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Investigation on Brake Disc Deformation Under Asymmetric Mechanical Loads
The motivation of this work is to identify thermal effects on the structural and contact behaviour of a disc-pad assembly using a finite element approach. The first analysis is performed on the disc-pad model without the presence of thermal properties. Structural performance of the disc-pad model such as deformation and von Mises stress is predicted. Next, thermomechanical analysis is performed on the same disc-pad model with the inclusion of convection, adiabatic and heat flux elements. The prediction results of temperature distribution, deformation, stress and contact pressure are presented. Comparison of the structural performance between the two analyses (mechanical and thermomechanical) is also made. From this study, it can assist brake engineers to choose a suitable analysis in order to critically evaluate structural and contact behaviour of the disc brake assembly.
Disc brake squeal can be classified as a form of friction-induced vibration. The elimination of brake squeal noise is very important as the problem causes discomfort of the vehicle occupant as well as pedestrians. This paper presents a new methodology for predicting disc brake squeal using finite element analysis considering both thermal effects and the structural compliance of brake components. An integrated dynamic study of non-linear contact pressure analysis and a fully coupled transient thermal analysis under variation of contact algorithm are performed before executing the instability study of a typical passenger car brake system using the complex eigenvalue analysis method. Based on the results of this exercise, a parametric study on the materials of brake components is carried out to define suitable design guidelines to reduce or to eliminate squeal problems.
The thermal distortion induced by thermoelastic instability (TEI) results in hot spots on the surface of the brake disc. This can incur low-frequency vibration known as judder. In pad-induced squeal noise, mode coupling occurs owing to the variation in the friction coefficient between the disc and pad, inducing high-frequency noise. Through a coupled analysis of hot spots and squeal phenomena, an optimum disc and pad design can be designed for higher thermal and mechanical performance. In this study, numerical and experimental analyses are performed in accordance with disc thickness, pressurization type of caliper, and lining arc length, considering thermal and mechanical instability simultaneously. Thermal deformation and pressure distribution are calculated using a finite element analysis (FEA). For evaluating TEI performance, experiments are performed using a chassis dynamometer and a high-speed infrared camera, and the results are correlated with FEA results. A complex eigenvalue analysis is conducted to evaluate mechanical instability using an FEA. Modal testing and simulations are conducted to correlate a real model and an FE model, and the corrected simulation results are applied for a complex eigenvalue problem to analyse coupled modes according to rotor and pad shapes. The results on disc brake performances considering the disc and pad design are discussed in terms of hot spots and squeal problems.
This paper deals with the mechanisms of the appearance of hot spots in railway brake disks. From infrared measurements, a classification of hot spots is presented. The most damaging thermal gradients are identified as macroscopic hot spots uniformly distributed on the friction surface. An explanation of the origin of these macroscopic hot spots is presented based on a scenario of progressive distortion of the disk. Three major effects have to be considered: the structural response of the disk and the pads; realistic boundary conditions, both geometric and loading; and the thermoelastoplastic behavior of the material. Finite element simulations based on the proposed model agree with experimental observations.
High-speed testing of a brake system using two different calipers and two different discs on a special test rig with a swinging caliper head mounting shows similar results with regard to hot judder. Holographic interferometry is used in an attempt to observe the disc mode shape during judder and although the classical fringe pattern was not obtained for the disc some useful and complementary information was forthcoming.
Disc run-out measurements show that the disc takes up a permanent and increasing deformation with a two-diameter mode formation. This deformation is seen to give a brake pressure fluctuation that results in judder - the pressure being detected using a pressure transducer fitted at the caliper and the mechanical judder with an accelerometer mounted on the caliper body. The two signals allow the degree of phase shift to be estimated.
A ""strobing'' effect, resulting from the combination of speed and video recording frequency, shows two hot-spots moving with the disc. Subsequent holographic results confirm the positions with the appearance of thermal fringes ""just off'' the disc surface. The images also show a classical two-diameter mode order of vibration on the disc center. The holographic recordings also indicate pressure waves being emitted from the trailing end of the pads with the mode shape of the pads confirming high amplitude bending; the existence of the pressure waves inferring the pads lose contact with the disc surface.
Other observations include hot spotting on the disc surface and a surface scan of the disc face indicates a depression at these positions. A metallurgical analysis of the blue spot is currently being carried out to determine if a material phase change has occurred within the disc body and this will be included in the paper.
The introduction of a pressure fluctuation absorber shows an improvement in the pressure amplitude and subsequent vibration signal and the possible development of this system is discussed
Finite element model (FEM) of three dimensional transient thermo-mechanical coupling dynamics was established for a ventilated disc brake, in which two rigid surfaces were used to simulate the normal force applications on both piston-side and finger-caliper-side, with different shapes according to their respective structures. Based on this model, the contact pressure between the disc and the two pads, the thermo-mechanical coupling characteristics of two pads were calculated under a certain emergency braking condition. The analysis results show that load transfer effect caused by friction force leads to the excursions of the pressure distribution center in the direction of entering friction area. However, the time-varying contact pressure distributions on both sides are different, which are affected by brake structures, dynamical normal forces and friction forces, and the disc thermal distortion. Thecomplicated temperature distribution of brake pads is caused by the varying frictional heat, the circular temperature gradients of the disc and the radial contact pressure gradients. The linings generate mainly pressure stress whose distribution is similar to that of contact pressure. The lining deformation patterns and their changes are strongly influenced by disc thermal distortion.
During light to moderate braking at high speeds the appearance of hot spots can often be observed on the brake disc surfaces. Such hot spots are mostly periodically distributed in the disc's circumference. The occurrence of hot spots leads to negative effects concerning driving comfort. This paper presents a description of the cause-and-effect chain of the mechanisms leading to the emergence of hot spots and thermal judder during the braking process.
In an automotive disk brake system, when the disk shows TEI at the speed over the critical one, hot spot forms on the disk surface and local contact friction is developed between the disk surface and the pad due to the hot spot. This non-uniform contact friction worsens the local heat concentration on the disk surface and has a direct effect on the disk pad, through which it affects the oil pressure supplier of the disk. The pressure change in the oil pressure supplier then affects the hot spot on the disk surface through the disk pad. This shows that there is a correlation between the vibration caused by the contact friction and the pressure change. The correlation between the pressure change in the disk with hot spot and DTV is analyzed. Since the brake vibration has several periodic elements, the unique characteristics of the brake vibration are analyzed, removing those elements with cepstrum and lifter-spectrum.
Thermoelastic instability in automotive disk brake systems is investigated focusing on the effect of a finite disk thickness. A finite layer model with an antisymmetric mode of deformation can estimate the onset of instability observed in actual disk brake systems. Also some effects of system parameters on stability are found to agree well to experimental observations.
The motivation of this work is to identify the thermal effects on the structural and contact behaviour of a disc-pad assembly using a finite element approach. The first analysis is performed on the disc-pad model without the presence of thermal properties. Structural performance of the disc-pad model such as deformation and Von Mises stress is predicted. Next, thermomechanical analysis is performed on the same disc-pad model with the inclusion of convection, adiabatic and heat flux elements. The prediction results of temperature distribution, deformation, stress and contact pressure are presented. Comparison of the structural performance between the mechanical and thermomechanical is also made. Three disc-pad designs are assessed using the developed finite element model. © 2014. MechAero Foundation for Technical Research & Education Excellence.
This paper focuses on the study of the relationships between brake disc surface temperatures and disc distortion for various high-energy stop-braking conditions. An original thermal metrology method combining an infrared camera and a fibre-optic two-colour pyrometer was used to record the spatial and temporal variation in disc surface temperature during braking. Disc distortion was investigated in situ by means of a high-frequency displacement sensor. In addition, an optical trigger kept track of disc revolutions and enabled the synchronization of the IR camera, two-colour pyrometer and displacement sensor measurements. This experimental set-up was successfully used to determine the surface temperature and investigate thermal localization and waviness distortion during braking. The results were correlated with each other in relation to the level of energy dissipation. It was shown that the highest temperature was reached in the hot spot regions at an early stage of stop-braking. By contrast, the greatest disc distortion appeared much later, during the last stage of stop-braking.
Thermoelastic instability (TEI) results irt uneven heating of the rotor and the development of hot spots in automotive disk brake systems. The hat spots cause rotor distortion and thickness variation which can cause torque variation resulting in brake roughness or low frequency noise. Lee and Barber (1993, ASME J. Tribal., 115, pp. 607-614) developed an analytical model to predict the critical speed above which TEI would occur: This paper describes enhancements to the model to include the effects of caliper/pad stiffness, the pad friction material thickness, and the pad length. The effects of these changes on the predicted speed are calculated and compared to the original model. [S0742-4787(00)01402-8].
Hot spotting and judder phenomena were observed in automotive aluminum drum brakes. A vehicle judder test schedule was developed to determine the critical speed for thermoelastic instability (TEI). The brake material properties relevant to the TEI analysis were measured as a function of temperature. The critical speeds for the brake systems with different drum materials were determined by the judder schedule and they are compared with the analytical predictions of Lee (2000). The brake drums and linings were then modified and tested in order to investigate its effects on the hot spotting and judder propensity. The design modifications include brake linings with a different compound, stress-relieved drums, linings with a convex or concave surface finish, three-segmented linings, and linings with a circumferential groove. The linings with a circumferencial groove effectively reduce the size of hot spots and the best judder rating was achieved.
Thermal levels of hot spots formed during high-energy braking with an organic matrix composite pad facing a steel disc have been studied using an infrared camera and a two-colour pyrometer. The coupling of these two devices allowed us to follow the friction areas and measure the disc surface temperature by taking into account the non-uniformity and variation of the emissivity. It also provided information about thermal gradients on the rubbing surfaces. Hot spot evolutions have been compared to friction contact variations. Material analyses revealed the occurrence of severe damage beneath the hot spots, particularly in the form of plastic deformation and microcracks propagating perpendicularly and parallel to the rubbing surface. In addition, solid–solid phase transformations taking place beneath the hot spots were observed in the vicinity of the rubbed surface. Finally, frictional force variations and subsurface damage were correlated with the surface temperature.Research highlights▶ Measurement of hot-spotting temperature formed during high-energy braking. ▶ Coupling an infrared camera and a two-colour pyrometer, consideration of disc emissivity. ▶ Frictional forces and subsurface damage are correlated with the surface temperature. ▶ Plastic deformation and microcracks perpendicularly and parallel to the rubbing surface. ▶ Solid–solid phase transformations taking place beneath the hot spots were observed.
Review of The Hot-spots Caused by Friction
- D Meng
- L Zhang
Hot Judder - An Investigation of the Thermo-Elastic and Thermo-Plastic Effects during Braking
- J Fieldhouse
- D Bryant
Effect of Initial Surface Runout on the Thermal Mechanical Coupling Characteristics of Brakes
- D Meng
- L Zhang
Research on Temperate and Deformation Characteristics of Brake Disc under Asymmetric Thermal Load”
- L Zhang
- J Zhang
- D Meng