The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Aspects of Fracture Processes in Paper

Lauri Salminen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics, Helsinki University of Technology, for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 5th of June, 2003, at 12 o'clock noon.

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Dissertation is also available in print (ISBN 951-22-6555-9)

Abstract

The strength properties and fracture processes are studied in paper. This thesis deals with the fundamental structure and physical phenomenon of fracture. The methods employed are Monte Carlo simulations with a finite element model and experiments in fractography and acoustic emission.

It is still unclear how the mechanical properties of paper, particularly strength depend on the disordered geometry of the fiber network. The shrinkage during manufacturing process induces internal stresses, which are crucial to macroscopic properties. The limiting strength in paper depends on crack pinning effects and obeys extremal statistics. The local stress variations introduce crack pinning and affect the fracture line topography.

In the thesis geometrical effects of fiber network shrinkage are simulated and observed to follow a simple analytic expression. The shrinkage of fiber segments agrees qualitatively with microscopic measurements in literature. Extensive tensile strength distributions are obtained and compared with theoretical strength models. The strength of paper is found to be close to the Weibull and Duxbury distributions. Crack localization in tensile mode I loading is studied with initially notched strips. The resulting pinning probability agree with simple simulations and demonstrates that paper tolerates short order of fiber length notches. The fractal nature of paper crack line is analyzed in large samples. The geometry in fast crack propagation is found to be self-affine with a roughness exponent close to 0.6. The value is not in agreement with any fracture models. In addition systematic deviations from pure power law dependence is observed in the length-scale 5-20 mm.

Acoustic emission spectroscopy is employed to study paper fracture in tensile and peel tests. By acoustic emission the energy released in micro fracturing is measured. The energy statistics are observed to obey power law analogously to Gutenberg-Richter's law for eartchquakes. In the tensile test the exponent characterizing the energy distribution is 1.2 and in the peel test 2.0. In the tensile tests the inter-arrival time between events obey a power law (Omori's law) with an exponent close to 1.0. In the peel tests deviations from Omori's power law are found. These observations suggest that the two often simultaneously witnessed power laws do no have a common origin. The acoustic emission results give new insight to fracture processes in the presence of disorder.

This thesis consists of an overview and of the following 6 publications:

  1. Korteoja M., Salminen L. I., Niskanen K. J. and Alava M. J., 1998. Strength Distribution in Paper. Material Science and Engineering A240, pages 173-180.
  2. Salminen L. I., Alava M. J., Heyden S., Gustafsson P.-J. and Niskanen K. J., 2002. Simulation of Network Shrinkage. Nordic Pulp and Paper Research Journal 17, No. 2, pages 105-110.
  3. Rosti J., Salminen L. I., Seppälä E. T., Alava M. J. and Niskanen K. J., 2001. Pinning of Cracks in Two-Dimensional Disordered Media. European Physical Journal B19, pages 259-263.
  4. Salminen L. I., Alava M. J. and Niskanen K. J., 2003. Analysis of long crack lines in paper webs. European Physical Journal B32, pages 369-374.
  5. Salminen L. I., Tolvanen A. I. and Alava M. J., 2002. Acoustic emission from paper fracture. Physical Review Letters 89, 185503.
  6. Salminen L. I., Pulakka J. M., Alava M. J. and Niskanen K. J., Crackling noise in paper peeling. Submitted for publication.

Keywords: strength, paper, fracture processes, fracturing, mechanical properties, fiber network, Monte Carlo simulations, acoustic emission spectroscopy

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© 2003 Helsinki University of Technology

Last update 2011-05-26