Mechanical analysis of the bending behaviour of plant stems

• Two consecutive phases can be distinguished during bending of crop stems: ovalisation and buckling.
• Existing mechanical models have successfully been adapted to describe the bending behaviour of crop stalks during both phases.
• The influence of the stem diameter, the wall thickness and the deformation rate has been studied for three different crops.
• The presence of a core-rind structure increases the bending resistance of the stems.

In order to optimise the processing of stem crops, insight into the deformation behaviour of the crop and the interaction between crop and machine is essential. Most research in the area of mechanical and physical properties of crop stems is focused on characterising the agricultural products to the point of failure using mechanical parameters and empirical relations. No studies have been conducted on the processes which lead to failure of stems and on the processes that take place after failure. In this paper it is shown that the bending behaviour of wheat and barley stalks is very similar to that of steel tubes. Two consecutive phases can be distinguished: ovalisation and buckling. During ovalisation the forces on the wall tend to flatten the cross-section. When this process continues the flexural stiffness is reduced until the structure becomes unstable and buckles. The cross-section locally completely flattens. This deformed cross-section offers virtually no resistance to bending. Mechanical models described in literature have successfully been adapted to describe the bending behaviour of crop stalks during both phases (R2>0.98R2>0.98 for ovalisation and R2>0.97R2>0.97 for buckling). The crop species, growing conditions, stem diameter and wall thickness were found to influence the bending process significantly. The presence of a core-rind structure increases the bending resistance of the stems.