PARAMETERISATION AND IDENTIFICATION OF A RUBBER SEAL H. Ahmadian, J.E. Mottershead and M.I. Friswell* Department of Mechanical Engineering The University of Liverpool, UK * Department of Mechanical Engineering The University of Wales Swansea, UK 1. INTRODUCTION Parameterisation is a key issue in finite element model updating [1,2]. It is important that the chosen parameters should be able to clarify the ambiguity of the model, and in that case it is necessary for the model output to be sensitive to the parameters. Mottershead et al. [3] used geometric parameters, such as beam offsets, for the updating of mechanical joints and boundary conditions. Ahmadian ef al. [4,5] demonstrated the effectiveness of parameterising the modes at the element level. Ahmadian et al. (61 used both geometric parameters and element-modal parameters (i.e. the so-called generic element method) to update mechanical joints. Occasionally part of the structure is so ill-defined that no finite element model can be constructed with confidence. An example is the rubber seal which provides the connection between a vehicle window and the car-body structure. The seal has a complicated cross-sectional shape into which the window glass and the steel sheet are pressed to form the joint. Furthermore it is important to model the seal accurately because vibration of the window has a strong influence on the acoustics of the passenger compartment. IO such cases there seems to be no reasonable alternative to direct parameter estimation. One approach to the modslling of the rubber seal would be to use springs, spaced at regular intervals around the edge of the window glass. This lnetbod has been tried previously for a glued window joint [7], but leads to an unduly tine mesh throughout the region of the window. In this paper we show how a generic element approach can provide physical insight. Finally the physical parameters of an equivalent distributed stiffness are identified from measured natural frequencies to provide a full model of the rubber seal and window together. 2. EQUIVALENT MODEL FOR THE RUBBER SEAL The rubber-seal joint between the body structure and the window glass is shown in cross-section in Figure 1. The glass is held in place by the rubber seal which forms a press-fit with the vehicle body panel and the glass sheet. It is clear that a large number of finite elements would be necessary to accurately represent the geometry of the rubber seal. Therefore an equivalent model is sought which will represent the dynamics of the rubber seal with a much coarser mesh of elements. The glass and the body panels are regularly modellrd with plate elements having 3 degrees-of-freedom at each node. Thus, the equivalent rubber seal (ERS) element should have the same degrees-of-freedom at each node. In its most general form the element is chosen to have 4.nodes and 12.degrees-of-freedom as shown in Figure 2. We now consider various physical constraints which will allow us to fin the terns in the mass and stiffness matrices of the ERS element. i) Symmetry: rotation of the element by 180” about the x, y or z axes does not change the mass of stiffness matrices. 142