Air Coupled Ultrasonic Guided Wave Measurement for Board Crease Integrity
Air-coupled ultrasonic guided waves quantify paperboard crease ply delamination and micro-cracking non-destructively, preventing box compression failure.
Zero Group Velocity
Acoustic Phenomenon
Propagation of a wave at a frequency where the energy remains stationary despite local oscillations creates a resonance state. Achieving zero group velocity allows for the localized measurement of thickness and elasticity. This condition occurs in plates and shells when the phase velocity and frequency reach a specific point.
It enables testing in thin substrates without interference from edge reflections.
Theoretical Application
Laser pulse excitation can probe the internal structure of paper without physical contact. Observations of zero group velocity provide a highly localized reading of the sheet properties. This method avoids the averaging effect seen in traditional ultrasonic testing.
It is particularly useful for measuring the through thickness stiffness of lightweight tissues and thin films.
Mill Integration
Deployment of this technology on the paper machine enables the detection of internal flaws at an early stage. Calculating zero group velocity is unaffected by the high speed movement of the web or the vibration of the rollers. Engineers use the resulting data to optimize the refining of the pulp and the pressure in the wet press section.
This level of detail helps prevent the production of reels with weak internal bonds that might fail during subsequent coating or printing steps.