Full-scale Dynamometer Research Articles

The braking performance and failure mechanism of copper-based brake pads during repeated emergency braking at 400 km/h

Copper-based brake pads have played a pivotal role in the braking systems of commercial high-speed trains travelling at speeds ranging from 200 to 350 km/h. In this study, the braking performance and friction and wear mechanism of copper-based brake pads at 400 km/h were further evaluated using a full-scale dynamometer. The results show that under repeated emergency braking conditions at 400 km/h, the mean friction coefficient can be maintained at about 0.3 and remains stable within six braking cycles. Subsequently, a fade phenomenon in the mean friction coefficient emerges accompanied by a significant increase in braking distance, while the mean wear loss is relatively high throughout the entire testing process. Microstructural characteristics of the friction film and matrix indicate that strong oxidation plays an important role in the failure process of the brake pads. The friction film primarily composed of oxides exhibits numerous defects, which are susceptible to peeling off during braking, thereby compromising wear resistance. Additionally, as the braking cycles accumulate, oxidation gradually invades and weakens the matrix, leading to violent migration of the copper-rich friction film across the friction interfaces and resulting in the deterioration of friction performance. This work provides valuable insights into the failure behavior of copper-based brake pads at higher braking speeds and can provide potential direction for further improving their performance.

Numerical modeling of heat generation process in a disk brake at three thermal contact conditions over an extended braking distance

The article examines the thermal behavior of five newly developed organic composite friction materials combined with a cast-iron railway brake disk during repeated braking. Bench tests were carried out in accordance with the established program, taking into account 15 braking and cooling cycles. In the next stage, the obtained braking parameters were implemented into a 3-D FE model of the brake. The main goal of the study was to determine an influence of important assumptions taken in computer simulations regarding contact conditions, cooling conditions, and the number of required brake components. The contact conditions included: 1) perfect thermal contact, 2) contact of rough surfaces taking into account thermal conductance, and 3) separate heating. The total duration of the analyzed multiple braking reached 7695 s, while the maximum distance covered during this time was 203.1 km. The results of numerical calculations were compared with experimental data obtained on the full-scale dynamometer test stand.

Braking Performance of Fe-Based and Cu-Based Powder Metallurgy Pads Paired with C/C-SiC Disk on Full-Scale Dynamometer

In this study, the friction performance of copper-based (Cu-PM) and iron-based (Fe-PM) powder metallurgy pads was systematically compared and analyzed by performing a braking test paired with a C/C-SiC disk on a full-scale flywheel brake dynamometer, including coefficient of friction (COF), wear rate (Wr), and worn surface under dry and wet tests. The test procedure simulated the braking conditions on high-speed trains with initial brake speeds (IBS) from 50 km/h to 200 km/h, as well as under various brake clamping forces (FB). During dry tests, both pads exhibited high levels of COF. The mean value of average COF (µm) for the Cu-PM pad was 0.371, which was slightly higher than that of 0.358 from the Fe-PM pad. The stability of µm over the IBS ranged from 68% to 82% for Cu-PM pads under different FB, while it was from 82% to 85% for Fe-PM pads. However, during the wet tests, the mean value of µm dropped to 0.207 for Cu-PM pads, and with the minimum µm of 0.062; whereas, they were 0.344 and 0.188 for Fe-PM pads, respectively, showing a significant difference in the friction performance between the two pads. Additionally, the Wr of the Cu-PM pad was 0.110 cm3/MJ during dry tests, which was 17.3% lower than that of 0.133 cm3/MJ observed for the Fe-PM pad. After the brake test with the Cu-PM pad, the friction film primarily covered the Si/SiC-rich region of the disk, effectively inhibiting the plowing of SiC hard asperities onto the pad surface. However, the friction film predominantly covered the fiber-rich region of the disk after the test with the Fe-PM pad, and the exposure of hard Si/SiC-rich asperities would continuously plow the pad during braking and resulted in furrows on the surface of the Fe-PM pad. In terms of wear mechanisms, adhesion wear was the primary mechanism on the contact surfaces of the Cu-PM pad and the C/C-SiC disk, while abrasion dominated the interaction between the Fe-PM pad and the C/C-SiC disk. This study provides valuable insights for the application of C/C-SiC brake disks on trains.

Alloy selection for wear-resistant lightweight aluminium brake disc

Brake discs for the rear axle of a high-performance sports car have been manufactured in three different lightweight materials; AA6082, AA7075 and AA2024 aluminium alloys. These were subsequently coated with a Keronite plasma electrolytic oxidation (PEO) ceramic layer. Full-scale dynamometer tests have been conducted against a standard low-met brake pad using a modified AK master test at increasing levels of braking temperature until a physical limit is reached. Microstructural characterisation of coating and brake pad wear and tribo-layer have been studied using SEM and EDX. In addition, post-dynamometer-test friction discs have been exposed to salt fog testing. Dynamometer tests give confidence that PEO-coated aluminium brake discs are a viable technical solution for vehicle lightweighting. In particular, compelling performance with respect to good coefficient-of-friction (CoF), extremely high coating adhesion, minimal disc and pad wear, low disc run-out, good heat dissipation, and good corrosion resistance has been demonstrated for Mat A (AA6082) alloy discs. Despite higher room temperature strength, the resulting performances of Mat B (AA7075) and Mat C (AA2024) are poor at higher braking temperatures, resulting in excessive coating spallation and subsequent grain boundary attack followed by severe intergranular corrosion.

Adaptation of rectangular and trapezoidal time functions to simulate the rotational motion of the brake disc

Numerical modeling of the rotational motion of the brake disc during frictional heating requires the use of specialized software. In this paper, an alternative approach was proposed consisting in the simulation of the displacement of the heating region contained within the boundaries of the friction surface of the brake pad relative to the stationary disc. The moving heat source effect was obtained using rectangular time functions generated based on the operating parameters and dimensions of the brake components. The first part of the article discusses the mathematical basis for the construction of time curves in relation to the solid brake disc combined with a pad in the shape of the angular section of a ring. Braking simulations were carried out using the finite element method (FEM) based software for 3D, 2D axisymmetric and 1D models of the disc. The calculated temperature filed changes were confronted with the results obtained using the 3D model developed based on standard methods. The temperature on the contact surface of the disc in the position of the mean radius at the end of braking, determined using the 2D axisymmetric and the 3D model with adaptation of rectangular time functions as well as the 3D reference model agreed well and were equal to 157.83 °C, 157.87 °C, and 157.90 °C, respectively. In the second part of the study, the 3D model of periodic heating was extended to the disc brake of a railway vehicle. The results of the calculations for the ventilated disc and segmented brake pad were compared with the experimental data obtained during testing on a full-scale dynamometer test stand. The maximum temperature values obtained on the basis of the 3D-pulse, 3D numerical models and measured on the test stand were equal to 73.3 °C, 74.4 °C and 75.6 °C, respectively. The insignificant temperature difference, i.e. below 3 %, obtained using the 3D-pulse heating model and measured using a thermocouple, confirms the correctness of the developed algorithm for simulating the rotational motion of the disc as well as the calculation methodology incorporating the heat partition coefficient (HPC).

Morphology and Composition of the Third Body on the Friction Surface of an Organic Composite Railway Brake Shoe

Friction properties are significantly affected by third bodies (films formed on friction surfaces). However, the study of their composition and structure remains incomplete. For this reason, an organic composite railway brake shoe was tested at an initial braking speed of 125 km/h using a full-scale dynamometer. A third body with a thickness of ~120 μm was obtained, and its morphology and composition were analyzed using a multiple techniques. The results indicated that the third body had a layered structure. The upper surface was smoother than the lower surface. The carbon content on the upper surface decreased by 68.01%, and the iron content increased by 11.85 times in relation to that on the lower surface. Compared to the brake shoe, the iron content of the third body increased by 272.81%, and most of the iron was oxidized. Furthermore, the content of barium, calcium, and silicon decreased by more than 33%, and the crystalline structures of the inorganic filler materials, such as graphite and barium sulfate, were destroyed, with new crystalline structures appearing. Finally, the residual weight at 650 °C increased from 90.35% to 96.59%. This research could provide a reference for exploring the friction and wear mechanisms of organic composite railway brake shoes.

Full-scale dynamometer tests of composite railway brake shoes including latxa sheep wool fibers

The main target of the present work is to characterize the effect of the inclusion of natural sheep wool (SW) into a railway brake block composition and then to compare it to that of a set of three organic fibers commonly used in the friction material industry: aramid fiber (AF), cellulose fiber (CF) and polyacrylonitrile fiber (PAN). In order to achieve this, 4 versions of the same friction material with a fixed amount of each organic fiber were produced and one more sample was manufactured including no organic fibers. The characterization work consisted of friction tests on a full-scale railway test rig. Then, the samples were SEM analyzed in order to characterize the tested surface microstructure. It was found that all organic fibers helped achieve a more stable bedding, and showed lower friction in wet conditions. They also affected the recovery %. Pictures of the blocks were taken after certain phases of the test and, although the failure sequence remained the same for all samples, the organic fibers very much influenced the magnitude of the wear rates. Sheep wool led to better results than cellulose. No final conclusions could be drawn with respect to metal pick-up. SEM analysis evidenced primary and secondary plateaus, but no significant differences were observed depending on the fiber nature. Finally, a Life Cycle Assessment with a “from cradle to gate” perspective was carried out. Ecoinvent v3.5 database and CML and ReCiPe Endpoint methodologies were used to evaluate the environmental impact create by the five brake block materials. Overall, cellulose, PAN and sheep wool brake blocks show slightly lower environmental impacts that the base material or than aramid fibers. Therefore, Latxa sheep wool offers a good balance between low cost, adequate wear rates and environmental impact, making it a compelling substitute for cellulose fibers.

Evaluation of the wear resistance of aluminium-based hybrid composite brake discs under relevant city rail environments

Lightweight hybrid composite brake discs consisting of a A357/SiC aluminium matrix composite (AMMC) top layer and an AA6082 Al alloy bottom base are fabricated by friction-stir processing. These hybrid AMMC brake discs demonstrate excellent wear and friction performance in full-scale dynamometer braking tests under relevant environments for operating a city rail vehicle. Characterisation and analysis conducted on the tested brake discs correlate these performances to a series of improvements in microstructures and properties. They include the presence of thermally stable and hard SiC particles enhancing wear resistance, especially at elevated temperatures, the refinement of microstructures by the uniform distribution of these SiC particles as well as secondary particles resulting in minimised crack initiation, and the generation of gradient hardness and tensile properties leading to increased resistance to external impacts and reduced internal thermal stress. The excellent performance of the lightweight hybrid composite brake discs confirms its high technology readiness level to replace heavy iron and steel brake discs in city rail vehicles.

Full-Scale Dynamometer Tests of Composite Railway Brake Shoes: Effect of the Resin-Rubber Ratio on Friction Performance and Wear

Abstract Designing a friction material for a brake system entails considering the effects of each constituent and the interactions that they may present between them. In the present work, a characterization of the influence of the resin-rubber ratio in a brake block material is carried out. Railway brake shoes were produced and tested in a full-scale railway dynamometer in demanding conditions. The brake blocks had also their physical and mechanical properties tested. The progressive addition of resin was proven to heavily affect the friction level in dry and wet conditions. Interestingly, the use of 5% of resin showed significantly higher friction in wet conditions. This composition also presented a more severe metal pick-up. The nature of the binder also affected wear-rates (which were lower for lower resin contents) and the wear mechanism. The sample using only rubber presented thermal cracks and heavier delamination as specific failure modes. Differences in the microstructure of the friction materials were observed depending on the binder. About 5% of resin appears as a very interesting choice to avoid friction loss in wet environments without incurring high wear-rates, as long as metal pick-up is diminished by different means. Otherwise, 100% of rubber as a binder grants the instantaneous friction stability that is often threatened by thermal fade.

Identification of a dynamic friction model for railway disc brakes

Braking forces occurring during emergency brake applications of passenger trains are generated by disc brake units. The acting friction forces depend on the frictional properties between disc and brake pad and are influenced by relative velocity, temperatures and normal pressure of the contacting surfaces. In this work a mathematical model is developed which aims to link these influencing variables to the instantaneous acting friction coefficient in order to include the characteristic behavior of friction forces in the calculation of longitudinal dynamics of railway vehicles. The model is identified by the use of data recorded on a full-scale dynamometer test rig and verified regarding the estimation of the brake distance.

Establishing a Correlation Between Friction Performance and Tribolayer Formation Using Engineered Mineral Fibers

Abstract This article relates the friction performance to the tribolayer formation when using engineered mineral fibers in the friction formulation. A nonasbestos organic (NAO) nonsteel copper-free passenger car disc pad (PC-DP) nonsteel Cu-free formulation is used to investigate the impact of the fibers on the third body formation and its influence on performance and wear. Full-scale dynamometer performance (AK-Master) test results show the pronounced effect of the fiber length on the friction performance. While a similar overall coefficient of friction (CoF) of around 0.33 is observed in all three cases, more stable friction behavior is observed for the longer fibers, with less variation. This behavior is even more observed in the fade and high-temperature sections. Especially the use of fiber spheres shows quite some unique features over the other tested mineral fibers, especially with respect to pad/disc wear. To understand these differences in performance, the transfer layer and tribolayer on the disc and brake pad are investigated, respectively, by means of scanning electron microscopy (SEM). Formulations showing excellent performance results so far show the formation of a smooth transfer film on the disc. No grooves are visible, and a continuous iron oxide layer is formed at the surface. The tribolayer of the corresponding brake pads nicely shows the formation of a compacted thin layer at the surface. In addition, the importance of mineral fibers in the formulation is highlighted as they are at the core of the tribolayer formation. This study is a step forward toward understanding the role of mineral fibers in a friction formulation and shows its importance toward reducing nonexhaust emissions.

A Comparison of 3D and 2D FE Frictional Heating Models for Long and Variable Applications of Railway Tread Brake.

The article proposes two 3D and 2D numerical FE models of frictional heating for the estimation of temperature distributions in railway tread brake in 1xBg configuration during repeated long-term braking. The results of computations were compared with the time courses of temperature measured using thermocouples throughout the duration of the tests on a full-scale dynamometer for two different brake shoe materials in combination with a steel wheel. The resulting temperature distributions calculated using the proposed models agreed well with the experimental measurements, and the maximum difference in temperature values does not exceed 20%. It has been proven that 2D FE model can be as efficient as 3D model to estimate the temperature distribution during long-term and variable braking in the considered friction node. The differences in the calculation of the temperature values using these models did not exceed 3%, and the calculation time for the 2D model, compared to the 3D model, was shorter approximately 85 times for the braking cycle lasting 5032 s, and approximately 45 times for the braking cycle lasting 3297 s.

Perancangan Dinamometer Skala Kecil untuk Pengujian Karakteristik Gesek Bahan Blok Rem Kereta Api

The brake blocks are components of the railway braking system that are useful for reducing speed or stopping the railway. In the development of railway brake block material needs a device that capable of testing the characteristics of the brake block material. One of the important characteristics of the brake block material is the frictional characteristic. The method of testing the frictional characteristics of existing brake blocks include using the pin on disc method, direct testing on the railway and using a full-scale dynamometer. The full-scale dynamometer has advantages than other methods because it can test specimens according to the actual railway braking conditions in a short time and less preparations. However, with the same specimen size as the original, the cost of making the specimen and the full-scale dynamometer become expensive. Therefore, to reduce the cost of making device and test specimens, this study will design the dynamometer small-scale brake block testing. This paper use Pahl and Beitz design method which divides the design into four stages: clarification of the task, conceptual design, embodiment design, and detail design. This dynamometer is designed by considering function designs, structures, and forces that are adapted to the brake block contact pressure according to the original conditions. This dynamometer is designed to vary material, wheel speed, and contact presssure with the output of measurement of friction coefficient of brake block material. Brake block test specimens have a reduced size with a 1: 4 scale from their original size. The results obtained from this research are engineering drawings and dynamometer test model of railway braking.

Full-Scale Dynamometer Test of Composite Railway Brake Shoes – Study on the Effect of the Reinforcing Fibre Type

Abstract When designing or developing friction materials, it is crucial to predict how the modification of the formulation will affect their properties. Fibres are introduced in the composition of the phenolic-based brake friction materials to improve their mechanical strength. Apart from reinforcing the composite, fibres can also affect its tribological and thermophysical properties. In this study two composite friction materials are compared. The difference between the materials was the type of reinforcing fibre used in the formulation – in one case it was glass fibre, in the other steel fibre. Thermal diffusivity of both materials was measured and thermal conductivity was calculated. Frictional characteristics determined by means of full-scale dynamometer tests are analysed and discussed. Substitution of glass fibre with steel fibre led to increase in the friction coefficient. Maximum average temperature below wheel surface, observed during the test of the material containing steel fibre, was lower as compared to the test results of the material with glass fibre in its formulation, despite higher heat flux in the course of brake applications. Thermal conductivity of the friction material was enhanced by including steel fibre in the formulation.

Mechanical and tribological behaviors of copper metal matrix composites for brake pads used in high-speed trains

Abstract As one of the most important components in high-speed trains, demands are improved on the mechanical and tribological properties of materials for brake pads. In this study, a newly developed copper metal matrix composite (Cu-MMC) for the aforementioned brake pads was fabricated by powder metallurgy route. The microstructure and mechanical properties of Cu-MMC were investigated. Cu-MMC was tribo-evaluated by a full-scale dynamometer, and special attention was paid on the braking performances during emergency stop-braking at initial speeds from 300 to 380 km/h. Examination and analysis of the worn surface and subsurface corroborated the wear mechanism. The results indicate that Cu-MMC exhibits excellent properties and can meet the technical requirements, so it holds great promise for applications in high-speed trains.

Full-ceramic brake systems for high performance friction applications

Ceramic matrix composites (CMC) are promising materials for friction applications. Typically, C/SiC rotors are combined with organic pads (Low Met). Within this work, two types of CMC brake pads were studied, C/SiC SF as well as C/SiC SF 11Coke with 11vol.% coke as filler, accompanied with the Low Met reference pads. Small brake pads (400mm2) were applied on a lab-scale and larger pads (12,000mm2) on a full-scale inertia dynamometer. In both cases, the corresponding rotor was a C/SiC disc with a SiC-rich friction layer. Sliding speeds up to 42.6m/s and braking pressures up to 8MPa were applied. The smaller CMC pads show lower wear and higher coefficient of frictions (COF), compared to the reference. This frictional behavior was confirmed on the full-scale dynamometer with mean COFs up to 0.45. However, the wear of the large C/SiC pads was somewhat higher on the full-scale dynamometer.

Friction surface structure of a Cf/C–SiC composite brake disc after bedding testing on a full-scale dynamometer

We have examined friction surface structure of a carbon ceramic brake disc tested on a full-scale dynamometer with microscopy techniques. The bedded friction surface is composed of two types of regions: transferred materials (TM) and SiC. The TM regions were formed through the deposition of wear debris into surface voids, followed by compaction and crystallite refinement during braking. A thin friction layer (FL) was developed on top of TM and SiC regions with nano-sized copper/iron oxide crystallites as the primary constituent. Analysis shows that debris generated from pad is the main source of TM and FL. No evidence shows chemical diffusion bonding between TM and composite constituent. On silicon carbide surface, dislocations were activated as the sources of surface fracture.

Development of a magnetorheological brake with a slotted disc

A conventional disc wears out and the brake pollutes the environment. Brake pad dust is reported to be the largest source of environment pollution1. The particles emanating because of wear of the brake pad pollute the environment2,3,4. In addition to pollution caused by wear particles, the friction-induced noise between the brake pad and the disc is also a major concern5. Also, localized heating occurs in a conventional disc brake. To tackle both of these problems, conventional disc brakes can be replaced with manetorheological fluid brakes. Magnetorheological fluids are materials having a shear yield stress which is a function of the magnetic field. On the application of a magnetic field, magnetorheological particles become aligned and increase the shear resistance between relatively moving surfaces. The friction between the stator and the rotor increases and fulfils the braking function, which means that magnetorheological fluids can be used as brake friction materials. A magnetorheological brake consists of a rotating disc or discs immersed in a magnetorheological fluid and enclosed in an electromagnetic casing. The torque characteristics of the magnetorheological brake in the shear mode are controlled by regulating the yield stress of the magnetorheological fluid. An increase in the yield stress increases the braking torque, which means that, the higher the yield strength of the magnetorheological fluid, the better is the performance of the magnetorheological brake. However, the major disadvantage of the shear-mode-based magnetorheological brake is its high resisting torque even in the off-state viscosity, and such magnetorheological brakes cannot be recommended for automotive applications. To obtain the performance of a conventional disc brake, experimental studies on a conventional disc brake were performed using a full-scale dynamometer. In addition to wear particles, localized heating of the disc was observed. The disc–pad interfaces were modelled to simulate the disc temperature. The values of the maximum temperature, which were obtained from simulations as well as experiments, were compared. The simulations were extended to hypothetical 360° pads, and a significant reduction in the maximum temperature was noted. Based on the idea of 360° pads, a magnetorheological brake subjected to shearing was analysed. To perform experiments on a small-scale magnetorheological brake, a test set-up was designed and developed, and it was confirmed that a magnetorheological brake subjected to shearing provides a better torque than does a conventional disc brake of the same size. An ideal magnetorheological brake should exert a zero frictional torque in the off-state condition and a controllable frictional torque in the on-state condition. An attempt was made to design such a magnetorheological brake. To overcome the disadvantage of the shear-mode-based magnetorheological brake, a new design of magnetorheological brake with a slotted disc was proposed. The design and development of the proposed magnetorheological brake, incorporating the compression-assisted shear mode, was detailed. The results of the braking torque, the temperature distribution and the off-state viscosities were plotted. Finally, comparison between the proposed magnetorheological brake and a small-scale disc brake was presented. The results show that, with an increase in the braking pressure (from 2 bar to 6 bar), the maximum value of temperature rises because of enhancement of the localized heating in the disc. The temperature rise is less in the case of the extended brake pad (360°), which was used in the magnetorheological brake. The surface temperature of the magnetorheological brake increases with increasing magnetic field. The magnetorheological fluid with 50 wt % iron particles has a less viscous torque in the off-state condition and a high-field torque under the compression mechanism in the magnetorheological brake, which fulfils the criterion for an ideal magnetorheological brake to replace a conventional disc brake of the same size.

Friction and wear performance of nickel and molybdenum-reduction cast iron composite brake blocks including ceramic foams

High speed trains in the northern area of Japan use alloyed cast iron brake blocks including nickel (Ni) and molybdenum (Mo). Reducing the Ni and Mo contents of the alloyed cast iron brake blocks would be beneficial because they are rare metals with variable prices. Ni and Mo play important roles in increasing the friction coefficient and decreasing the wear of brake blocks at high speeds. Brake blocks with reduced contents of these metals must maintain a sufficient friction coefficient and wear characteristics at high speeds. Cast iron composite brake blocks with reduced Ni and Mo contents were prepared using two types of alumina: aluminum titanate and silicon carbide foams to maintain the friction coefficient and decrease the wear characteristics at high speeds. These blocks were tested on a full-scale dynamometer, and the friction and wear performance of one of the best performing blocks were tested under real operating conditions. The friction coefficients of all reduced Ni, Mo cast iron composite brake blocks were higher than those of conventional blocks at high speeds. The wear of the reduced Ni, Mo cast iron composite brake blocks containing alumina blended with aluminum phosphate (AlPO4) foam was significantly decreased as compared with that of conventional blocks. Under real operation, the friction coefficients of conventional and reduced Ni, Mo brake blocks were similar under normal braking conditions, but those of conventional brake blocks were lower than those of the reduced Ni, Mo blocks under emergency braking conditions. The lower friction coefficient of conventional brake blocks under emergency braking conditions is likely caused by the adhesion of wear particles to the wheel tread. While the wheel tread was not worn by conventional brake blocks, it was worn by the reduced Ni, Mo blocks under emergency braking conditions.

Infrared Thermographic Analysis of Railway Brake Disc during Braking

The temperature monitoring on the surface of the railway brake disc was performed using high-speed infrared (IR) camera. The railway brake disc was developed for disc braking of maximum train speed of 180 km/h. The braking tests were conducted with a full scale dynamometer, and a high-speed infrared camera was employed to monitor temperature evolution on the brake disc during braking operation. The high-speed IR camera provides the measurement of temperature change during braking as well as the images of temperature contour on the brake disc surface. In general, the hot spot generation has been considered the main degradation mechanism in railway brake disc. In this investigation, damage evolution due to generation of hot spots on railway brake disc was investigated using the infrared thermography method.