A Technique to Measure Strain Distributions in Single Wood Pulp Fibers


  • Laurence Mott
  • Stephen M. Shaler
  • Leslie H. Groom


Fibers, micromechanics, strain, digital image correlation, tensile testing, environmental scanning electron microscopy


Environmental scanning electron microscopy (ESEM) and digital image correlation (DIC) were used to measure microstrain distributions on the surface of wood pulp fibers. A loading stage incorporating a fiber gripping system was designed and built by the authors. Fitted to the tensile substage of an ESEM or a Polymer Laboratories MINIMAT tester, it provided a reliable fiber straining mechanism. Black spruce latewood fibers (Picea mariana (Mill) B.S.P.) of a near-zero microfibril angle displayed a characteristically linear load elongation form. ESEM was able to provide real-time, high magnification images of straining fibers, crack growth, and complex single fiber failure mechanisms. Digital images of single fibers were also captured and used for subsequent DIC-based strain analysis. Surface displacement and strain maps revealed nonuniform strain distributions in seemingly defect-free fiber regions. Applied tensile displacements resulted in a strain band phenomenon. Peak strain (concentration) values within the bands ranged from 0.9% to 8.8%. It is hypothesized that this common pattern is due to a combination of factors including the action of microcompressive defects and straining of amorphous cell-wall polymeric components. Strain concentrations also corresponded well to locations of obvious strain risers such as visible cell-wall defects. Results suggest that the ESEM-based DIC system is a useful and accurate method to assess and, for the first time, measure fiber micro-mechanical properties.


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