The maize stalk is a complex, multi-scale, hierarchical structure with spatially varying material properties. Stalk strength is dependent upon factors and interactions that occur at and across the nanoscale, microscale, mesoscale, and macro-scale. The bulk of our current understanding is based upon studies at the macro-scale (agricultural studies) and nanoscales (basic biology and genetics).The mode of failure of the maize stalk has been characterized at the macroscale as Brazier (localized) buckling that occurs near a node. This failure mode is shown in Figure 2 at right. While the macro-scale failure mode is well understood, there is little known about how this failure progresses as the macroscale. In other words, collapse could cause Brazier buckling, but it is also possible that in the course of Brazier buckling causes an increase of stresses within the rind tissue, leading to collapse of the structure.Brazier buckling of a hollow tube is governed by the degree of ovalization. At a critical degree of ovalization, catastrophic failure occurs. It has been shown that pith tissue strengthens the stalk by resisting transverse ovalization.Brazier buckling was induced in the laboratory using the long-span three-point bending test configuration. Supports and the central load were placed at nodes to minimize cross-sectional deformation. During each three-point bending test, an optics-based system was used to quantify the degree of stalk ovalization that occurred prior to buckling. Camera images were aligned with force/deformation data using a series of pre-programmed digital signalsThe optics system consisted of a single camera mounted perpendicular to the flexural axis of the stalk. The camera was mounted to the “head” of a universal testing machine such that the camera moved synchronously with the loading anvil. This insured that the camera perspective remained constant throughout the test. Stalks were lighted from above and below using LED light sources. Black fabric was mounted behind the stalk to provide high contrast between the stalk and the background.Image processing was used to remove the background from the collected images. The perpendicular width of the stalk cross-section was computed by finding the normal direction of each point along the top and bottom surface of the stalk, and then computing the width of the stalk in the normal direction.Significant cross-sectional ovalization was observed in some specimens. Ovalization was followed by compressional tissue failure at locations of high bending stress. In other specimens, compressional tissue failure occurred with relatively little ovalization. These results show that this method can be used to quantify the degree of ovalization and detect the onset of compressional tissue failure. The sample size in this exploratory study is insufficient to conclusively state which failure mode is dominant. Further studies are planned using this test method to provide an estimate of the proportion of these failure modes.

Development of a Method for Quantifying the Degree of Maize Stalk Ovalization Prior to Buckling Failure

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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