Modeling foam damping materials in automotive structures

Foam damping materials judiciously placed in automotive structures efficiently reduce the vibration amplitudes of large, relatively flat exterior body panels such as the hood, roof, deck lid (trunk) and door skin. These polymer foams (typically epoxy or vinyl) have mechanical properties that depend on the foam homogeneity, degree of expansion, temperature and frequency of excitation. Standard methods for determining true bulk mechanical properties, such as Young's modulus, shear modulus and damping terms, are discussed along with methods for determining engineering estimates of the properties as used in automotive applications. Characterizing these foam damping materials in a component or full body finite element structural model as discrete springs and dashpots provides an accurate and economical means to include these features. Example analyses of the free vibrations and forced response of a hood are presented accompanied by test data that demonstrate the accuracy of the structural model. A parametric study investigates the effect of foam material stiffness and damping properties on hood vibration amplitudes under dynamic air loading. A methodology is discussed to reduce the hood vibration level under cross-wind conditions to an acceptable level with the use of foam materials.