Paper stiffness monitoring using laser ultrasonics

The use of a laser ultrasonics method to induce and detect Lamb waves in paper was demonstrated. A pulsed Nd:YAG laser was used for Lamb wave excitation. Detection of out-of-plane and in-plane surface motions was accomplished using a CW Ar:ion laser and appropriate heterodyne interferometric systems. Two propagation modes, a fast mode and a slow mode, were observed with substantially different velocities, amplitudes, and frequency contents. They were found to correspond to the fundamental dilatational (S₀) and bending (A₀) modes for Lamb waves, respectively. Velocity measurements gathered along paper machine direction and cross-machine direction for the S₀ mode were shown to be in very good agreement with velocities obtained using a pair of contact bimorph transducers. Results are presented for various commercial paper grades ranging from fine papers to paperboards. The rationale for contactless real-time stiffness monitoring of a moving paper web is discussed