A Study of the Apparent Modulus of Elasticity and Maximum Contact Stress of Timothy Stem Nodes in Relation to Disinfesting Hessian Fly Puparia in Hay Rebaling Processing

To disinfest Hessian fly in timothy hay bales by mechanical compression, the crushing strength of timothy stem nodes was investigated using an Instron testing machine. Unbaled first-cut timothy hay of the 2007 crop year was used in the test. Before testing, stem segments with nodes were equilibrated to moisture contents of 12.19% and 14.13% wet basis (w.b.). According to its position in the hay stalk, the node on each stem was designated as a top, middle, or bottom node. The force-deformation curves for the nodes as affected by the node position on the stem, node size, and moisture content were obtained under a constant rate of deformation of 1 mm min -1 . Equations based on Hertz's contact stress theory for cylindrical bodies radially compressed between two parallel flat plates were employed for calculation of the apparent modulus of elasticity and maximum contact stress of the timothy stem nodes. The apparent modulus of elasticity and maximum contact stress values were affected by node moisture content, size, and position on the stalk. The uniaxial compression test showed that the nodes had no elastic behavior. The apparent moduli of elasticity of timothy stem nodes for this study ranged from 24.86 to 39.82 MPa at 12.19% moisture content and from 35.67 to 53.98 MPa at 14.13% moisture content. Maximum contact stress of 8.22 MPa with a standard deviation of 3.52 MPa was obtained for timothy stem nodes at moisture content of 14.13% and size less than 1.60 mm. The maximum contact stress values obtained for the stem nodes in this study indicated that sufficient pressures were applied to Hessian fly puparia in the field tests (12.41 MPa) within 99% probability. These pressures were influenced by the moisture content, node size, and node position. The apparent modulus of elasticity and maximum contact stress values increased with increasing moisture content and decreased with increasing node size, and these values decreased from top position to bottom position on the stem.