The results of the present work indicated change in morphology of martensite with increase in the distance beneath the top surface. The operating abrasive wear mechanisms involved ploughing, microcutting and indentation.     

Toughness and the transition between cutting and rubbing in abrasive contacts

The transition between rubbing and cutting when a facet-first pyramid is slid along the surface of a ductile material is investigated with new experiments under both constant depth of groove and constant load, and with a new ductile fracture mechanics analysis for the cutting regime. The critical attack angle at which the transition occurs is shown to be controlled by the toughness/strength (R/τ_y) ratio of the material, the depth of groove, friction and the face angle of the tool. The new analysis is contrasted with the Bowden and Tabor approach in Sedriks and Mulhearn [A.J. Sedriks, T.O. Mulhearn, Mechanics of cutting and rubbing in simulated abrasive processes, Wear 6 (1963) 457–466; A.J. Sedriks, T.O. Mulhearn, The effect of work-hardening on the mechanics of cutting in simulated abrasive processes, Wear 7 (1964) 451–459] and with other approaches for the same events, and it explains why certain assumptions in Sedriks and Mulhearn can be justified.The analysis shows that the vertical load during sliding is different from that required for indentation to the same depth below the surface. Consequently the depth of the groove during sliding under deadweight loading is different from the depth resulting from initial indentation. At very small attack angles, the depth can be smaller than the static indentation depth, but as the attack angle increases, so does the depth of groove formed by cutting.The relevance of the model to abrasive wear and polishing is discussed, along with implications for Krushschov–Babichev wear resistance diagrams and the Archard equation. The analysis explains why the specific energy in grinding increases at small depths of cut.Measurement of the critical attack angle in scratching with a facet-first indenter may possibly be a way to estimate the fracture toughness of small samples when other, more conventional, methods are difficult to carry out.     

Relationship between subsurface damage and surface roughness of glass BK7 in rotary ultrasonic machining and conventional grinding processes

Subsurface damage (SSD) induced during the abrasive machining process considerably influences the technological application of the optical components. However, to date, there is no rapid and effective method to inspect the depth of SSD. For the purpose of precise and nondestructive evaluation of the SSD depth generated in rotary ultrasonic machining (RUM) and conventional grinding (CG) processes, a theoretical model, ground on indentation fracture mechanics of brittle material, was proposed by analyzing the correlation between the median and lateral crack systems aroused by a sharp indenter. It was found that the SSD depth was nonlinear monotone increasing with square of surface roughness (SR), namely, SSD?=?χSR²?+?l. Utilizing this model, the SSD depth could be quickly and precisely predicted through the SR (p–v value.) of the machined surface, geometrical features of the abrasive, and the material mechanical properties. To validate the feasibility of this method, both RUM and CG tests were conducted on the BK7 glass specimens with a Sauer Ultrasonic 20. Subsequently, the SSD of these specimens was exposed with the polishing–etching technique. The measurement results of SSD depth were consistent with the prediction values of this model, which reflected the feasibility of using this model to rapidly and accurately predict the SSD depth.     

螺杆转子砂带磨削装置开发及材料去除率预测

为实现螺杆转子表面磨削材料的均匀去除,开发了一种新型砂带磨削装置,该装置由接触轮式及自由式砂带磨削工具组成。针对磨削工具与工件的接触特点分别采用半解析法及几何近似法建立接触模型,获得接触区域内接触应力分布规律。提出了基于ThunderGBM算法的材料去除率预测模型,以接触应力及磨削工艺参数作为输入,对螺杆转子砂带磨削材料去除率进行了预测,然后针对五头螺杆转子设计并实施了磨削实验。实验数据与数值计算结果的对比表明提出的去除率预测模型具有较高的准确性与有效性。     

Multiscale characteristic lengths of abraded surfaces: Three stages of the grit-size effect

The abrasion mechanisms in polishing titanium-alloy samples with different grades of silicon carbide-coated abrasives were characterized using a novel multiscale analysis of the extreme amplitudes of the peaks and valleys (EAPV) of surface roughness. Two stages of roughness were found: a fractal stage (l∈10-160 μm), where EAPV values versus the observation length l were linked to the fractal dimension D (EAPV∝l^2−D), and a stochastic stage (l>160 μm), where EAPV was modeled by the extreme-value theory, allowing the prediction of EAPV values versus observation length. Three regimes of abrasion were found: for grit particles of diameter d>100 μm, EAPV values did not depend on the observation scale and were consistent with Archard’s model. For particle sizes 10 μm<d<100 μm, the EAPV diminished with d regardless of scale, representing the “grit-size effect”. For particles of diameter d<10 μm, the EAPV dramatically decreased at all scales and was independent of d because of adhesive wear. We show that Regimes one and three were dominated by the valleys due to cutting and adhesive wear, respectively, whereas Regime two (the grit-size effect) was dominated by wear peaks due to clogging and deterioration of the abrasive surface, which led to a lower indentation of the abrasive. This contribution proves that the bifractal characteristic previously observed in abrasion can be explained by a single fractal power law and, above a threshold of l>160 μm, by a cumulative-damage model, with a probability proportional to the length of the sample but always uncorrelated with scale.     

Evaluating the role of particle distribution and shape in two-body abrasion by statistical simulation

Previous work suggests that particle shape—specifically, its variation with particle size—is unlikely to be the dominant cause of the particle size effect (PSE) encountered in abrasion. In theory, the statistically similar nature of particle geometry implies that wear rate in two-body abrasion should be independent of particle size when all other conditions are unchanged and boundary effects are negligible. In practice, however, the severity of wear can be influenced substantially by the cumulative effect of relatively small variations of each governing factor. This paper deals with numerous issues related to the shape of abrasive particles and surfaces, with the view of understanding how shape contributes to wear and the manifestation of the PSE. It has been discovered, for example, that among the various solids of revolution used in the past to model the asperities of particles, the most representative possesses a power-law generatrix. Particle shape alone, however, constitutes a modest part in determining the properties of abrasive tools, such as grinding wheels and abrasive coated paper. Consequently, the density and distribution of the particles on the surface must also be given due consideration. To this end a statistical simulation of two-body abrasion has been developed. The effects of varying asperity shape and distribution on wear rate are clearly demonstrated and compared to results obtained from two-body experiments.     

Crack initiation threshold as a parameter for characterizing fused alumina abrasive grains

The crack initiation threshold F_c, which is determined using the Vickers indentation technique, was used to estimate abrasion strengths of different fused alumina abrasive grains. Assuming Weibull distributed strength, the dependence of the crack initiation probability on the indentation load F according to F^−1/4 was verified. The crack initiation thresholds of Al₂O₃ crystals correlate with the strengths of the abrasive grains which were determined by static and dynamic methods.

The contents of chromium, iron, manganese and titanium in the alumina crystals were estimated by electron paramagnetic resonance and energy dispersive X-ray spectrometry measurements; the results indicate that the crack initiation thresholds are not essentially determined by these impurities.     

Modeling surface roughness for polishing process based on abrasive cutting and probability theory

The surface roughness is a variable used to describe the quality of polished surface. This article presents a surface roughness model based on abrasive cutting and probability theory, which considers the effects of abrasive grain shape, grit and distribution feature, pressure on surface roughness. The abrasive grain protrusion heights are thought to close to Gaussian distribution, and then the relationship between the indentation depth and the pressure based on Hertz contact theory is obtained. Surface roughness prediction model is established by calculating indentation depth of the abrasive grains on workpiece surface. The maximum surface profile height (R_y) is approximately equal to the maximum indentation depth of the abrasive grain. The arithmetic average surface roughness (R_a) is equal to the average indentation depth of the abrasive grain. The effects of process parameters such as pressure and grit on R_y and R_a were simulated and analyzed in detail.     

氮化硅陶瓷球研磨去除机制试验与仿真研究

为研究研磨过程中氮化硅陶瓷球的材料去除形式及磨损行为,结合陶瓷材料动态压痕断裂力学理论,进行陶瓷球研磨加工试验,采用超景深三维显微镜和扫描电镜对研磨后陶瓷球表面进行观察,同时建立单颗金刚石磨粒冲击作用有限元模型并进行仿真研究。试验结果表明:氮化硅陶瓷球表面材料去除以脆性断裂去除和粉末化去除为主,陶瓷球表面残留有大量贝壳状缺陷和呈簇状随机分布的粉末化材料区域;研磨过程中,陶瓷球表面存在擦伤、划伤和凹坑等缺陷;磨粒冲击作用时,表面材料会受微切削作用产生破碎去除,同时也会受挤压作用产生脆性断裂去除,当磨粒以滚动方式作用在陶瓷球表面时,陶瓷球表面更容易形成粉末化去除,且材料去除率更高。仿真结果表明:各磨粒冲击作用方式产生的最大等效应力由大到小的顺序为滚动磨粒变切深、滚动磨粒定切深、磨粒挤压、滑动磨粒定切深,其中,滚动磨粒变切深产生的亚表面裂纹最深。     

Influence of abrasive hardness on the wear resistance of high chromium irons

The resistance to abrasive wear was determined for a series of alloyed white cast irons in a high stress abrasion test which utilizes a specimen in sliding contact with bonded abrasives. These were conducted on silicon carbide, alumina and two sizes of garnet abrasive.The results indicate that the hardness, or type, of abrasive used in the test significantly influenced the wear rate of white irons, i.e. the rate of wear increased with increasing hardness of the abrasive. Also, the results indicate that the type of abrasive used in the test was a significant factor in ranking white irons for resistance to high stress abrasion. When tested on silicon carbide or alumina abrasive, as-cast austenitic irons exhibited lower rates of wear than heat treated martensitic irons; when tested on garnet, an abrasive of lower hardness, those irons with martensitic matrix microstructures exhibited the same or less wear than irons with austenitic matrix microstructures. It was also evident that heat treated irons with martensitic matrix microstructures exhibited varying degrees of resistance to abrasive wear depending on cooling rates and alloy content.     

Tribological problems of mineral comminution

A laboratory method has been developed which involves the use of three new rigs simulating the grinding action in the majority of existing types of mills. All three rigs combine abrasion and erosion with comminution processes. The object of this investigation was to develop a test procedure that would be suitable for general use in estimating the abrasiveness of particular minerals, and to study abrasive wear in various grinding systems. Three separate industrial problems involving coal grinding have been investigated by means of the novel apparatus. The results from the relatively simple laboratory procedures, designed and operated according to the principles of similarity, may be used to predict the service life of grinding machine elements. The tribo-testing procedure and apparatus can be used to evaluate the abrasiveness of any granular mineral, and for testing the wear resistance of any material in abrasive or erosive action. A second paper (see p. 257) publishes results obtained using the apparatus.     

A study of carbide removal mechanisms during quartz abrasion: II: Effect of abrasive particle shape

The effect of abrasive particle shape on fracture and pit formation in carbides during low stress abrasion of high chromium white cast irons by AFS 50–70 Ottawa quartz test sand is described in this paper. Abrasion tests were performed using a rubber wheel abrasion test apparatus and the abraded surfaces were examined in the scanning electron microscope. The shapes of the fresh and used abrasive particles were analyzed using the scanning electron microscope. Scratch tests were performed in situ in the scanning electron microscope using individual grains of fresh semiangular quartz particles and fractured angular quartz particles as scratch tools. Hertzian elastic theory was applied to gain a semiquantitative understanding of the effect of the radius of curvature of the abrasive particle tip on carbide fracture.The results of this study indicate that fracture of the carbides is initiated by the action of an extremely small portion of the abrasive particles which fracture during abrasion and therefore become very sharp and angular. Further crack extension and subsequent pit formation in carbides can be caused by the action of fresh rounded abrasive particles.     

Characterization of abrasive grain’s behavior and wear mechanisms

Grinding is a finishing process largely used in motor industry, aeronautics, space industry and precision cutting tool manufacturers. The grinding process can be summarized by the action of a grinding wheel on a workpiece. The wheel is constituted by abrasive grains. Thus grinding is in fact the action of grains on the workpiece. The grain behavior changes according to numerous parameters (geometry, mechanical characteristics, wear mechanisms). In some cases abrasive wear is observed while micro-cutting is obtained in some other cases.In this paper two useful and complementary experimental approaches for the interface physics understanding is presented. The study of the cutting power is carried out using a high-speed scratch test device in order to understand the grain behavior and the wear mechanisms for several wheel surface speeds. In this paper an approach for the specific abrasion energy computation is also presented.     

Abrasive wear behaviour of bamboo

Abrasive wear of bamboo (Phyllostachys pubescens) against a free abrasive consisting of quartz sand (96.5%-wt) and bentonite (3.5%-wt) was studied on a rotary-disk type abrasive wear tester (a free abrasive wear tester) and the micromorphology of the abraded surfaces were examined by scanning electron microscopy. It was observed that the bamboo fibre (vascular bundle) orientation with respect to the abrading surface had an important influence on the abrasive wear performance. The normally oriented specimens gave much higher abrasion resistance than the parallel-oriented ones, the surface layer than the inner layer, and the vascular bundle than the matrix tissue.     

Probabilities of abrasive tool grain wearing during grinding

Functional dependencies are proposed for calculating the probabilities of abrasive tool grain wear as a result of abrasion, chipping, and tearing form a complete group. Objective laws of the changing probabilities of wear due to a load acting on the grain tip are considered. The effect of the grinding wheel hardness on the probability of wear is investigated.