Kao D., Pericleous K., Graham D., Knight B.

In this paper, a coupled mechanical-acoustic system of equations is solved to determine the relationship between emitted sound and damage mechanisms in paper under controlled stress conditions. The simple classical expression describing the frequency of a plucked string to its material properties is used to generate a numerical representation of the microscopic structure of the paper, and the resulting numerical model is then used to simulate the vibration of a range of simple fibre structures when undergoing two distinct types of damage mechanisms: (a) fibre failure, (b) bond failure. The numerical results are analysed to determine whether there is any detectable systematic difference between the resulting acoustic emissions of the two damage processes. Fourier techniques and are then used to compare the computed results against experimental measurements. Distinct frequency components identifying each type of damage are shown to exist, and in this respect theory and experiments show good correspondence. Hence, it is shown, that although the mathematical model represents a grossly-simplified view of the complex structure of the paper, it nevertheless provides a good understanding of the underlying micro-mechanisms characterising its properties as a stress-resisting structure. Use of the model and accompanying software will enable operators to identify approaching failure conditions in the continuous production of paper from emitted sound signals and take preventative action.