RATCHETING EVALUATION OF BIOLOGICAL TISSUES OVER ASYMMETRIC LOADING CYCLES

This study intends to evaluate the ratcheting response of biological samples prepared from bovine and porcine trabecular bone, articular cartilage, meniscus, and skin tissues and tested under asymmetric (nonzero mean stress) cycles. Meniscus and skin samples were tested with stress ratios of [Formula: see text] and [Formula: see text], respectively, while other tissues were tested at [Formula: see text]. Experimental ratcheting data and related influential parameters including stress level, stress rate, and testing frequency were discussed. A parametric ratcheting equation was further calibrated to estimate the ratcheting response of tissues. The predicted ratcheting data were found to be in close agreement with the reported experimental data.

Ratcheting response of biological tissues over asymmetric loading cycles

The purpose of this study is to examine the ratcheting phenomenon in a variety of biological tissues including the trabecular bone, meniscus, articular cartilage and skin, and propose a parametric model to predict the ratcheting strain of these tissues. Furthermore, utilizing experimental data, and the influence of different mechanical and biological parameters on the ratcheting strain are discussed. The dependency of ratcheting on frequency, stress rate, stress variation, physiological environment, and tissue sites is demonstrated. Besides, stiffness of the toe and linear regions in each cycle, and the modulus of the failure region of the stress-strain curve are computed. The energy dissipation in different cycles at two frequencies of 1 Hz and 10 Hz is discussed. A parametric model was employed to predict ratcheting behavior of the said biological tissues. The model predictions of the strain accumulation in tissues are found in agreement with the experimental data.

Ratcheting response of biological tissues over asymmetric loading cycles

The purpose of this study is to examine the ratcheting phenomenon in a variety of biological tissues including the trabecular bone, meniscus, articular cartilage and skin, and propose a parametric model to predict the ratcheting strain of these tissues. Furthermore, utilizing experimental data, and the influence of different mechanical and biological parameters on the ratcheting strain are discussed. The dependency of ratcheting on frequency, stress rate, stress variation, physiological environment, and tissue sites is demonstrated. Besides, stiffness of the toe and linear regions in each cycle, and the modulus of the failure region of the stress-strain curve are computed. The energy dissipation in different cycles at two frequencies of 1 Hz and 10 Hz is discussed. A parametric model was employed to predict ratcheting behavior of the said biological tissues. The model predictions of the strain accumulation in tissues are found in agreement with the experimental data.

Stirrup forces during approach, take-off and landing in horses jumping 70 cm

Stirrups aid the rider to stabilise their lower leg allowing it to be used effectively for communication and in maintaining their position in the saddle. Relatively few studies have investigated stirrup forces and to the best our knowledge no studies have reported stirrup forces in jumping. The aim of the present study was to measure stirrup forces in five showjumping horses ridden by the same professional rider. All horses were in regular training and competition jumping at least 30 cm higher than the fence used for the study. The fence chosen was a 70 cm upright with a pole at the top and a groundline. Right and left stirrup forces were measured using wireless load cells placed between the stirrup leathers and the stirrup. The signals were transmitted and digitised at 100 Hz and synchronised with video from a webcam using an inertial measurement unit. After warming-up, including over jumps, each horse attempted the jump three times from each rein in canter (3 horses left then right rein; 2 horses right then left rein). Mean peak total (sum of left and right) stirrup force for the approach (n=5 strides per horse per jump), take-off and landing phase of the jump was 1,034±110, 1,042±284 and 1,447±256 N (range 905 to 1,815 N), respectively (mean ± standard deviation). There was no significant difference between right or left mean peak stirrup force during approach or take-off, but mean peak force was consistently higher on the right stirrup during the early phase of landing on either the right or left rein (right: 827±320 N; left: 615±336 N; P<0.05). In conclusion, the mean total peak stirrup forces measured in the present study in the same rider jumping five different horses over a 70 cm single upright fence are similar to previous reports of peak stirrup forces in gallop and consistent with observations of asymmetric loading of the saddle and horses’ backs by riders.

MODELING THE PROCESS OF DISINTEGRATION OF SOLID MATERIALS BY ASYMMETRIC LOADING IN CRUSHING MACHINES IN ORDER TO FIND WAYS TO REDUCE ENERGY COSTS

Subject of study. the processes of cracking and destruction of rocks under the action of the working bodies of machines for disintegration. Methodology. A complex method of generalizing the laws of the theory of elasticity and plasticity was used; theoretical and experimental confirmation of the regularities of the distribution of contact normal and tangential stresses, equations of the limiting state of materials based on the Coulomb strength criterion; slip line theory; comparison of theoretical results with experimental diagrams "normal stresslongitudinal deformation" of samples; facts and phenomena of destruction of rocks; generalization of the theoretical regularities arising from the power contact of the tool with the rock in crushers. Purpose. Reducing energy consumption and increasing the efficiency of rock disintegration by controlling its stress-strain state in crushers on the basis of mathematical modeling and using the established regularities of stresses and deformations in rocks when interacting with a working tool. Output. In the contact area, with an increase in the tangential load, the zone of uniform compression of the material decreases, the depth of the most stressed point approaches the contact surface. There is a significant zone of shear deformations, which are the decisive factor in crack initiation. The development of the crack in depth and complete destruction occurs along the shear lines. Such conditions of rock loading are observed in jaw crushers with complex jaw movement, in cone crushers, in roller crushers with different roll rotation speeds and correspond to the model of the most effective sliding compression. The creation of asymmetric loading conditions using the forces of contact friction, frictional and strength characteristics of the destroyed material can reduce the energy consumption of disintegration.

CYCLIC TORSION WITH INTERNAL PRESSURE OF PRE-STRETCHED THIN-WALLED CYLINDERS

The article considers the following two tasks. 1) Strength calculations are carried out for a long thin-walled cylinder, the ends of which are not closed during reloading. At the first loading, the thin-walled cylinder is stretched by a longitudinal force leading to longitudinal plastic deformations. When reloaded, the cylinder twists and at the same time uniform pressure acts from the inside. It is noted that, in particular, the shaft of wind generators is subjected to such loading. It is proved that the equation of the yield curve in the plane of the normal and tangential stresses of the stress space is an ellipse, whose semi axes are a function of the mechanical characteristics of the material and the residual stresses after the first loading. 2) We consider cyclic twisting with internal pressure of pre-stretched thin-walled cylinders. The yield surface equation is obtained for a thin-walled cylinder under repeated loading τ + p. which stretched upon first loading. It is proved that the yield surface equations also yield the fatigue surface equations if the yield stress σ_(T.) is replaced by the fatigue stress σ_(0.) Expressions are found for the number of cycles required for fatigue failure under an asymmetric loading cycle under repeated loading. It is proved that in order to increase the number of cycles required for fatigue fracture, during the first loading, when the material is strengthened due to plastic deformation, and with repeated cyclic loading, the same stress components must take place. It is proved that if during repeated loading the end of the loading path is inside the endurance surface, then the number of cycles required for fatigue failure in an asymmetric loading cycle tends to infinity. Keywords: Initial loading, reloading, cyclic loading, strength, fatigue failure, plastic deformation, residual stress, yield surface.