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The Cause of Pressure Sores
Published in J G Webster, Prevention of Pressure Sores, 2019
Shear is defined as a mechanical stress that is parallel to a plane of interest (Bennett and Lee 1985). An example of shear occurs in the sacral region when a person is seated with his back at a 45° angle. The deeper fascia slides downward with the bone while the superficial sacral fascia remains attached to the dermis. This produces stretching and avulsion of the perforating arteries that supply the skin for underlying fascia and muscle (Maklebust 1987). If the skin becomes sufficiently ischemic, a pressure sore will form with wide undermining around its base (Reichel 1958). Since the microcirculation below the skin’s surface is a network of small vessels oriented in all directions, it is possible that this network responds the same to shear and normal forces (pressure). Bennett et al (1979) showed that when enough shear is present only half as much pressure is necessary to cause vascular occlusion.
A new approach to the concept of tectonics
Published in Paulo J.S. Cruz, Structures and Architecture: Bridging the Gap and Crossing Borders, 2019
Depending on the forces acting on an element or a structure, a distinction is made between several types of strain: compressive strain, tensile strain, bending strain and shear strain. Expressing type of strain means direct expression of deformations that occur in compression, tension or bending. In some types of constructions, that expression is intrinsic: tensile membranes, cables, air-supported structures, suspended bridges, cable-stayed beams etc. Frei Otto’s Olympic Park in Munich and the catenary roof of the Lowara Office Building by Renzo Piano both have, as tectonically dominant, the expression of the mechanical function type of deformation. It is interesting to find the National Nederlanden Building by Frank Gehry is an atectonic manifest of deformations on the macro-tectonic level, in a loadbearing structure coming from the action of unexciting forces. The system of nonloadbearing elements can suggest visual deformations that have tectonic statement. Sometimes components of buildings are used to convey the idea of nonexistent deformation in the load-bearing structure, which is the case with the enclosure of Calatrava’s the Turning Torso skyscraper. Similar atectonic use of a building’s skin for expression of deformation in structure is traceable in many of the neo-futurist buildings of Zaha Hadid as well.
Introduction to Mechatronic Systems
Published in Bogdan M. Wilamowski, J. David Irwin, Control and Mechatronics, 2018
Depending on the direction of the applied force on a particular area, there are three kinds of stresses: compressive stress, tensile stress, and shear stress. Compressive stress (or compression) is the stress state caused by an applied load that induces a reduction in the length of a material along the direction of the applied load. A simple case of compressive stress is one-dimensional compression induced by the pushing coaxial forces. The compressive strength of materials is generally higher than that of tensile stress. Tensile stress is the stress state caused by an applied load that tends to elongate a material in the same direction as of the applied load; in other words, it is the stress caused by pulling the material. Shear stress is the stress state caused by a pair of opposing forces acting along parallel lines of action through the material; in other words, it is the stress caused by sliding faces of the material relative to one another. An example is cutting paper with scissors.
Effects of fatigue on hamstrings and gluteus maximus shear modulus in hip extension and knee flexion submaximal contraction task
Published in Sports Biomechanics, 2023
Ricardo Pimenta, Pedro Almeida, José P. Correia, Paula M. Bruno, João R. Vaz
being E the Young’s modulus and G the shear modulus (Shiina et al., 2015; Sigrist et al., 2017). The shear modulus is the ratio of shear stress over shear strain. Since muscle strains occur due to a shear force on the muscle fibres, shear modulus is often used in muscle rigidity analysis and consequent injury. For the shear modulus calculation, each clip exported from Aixplorer’s software was sequenced in.jpeg images. Image processing converted each pixel of the colour map into a value of the Young’s modulus based on the recorded (ultrasound) colour scale. The largest ROI in the elastogram window was determined manually by an experienced examiner avoiding aponeuroses and tissue artefacts (e.g., vessels) and the values were averaged to obtain a representative muscle value. Within each trial, the most stable values with ~ 20 s duration were averaged and divided by 3 to estimate the muscle shear elastic modulus (Bercoff et al., 2004).
Influence of bedding and jointing sand on the shear strength characteristics of Interlocking Paver Blocks – bedding sand interface
Published in International Journal of Pavement Engineering, 2022
Arjun Siva Rathan RT, Sunitha V, Murshida P, Anusudha V
Small scale direct shear test was used to find the shear behaviour of the bedding and jointing sand gradation. The shear box size was of 60 mm *60 mm *25 mm. The test procedure adopted was as per IS 2720 Part 13 (1986). The sand was mixed with an Optimum Moisture Content of 6%. The normal stresses used for the study were 50, 100, 150 and 200 kPa. The readings were recorded digitally using a data logger that is connected with two LVDTs for displacement measurement and a load cell for load measurement. The test was stopped either when the shear displacement reaches 25 mm or when steady state was reached. The shear modulus was calculated by the ratio of shear stress and shear strain considering the slope in the elastic region. The secant modulus was calculated from the derived shear modulus and Poisson’s ratio. The Poisson’s ratio adopted throughout the study was 0.35.
Effect of convection on boundary layer structures in finite thermoelasticity
Published in Journal of Thermal Stresses, 2019
Now, if we fix b = 1, and vary the value of t (or vary the convection coefficient h), we again obtain a boundary layer growth as can be seen in Figure 17. As the value of t (or h) increases, also increases. In fact, when the convection coefficient is increased, more heat is lost through convection, and this causes the temperature θ to drop. This in turn leads to a decrease in the shear modulus μ, as confirmed by Eq. (42). Now, by definition, the shear modulus is the ratio of the shear stress to the shear strain. Since the shear stress remains constant in this case, the shear strain has to increase. This in turn means that the deformation will be greater for higher values of h. Hence, the increase in is justified, as per Eq. (2).