Single Grit Scratch Research Articles

Fiber orientation effects on grinding characteristics and removal mechanism of 2.5D Cf/SiC composites

Carbon fiber reinforced silicon carbide (Cf/SiC) composites are widely used in aerospace for their excellent mechanical properties. However, the quality of the machined surface is poor and unpredictable due to the material heterogeneity induced by complex removal mechanism. To clarify the effects of fiber orientation on the grinding characteristics and removal mechanism, single grit scratch experiments under different fiber orientations are conducted and a three-phase numerical modelling method for 2.5D Cf/SiC composites is proposed. Three fiber cutting modes i.e., transverse, normal and longitudinal, are defined by fiber orientation and three machining directions i.e., MA (longitudinal and normal), MB (longitudinal and transverse) and MC (normal and transverse), are selected to investigate the effect of fiber orientation on grinding force and micro-morphology. Besides, a three-phase cutting model of 2.5D Cf/SiC composites considering the mechanical properties of the matrix, fiber and interface is developed. Corresponding simulations are performed to reveal the micro-mechanism of crack initiation and extension as well as the material removal mechanism under different fiber orientations. The results indicate that the scratching forces fluctuate periodically, and the order of mean forces is MA > MC > MB. Cracks tend to grow along the fiber axis, which results in the largest damage layer for transverse fibers and the smallest for longitudinal fibers. The removal modes of transverse fibers are worn, fracture and peel-off, in which normal fibers are pullout and outcrop and the longitudinal fibers are worn and push-off. Under the stable cutting condition, the change of contact area between fiber and grit leads to different removal modes of fiber in the same cutting mode, and the increase of contact area results in the aggravation of fiber fracture.

Investigation of the effects of reinforcement particles in polymers on their grindability in single grit scratch test

Recently, polyether ether ketone (PEEK) and its particle-reinforced composites have found a special place in various industries due to their high strength-to-weight ratio, anti-allergic properties, high resistance to buckling, and other superior properties. These materials have widely been used in the aerospace and medical industries and are commonly finished in the final step through the grinding process. In this work, the single grain scratch test was performed to fundamentally study the grinding process and material removal mechanism of PEEK and its particle-reinforced composites. In this method, the material removal is realized by a single grit penetrating the workpiece. It was found that the amount of flowed or piled-up (side flow) materials around the scratch in the pure polymer is higher than the other two composites. Pure polymer with 2.5 J/mm3 and glass fiber-reinforced composite with 1.1 J/mm3 resulted in the highest and lowest specific energies, respectively. The specific energy of all three materials is reduced significantly by increasing the grit penetration depth, showing the significant role of the size effect in grinding these materials. The main reasons for the different results are the high heat transfer coefficient and higher strength of CFRP composite compared to PEEK and PEEK GF 30.

Modeling Grinding Processes—Mesh or Mesh-Free Methods, 2D or 3D Approach?

The objectives of this study are mainly two: (1) to validate whether a single grain scratch process can be modeled in two dimensions under the assumption of plane strain, and (2) to select the best discretization approach to model a single grain scratch process. This paper first focuses on the simulation of the orthogonal cutting process (aluminum alloy A2024 T351) using two mesh-based discretization approaches, the pure Lagrangian method (LAG) and the arbitrary Lagrangian–Eulerian method (ALE), and two particle-based approaches, the particle finite element method (PFEM) and smooth particle hydrodynamics (SPH), for both positive and negative rake angles. Benchmarking of the orthogonal cutting models at a rake angle of γ=20∘ is performed with the results of the process forces (cutting and passive forces) of a turning experiment from the literature. It is shown that all models are able to predict the cutting forces, but not the passive force. The orthogonal cutting model is further extended to simulate the cutting mechanism with negative rake tool geometries typically found in grinding and single grit scratching processes. The effects of the negative rake angles on the discretization approaches are studied. The calculated process forces are also compared to the measurements of the single grit scratch process performed at our laboratory. The 2D orthogonal cutting models significantly overestimate the process forces. One of the reasons why the orthogonal 2D cutting model is inadequate is that it cannot describe the complex mechanisms of material removal such as rubbing, plowing, and cutting. To account for these phenomena in LAG, ALE, and SPH discretization approaches, a 3D scratch model is developed. When comparing the process forces of the 3D model with the experimental measurements, all three discretization approaches show good agreement. However, it can be seen that the ALE model most closely matches the process forces with the experimental results. Finally, the ALE 3D scratch model was subjected to sensitivity analysis by changing the cutting speed, the depth of cut and the tool geometry. The results clearly show that the ALE method not only predicts the process forces and form the trends observed in the scratching experiments, but also predicts the scratch topography satisfactorily. Hence, we conclude that a 3D model is necessary to describe a scratch process and that the ALE method is the best discretization method.

Understanding the surface generation mechanism during micro-scratching of Ti-6Al-4V

A lot of grinding research in the past has been emulated using single grit scratching. This limits studying surface generation mechanism due to overlapping scratches inherent to the grinding process. It not only affects the surface generation mechanism, but also limits studying elastic recovery which has a significant effect on the final form and topography including the roughness generated after grinding. Taking Ti-6Al-4V as a testbed, this investigation was directed on comparing single scratches with adjacent scratches, which are often referred to as double scratches in literature. During the experiments, the effect of scratch speed and the scratch separation distance on the cutting forces, material pile-up area ratio (PA), groove depth, material removal area (MRA), and sub-surface deformation were investigated. It was found that the scratch speed significantly affects the pile-up and groove depth because of phenomena like stress build-up and elastic recovery. A complementary analytical model of ploughing and shearing forces developed here led to the observation that the minimum chip thickness can only be around 0.1 to 0.2 times the radius of the indenter. Elastic recovery was found to be larger at the middle of the scratch, which was confirmed by the stress-strain relations derived from the FEM simulations. It was also observed that the spacing between two subsequent scratches can significantly affect the scratched surface quality. These experiments show that the severe ploughing and mid-region ridge features can be avoided by having the scratch pitch to be at least one third of the grit tip radius.

Effect of wood porosity on surface creation process by single-grit scratching method

Effect of wood porosity on surface creation process by single-grit scratching method

Investigation of conditions leading to critical transitions between abrasive belt wear modes for rail grinding

In abrasive belt rail grinding, understanding the abrasive belt wear mechanisms is important for improving grinding accuracy and abrasive belt service life. Taking a single ceramic grit in a stable grinding state as an example, this study investigated the critical transition conditions and recognition methods between the different wear modes. Based on the crack growth equation, the critical transition condition from abrasion to fracture was modeled by calculating the stress around the cracks hiding in the grit tip. The transition condition between fracture and pull-off was determined by comparing the external stress around the grit root and the bonding strength of the resin binder. These critical conditions were verified by single grit scratching experiments and the improved Sc wear map method.

Experimental study of single grit scratch test on carbon fiber-reinforced polyether ether ketone

Carbon fiber-reinforced Polyether Ether Ketone is widely used in aerospace and medical industries due to its unique mechanical and physical properties. The use of CFR-PEEK in the industry is growing by replacing materials such as aluminum (in aerospace and mechanical industry), titanium, and stainless steel (in the medical industry). In this research, the grinding process of this material is studied through single grit scratch test, which is a common method to study the fundamentals of chip formation in the grinding process. In this method, one single grit as a representative of whole grinding wheel grits engages with the workpiece under the conditions identical to grinding. Investigating the corresponding phenomena during the abrasive-material engagement gives a better understanding of the grinding process. The experimental results showed that by increasing the cutting speed, vc, the specific energy, ec, increases. It is worth to mention that the minimum specific energy in maximum depth is converged to ec = 1.2 J/mm3. The portion of plowing and rubbing are reduced by a rise in the depth of cut. In low cutting depths, tangential force to normal force ratio is low and by an increment in depth, this ratio increases as well.

A comparative study of an isotropic and anistropic model to describe the micro‐indentation of TWIP steel

AbstractIn grinding, the crystal grain size of the workpiece material is relatively same range compared to the removal depth. This raises a question if an anisotropic material model, which considers the effect of the crystal grain size and orientations, would better predict the process forces when compared to an isotropic material model. Initially, a simple micro‐indentation process is chosen to compare the two models. In this work, a crystal plasticity model and an isotropic Johnson‐Cooke plasticity model are employed to simulate micro‐identation of a twinning induced plasticity (TWIP) steel. The results of the two models are compared using the force‐displacement curves from the micro‐indentation experiments. In the future, the study will be extended to describe the material removal process during a single grit scratch test.

Evaluation of crack growth stage of sapphire under scratching based on AE signals

For hard-brittle materials, crack initiation and growth is an important factor to determine the surface quality of components during machining process. In this paper, the crack growth stage of sapphire was investigated based on acoustic emission (AE) signals through single-grit scratch experiments. The AE waveform indexes, i.e. fractal and frequency characteristics, were fully analyzed and correlated with groove microstructure of specimen after tests. The research shows that the specimen undergo four stages of crack growth successively as scratch depth increasing. It is found that the fractal dimension (FD) and frequency characteristics of AE signals are distinctive to various damage behaviors and be used to evaluate crack growth stage induced in machining process effectively. Especially, the FD distribution is sensitive to the occurrences of the critical damages and removal modes resulting from crack growth in scratch procedure. This paper can shed light on the control of surface integrity and subsurface damage of hard-brittle materials in ultra-precision manufacturing.

Numerical investigation of crack initiation, propagation and suppression in robot-assisted abrasive belt grinding of zirconia ceramics via an improved chip-thickness model

Motivated by the prevailing assisted techniques in wheel grinding of brittle and hard materials, an attempt is made in this paper to identify the feasibility of robot-assisted abrasive belt grinding of zirconia ceramics. Owing to the flexible machining system, the challenge of this attempt resides in achieving the required profile accuracy and surface quality, in which the evolution of grinding-induced micro-cracks is prioritized. The single-grit scratching simulation based on an improved chip-thickness model that incorporates elastic modules of tool-workpiece engagement is employed to explore the damage mechanism in terms of the initiation, propagation and suppression of micro-cracks. The simulation results demonstrate that the critical depth of cut for brittle-to-ductile transition of zirconia ceramics is determined as 0.42 μm according to the tentative maximum undeformed chip thickness (UCT) values. In ductile-regime grinding, the zirconia surface morphologies are independent of the abrasive particle velocity. Lateral cracks begin to initiate especially when the maximum UCT exceeds 0.42 μm, and the brittle removal becomes dominant. In brittle-regime grinding, high abrasive particle velocity could help substantially enhance the workpiece surface integrity by suppressing the median/radial cracks that initiate once the maximum UCT approaches 0.8 μm. Experiment concerning the force-controlled robotic belt grinding of zirconia ceramics is conducted to verify the simulation results via the microscope observation of ground surface morphologies. The findings are likely to provide experimental evidence on the feasibility of belt grinding of brittle and hard materials with a flexible industrial robot.

Elastic recovery of monocrystalline silicon during ultra-fine rotational grinding

Micromachining of brittle materials like monocrystalline silicon to obtain deterministic surface topography is a 21st Century challenge. As the scale of machining has shrunk down to sub-micrometre dimensions, the undulations in the machined topography start to overlap with the extent of elastic recovery (spring back) of the workpiece, posing challenges in the accurate estimation of the material's elastic recovery effect. The quantification of elastic recovery is rather complex in the grinding operation due to (i) randomness in the engagement of various grit sizes with the workpiece as well as (ii) the high strain rate employed during grinding as opposed to single grit scratch tests employed in the past at low strain rates. Here in this work, a method employing inclination of workpiece surface was proposed to quantify elastic recovery of silicon in ultra-fine rotational grinding. The method uniquely enables experimental extraction of the elastic recovery and tip radius of the grits actively engaged with the workpiece at the end of the ultra-fine grinding operation. The proposed experimental method paves the way to enable a number of experimental and simulation endeavours to develop more accurate material constitutive models and grinding models targeted towards precision processing of materials. It can also be shown that using this method if the tip radius distribution of active grits is measured at different time instances, then this data can be used to assess the state of the grinding wheel to monitor its wear rate which will be a useful testbed to create a digital twin in the general framework of digital manufacturing processes.

Evaluating Onset Times of Acoustic Emission Signals Using Histogram Distances

Determining the onset of a transient signal, for example in seismograms, acoustic emission (AE) signals, or ultrasonic signals is very important in non-destructive process monitoring and geophysics. In some cases, the AE from a malfunction is relatively weak, with a low signal-to-noise ratio. Thus, the signals are often hidden, making it rather difficult to separate them from the background noise. The present work proposes a new method of evaluating onset times based on bin-to-bin histogram distances measured with the Bhattacharyya coefficient. A criterion and a standard procedure for determining an onset are formulated. A key parameter, window length, was discussed in detail. Tests on AE signals from a pencil-lead break, single-grit scratching, and filament breakage reveal the feasibility and effectiveness of the proposed method. This method is believed to be especially appropriate when the emission signal strength of the target malfunction is lower than environmental AE signals generated by other stationary sources. It can also be used as an alternative method for identifying onsets in AE signals in process monitoring and other fields.

Numerical and Experimental Analysis of Material Removal and Surface Defect Mechanism in Scratch Tests of High Volume Fraction SiCp/Al Composites.

This paper addresses a comprehensive and further insight into the sensitivity of material removal and the surface defect formation mechanism to scratch depth during single-grit scratch tests of 50 vol% SiCp/Al composites. The three-dimensional (3D) finite element model with more realistic 3D micro-structure, particle-matrix interfacial behaviors, particle-particle contact behaviors, particle-matrix contact behaviors and a Johnson-Holmquist-Beissel (JHB) model of SiC was developed. The scratch simulation conducted at scratch velocity 10 mm/min and loading rate 40 N/min revealed that the scratch depth plays a crucial role in material removal and the surface forming process. Brittle fracturing of SiC particles and surface defects become more deteriorative under a large scratch depth ranging from 0.0385 to 0.0764 μm. The above phenomenon can be attributed to the influence of scratch depth on SiC particles’ transport; the increase in the amount of SiC particle transport resulting from an increase of scratch depth raises the occurrence of particle-particle collision which provides hard support and shock for the scratched particles; therefore, brittle fracturing gradually becomes the major removal mode of SiC particles as the scratch depth increases. On the deteriorative surface, various defects are observed; i.e., lateral cracks, interfacial debonding, cavies filled with residually broken particles, etc. The von Mises stress distribution shows that SiC particles bear vast majority of load, and thus present greater stress than the surrounding Al matrix. For example: their ratio of 3 to 30 under the scratch depth of 0.011 mm. Namely, SiC particles impede stress diffusion within the Al matrix. Finally, the SEM images of the scratched surface obtained from the single-grit scratch experiments verify the numerical analysis’s results.

Cutting Characteristics for Sugar Maple Using Single Grit with Spherical Cone and Triangular Pyramid Geometries.

Abrasive belt sanding plays an important role in wood processing. The abrasive grits on the belt perform similar to small cutting tools with negative rake angles. In this study, a series of single-grit scratching tests were carried out on Sugar maple workpieces to investigate the cutting characteristics of two different abrasive-grit shapes. The spherical cone grits had two kinds of included angles, and the triangular pyramid grits provided two cutting forms: one main cutting edge and two side cutting edges as well as two main cutting edges. Both scratching along and across the wood grain direction were conducted. In all cases, the material deformation and surface creation were analyzed, as well as cutting force. Several physical cutting models were established to help further understand the cutting mechanism. A new method was proposed to evaluate the energy consumption of single-grit scratching. The results showed that triangular pyramid grits with sharp cutting edges could sever wood fibers more efficiently, while spherical cone grits are prone to make material plastic deformation mainly manifested as superficial densification and pile-up. When scratching along the wood grain, the triangular pyramid grit with two main cutting edges showed the best cutting performance with better surface quality. It was also shown that the cutting force ratio of spherical cone grits was apparently less than that of triangular pyramid grits. The overall cutting power for spherical cone grits was remarkably higher than that for triangular pyramid grits for both scratching along and across the wood grain, which indicates that triangular pyramid grits have higher cutting efficiency and power utilization.

Fundamental study on damage-free machining of sapphire: Revealing damage mechanisms via combining elastic stress fields and crystallographic structure

This paper aimed to reveal the damage mechanisms of sapphire by single grit scratch tests on C-plane, which would establish the basis of damage-free machining. The deformation and damage evolution experienced five stages with increasing cutting depth resulting from the combined effect of principle stresses and activated slip/twining systems. It was proved that subsurface and surface damages occur in ductile removal regime of sapphire, which were dominantly induced by high residual stress and the rhombohedral twinning on (101‾2) and (1‾102), respectively. Three critical cutting depths for suppressing subsurface damage, surface damage and surface roughness were determined, respectively. These conclusions were also supported by the acoustic emission (AE) features both in time and frequency domains during scratching. This paper also undertook the theoretical study. An improved elastic stress field model has been established to understand the deformation and damage evolution in relation to cutting depth.

Numerical and experimental investigation on grinding-induced exit burr formation

Exit burrs are easily generated on workpiece edges. In grinding process, particularly, exit burrs could deteriorate the highly required precision and even affect the functionality of components. Therefore, to understand the burr formation mechanism and control it in grinding process is of great significance. To this end, this paper investigates the features and mechanism of exit burr formation, as well as the influence of grinding conditions including grinding speed, grinding depth, wheel granularity, and grinding pass sequences on the exit burr formation of 12CrMoV stainless steel using the Al2O3 grinding wheels. A 3D finite element (FE) model of single-grit scratching process was developed for explaining the exit burr formation procedures. The effects of different grinding conditions on the exit burr sizes were analyzed, and the characteristics of the exit burr sizes and hardness were observed via experiments. It was found that the whole procedure of exit burr formation during grinding could be divided into six stages: steady-state grinding, pre-initiation, appearance of the fourth deformation zone, formation of plastic deformation band, burr development, and finish of burr formation. The burr sizes were strongly determined by the wheel granularity and grinding depth, rather than the grinding speed. For the same required grinding depths, using whether single pass or multiple passes of the grit has a very small impact on the exit burr sizes. The hardness of the burr root part is relatively stronger than that of the burr rollover part, while the burr hardness would increase when decreasing the grinding depth.

Evaluation of grinding strategy for bioceramic material through a single grit scratch test using force and acoustic emission signals

A specified geometry of diamond indenter with different rake angles such as 0°, −2° and −4° were used in single scratch experiments to study the mode of material removal on both the sintered and pre-sintered yttria-stabilized tetragonal zirconia (Y-TZP). To evaluate these features during the scratch experiment, the generated forces and the acoustic emission signals were correlated with the surface morphology of scratches. The results indicated that the forces generated by the higher negative rake angle (−4°) increase with reduced chipping in comparison with other rake angles.The material removal above 5 μm scratch depth for sintered zirconia resulted in a brittle fracture, whereas a quasi-plastic behavior was observed with microcracks leading to a clumsy chip in pre-sintered zirconia. This was analyzed from the morphology of the chips and the scratches produced. The formation of chipping was reduced with the increase in negative rake angle, indicated by the changes in the features of the acoustic emission signal. The major frequency peaks were formed at a lower level below 250 kHz denoted the predominant brittle fracture. The acoustic emission burst occurred in the sintered component, which indicated the cracking and chipping formation was more compared to the pre-sintered block.

Effect of cutting tool geometries on the ductile-brittle transition of monocrystalline sapphire

This paper investigates the effect of cutting tool geometries on the initiation of ductile-brittle transition (DBT) of monocrystalline sapphire with a series of single-grit scratching tests. According to the method of single factor experiment, four typical geometrical parameters, namely rake angle, apex angle, opening angle, and truncated section, were evaluated in 7 scratching tests. The scratching force, groove topography and critical cutting depth for DBT (DBdc) have been measured by a dynamometer, SEM, and AFM, respectively. Furthermore, a new theoretical model of DBdc with respect to the rake and apex angle is established. The predicted DBdc agrees well with experimental results. The mechanism of the initiation of DBT is further investigated by the coupled SPH-FEM simulation. The research results reveal that all of the four geometrical parameters are the dominant factors with significant impact on the initiation of DBT of sapphire. Without exceeding the critical value, an increase in the negative rake angle contributes to an increased DBdc. The apex angle and opening angle are proved to adversely affect the DBdc and give rise to DBT at a smaller cutting depth, though the mechanisms associated with these two parameters are different. The parameter of the truncated section also has a significant effect where the DBdc decreases abruptly and severe brittle fracture is present with an increasing dimension of the truncated section.

Research on the Size Effect of Specific Cutting Energy Based on Numerical Simulation of Single Grit Scratching

A method of research on the size effect of the specific cutting energy based on the numerical simulation has been proposed. The theoretical model of the research on size effect of specific cutting energy using single grit scratching simulation has been presented. A series of single grit scratch simulations with different scratching depths have been carried out to acquire different material removal rates. Then, the specific cutting energy has been calculated based on the power consumed and the material removal rate. The relationship between the specific cutting energy and the material removal rate has been given which agrees well with that presented by Malkin. The simulation results have been analyzed further to explain the size effect of specific cutting energy.

Research on the Single Grit Scratching Process of Oxygen-Free Copper (OFC).

Single grit scratching is a basic form of material removal for many processes, such as grinding single point diamond turning and coating bonding performance tests. It has been widely used in the study of micro-scale and nano-scale material removal mechanisms. In this study, single grit linearly loading scratching tests were carried out on a scratching tester. A Rockwell indenter made of natural diamond was selected as the tool used, and the material of the workpiece was oxygen-free copper. Scratch topography was measured using a super-depth microscope to analyze the material deformation of the scratching process. A single grit scratching simulation has been developed by AdvantEdge™ to comprehensively study the material deformation of scratching processes. A material constitutive model and friction model were acquired using a quasi-static uniaxial compression experiment and a reciprocating friction test, respectively. These two models were used as the input models in the finite simulations. The simulated scratching forces aligned well with the experimental scratching forces, which verified the precision of the simulation model. Since only the scratching force could be obtained in the scratching experiment, the plastic strain, material flow, and residual stress of the scratching were further analyzed using simulations. The results showed that the plastic strain of the workpiece increased with the increase in scratching depth, and further analysis showed that the workpiece surface was distributed with residual compressive stress and the sub-surface was distributed with residual tensile stress in single grit scratching.