Constant Normal Force Research Articles

Influence of the thermal shrinkage-induced volume contraction on the yielding behavior of waxy oil

The present article investigates the effect of thermal shrinkage on the yielding behavior of waxy oil gel. The paper compares constant gap and constant normal force measurement protocols to examine the effect of measurement protocol on the rheological behavior of thermoreversible gel. The findings of this study reveal that the extent of stress overshoot and yield stress in a constant shear-rate start-up flow shows a lower magnitude in a constant gap protocol compared to a constant normal force protocol. The decrease in gel strength in the former protocol is mainly attributed to the formation of voids. These voids cause localized fractures within the crystal network. In contrast, in the constant normal force measurement protocol, gel contraction is compensated by utilizing a variable gap setting. Variable gaps compensate for the lower specific volume of the gel after crystallization, minimizing void formation and subsequent rupture of the crystal network. Hence, the gel network formed using the constant normal force protocol is more homogeneous, eliminating the uncertainties in yield stress measurement. Finally, the effect of thermal history, wax content, and aging period on the yield stress values highlights notable findings. Contrary to the conventionally accepted results, the aging period is found to impact the yield stress negatively, and a nonmonotonic relationship between the cooling rate and yield stress is noticed under the constant gap protocol. Thus, the results obtained under the constant normal force protocol are more reliable and can help in developing a fundamental understanding of the yielding behavior.

An MFC-based friction damper with adjustable normal force: conception, modelling, and experiment

For classic friction dampers, the normal force on the contact interface is designed to be constant and cannot be easily adjusted post-manufacturing. The damping performance will attenuate when the working condition deviates from the target one. To overcome this issue, semi-active friction dampers are developed, where the normal force can be regulated by actuators to match the current working condition. However, the additional mass brought by the actuators limits their application prospects in the aerospace field. In this work, we propose a macro-fiber-composite-based adjustable friction damper (MFC-AFD) for typical thin-walled structures in aircrafts, which has the advantages of being lightweight, easy to install, and inherently fail-safe. The idea is to use the embedded MFC to drive the flexible structure of the damper to deform, thereby regulating the normal force. By adjusting the constant component of the normal force, the effective working range is expected to be enlarged, especially under non-target working condition. By controlling the second harmonic component, the optimal damping performance is expected to be further improved. We conduct the research using a combination of numerical simulations and experiments. To accurately and efficiently predict the damping performance of the MFC-AFD, a homogenization modeling method of MFC is proposed, and the corresponding experimental calibration technique is given. The harmonic balance-based nonlinear solver specifically for calculating the steady-response of electromechanically coupled structures is developed. Concerning the constant normal force control, a step-back to zero control law is established to eliminate the hysteresis behavior of MFC. In terms of the time-varying normal force control, a closed-loop control strategy in the frequency domain for the second harmonic component is proposed and realized. The vibration reduction effects of the MFC-AFD adopting the constant and time-varying normal force control laws are validated experimentally through a cantilever beam. Experiment results are in line with our expectations. When the vibration level is nearly 10 times of the target one, the reduction ratio recovers to 75% (i.e. close to the target working condition) by using the constant normal force control. There is an additional 32% vibration reduction compared to the optimal constant normal force when the time-varying normal force is properly controlled.

An Angle‐Sensitive Microcolumn‐Based Capacitive Shear Force Sensor for Robot Grasping

AbstractShear force sensors play an indispensable role in tactile perception for robot manipulation tasks. However, recent advancements in shear force sensors have been hindered by issues such as direction sensitivity and integration limitations. This paper proposes a microcolumn array dielectric layer produced using photolithography technology that enables tunability of sensor sensitivity and detection range by adjusting the aspect ratio and interval of the microstructures. Meanwhile, the impact of five constant normal force couplings on the sensitivity of shear force perception is investigated. The structure array with a 1:2 aspect ratio and 600 µm interval demonstrates an ultrahigh sensitivity of 6.189 N−1 and outstanding linearity (R2 = 0.9873) within the range up to 0.1 N. The sensor exhibits low hysteresis and robust stability over 3000 cycles. Additionally, it exhibits remarkable anisotropic direction sensitivity, enabling accurate positioning within a quarter‐circle angle. An intentionally designed orthogonal array is employed to extend the shear angle range up to 360°. Owing to the high performance of the sensor, it is further integrated onto a gripper to facilitate the grasping operation and effectively capture delicate movements. The experimental outcomes highlight that the designed sensor holds promise for applications in robotic applications and electronic skin domains.

A study of rotary cutting machine (RCM) performance on Korean granite

PurposeDespite the many advantages this type of equipment offers, there are still some major drawbacks. Linear cutting machine (LCM) cannot accurately simulate the true rock-cutting process as 1. it does not account for the circular path along which tunnel boring machine (TBM) disk cutters cut the tunnel face, 2. it does not accurately model the position of a disk cutter on the cutterhead, 3. it cannot perfectly replicate the rotational speed of a TBM. To enhance the knowledge of these issues and in order to mimic the real rock-cutting process, a new lab testing equipment was developed by Hyundai Engineering and Construction.Design/methodology/approachA new testing machine called rotary cutting machine (RCM) is designed to simulate the excavation process of hard-rock TBMs and includes features such as TBM cutterhead, RPM simulation, constant normal force mode and constant penetration rate mode. Two sets of tests were conducted on Hwandeung granite using different disk cutter sizes to analyze the cutting forces in various excavation modes. The results are analyzed using statistical analysis and dimensional analysis. A new model is generated using dimensional analysis, and its results are compared against the results of actual cases.FindingsThe effectiveness of the new RCM test was demonstrated in its ability to apply various modes of excavation. Initial analysis of chip size revealed that the thickness of the chips is largely dependent on the cutter spacing. Tests with varying RPM showed that an increase in RPM results in an increase in the normal force and rolling force. The cutting coefficient (CC) demonstrated a linear correlation with penetration. The optimal specific energy is achieved at an S/p ratio of around 15. However, a slightly lower S/p ratio can also be used in the design if the cutter specifications permit. A dimensional analysis was utilized to develop a new RCM model based on the results from approximately 1200 tests. The model's applicability was demonstrated through a comparison of TBM penetration data from 26 tunnel projects globally. Results indicated that the predicted penetration rates by the RCM test model were in good agreement with actual rates for the majority of cases. However, further investigation is necessary for softer rock types, which will be conducted in the future using concrete blocks.Originality/valueThe originality of the research lies in the development of Hyundai Engineering and Construction’s advanced full-scale laboratory rotary cutting machine (RCM), which accurately replicates the excavation process of hard-rock tunnel boring machines (TBMs). The study provides valuable insights into cutting forces, chip size, specific energy, RPM and excavation modes, enhancing understanding and decision-making in hard-rock excavation processes. The research also presents a new RCM model validated against TBM penetration data, demonstrating its practical applicability and predictive accuracy.

A novel test apparatus to study the mechanism of harmonic normal force on fretting wear

Fretting wear significantly affects the hysteresis behavior of the contact surface and leads to joint degradation due to material removal. Although numerous tests have been conducted to investigate fretting wear under constant normal force, the applied mechanical load on the contract surface more or less deviates from that in the operational condition. The normal force is typically time-varying due to the coupling of tangential and normal vibration, rather than remaining constant. The time-varying normal force can be regarded as the superposition of harmonics for steady-state vibration which is widely encountered in engineering. In this paper, we investigate how would the harmonic component of the normal force affect the evolution of interface morphology and contact parameters with fretting wear. To do that, we design a novel fretting wear test apparatus that generates the stable harmonic normal force. The normal force is controlled by a closed-loop control system, ensuring its consistency throughout the entire wear test. In addition, we develop various subsystems to realize different functions like support, constraint and measurement, enabling the acquisition of hysteresis loops under different normal forces. The critical performances of the test apparatus (e.g. force transfer, closed-loop control and repeatability) are also demonstrated. Results indicate that the harmonic normal force can significantly influence the hysteresis loop and the energy dissipation distribution on the contact surface. In comparison to the constant normal force, the worn surface has a higher degree of non-uniformity, characterized by alternating pits and prominences. Furthermore, the friction coefficient exhibits an outstanding upward trend after reaching a steady state, while the tangential contact stiffness continuously weakens as fretting wear progresses. These unique evolution characteristics could be attributed to the non-uniform energy dissipation caused by the harmonic normal force acting across the fretting space. This viewpoint can be partly validated through observing the interesting hysteresis loops, which are captured by the test apparatus.

Effects of feed direction on material removal behavior in belt grinding of titanium alloys

For abrasive belt grinding, although machine processing selection has been well studied to achieve good machinability, there is still a lack of understanding regarding the influence of feed direction and its mechanism, even though it is a common matter. This paper investigates the differences between up- and down-grinding in belt grinding of TC4, where the grinding forces, material removal performance, surface quality, and chip morphology are captured for the grinding process evaluation and mechanistic analysis In up grinding, it was observed that approximately 21.1 % more material removal can be achieved due to the larger penetration depth of scratching grains when maintaining a constant normal grinding force. Furthermore, the higher tangential grinding force resulting from increased cutting behavior and an additional sliding phase leads to more severe abrasive belt wear, as evidenced by a 22.7 % higher grinding ratio. Comparatively, down grinding mode generates a machined surface with lower roughness in the transverse and longitudinal direction, which can be explained by the minimum chip thickness effect and undeformed chip thickness model. The microscopic characteristics of chips and ground surface provide more evidence on the different chip formation mechanisms between two grinding modes.

Comprehensive investigations into the force and thermal characteristics of belt grinding Inconel 718 under constant normal forces

Belt grinding is an efficient and widely used method for machining precise high-strength components made from the superalloy Inconel 718. By integrating robots with a force sensor, normal forces can be kept to achieve high-accuracy grinding under varying processing conditions. However, the force and thermal characteristics of belt grinding Inconel 718 under constant normal forces, which significantly affect the quality of ground workpieces, are still unclear. In this study, four characteristics including grinding effects, dynamic grinding forces and force ratios, specific grinding energy as well as dynamic grinding temperature distributions, are comprehensively investigated. The grinding effects are determined by the single-grain scratch test, and the material springback mechanisms are revealed. The influences of grinding parameters on grinding forces, specific grinding energies and grinding temperatures are analyzed in detail. Additionally, a new finite element model is established to obtain the dynamic temperature distributions of grinding surfaces. This study aims to improve understanding of the process of constant-force belt grinding Inconel 718 and lays foundations for further research on grinding mechanisms and modeling.

Non-invasive rust detection of steel plates determined through interfacial modulus

Initial methods to detect rust in pipelines have been conducted through invasive probes and sectioning off parts of the facility as the plant is running. These methods greatly increase the costs overall. The need for a feasible solution to this issue lies in the detection of rust formation through a non-invasive method. This study’s objective is to measure rust formation through droplet motion on the outer layer of pipelines. Multiple experiments are conducted using carbon steel sheets whose bottom layer has been exposed to acid for different durations of time. As rust formation in the metal is a voltaic phenomenon, it would mean that the acid corrosion of the bottom layer would adversely affect the top layer of the substrate. Consequentially, droplet motion and the droplet’s contour would change in different corrosive scenarios which we could then detect with novel parameters in our lab. One such parameter is the Interfacial Modulus (GS), which describes the initial resistance of the solid’s outer layer towards the liquid. We can understand this parameter with the aid of the novel device, known as the Centrifugal Adhesion Balance (CAB). As we cause the drop to slide across the substrate at constant normal force condition, we observe the difference in the contour of the drop as it slides across the substrate. The real-time change in contact angles at each edge of the drop, along with its change in external lateral force, causes a change in the GS values, which varies in different corrosive scenarios.

Robotic Polishing of Unknown-Model Workpieces With Constant Normal Contact Force Control

Industrial robots are increasingly being considered for application in performing polishing operations. Most research has been performed on workpieces with a priori (CAD) model, and used impedance control or hybrid force-position control to achieve constant force operation. For workpieces without priori models, it is still difficult to maintain a constant normal contact force during the polishing process. To address this challenge, this paper proposes a cooperative force-position control method consisting of three parts: constructing a model between contact forces and robotic poses during polishing operations and obtaining a real-time pose deviation of the current tool-workpiece contact state and the desired (normal) contact state; based on the deviation, a method of real-time adjustment of 2D predefined polishing paths to 3D actual polishing paths is proposed; finally, constant normal contact force control is achieved by a direct force control algorithm based on the results obtained from the above method. The proposed method is based on a macro-mini robotic system with an active force control polishing device, and its effectiveness was tested by various unknown-model workpiece polishing scenarios. The results demonstrate that the proposed method can achieve stable normal force tracking and high-quality surfaces during unknown-model workpiece polishing.

Frictional Energy Dissipation in Partial Slip Contacts of Axisymmetric Power-Law Graded Elastic Solids under Oscillating Tangential Loads: Effect of the Geometry and the In-Depth Grading

Due to the rapid development of additive manufacturing, a growing number of components in mechanical engineering are made of functionally graded materials. Compared to conventional materials, they exhibit improved properties in terms of strength, thermal, wear or corrosion resistance. However, because of the varying material properties, especially the type of in-depth grading of Young’s modulus, the solution of contact problems including the frequently encountered tangential fretting becomes significantly more difficult. The present work is intended to contribute to this context. The partial-slip contact of axisymmetric, power-law graded elastic solids under classical loading by a constant normal force and an oscillating tangential force is investigated both numerically and analytically. For this purpose, a fictitious equivalent contact model in the mathematical space of the Abel transform is used since it simplifies the solution procedure considerably without being an approximation. For different axisymmetric shaped solids and various elastic inhomogeneities (types of in-depth grading), the hysteresis loops are numerically generated and the corresponding dissipated frictional energies per cycle are determined. Moreover, a closed-form analytical solution for the dissipated energy is derived, which is applicable for a breadth class of axisymmetric shapes and elastic inhomogeneities. The famous solution of Mindlin et al. emerges as a special case.

The effect of dynamic normal force on the stick–slip vibration characteristics

In the experiment, we observed such a phenomenon: the alternating normal force changes the vibration state of a friction system. A single-degree-of-freedom mathematical model was used in this paper to discuss the effects of a constant and alternating normal force on the stick–slip vibration characteristics for different dynamic and static friction coefficients. Under the condition that the applied constant normal force continues to increase, the vibration amplitude of the system, the amplitude of the limit cycle, and the adhesion time of the system increase. When the difference between the dynamic and static friction coefficients (DSFCs) is small, the system has a complete and clear limit cycle. When the dynamic friction coefficient is reduced, the difference between DSFCs increases, and the limit cycle of the system is deformed. The friction system has more abundant dynamic vibration characteristics under an alternating normal force than a constant normal force. The vibration state of the system presents a single-cycle stick–slip vibration when the alternating normal force excites the multi-order harmonic response of the friction system, and the excitation frequency of the alternating normal force is the same as the main response frequency of the system with the highest energy or the low-order even-order main frequency. In contrast, the system exhibits various vibration modes when the excitation frequency of the alternating normal force is dissimilar to the main frequency of the system's highest energy response or is consistent with the odd-order main frequency. In addition, increasing the difference between DSFCs or using very high excitation frequencies and excitation amplitudes increases the likelihood of the system entering a chaotic vibration state.

Frequency-Dependent Behavior of Electrostatic Forces Between Human Finger and Touch Screen Under Electroadhesion.

The existing lumped parameter circuit models do not capture the true (experimentally observed) behavior of electrostatic forces between human finger and a touch screen under electroadhesion, changing as a function of stimulation frequency. In order to address this problem, we first conducted an experiment to measure the voltage-induced frictional forces acting on the finger of a user sliding on a touch screen under constant normal force for stimulation frequencies ranging from 1 to 10 6 Hz. The steady-state values of coefficient of sliding friction for those frequencies and the value for voltage-free sliding (no electroadhesion) were utilized to estimate the magnitude of electrostatic force as a function of frequency. The experimental data shows that electrostatic force follows an inverted parabolic curve with a peak value around 250 Hz. Following the experimental characterization of electrostatic forces, an electro-mechanical model based on the fundamental laws of electric fields and Persson's multi-scale contact mechanics theory was developed. Compared to the existing ones in the literature, the proposed model takes into account the charge accumulation and transfer at the interfaces of finger and touch screen. The model is in good agreement with the experimental data and shows that the change in magnitude of electrostatic force is mainly due to the leakage of charge from the Stratum Corneum (SC) to the touch screen at frequencies lower than 250 Hz and electrical properties of the SC at frequencies higher than 250 Hz.

Electro-lubrication in Janus transition metal dichalcogenide bilayers

Lubrication induced by a vertical electric field or bias voltage is typically not applicable to two-dimensional (2D) van der Waals (vdW) crystals. By performing extensive first-principles calculations, we reveal that the interlayer friction and shear resistance of Janus transition metal dichalcogenide (TMD) MoXY (X/Y = S, Se, or Te, and X ≠ Y) bilayers under a constant normal force mode can be reduced by applying vertical electric fields. The maximum interlayer sliding energy barriers between AA and AB stacking of bilayers MoSTe, MoSeTe, and MoSSe decrease as the positive electric field increases because of the more significant counteracting effect from the electric field energy and the more significant enhancement in interlayer charge transfer in AA stacking. Meanwhile, the presence of negative electric fields decreases the interlayer friction of bilayer MoSTe, because the electronegativity difference between Te and S atoms reduces the interfacial atom charge differences between AA and AB stacking. These results reveal an electro-lubrication mechanism for the heterogeneous interfaces of 2D Janus TMDs.

A Study of Effect of Various Normal Force Loading Forms on Frictional Stick-Slip Vibration

In this work, a comparative study is performed to investigate the influence of time-varying normal forces on the friction properties and friction-induced stick-slip vibration (FIV) by experimental and theoretical methods. In the experiments, constant and harmonic-varying normal forces are applied, respectively. The measured vibration signals under two loading forms are compared in both time and frequency domains. In addition, mathematical tools such as phase space reconstruction and Fourierspectra are used to reveal the science behind the complicated dynamic behavior. It can be found that the friction system shows steady stick-slip vibration, and the main frequency does not vary with the magnitude of the constant normal force, but the size of limit cycle increases with the magnitude of the constant normal force. In contrast, the friction system under the harmonic normal force shows complicated behavior, for example, higher-frequency larger-amplitude vibration occurs and looks chaotic as the frequency of the normal force increases. The interesting findings offer a new way for controlling FIV in engineering applications.

Stick-slip Instability in a Compliant Bistable Double-Slider Mechanism

This paper investigates friction-induced instability in a Single Degree-of-Freedom, pseudo-rigid-body representation of a bistable compliant mechanism composed of two sliders connected with a massless rigid link. The friction force is a function of the state variables through Stribeck effect and variable contact force due to the structural nonlinearity of the mechanism. A Constant normal force ensures the mass-belt contact during oscillation. It is shown that the steady-state response of the vibrating mechanism depends on the belt velocity, applied normal load, and stiffness. The applied normal force and belt velocity, as bifurcation parameters, are used to define the number and location of equilibrium points and their corresponding stability.

Comparison of wheel load application methods in single-wheel testbeds

This work reviews wheel load application methods in single-wheel testbeds (SWTBs), used widely to study terramechanics. Normal force oscillations have been reported in SWTBs, and visible in time-series data when provided. This has usually been attributed to grousers, but is also visible for wheels without grousers suggesting other system dynamics (e.g. friction in the vertical axis) are the cause. Furthermore, accelerations related to periodic vertical displacements of the wheel (whether caused by grousers or otherwise) cannot explain more than a small fraction of the amplitude of normal force variations. In this work, a 4-bar mechanism was developed that balances the normal load and ensures that a constant (not oscillating) normal force is achieved throughout terramechanics experiments. System behaviour with and without the 4-bar mechanism was assessed, highlighting the need for a 4-bar mechanism or other method of balancing normal loads if a constant normal force is desired. No significant effects on average experimental results were observed due to normal force oscillation, but artifacts were observed in the time-series data. Comparisons of results with a system where wheel load is applied pneumatically and characterization of friction in each testbed provide further support to the conclusion that normal force oscillations are mainly caused by friction in the vertical axis.

Effect of cyclic shearing on contact surfaces of geo-materials under constant normal force

A large amount of research has been done on conforming surfaces in rock joints as well as on the contact between individual grains; however, not much exist in nonconforming contact surfaces subjected to friction, such as flat contacts between ballast particles, stone columns, or riprap; applications that involve the use of coarse gravel subjected to low vertical stresses. Therefore, this article aims to study changes in contact properties between nonconforming flat contacts between large geo-material particles that have been subjected to cyclic shearing under a constant low vertical force, using a direct shear apparatus. Two different silica carbide sandpapers that do not loose particles were used, to simulate different morphologies, a nominally fine and a coarse surface texture. The results show a passivation of the shear strength where a constant value of friction coefficient is reached after around 15 to 17 cycles for all tests, except the tests at the lowest vertical force. For the tests at the highest vertical force, similar friction coefficients were determined for the coarse and fine surfaces. The mass broken during the 10th and 20th cycles was collected at the end of the tests and seemed to show a linear relationship with the vertical force used in the test. Particle analysis, determined via microscopy, shows that the grading is dependent on the initial topography of the surfaces. Despite being subjected to 10 and 20 cycles of shearing, the broken particles look similar in shape with sharp, jagged edges and having different shapes and roundness values with a large variation, indicating that the breakage was not enough to fill in the space between the particles.

Interfacial Friction Anisotropy in Few-Layer Van der Waals Crystals.

Friction anisotropy is one of the important friction behaviors for two-dimensional (2D) van der Waals (vdW) crystals. The effects of normal pressure and thickness on the interfacial friction anisotropy in few-layer graphene, h-BN, and MoSe2 under constant normal force mode have been extensively investigated by first-principle calculations. The increase of normal pressure and layer number enhances the interfacial friction anisotropy for graphene and h-BN but weakens that for MoSe2. Such significant deviations in the interfacial friction anisotropy of few-layer graphene, h-BN and MoSe2 can be mainly attributed to the opposite contributions of electron kinetic energies and electrostatic energies to the sliding energy barriers and different interlayer charge exchanges. Our results deepen the understanding of the influence of external loading and thickness on the friction properties of 2D vdW crystals.

Innovative squeak noise prediction: An approach using the harmonic balance method and a variable normal contact force

A flawless interior acoustics is a customer requirement in automotive industry. Any disturbing noise e.g. squeak or rattle must be avoided to meet the customer’s expectations regarding quality. This work presents an improved approach to predict squeak noise. For this purpose, a test rig generating the underlying stick-slip phenomenon by harmonic excitation is used and a corresponding finite element model is built describing the system’s dynamic behavior. The Harmonic Balance Method is applied for solving the nonlinear equations of motion. The major difference to previous approaches consists in the implementation of a contact algorithm allowing variable normal forces in the contact domain. The calculated nonlinear system response is compared to existing methods using a constant normal force. The proposed method is capable of predicting odd harmonics, which is a significant improvement in assessing squeak noise. Furthermore, good agreement between simulation and experiment is observed which validates the approach.

Assessment of micro-abrasive wear tribological properties of H10 tool-steel under conditions of “constant normal force ⇒ variable pressure” and “constant pressure ⇒ variable normal force”

In industrial applications, tools of H10 tool-steel used in mechanical and metallurgical manufacturing processes, such as rolling, extrusion, forging, sheet metal stamping, milling and turning, frequently present significant levels of micro-abrasive wear, which occurs under specific circumstances of “constant normal force”, resulting in a “variable pressure” physical condition, or “constant pressure”, resulting in a “variable normal force” physical condition. Consequently, results of micro-abrasive wear generated under these tribological conditions can help engineers design materials and tools with a longer working life span when they are working under mechanical and metallurgical manufacturing processes. Observing the relevance of this subject, the purpose of this work is to assess the micro-abrasive wear tribological properties of H10 tool-steel submitted to experimental conditions of “constant normal force ⇒ variable pressure” or “constant pressure ⇒ variable normal force”. Ball-cratering wear tests were conducted on quenched and tempered AISI H10 tool-steel; the counter-body was a quenched and stress-relieved AISI 52100 bearing steel sphere and the abrasive slurries were prepared mixing abrasive particles of silicon carbide with distilled water. During the micro-abrasive wear tests, the abrasive slurry was, continuously, agitated and fed between the sphere and the tested material; values of coefficient of friction were, simultaneously, acquired, as well. After the tests, each wear crater was analyzed by optical microscopy, SEM and CAD software. The results were discussed based on the behavior of the “Steady-State Wear Regime”, “wear volume”, “wear rate”, “micro-abrasive wear modes”, “pitch between grooves”, “coefficient of friction” and, finally, validated by ANOVA. With the attainment of the Steady-State Wear Regime, it was possible to present the following conclusions: (i) wear volume and wear rate increased under conditions of “constant pressure ⇒ variable normal force”, (ii) there was the predominance of “grooving abrasion”, with the action of “micro-rolling abrasion” wear phenomenon, (iii) the pitch between grooves presented, approximately, the same value of the average abrasive particles size and (iv) the coefficient of friction remained in a tribological condition classified, statistically, as “constant”.