  Atomic force microscopy-based nanoindentation (AFM-NI) enables
characterization of the basic mechanical properties of wood pulp fibers in
conditions representative of the state inside a paper sheet. Determination of
the mechanical properties under different loads is critical for the success of
increasingly advanced computational models to understand, predict and improve
the behavior of paper and paperboard. Here, AFM-NI was used to indent fibers
transverse to and along the longitudinal axis of the fiber. Indentation moduli
and hardness were obtained for relative humidity from 25 % to 75 %. The
hardness and the indentation modulus exhibit moisture dependency, decreasing by
75 % and 50 %, respectively, over the range tested. The determined indentation
moduli were combined with previous work to estimate the longitudinal and
transverse elastic modulus of the fiber wall. Due to the relatively low
indentation moduli, the elastic constants are also low compared to values
obtained via single fiber testing.
