Acoustic Measurement Of Dynamic Characteristics Of Thin Sheets

The same as many other materials, thin sheets (Paper Sheet, Low Density Polyethy-lene and the laminate from the above two together with Aluminum) exhibit a non-equilibrium dynamics under outer dynamic strain or extension. Because they are vis-coelastic, and they behave under a nonlinear, non-Hookean characteristic. Applying a constant strain or extension on an ideal thin film, a stress relaxation phenomenon happens in time advance.In terms of characterization of the material stress relaxation, the most direct ap-proach is to measure the shifting of the stress on material. In practical, it suffers many difficulties. The article attempts to apply a new diagnostic method to assess the prop-erty. That is to measure the resonance of the membrane by acoustic diagnostic method when applying a dynamic strain on it, which is one of the easiest ways and can be used in most of the cases. It has an obvious advantage to be used in non-destructive measurement in engineering fields. In this topic the dynamic characteristic, which is the slow shifting of resonance value, will be measured and its trend will be observed by test result and the figures.In the method a low frequency sound (under 1 kHz) is provided, which is used for its good propagation through materials. And it is because that the sound waves can excite vibrations remotely as well. Based on the remote excitation, the characteristic of specimen vibrated by sound waves are studied. The basic principle under the mea-surement is that modal parameters are according to the properties of specimen. This work is based on the non-linear relationship between resonance and strain or tension applied on thin sheet, and transfer it into linear relationship, then study the dynamic characteristic of stress relaxation. It estimates the relaxation trend of thin film from an experimental measurement.In addition, a dynamic characteristic of membrane suffering to periods of dynamic outer tension is studied. And a phenomenon of recover during constant extension after periodic dynamic strain is included in the work.