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Acoustic emission applied to mechanically loaded paper

The paper industry is one of the most rapidly changing industries today. Driven by the pressures and demands of Information Technology, its products must meet ever greater demands placed upon it by the customer, involving new paper types and qualities.
Paper is a complicated material, or actually a complicated structure in that it involves a number of different deformation processes, for example plasticity, viscoplasticity, viscoelasticity and damage. Until now, much of the research has mainly been focused on plasticity and viscoelasticity and not much has been done in investigating the influence of damage.
In order to account for damage, one must know what particular damage mechanisms are active during the mechanical loading of paper. At least two different mechanisms can be identified regarding paper, i.e. fibre - fibre bond failure and fibre rupture. Since each of these mechanisms will have an entirely different effect on the mechanical properties it is obvious that it is crucial to be able to identify what are the active damage processes.
It has been known for some time that paper, while being loaded mechanically, will emit stress waves that can be detected using appropriate sensors. In fact, this is the essence of the Acoustic Emission (AE) testing method.

The overall objective with this project is to investigate, by use of Acoustic
Emission (AE) monitoring, the micro mechanics behind the behaviour of paper under
mechanical loading with a view to enabling the development of new and value added
paper production.
There are two objectives for this project. The first is to try to identify from the AE signals what is specific for a certain damage mechanism. This will be done by comparing AE signals from specimens in situations where one can be certain that a specific damage mechanism is active.
The second aim is to study the applicability of continuum damage mechanics to paper.

Progress to Date
At a project meeting in Greenwich, March 2004, it was argued that the signals detected on the surface could have been altered on their way from the paper through the grips and to the sensor. It was therefore decided to do the testing in a different way i.e. to use tensile specimens with side grooves and with a certain amount of coloured fibres.
To promote fibre fracture, starch was added to the mechanical pulp to increase the bond strength and the above procedure was repeated. It was now found that a certain amount of fibre fractures occurred and based on some very simple assumptions, the number of acoustic events caused by fibre fractures could be estimated. With this experimental procedure, we strongly believe that we have solved the problems encountered in connection with the zero - span and the tension tests in the thickness direction.
As for the continuum damage mechanics part of the project, we believe that we already have proven the applicability of continuum damage mechanics to paper. For example, this model has been applied to explain a phenomena observed when studying how cracks propagate in embossed (with small indentations in a regular pattern) tissue paper. It is observed that these embossings do have a screening effect on the mechanical fields in the vicinity of a growing macro crack. Introducing an initial damage in the embossed regions and then performing the calculations with a suitable damage model can explain this phenomenon. Using damage mechanics in this context is, to our knowledge a completely new and physically sound approach.
The direct coupling between damage and number of acoustic events in isotropic, mechanical pulp hand sheets has been shown. This is also new as is the determination of the gradient sensitivity parameter, which is shown to be of the same order of magnitude as some average fibre length.
A unique ingredient in the project is the attempts to study in detail the paper structure using absorption x-ray micro-tomography. This study will hopefully contribute much to the understanding of the processes giving rise to acoustic emission. This study is undertaken since Partner 3 has got access to this equipment in ESRF, Grenoble, completely free of charge.


Scientist responsible for the project

Professor PER GRADIN
Holmgatan 10
851 70 Sundsvall
Sweden - SE

Phone: +46 60148946
Fax: +46 60148820


Project ID QLRT-2001-00772
Area 5.3.2
Start date 01 January 2003
Duration (months) 36
Total cost 1 275 243 €
Total EC contribution   893 463 €
Status Ongoing
Web address of the project

The partners

  • UNIVERSITY OF GREENWICH, School of Computing and Mathematical Sciences, United Kingdom (The) - GB

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