A technique to measure strain distributions in single wood pulp fibers

Abstract

Environmental scanning electron microscopy (ESEM) and digital image correlation (DIQ 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 lalpwood fibcrr (Yjcea ,narrona (MIIIJ B S P ) o f a near-7ero mlcrofibnl angle d~splayed acharactcnsr~callyl~ncar load elonkal~on form ESFM was ablc lo provlde real-tme, h~gh magnlficatlon images of straining fiben, crack growth, and complex single fiber failure mechanisms.Digital images of single fiben 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 offactors including the action ofmicrocompressive defectsand straining of amorphous cell-wall polymeric components.Strain concentrations also corresponded well to locations of obvious strain risen 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 micromechanical properties.