S. James

A methodology for the determination of dynamic instabilities in a car disc brake

The dynamics of a car disc brake system is investigated by a combined analytical and numerical method. The disc is rotated past the stationary pads and calliper in sliding friction at constant speed. The modal data of the disc are obtained by means of modal testing whereby a dynamic model for the disc is derived based on the thin plate theory. Then the pads, calliper and mounting are analysed by means of the finite element method. Finally the equations of motion for the whole disc brake system are established through the interfaces between the pads and the disc. The stability of the vibrating system is studied by the method of state space.

Finite-element modelling and updating of laser spot weld joints in a top-hat structure for dynamic analysis

Abstract Spot welds made by resistance spot welding are used extensively in automotive engineering. However, owing to increasing demands in the use of advanced and lightweight materials, laser welding has become a popular alternative for producing spot welds. Because of the complexity and uncertainties of laser welds and thus formed structures, the finite-element (FE) modelling of the welds for dynamic analysis is a research issue. This article first outlines some of the existing modelling works of spot welds. Then, a hat-plate structure used for this study is described and its FE representations are explained. The welds are modelled using CWELD elements in MSC/NASTRAN and their feasibility for representing laser spot welds is investigated. Numerical results for the initial FE model differ considerably from that of their experimental counterparts; hence, a model updating procedure is carried out to minimize the discrepancy between the two sets of results. In this work, the updating is posed as an optimization problem and is performed using the structural optimization capability (SOL 200) in MSC/NASTRAN. Two stages of updating are conducted, that is (a) updating FE models of individual components and (b) updating an FE model of the welded structure. Crucial steps in updating are explained. It is found that by selecting the right updating parameters, the CWELD element can be used to represent laser spot welds with good accuracy.

Finite element analysis of wear and its effect on squeal generation

Abstract In the past, wear at the pad interface of disc brakes has rarely been accounted for in studies of brake squeal using the finite element method, and its effect on disc brake squeal has been investigated largely through experimental methods. In the present paper, wear taking place at this interface over time is simulated using a modified wear rate formula. The surface topographies of two new and unworn pairs of brake pads are measured. The same brake pads are tested under braking applications of three time durations. For each braking application, the static contact pressure distribution is measured using pressure-indicating film. The results are used to compare with the simulated results predicted by the three-dimensional finite element model of a real disc brake. The paper also investigates squeal generation due to the above braking applications using complex eigenvalue analysis that is available in a commercial software package. It is found that the predicted unstable frequency is very close to the observed squeal frequency and that they take place in the same braking duration.

Active vibration control experiments on an AgustaWestland W30 helicopter airframe

This article describes the practical application of a vibration control technique, developed by the authors and known as the receptance method, to the AgustaWestland W30 helicopter airframe in the vibration test house at Yeovil. The experimental work was carried out over a total of 5 days in two visits to the Yeovil site during February and March 2011. In the experiments, existing electro-hydraulic actuators were used; they were built into the airframe structure and originally designed for vibration suppression by the methodology known as active control of structural response developed at the AgustaWestland Helicopters site in Yeovil. Accelerometers were placed at a large number of points around the airframe and an initial open-loop modal test was carried out. In a subsequent test, at higher actuator input voltage, considerable non-linearity was discovered, to the extent that the ordering of certain modes had changed. The vibration modes were, in general, heavily damped. Control was implemented using measured frequency response functions obtained at the higher input level. After acquiring the necessary measurements, simulations were carried out and the controller was implemented using MATLAB/Simulink and dSPACE. The closed-loop poles were mostly assigned with small real parts so that the system would be lightly damped and sharp peaks would be clearly apparent in the measured closed-loop frequency response functions. Locations of the open- and closed-loop poles in the complex s-plane were obtained to verify that the required assignment of poles had taken place.

Model updating for a welded structure made from thin steel sheets

Dynamic behaviour of a welded structure made from thin metal sheets which has a large flat surface and has been assembled together by a number of scattered spot welds is investigated. An impact hammer and roving accelerometers are used in the modal tests to provide data to update the FE model. NASTRAN Solution 103 is used to compute natural frequencies and modes of interest. The large error of the initial FE model is largely reduced mainly by adjusting a crucial parameter of the bending moment of inertia ratio, 12I/T**3.

Vibration of a continuous beam excited by a moving mass and experimental validation

The work presented in this paper deals with the vibration of a continuous slender beam excited by a mass moving at various speeds along it. An experimental model is designed and set up to study this problem. This model, which consists of a four-span continuous beam traversed by a moving mass at a constant speed, is used to build a theoretical model for the supporting structure. A series of tests designed to assess the accuracy of the model are carried out. The final section of the paper is dedicated to the numerical and experimental results and discussion.

FE Model Updating for Damage Detection – Application to a Welded Structure

Finite Element (FE) model updating is initially developed to update numerical models of structures to match their experimentally measured modal properties (i.e., natural frequencies and modes). In FE model updating, uncertain physical parameters of a structure are modified so that the discrepancies between the numerically estimated and experimentally measured modal properties are minimized. The process of updating is employed not only in parameter identification; it can also be developed for structural damage identification. In this work, a welded structure that is intended to represent a common configuration used in automotive body construction is investigated. It is known that presence of any damage in the welds of such a structure could affect its dynamic behavior. So, in theory modal test data can allow damage to be assessed accurately. As a typical automotive body contains thousands of welds, the effects of damage in the welds could be influential. The FE model updating process using experimental data is presented. It is carried out using NASTRAN optimization code. The procedure aims to adjust the uncertain properties of the FE model (from the weld joints) by minimizing the differences between the measured modal properties and the corresponding numerical predictions. The initial parameter values used in the numerical model are the nominal values. The procedure brings the numerical results of the structure as close as possible to the experimental ones, according to an objective function, therefore altering some of the FE model parameters of the structure. It may be concluded that when the identified values of certain parameters deviates from the nominal values to certain extent, there is a fault or damage at that particular joint.

Modal testing and finite element model updating of laser spot welds

Spot welds are used extensively in automotive engineering. One of the latest manufacturing techniques for producing spot welds is Laser Welding. Finite element (FE) modelling of laser welds for dynamic analysis is a research issue because of the complexity and uncertainty of the welds and thus formed structures. In this work, FE model of the welds is developed by employing CWELD element in NASTRAN and its feasibility for representing laser spot welds is investigated. The FE model is updated based on the measured modal data of hat-plate structures and cast as a structural minimisation problem by the application of NASTRAN codes.

The Development of a Reference Material for Calibration of Full-Field Optical Measurement Systems for Dynamic Deformation Measurements

A reference material is defined as material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process. Reference materials provide a simple definition of the measured quantity that can be traced to an international standard and can be used to assess the uncertainty associated with a measurement system. Previous work established a reference material and procedure for calibrating full-field optical systems suitable for measuring static, in-plane strain distributions. Efforts are now underway to extend this work to the calibration of systems capable of measuring three-dimensional deformation fields induced by dynamic loading. The important attributes for a dynamic reference material have been identified in a systematic and rational fashion, which have been subsequently translated into a generic design specification. Initial prototypes of candidate designs have been produced and evaluated using experimental modal analysis and digital speckle interferometry, and the results have been compared with finite element analyses. Based on the outcome of this initial evaluation, further refinements in design and manufacturing are proposed.

Car Disc Brake Squeal: Theoretical and Experimental Study

This paper presents a numerical method for the calculation of the unstable frequencies of a car disc brake and the analysis procedure. The stationary components of the disc brake are modelled using finite elements and the disc as a thin plate. This a pproach facilitates the modelling of the disc brake squeal as a moving load problem. Some uncertain syste m parameters of the stationary components and the disc are tuned to fit experimental res ults. A linear, complex-valued, asymmetric eigenvalue formulation is derived for disc brake squeal. Predicted unstable frequencies are compared with experimentally established squeal frequencies of a realis tic car disc brake.