Harvesting System Design and Performance - Engineering and Science of Biomass Feedstock Production and Provision - page 84

Table 5.2 Biomechanical properties of bioenergy crops
Mechanical properties Switchgrass Miscanthus Alfalfa Maize Sunflower Rice straw

UFTS (MPa) 97.8 (Alamo) 9-36 55-69 2.8-8.7 10-13.3

89.7 (Kanlow)

UFSS (MPa) 20.5(Alamo) 0.4-18

17.9 (Kanlow)

MOE (GPa) Internodes 4.5 6.8-17.2

Nodes 5.8

FR (Pa) Internodes 1.0-2.6

Nodes 2.9-3.6

SCE(kNm −1 ) KBA30° 6.3
KBA45° 10.1
Reference [ 12 , 13 ] [ 11 ] [ 14 ]
UFTS, ultimate failure tensile stress; UFSS, ultimate failure shear stress; MOE, modulus of elasticity;
FR,lexuralrigidity;SCE,speciiccuttingenergy;KBA,knifebevelangle

of the optimum way depends on the biomechanical properties of the crop and the
cutting parameters [ 9 , 10 ]. Mostly, the design of cutting devices aims to minimize
energy consumption while maintaining the desired quality of cut. Table 5.2 sum-
marizes the biomechanical properties of different crops. Tensile failure stress of the
maize stem was lower than for the switchgrass. The elastic modulus of Miscanthus
was lower than that of maize crop, and it varied from 2 to 8 GPa with harvest time
and node number [ 11 ]. Flexural rigidity of the Miscanthus stem internodes decreased
linearly with higher internode number, and for the nodes it decreased exponentially.
The elasticity decreased linearly from the lower to the upper part of the Miscanthus
stems, but it did not vary in a systematic pattern with respect to harvest time [ 11 ].
Figure 5.1a shows that the shearing stress (curve 1) and maximum shearing force
(curve4)requiredtocutMiscanthusstemswereinverselyproportionaltotheheight
of cut from the stem base [ 9 ].

5.2.3 Cutting Mechanics

The cutting mechanics of agricultural materials differ significantly from metals or
plastics because agricultural materials are viscoelastic, meaning they do not possess
a strictly defined relationship between stress and deformation. Deformation in plant
materials is a function of time (creep), and their modulus of elasticity is not constant
[ 15 ]. The plant materials also behave differently under tensile and compressive
forces as well as static and dynamic loading. Although the cutting mechanics of
plants are difficult to predict theoretically, plants are often viewed as bundles of
fibers of high tensile strength bound by materials of much lower strength. The diam-
eter of the bundle of structural fibers rather than the outside diameter of the stem
determines the bending and tensile strength of the stem. Thicker stems, such as
those found in Miscanthus and corn, are often composed of strong node and weak