Page 383 - Biosystems Engineering
P. 383
360 Cha pte r T w e l v e
Density (kg/m 3 ) 1000.00
800.00
600.00
400.00
200.00
0.00
24.000 26.400 28.800 31.200 33.600 36.000 38.400 40.800 43.200 45.600 48.000
Distance (mm)
FIGURE 12.4 Density profi le of southern pine. (Courtesy of Chuck Dawson, Quintek
Measurement Systems, Inc.)
Nanoindentation
Nanoindentation testing is a technique that determines the mechani-
cal properties of a material in the micron or submicron scale. The test
involves penetrating a sample material using an indenter, whereas
the penetration depth and load are recorded so that the stiffness and
hardness of the indented location can be subsequently calculated.
The indenter head can have a radius of 100 nm (in the case of the
Berkovich indenter), and penetration can be up to 1 or 2 μm deep,
with the resulting indent having a linear dimension on the order of
micrometers. This dimension is in the same order of magnitude of the
thickness of the wood cell wall. The wood cell walls were reported to
be 5 to 6 μm and 9 to 13 μm thick, respectively, for earlywood and
latewood of loblolly pine (Barefoot et al. 1965). Therefore, the local
mechanical properties of wood cell walls can be probed using nanoin-
dentation tests. More specifically, the test detects the mechanical
properties of the cell wall S2 layer, which constitutes about 80 percent
of the total cell wall thickness and is the major contributor to the
mechanical properties of wood cell walls. Figure 12.5 shows two
indent marks on the cell wall of the Keranji hardwood (Dialium spp.).
The micron spatial (lateral) resolution in nanoindentation tests
renders the foregoing technique very useful in the investigation of
the wood cell-wall level as a result of growth processes or utilization
operations. To date, a few studies have used nanoindentation to
investigate the effects of seasonal growth response (earlywood ver-
sus latewood) (Wimmer et al. 1997), cell wall lignification (Gindl et al.
2002), growth ring (Tze et al. 2007), and wood species (Wu et al. 2009)
on the mechanical properties of single cell wall. An attractive feature
demonstrated in these studies is that the measurements were made
without requiring chemical or mechanical modifications to isolate
individual wood fibers as required in single-fiber tensile tests. These
chemical and mechanical modifications change the mechanical prop-
erties of the wood fibers in poorly defined ways.