Abstract

Non-metallic materials have frequency dependent dynamic properties which must be characterised for use in computer simulations. The characterisation of such properties is important as most modern structures utilise these materials. Hence, a novel test method has been developed, which combines vibration testing with finite element analysis, to yield dynamic modulus of elasticity and damping. Material properties can be measured in the frequency range 2Hz - 2000Hz. The test method involves a can¬tilever beam. Two samples of the test material sandwich the root of the beam and are held in place between inertial masses. Experimental modal analysis techniques, where an instru¬mented hammer vibrates the beam, are used to exercise the material. Acceleration and force time histories are measured and processed to obtain a spectrum, from which the natural frequencies of the test setup are derived. The modulus of elasticity of the material is found by constructing a finite element model of the test setup and tuning the simulated response with that of the experiment. This is done for each of the bending modes, resulting in the measurement of elastic modulus as a function of frequency. Damping properties are extracted by applying data fitting techniques to the time histories and spectrum. These are converted into material properties by simulating and accounting for the energy balance between the samples and test setup. Doing so also eliminates the need to estimate damping ratio at each frequency, as it can be calculated using the strain energies in undamped modes. It is important that the sample is gripped in a manner that exercises it effectively and is simple to simulate. Eliminating slipping, and thus difficult to model friction, is a key concern and has been investigated in depth. The best solution found is an axisymmetric bolting arrangement which holds the samples in place. Additionally, the test setup utilises suspension, together with inertial masses and an orthogonal layout to isolate against external vibrations. Polymers and rubbers, which exhibit complex frequency dependent behaviour, have been validly characterised using this method. The damping material Sorbothane has also been characterised and produced results that aligned within manufacturer specifications. This method proves to be a reliable procedure for dynamic material property testing.