Effect of presence of SiC and operating frequency on the damping behaviour of pure magnesium N. Srikanth, D. Saravanaranganathan and M. Gupta The objective of the present work was to investigate the effect of presence of SiC reinforcement and the vibration frequency on the overall damping characteristics of pure magnesium. The testing method uses a combination of the modified free – free beam method coupled with a circle fit approach. The effect of frequency on the damping property was studied by adding end masses to the specimen so as to alter the resonant frequency of the suspended beam. In the present study, the results are compared against a monolithic magnesium sample. The results revealed a higher damping capability of the composite specimen at all tested frequencies when compared to monolithic magnesium. Both monolithic and reinforced magnesium showed a progressive decrease in damping with an increase in frequency. An attempt is made to rationalise the damping behaviour of the composite in terms of the presence of a process induced plastic zone at the matrix/particulate interface and the operating frequency. MST/6052 Keywords: Damping, Internal friction, Loss factor, Plasticity, Anelasticity The authors are in the Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576. The corresponding author is Professor M. Gupta (mpegm@nus.edu.sg). Manuscript received 25 November 2003; accepted 4 June 2004. # 2004 Institute of Materials, Minerals and Mining. Published by Maney on behalf of the Institute. Introduction Knowledge of mechanical properties of composites and their constituents is fundamental to analysis and design of particulate reinforced metal matrix composites (PRMMCs). Among the various mechanical properties of a material, damping capacity and dynamic modulus remains of prime importance for a design engineer in the field of semicon- ductor equipment design, aerospace, sports and military applications. 1,2 Testing is usually time consuming and costly and composite specimens with specific configurations must be made prior to testing. Simple, fast and reliable testing methods are therefore needed for industrial applications. In a previous study, the authors used a free – free type suspended beam technique combined with a frequency domain based circle fit approach. 3–5 A literature review revealed that no such method has been extended so far to determine the effect of frequency on the damping behaviour of metallic composite material. Accordingly, in the present study, the feasibility of such an approach is investigated to understand the effect of frequency on the damping loss factor by altering the specimen’s resonant frequency using end masses. In pure metallic material, various factors contribute to the total damping capacity, for example, crystalline defects, dislocation motion, grain boundary sliding, elastothermo- dynamic and magnetoelastic effects. 1 Among the various lightweight materials, magnesium remains a popular choice because of its high damping capacity. Studies by various researchers showed that addition of hard reinforcing particles such as SiC in a ductile metallic matrix (e.g. Mg or Al), helps to improve the overall damping capacity and stiffness. 1–5 Such an improvement helps to reduce the vibration amplitude of mechanical systems significantly. This helps in avoiding fatigue damage, structural failure under resonance and excessive noise radiation from the structure. In order to improve the service reliability of components under such conditions, studies should include the effect of operation parameters, such as frequency, temperature and strain amplitude, on the materials characteristics. Studies by other researchers have also shown that the damping characteristics of a metallic material depend on the operating vibration frequency. 6 Therefore, in the present study an attempt is made to understand the effect of frequency on the damping characteristics of a SiC reinforced Mg composite using the end mass, attached, suspended beam method, followed by damping capacity determination using a circle fit technique. The results are further compared against a monolithic Mg specimen. Theoretical background FREE – FREE BEAM TYPE METHOD The flexural vibration of a long slender specimen in the transverse direction can be mathematically described by the Bernoulli – Euler equation as follows 7 EI cA L 4 y Lx 4 z L 2 y Lt 2 ~0 : : : : : : : : : : : : (1) where, y is the transverse displacement, t is time, x is the position along the beam, c is the density, A is the cross sectional area of the beam, E is the Young’s modulus, L is the length of the beam and I is the second moment of the cross sectional area. For practical purposes, the round specimen can be assumed to be slender if the length/ diameter ratio is greater than 25. Equation (1) is a fourth order differential equation, which can be solved to arrive at the natural frequency of vibration of the beam for various end support conditions. 7 For example, under the free – free support condition with end mass at each end, the beam is suspended at the vibration nodal points by nylon strings, and the boundary conditions at the two ends can be expressed as follows 8 at x~0[ m e L 2 y(0,t) Lt 2 ~{EI L 3 y Lx 3 and L 2 y(0,t) Lx 2 ~0 at x~L[ m e L 2 y(L,t) Lt 2 ~EI L 3 y(L,t) Lx 3 and L 2 y(L,t) Lx 2 ~0 0 @ 1 A (2) DOI 10.1179/026708304225022269 Materials Science and Technology November 2004 Vol. 20 1389