STRAIN-RATE-DEPENDENT MECHANICAL-PROPERTIES OF THE EQUINE HOOF WALL

The mechanical properties of fully hydrated equine hoof wall were exam ined at various loading rates in compact tension (CT) fracture, tensil e and three-point bending dynamic tests to determine possible effects of hoof wall viscoelasticity on fracture toughness and tensile paramet ers. Four cross-head rates were used in CT tests: 1.7X10(-5), 1.7X10(- 3), 1.7X10(-2) and 2.5 m s(-1); four strain rates were used in tensile tests: 1.6X10(-3), 3.2X10(-2), 0.33 and 70 s(-1). Speeds for the high est test rates were achieved using a large, custom-built impact pendul um, Bending test frequencies ranged from 0.04 to 200 Hz. In CT tests, both the initial modulus E(i) and the stress intensity factor K rose w ith increasing strain rate (from 0.38 to 0.76 Gpa for E(i) and from 0. 71 to 1.4 MN m(-3/2) for K), whereas the fracture toughness parameter J remained constant at 12 kJ m(-2). All tensile parameters except ulti mate strain were sensitive to strain rate. E(i), total energy to break age and maximum stress rose with increasing strain rate from 0.28 to 0 .85 GPa, from 5.4 to 9.7 MJ m(-3) and from 17 to 31 MPa, respectively. Data from low-amplitude dynamic tests agreed well with E(i) trends fr om CT and tensile tests. Direction of crack growth differed through th e thickness of the wall, the pattern of which resembled a trilaminar p ly. Although scanning electron microscopic examination of fracture sur faces revealed a decreasing pseudo-ductile behaviour with increasing s train rate, and ultimate tensile parameters are positively affected, e quine hoof wall viscoelasticity does not appear to compromise fracture toughness at high strain rates.