Resonance apparatus for damping measurements
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INTRODUCTION
THE resonance apparatus has been used extensively to characterize polymeric materials. Numerous articles tl-51 have been published describing the apparatus and results for polymers. However, very little work has been done on using the resonance apparatus to characterize metallic materials. The purpose of this article is to illustrate the use of the resonance apparatus in characterizing a high-loss polymeric material and a low-loss metallic material. The polymeric material was an epoxy polymer, developed here. Commercially available aluminum rods were used. Also, the accuracy and precision of the instrument were determined. The accuracy was determined by comparing modulus and loss factor results of aluminum with literature values. The precision was estimated by comparing the results of four different lengths of aluminum rods. In general, instruments for measuring dynamic mechanical properties are subject to various measurement errors. Some of these errors are related to the test fixture and others are related to sample dimensions. A brief description will be given of these errors for a popular commercial instrument and for the resonance apparatus. Also, some of the advantages that are unique to the resonance apparatus will be presented. For some commercial instruments, the test sample is clamped into a cantilever fixture. Errors in modulus were found to be 15 pct in measuring ceramic aluminum composites t61 and as large as 200 pct in soft polymers. The errors are attributed to the clamping fixture. When the sample stiffness is comparable to the stiffness of the clamping fixture, the modulus obtained is a combination modulus of the test sample and fixture. To minimize this problem, the test sample must be made thinner in order to be more compliant. Another error is due to either clamping too lightly or too hard on the test sample in the clamping fixture. If the clamping pressure is too low, low modulus values are obtained. This problem is GILBERT F. LEE, Research Materials Engineer, is with Naval Surface Warfare Center, Silver Spring, MD 20903-5640. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee. METALLURGICAL AND MATERIALSTRANSACTIONS A
easily solved for metallic materials by increasing clamping pressure. However, if the clamping pressure is too high, the clamps will distort the ends of polymeric samples and lead to erroneous results. Thus, a clamping fixture is acceptable for metallic samples but not for soft polymers. In contrast to those instruments that clamp the sample, the resonance apparatus is not subject to errors due to clamping. Test samples are adhesively bonded to mounting blocks. The adhesive layer is very thin to minimize its effect on the measurements. There are two errors that are worth noting with the resonance apparatus. One error is due to an accelerometer cable attached to the sample, which can lead to high loss fa
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