Ultrasonic Velocity
Measurements of the interval required for an acoustic signal to travel through a known distance of paper or board reveal the elastic properties of the material. This time of flight calculation is the basis for non-destructive testing of mechanical strength. By sending a pulse from a transmitter to a receiver, a mill can determine the modulus of elasticity without breaking the sample.
The speed of the sound wave changes depending on the density and bonding of the fibers.
Stiffness Measurement
Propagation speeds are higher in the machine direction than in the cross direction due to the orientation of the wood fibers. Using time of flight data allows technicians to map the fiber alignment across the width of the paper machine. This information is vital for controlling the dimensional stability of the sheet.
If the sound travels too slowly, it may indicate weak internal bonding or low density.
Directional Analysis
Automated sensors on the production line provide real time feedback to the operators. Results from time of flight testing correlate closely with traditional destructive tests like tensile strength and ring crush. Because the test is instantaneous, it allows for much tighter control over the manufacturing process.
It works on the principle that stiffer materials transmit sound waves faster. This technology is common in the production of high performance linerboard and sack kraft paper.
