Deformation and Fracture of Ivory

Abstract

Ivory does not deform in a completely elastic manner prior to fracture, but instead plastic microyielding commences at a very small applied stress. The nature of the plastic deformation is determined by the orientation of the test specimen relative to the growth ring structure. Specimens oriented with the tension axis parallel to the tusk length (designated longitudinal) exhibit both a larger microscopic yield stress (the stress to produce a plastic strain of 2×10−6 in./in.) and a higher rate of strain hardening than specimens oriented perpendicular to the tusk length (designated transverse). Prestraining has no effect on the rate of strain hardening of longitudinal specimens, but progressively reduces the measured rate of strain hardening of transverse specimens. A feature common to the deformation of both longitudinal and transverse specimens is the considerable anelastic contraction which occurs after unloading. The fracture characteristics, which are also a function of specimen orientation, are correlated with the relative planes of weakness provided by the interfaces between growth rings.