P.C. Brooks

Review of life assessment techniques applied to dynamically loaded automotive components

Structural optimisation based on fatigue life of dynamically loaded structures of realistic complexity is rarely attempted due to computational costs. Very efficient stress analysis and fatigue life assessment techniques are needed if this is to become routine. For the first time, this paper compares several approaches to fatigue life prediction using a real automotive engineering case study, taking into account that optimisation based on fatigue life requires accurate relative distribution rather than exact values. The paper concludes that although both the quasi-static and frequency domain approaches are potentially more efficient than transient dynamic analysis, parameter sensitivity of the frequency domain approach may preclude its eventual use.

A PREDICTIVE TOOL TO EVALUATE DISC BRAKE SQUEAL PROPENSITY PART 1: THE MODEL PHILOSOPHY AND THE CONTACT PROBLEM

This paper is the first in a series of three that describe an integrated approach to the analysis of disc brake squeal. The technique is based around the use of a parametric finite element representation of the disc-pad assembly. In this paper, the problem is introduced together with the philosophy that underpins the model. The procedure adopted to solve the non-linear contact problem, which occurs at the friction pair interface, is illustrated and typical results presented that show how the contact behaviour is influenced by change in interface friction levels, hydraulic actuation pressure and loading mechanism.

A predictive tool to evaluate disk brake squeal using a fully coupled thermo-mechanical finite element model

This paper presents a new integrated approach to the analysis of brake squeal modelling. It focuses on developing a validated thermo-mechanical finite element model by taking full account of the effect of thermal loading on the structural response of the brake. An integrated study involving time-dependent non-linear contact and a fully coupled transient thermal analysis are carried out to provide the contact and temperature distribution within the brake before executing an instability study using the complex eigenvalue technique. The results, in turn, demonstrate the fugitive nature of brake squeal through the system eigenvalues that are extracted throughout the braking period.

The development of an optimisation algorithm based on fatigue life

A new structural optimisation algorithm based on fatigue life is introduced. The two key decisions in implementing this algorithm are firstly what assumption about the stress state (uniaxial or multiaxial proportional or non-proportional) should be made and secondly what method of stress calculation should be used (quasi-static, transient dynamic or harmonic modal stress analysis). The paper investigates the effects of different assessment strategies on the predicted fatigue life of a lower suspension arm, the properties of which are modified to generate different degrees of interaction between the arm natural frequencies and the frequency range of the applied forcing functions. The results of this investigation are used to derive a new form of structural optimisation algorithm which is more robust and efficient than the original.

A predictive tool to evaluate disc brake squeal propensity Part 3: Parametric design studies

This paper is the last in a series of three that present an integrated numerical approach to the analysis of disc brake squeal. It focuses on the application of the technique to the solution of a number of generic classes of problem encountered by the brake system engineer and the results are related to real world experience. The ability of the methodology to find direct use in such a broad area of application lies in the use of fully parameteriSed solid models to define the initial mechanical system. This flexibility is a powerful feature of the approach that ensures its continued use as a predictive tool. The paper concludes with a critical review of the technique and points to a number of refinement routes

Thermo-Mechanical Contact Analysis of Car Disc Brake Squeal

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.

Evolutionary structural optimisation of dynamically loaded components in consideration of fatigue life

This paper presents an evolutionary structural optimisation strategy based on the concept of repeatedly removing sets of finite elements with the highest fatigue life. The practical application of the proposed strategy required the development of: the most appropriate stress calculation method, a suitable fatigue life evaluation technique and code to select and remove elements with the highest fatigue life. Results obtained using code written to implement the proposed optimisation strategy appear to be very promising, compared to corresponding results obtained using other optimisation methods. The proposed technique was found to be particularly powerful in optimising dynamically loaded components starting from basic ground structures.

A PREDICTIVE TOOL TO EVALUATE DISC BRAKE SQUEAL PROPENSITY PART 2: SYSTEM LINEARISATION AND MODAL ANALYSIS

This paper is the second in a series of three that together describe an integrated theoretical approach to the analysis of disc brake squeal. Within this paper the specific problems of system linearisation and complex modal analysis are addressed. The likelihood of squeal noise occurrence is quantified by the definition of a single indicator derived from the system eigenvalues. Mode shape analysis permits study of the motion linked to each mode and a technique is presented that reconstructs a true picture of the complex mode through the combination of its real and imaginary components. Typical results are presented for stable and unstable modes of vibration which lead to the proposal of a mechanism that gives rise to the unstable squeal vibration.

Dynamic Simulation of Vehicle Suspension Systems for Durability Analysis

Optimisation for fatigue life is currently carried out manually, which is time consuming and may not achieve the best design. To shorten the design process, software for automated durability optimisation is being developed at the University of Leeds and is initially aimed at the automotive industry. A key aspect of the optimisation process is to obtain accurate load histories for the particular component to be optimised. If this is to be done early in the design process, before a rolling chassis prototype is available, then simulation must be relied upon to obtain these load histories. In the case of suspension components either a quarter vehicle model (QVM) or a full vehicle model (FVM) can be used as the basis for the multi-body system (MBS) simulation. This paper compares the suitability of the QVM and FVM for durability analysis. Results are presented for both a simplified vehicle suspension and for a more realistic suspension. Step inputs representing a kerb and a simplified pothole were applied to one wheel only. For the simplified suspension case, the local displacement of the body was less for the FVM than for the QVM. This indicates that the dynamic response at the other wheel stations contributes to the behaviour of the wheel station directly subjected to the step or pothole input. The study was repeated with a more realistic suspension model in the QVM and at one wheel station of the FVM, with similar results to the simplified suspension case. This study has shown that coupling exists between the four wheel stations of the FVM even when the suspension is independent. This coupling can affect the load histories applied to a particular suspension component, which may then affect its calculated durability. The strength of this coupling is such that the use of the QVM for durability analysis becomes questionable and the FVM should be used as the default.

An Instrumented Walking Aid to Assess and Retrain Gait

An instrumented walking aid, the iWA system, has been developed to measure kinematic and kinetic properties of walking aid (WA) use and deliver feedback to improve gait. The clinical requirements, technical specification and design of the system are developed through clinical collaboration. The development of the system is described, including hardware components and data analysis used to process the measured data for assessment. The system measurements are validated under controlled laboratory conditions. The iWA system is evaluated in a typical U.K. clinical environment by a participant in a rehabilitation session. The resultant data successfully capture the quality of the participants WA use and agree with clinical opinion, supporting the efficacy of this approach.