EFFECT OF MOISTURE CONTENT AND ORIENTATION ON FORCE-DEFORMATION BEHAVIOR OF PIGEON PEA GRAINS DURING UNIAXIAL COMPRESSION

The force-deformation behavior of pigeon pea grains and its correlation with moisture content was studied. Hertz's theory of contact stresses was applied to predict the splitting and failure of cotyledons during uni-axial compressive loading between two rigid parallel plates. The apparent modulus of elasticity of pigeon pea decreased from 280.52 to 9.29 MPa when force was applied normal to the hilum joint (horizontal loading), whereas it decreased from 208.08 to 33.37 MPa when force was applied in the direction of the hilum joint (vertical loading) with an increase in moisture content from 5.28% to 34.568% (d.b.). At higher moisture contents (15.17% and above), the grains yielded after considerable deformation, thus showing a change in nature from brittle to ductile. Distribution of stresses below the point of contact were calculated and plotted to predict the location of the critical point, which was found at 0.45 to 0.75 mm and at 0.50 to 0.67 mm below the contact point in vertical and horizontal loading, respectively, depending on moisture content. The bio-yield of pigeon pea grains was due to breakage of the cotyledons in horizontal loading, whereas it was due to separation of the cotyledons in vertical loading. Separation of the cotyledons from each other was initiated before yielding of the cotyledons at lower moisture contents, and thus splitting of the seed took place.