DESIGN COMPLEXITY AND FRACTURE CONTROL IN THE EQUINE HOOF WALL

Morphological and mechanical studies were conducted on samples of equi ne hoof wall to help elucidate the relationship between form and funct ion of this complex, hierarchically organized structure. Morphological findings indicated a dependence of tubule size, shape and helical ali gnment of intermediate filaments (IFs) within the lamellae on the posi tion through the wall thickness. The plane of the intertubular IFs cha nged from perpendicular to the tubule axis in the inner wall to almost parallel to the tubule axis in the outer wall. Morphological data pre dicted the existence of three crack diversion mechanisms which might p revent cracks from reaching the sensitive, living tissues of the hoof: a mid-wall diversion mechanism of intertubular material to inhibit in ward and upward crack propagation, and inner- and outer-wall diversion mechanisms that prevent inward crack propagation. Tensile and compact tension fracture tests were conducted on samples of fully hydrated eq uine hoof wall. Longitudinal stiffness decreased from 0.56 to 0.30 GPa proceeding inwardly, whereas ultimate (maximum) properties were const ant. Fracture toughness parameters indicated that no compromise result s from the declining stiffness, with J-integral values ranging from 5. 5 to 7.8 kJ m(-2) through the wall thickness; however, highest toughne ss was found in specimens with cracks initiated tangential to the wall surface (10.7 kJ m(-2)). Fracture paths agreed with morphological pre dictions and further suggested that the wall has evolved into a struct ure capable of both resisting and redirecting cracks initiated in nume rous orientations.