Noncontact determination of the bending stiffness of paper using laser ultrasonics and wavelet analysis: effects of moisture and temperature

Laser ultrasonics is used to generate and detect the A₀ Lamb mode in copy paper. The frequency dependence of the group velocity of the A₀ mode is found from the wavelet transform of the measured signal. A best fit of the dispersion curve against predictions based on orthotropic plate theory yields an estimate of the bending stiffness of the sample. The sample is enclosed in an environmental chamber so that the effects of temperature and humidity can be controlled. Results indicate that the bending stiffness decreases with increasing temperature at a rate of 4×10^-6 N.m/°C, and that it decreases with increasing percent moisture content (PMC) at a rate of 2×10^-5 (N/m)/PMC. Three different methods of signal analysis are compared: the wavelet transform, the short-time Fourier transform, and the phase-unwrapping method. Calibrations are made using a silicon wafer and sources of error are discussed