Elastic Propagation
Mechanical energy travels along the outer boundary of a solid medium through surface rayleigh waves. These disturbances involve particle motion following an elliptical path as the wave advances. Density and elastic modulus determine the velocity of these oscillations.
Attenuation occurs rapidly with depth because the energy stays concentrated near the free boundary.
Structural Integrity
Ultrasonic testing equipment sends acoustic pulses into paper board stacks to identify voids or density variations. Sensors detect surface rayleigh waves after the signal hits a discontinuity in the substrate. Calibration against reference samples allows operators to convert time of flight data into absolute stiffness values.
Accurate measurements require tight coupling between the transducer and the material surface to prevent signal loss. Thin film coatings on packaging substrates change the velocity profile of these signals depending on the thickness of the application. High frequency inputs permit finer resolution of surface defects in laminated products.
Dynamic Impedance
Elastic moduli calculations rely on the specific relationship between the velocity of these waves and the shear modulus of the paper matrix. Increased grammage shifts the dispersion pattern of the signal across the sheet. Machines monitoring the conversion process track deviations to ensure consistent tensile properties across the production web.
Changes in moisture content modify the velocity which shifts the resonance frequency detected by the transducer. Every measurement relies on the acoustic properties of the cellulose network to infer the structural state of the stock.
