Low Frequency Acoustic Excitation and Laser Sensing of Vibration as a Tool for Remote Characterization of Thin Sheets

Abstract: There is a need to monitor the existence and effects of damage in structural materials. Bulk components provide a much publicized example, but the need exists in a variety of other structures, such as layered materials used in food packaging industries. While several techniques and models have been proposed for characterization and condition monitoring of bulk materials, less attention has been devoted to thin films having no bending rigidity. This study is therefore devoted to the development of a new method for remote acoustic non-destructive testing and characterization of thin films used in food packaging materials or similar structures. A method for assessing the strength in the presence of crack of thin layers used in food packaging is first presented using a modified Strip Yield Model. Resonance frequency measurement is then introduced and it is shown, at low frequency range, less than 2kHz, that a change in the physical properties such as a reduction in stiffness causes detectable changes in the modal properties, specifically in the resonance frequency. This observation leads to the implementation of a method for damage severity assessment on sheet materials, supported by a new theory illustrating the feasibility of the detection of inhomogeneity in form of added mass, as well as damage severity assessment, using a measurement of the resonance frequency shift. A relationship is then established between the resonance frequency and the material’s elastic property for single layers as well as for laminates, which yields a new modality for sheet materials remote characterization. Further, the method has allowed demonstrating that thin sheets having no bending stiffness exhibit a slow non-equilibrium dynamics when slightly loaded within their elastic region and monitored at constant strain. We found that the resonance frequency shifts downward in response to a conditioning strain and to the number of cycles. This is an indication of a long-time slow dynamics relaxation, similar to that observed on bulk materials of many types. Differences and similarities in the setup as well as features observed are pointed out in this work. This type of measurement is important for the fundamental understanding of material dynamics and for further development of theories on thin sheets dynamics. The result of this study is the foundation of a method, based on low-frequency acoustic excitations and laser detection, for non-contact nondestructive testing and characterization of sheet materials.