Investigation of acoustic and dynamic properties of weatherstrips used in vehicle doors

Abstract

Today, a rapid transformation is taking place in the automotive industry. Users' expectations of vehicles are changing and diversifying. Increasing expectations for acoustic performance and redefining sealing elements to meet these expectations are among these changes. Acoustic harmony inside the vehicle, provided with a balance between the drivetrain, road and wind noise is no longer valid. Preventing wind noise, which is no longer adequately masked, has become very important for vehicle acoustics studies. Another factor that causes wind noise to become dominant is that the average cruising speed of vehicles is increasing day by day. With the increase in mobility and new technologies, cruise speed is increased, which makes wind noise important to be reduced. As a result of new technological innovations, changes have occurred in the vehicle architecture, and different aerodynamic noise sources have been reduced with applications such as mirrorless vehicles. It has become more important to prevent the transmission of wind noise into the interior cabin. Door closure and glass run channel systems are two important paths for noise transmission into the cabin. Considering the changes that have taken place in the last ten years, door closure systems need to be improved in terms of their acoustic performance. Another significant trend is electrification in the automotive industry. Within the scope of both electrification and sustainability studies, the weights of vehicles and sub-systems such as doors, hoods and trunks are gradually decreasing. This situation caused the increase of tolerance for mounting the subsystems to the body, and the vibration amplitudes are increased. In order to investigate and improve the acoustic performance of the sealing systems, it is necessary to reveal both the polymer material properties of these structures and the shape properties with unique geometries, and the dynamic properties related to those need to be analysed. The materials used in sealing systems are polymers with high elasticity, having low elastic modulus and high yield strains. With these properties, they are generally expressed as elastomers. Elastomer materials show nonlinear stress-strain behaviour. In addition to these features, they are viscoelastic materials and may have very high damping values. Since these materials are mainly produced by the extrusion method in sealing systems, they have non-isotropic properties. Their structures mainly depend on polymer compounding recipes for each product and their mechanical properties vary significantly according to process conditions. All these properties specific to elastomers are critical features for investigation of acoustic performance studies. This thesis investigated acoustic properties of sealing systems and the specific properties of polymer materials used in these systems. Analytical methods, numerical calculation methods and experimental methods are utilized to carry out these studies. Acoustic performance properties determined depending on both the material structure and the geometric structure have been investigated in detail. The first phase studies aim to develop a numerical modelling method that can be used in both academic and industrial fields for sealing system investigations. For this purpose, two different finite element models of the sealing system were developed. In the first model developed, the deformation after door closing was obtained using the materials' non-linear hyperelastic properties. Then, a dynamic material model was created to calculate modal properties and acoustic performance using deformed geometry. The noise source, transmission path, air cavity and elastomer material properties are included in this model by considering the theoretical principles. An acoustic measurement setup has been developed to be used in the validation studies of this model. With this setup, the sound transmission loss values of a sealing profile were measured by representing different door gaps on the closure of vehicles. After the validation of the finite element model, parametric studies were carried out and the factors affecting the acoustic performance were examined. The modal behaviour of the partitions forming the geometry of the sealing elements was investigated. The relationship between dynamic vibration behaviour and acoustic properties was revealed. While carrying out these studies, the original structure of elastomer materials was considered, and hyperelastic and viscoelastic material measurements were made. An intermediate material validation step is described using impedance tube measurements for viscoelastic properties, increasing the study's novelty. In the second stage of studies, the viscoelastic properties of elastomer materials and the effect of these properties on sound transmission were investigated. The properties of porous and non-porous materials used in sealing elements were measured by the dynamic mechanical analysis method and results are explained. A characterization method has been developed by utilizing the relationship between sound transmission properties and their dynamic behaviour. In this characterization method, the elasticity and damping properties of the material, which vary according to the frequency spectrum are obtained using impedance tube measurements. The theoretical models for one-dimensional and two-dimensional sound transmission are explained, then the vibration equations of circular plates are presented. The surface integral was calculated with the analytical expressions obtained in closed form, the mechanical impedance was obtained depending on the frequency by using the plane wave approach, which is valid for the impedance tube, and the properties of the materials were determined by the reverse calculation method. As a result of the studies carried out in the second stage, it was determined that the influential factor in sound transmission in the sealing system is the resonance characteristics of the material walls. It has been determined that the dynamic behaviour in non-porous materials complies with the results of the circular plate theory. On the other hand, it was observed that the higher modes in porous materials are located between the modes of plate and membrane theory. These observations are included in the method by expressing them in terms of empirical coefficients of the calculations. A step-by-step calculation method is described for practical use as a test method for elastomers. The material test results were compared and the developed method was validated accordingly. This method, which is valid for the audible frequency range, is expected to be an alternative to the expensive and complicated material testing methods in acoustic studies. In the studies carried out in the third phase, the acoustic optimization of the sealing systems was performed. Both material properties and shape properties were examined, and the statistical design of experiment method was utilized. In this study, outputs obtained by numerical calculations are used as response data. Firstly, experimental validation of the numerical calculation methods is shown. Afterward, the effects of material properties on acoustic performance were investigated with both the full factorial design and the Taguchi experimental design method. Within the scope of this study, all dimensional variables that define the shape properties of a sealing element are expressed by parametric entities, then the effect of each parameter is analysed by the design of experiments method. In the studies, a robust optimization method is also considered by including both controllable variables (material and geometry) and uncontrollable variables (noise spectrum and door gap tolerance). As a result of the optimization, the parametric values in which respond best parameters are determined.