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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.
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