An automated instrument for the measurement of mechanical properties of thin materials

Flexural rigidity and shear rigidity of sub-millimeter thin webs can be determined by analysis of the zero order antisymmetric (A(0)) Lamb wave in the MHz range, even when the web thickness is unknown. Using Laser Ultrasonics to excite and detect Lamb waves in webs is advantageous because it is a non-contact technique suitable to both on-line and off-line measurements. The authors have developed an interferometer using an undoped Gallium Arsenide photorefractive crystal. They employed a two-wave mixing technique that is sensitive to displacement amplitudes in the nanometer range. The interferometer is part of a laboratory instrument in which both the generation and the detection laser beams are carried through fiber optics. The instrument uses several motorized translation stages to manipulate the position and orientation of the web sample. The instrument is computer-controlled and fully automated using LabVIEW for routine testing of foil samples. The results of measurements demonstrating the wide bandwidth of the instrument and the dispersion of the waves are presented. The probed materials were as different as paper, paperboard, non-woven and brass foils.