Strain Rate and Load
Michael “Mick” Peterson, PhD, University of Maine, Orono Maine
C. Wayne McIlwraith, BVSc (DVM), PhD, Diplomate ACVS
Raoul Reiser, PhD, Colorado State University, Fort Collins CO
ACVS Symposium Equine and Small Animal Proceedings
October 27, 2005
The demands on the track during the different phases of the gait are quite different and in some ways contradictory. During the impact phase of the gallop the loading of the foreleg is nearly vertical. The total load on the soil is small since it is primarily due to the mass of the hoof. However, the loading rate during the impact phase is high. As the animal moves into the stance phase, the loading rate is reduced since the hoof has contacted the soil, and the weight of the horse is being transferred to the hoof. The deceleration of the hoof occurs during the impact with the top layer of soil. As the body weight is transferred to the hoof, the role of the stiffer lower soil becomes dominant. During the stance phase the vertical load increases to the peak of up to 2 times body weight. During the stance phase a horizontal component is introduced which is associated with the transition from forward motion of the hoof during the swing into the propulsive phase of the gait. This horizontal component occurs because of the need to match the hoof speed to that of the ground during the propulsive phase. Initially, this horizontal component is in the direction opposite to that of the motion of the horse. During breakover, this horizontal load reverses direction in order to provide the propulsive force. The propulsive phase is followed by vertical unloading. As the propulsive phase ends, acceleration occurs during the swing phase to catch up with the forward motion of the horse.
Once the horse has moved fully into the stance phase, the shear strength of the soil becomes the dominant issue. Minimum shear strength for the soil is a key design criterion for a track surface. However, recent work has suggested that the horizontal deceleration of the hoof during initial contact with the soil may also be a critical issue [Nunamaker 2003]. Thus, a conservative design for a track surface may not exist, since an optimal soil would fail in shear during the initial stance phase and should not fail during the propulsive phase of the gait. Thus both shear strength and vertical stiffness are important and also are strain rate dependent. A measure of these characteristics must replicate the vertical and horizontal velocity for the hoof at the gallop. The gait information does not exist in the form needed for the apparatus design [Reiser et. al. 2000], so for initial system design the highest vertical velocity of a hoof, -5.0 m/s, reported for a horse trotting at 10m/s was used [Johnston 1991].
While much work has been done in the past to characterize track surfaces, most of it has not adequately taken into account the non-linear, elastic, plastic material characteristics of soil. For example, systems have been developed for measuring the vertical properties (or hardness) of the soil [Clanton et. al. 1991, Oikawa et. al. 2000, Ratzlaff et. al. 1997]. In some previous tests, loads as small as 10 kg dropped from heights of less than 1 meter were used [Pratt 1985]. This type of test represents the impact phase of the gait and does not address the shear strength of the soil. When smaller loads are used the harrowing of the track is overstated. Additionally, the shear strength was not considered in many of these investigations. Clanton et al  performed one study on shear properties as well as work by Pratt using a shear vane test. In these tests the characteristics of soil were simplified since strain rates were not replicated.
The device that has been developed is an adjustable speed drop hammer, which impacts the soil at an angle and thus experiences the same horizontal deceleration that a hoof impacting the soil will experience. This horizontal deceleration serves as a test of the horizontal shear strength of the soil. The machine uses a synthetic hoof that impacts at the appropriate angle to the soil. The speed of the hoof at impact replicates the velocity of impact of the hoof, with a secondary loading of the hoof through an adjustable gas spring. The adjustable gas spring replicates the compliance of the leg with adjustability built into the design and because current information on the leg is incomplete. A stiff mass is attached above the hoof, replicating the mass of the hoof, which initially impacts the track. Attached to the mass is a three-axis 100 g accelerometer. Load is transferred into a gas spring from the hoof mass using a dynamic load cell with a DC to 36 kHz bandwidth. The position of the hoof on the drop rail is determined using a string potentiometer. The redundant data from the acceleration and the velocity is used to estimate the penetration into the soil and to calculate the velocity of the hoof at impact. Unlike the actual hoof, the angle of the device is fixed during impact. The system replicates the strain rate, the loads and the hoof velocity of the horse. Comparison of data from a number of racing venues and types of track surfaces suggest that peak loads on the forelimb of a horse during the gallop vary dramatically. Much of this variation is not predicted by lower strain rate and lower load test methods. Calibration is based on high-speed video data and should also include acceleration data obtained from instrumented horseshoes when it becomes available.
Based on these measurements, maintenance can be performed using methods familiar to many track superintendents. The method of improving track characteristics would depend on the climate and design of the track. For example, if a track with high sand content was found to have low shear strength, the track could be improved by increasing the clay content, introducing fibers or, in some cases, possible by increasing the average moisture content. On a sand track, in the case of a sand track without a pad, excessively peak loads could result from excessive moisture content or on a track with a pad it may indicate too high of a high clay content. The effects of changes could be monitored using quantitative data and inconsistent track surfaces could be reduced if quantitative information is available from this type of testing.
Clanton, C., C. Kobluk, R. A. Robinson and B. Gordon, 1991 “Monitoring Surface Conditions of a Thoroughbred Racetrack” JAVMA Vol. 198(3), p. 613-620.
Johnston C, G. Hjerten, and S. Drevemo, 1991, “Hoof landing velocities in trotting horses” Equine Exercise Physiology Vol. 3, p. 167-172.
Nunamaker, D, 2003, e-mail communication, November 3, 2003.
Oikawa, M., S. Inada, A. Fujiswa, H. Yamakawa and M. Asano, 2000, “The Use of a Racetrack Hardness Measurement System” Equine Practice, Vol. 22 (4), p. 26-29.
Pratt, G.W., 1985, “Racetrack Surface Biomechanics” Equine Vet. Data, Vol.6 (13), p. 193-202.
Reiser, Raoul, M. L. Peterson, C.W. McIlwraith and B. Woodward, 2000, “Simulated Effects on Racetrack Material Properties”, Sports Engineering Vol. 3 (1), p. 1-11.
Ratzlaff, M. H., M. L. Hyde, D. V. Hutton, R. A. Rathgeber and O. K. Balch, 1997, “Interrelationships Between Moisture Content of the Track, Dynamic Properties” J. Equine Vet. Sci. Vol. 17 (1), p. 35-42.
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