THERMOMECHANICAL FEM ANALYSIS OF A DISC BRAKE

Project: Thermo-elastic instability phenomenon of automotive disc brakes
Sponsor:
University: LSU, Mechanical Engineering
Principal Investigator: Dr. Michael Khonsari
Graduate student: Serhan Sezer contact information: Dr. Michael Khonsari

Introduction:

Not only are brakes in cars expected to work properly with a minimum amount of service and stop the vehicle in a specified distance but also they are designed to minimize the customer annoyance problems such as noise and roughness. The purpose of brake is to reduce the velocity or to maintain it when the vehicle is driving downhill. Noise occurs at different times in the driving cycle and the pitch varies from very high to very low frequency. Roughness, however, occurs when the brake puts out uneven torque for each revolution of the wheel during a stop. In other words roughness, judder, is a pulsation caused by uneven torque at the brake pad/rotor interface during a single revolution of the rotor as mentioned above. This pulsation is sensed by the driver as steering wheel circumferential vibration, brake pedal pulsation, seat vibration, or in extreme cases whole vehicle pulsation. The torque variation can be caused by either circumferential thickness variation or circumferential coefficient of friction variations in the rotor. The thickness variation can be either due to wear or it can be induced by uneven thermal expansion. Braking performance of a vehicle can be significantly affected by the temperature rise in the brake components. High temperatures during braking may cause brake fade, premature wear, brake fluid vaporization, bearing failure, thermal cracks, and thermally excited vibration. Therefore, it is important to predict the temperature rise of a given brake system and assess its thermal performance in the early stage.

Uneven heat distribution, thermoelastic instability, is one of the methods of inducing this thermal thickness variation, a reason of non uniform brake behavior and can be a cause of judder (noise and roughness). Thermoelastic instability occurs when a pressure perturbation in the system causes more energy to be induced at one point in the rotor. Since more energy enters the rotor at one point, this spot becomes hotter than the adjacent material noting that these pressure perturbations are almost always present. Thus this area expands more than the adjacent material causing a minute thickness variation. As the pad slides over this area the fact that it is now thicker causes higher pressure at this point and hence more energy to be input. If the rotor is below the critical sliding speed for TEI, this 'hot spot' dissipates its energy into the surrounding material faster than additional energy is input to the spot. If the rotor is above critical speed, then the increased pressure at the hot spot causes energy to be input to the region faster than it is dissipated to the surrounding material. This results in a positive feedback, and the hot spot grows exponentially. The hot spot continues to increase in temperature and thickness until equilibrium is reached between the energy entering the spot and dissipation to the surrounding media. The thickness variation caused by this hot spot can contribute to torque pulsations resulting in judder or low frequency noise problems. As we can see explanations above that there are, arguably, two primary causes of brake roughness: 1 rotor thickness variation from wear, rust, or uneven thermal distortion and/or 2 coefficient of friction (mu) variation due to uneven lining transfer, rust, or variations with temperature. These two causes are closely coupled and roughness initiated by one may quickly involve the other due to thermal phenomenon. Eventually, the area with the highest torque receives the greatest amount of energy input during a given revolution of the rotor, and this area of the rotor gets hotter than the surrounding areas. Thermal growth causes it to become thicker than other regions of the rotor and that results in even higher torque variation. Eventually, this process continues until the brake is released or equilibrium is reached between the uneven energy into the rotor surface, the conduction within the rotor between the hot and cool regions, and friction behavior.

Project Description:

A static structure and transient thermal analysis of a car disk brake under contact conditions are performed using Pro/Mechanica; researches done about Thermoelastic Instability are used as a guide to design a disk brake model with the friction surfaces. However, boundary conditions integrated from numerical solutions of the problem are simplified for the software and applied for the static conditions. A comparison between numerical solutions and Pro/Mechanica solutions is presented. Further considerations about improving the model and increasing the accuracy of the solutions are discussed. The necessary assumptions are made in the analysis, such as neglected wear and solid disk brake model. As a current study of this problem Ansys Simulations with less assumption and less program restrictions have been performed for the thermo-mechenical case. Temperature distribution obtained by the transient thermal analysis is used in the calculations of the stresses on disc surface.

The objectives of this research:

1 .Investigate the effects of the rotating speed of the disk and the material properties on thermoelastic behaviors


2. To reduce the solution time for the problem in FEA by utilizing ANSYS's contemporary advantages in contact and thermo-mechanical problems.

 


Fig 1.Von Mises stress due to the only braking momentPublication's Results (Ansys5.0)
Uncoupled Von_Mises Stress Distrubition

Fig 2. Ansys8.0/Workbench's Resul Von-Mises Stress Distribution due to only braking moment

Fig 3. Publication's Results (Ansys5.0)Uncoupled Thermal Transient Analysis
Units: Celcilus&

Fig 4. Cosmos/Works-Ansys8.0 Results (Current Analysis)
Uncoupled Thermal Transient Analysis
Units: Kelvin

 

click here to see the animation

 

 


 

 

 

 

 

 

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