Acoustical Measurements of Material Nonlinearity and Nonequilibrium Recovery

Sammanfattning: A damaged material or a material with non perfect atomic geometry, dislocations or cracks, exhibits two types of characteristic responses to acoustic excitations. First is the fast nonlinear dynamics response that is present as long as the material is excited. As soon as the excitation stops the response disappear. Second is the Slow Dynamics, which detects alterations of the material properties. The properties are affected by, for example, a mechanical pulse, changes in temperature, pressure or humidity. When the cause of alteration stops the material is recovering towards its equilibrium state. This recovering can exist over a long period of time, much longer than the vibration from a mechanical pulse. The techniques used here, both the fast and Slow Dynamics, have been used for NonDestructive Testing to detect damage in objects. All of them are suitable for this purpose, but for different material and geometry different techniques can be advantageous. They offer the possibility to use relatively low frequencies which is advantageous because attenuation and diffraction effects are smaller for low frequencies. Therefore large and multi-layered complete objects can be investigated. Sometimes the position of the damage is required, but it is in general difficult to limit the geometrical extent of low-frequency acoustic waves. A technique is presented that constrains the wave field to a localized trapped mode so that damage can be located. The existence of trapped modes is shown using an open resonator concept and the localization is shown to be successful. The problem with intermittent and changing amplitudes, even when very small, is that the material is really never at equilibrium, or even at steady state. The measurement signal influences the outcome. The material is affected by its strain history and its constantly changing state, the fast and Slow Dynamics are hard to separate. A measurement technique keeping the internal strain constant has been used to minimize the influence of Slow Dynamics allowing observations of only nonlinearity. The influence of temperature is also studied with this technique.