shear stress

Examples of shear stress in the following topics:

• What is a Fluid?

  • A fluid is a substance that continually deforms (flows) under an applied shear stress.
  • A fluid is a substance that continually deforms (flows) under an applied shear stress.
  • These properties are typically a function of their inability to support a shear stress in static equilibrium.
  • Solids can be subjected to shear stresses, and normal stresses—both compressive and tensile.
  • Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

• Poiseuille's Equation and Viscosity

  • Mathematically, viscosity is a proportionality constant relating an applied shear stress to the resulting shear velocity and is given, along with a representative diagram, (see ).
  • As shown, when a force is applied to a fluid, creating a shear stress, the fluid will undergo a certain displacement.
  • Different fluids exhibit different viscous behavior yet, in this analysis, only Newtonian fluids (fluids with constant velocity independent of applied shear stress) will be considered.
  • The variation in velocity between adjacent parallel layers is due to the viscosity of the fluid and resulting shear forces.
  • A proportionality constant relating an applied shear stress to the resulting shear velocity.

• Stress and Strain

  • The ratio of force to area $\frac{F}{A}$ is called stress and the ratio of change in length to length $\frac{\Delta L}{L}$ is called the strain.
  • Deformations come in several types: changes in length (tension and compression), sideways shear (stress), and changes in volume.
  • The ratio of force to area $\frac{F}{A}$ is called stress and the ratio of change in length to length $\frac{\Delta L}{L}$ is called the strain.
  • Stress and strain are related to each other by a constant called Young's Modulus or the elastic modulus which varies depending on the material.
  • Using Young's Modulus the relation between stress and strain is given by: $\text{stress} = Y\cdot\text{strain}$.

• Platelets and Coagulation Factors

  • Non-physiological flow conditions (especially high values of shear stress) caused by arterial stenosis or artificial devices (e.g. mechanical heart valves or blood pumps) can also lead to platelet activation.

• Viscosity

  • The resulting stress is measured as deformation, in the case of liquid, as the ease of flow.
  • There are several ways in which the stress changes with the strain.
  • In other systems, it increases or decreases with shear rate.
  • In the latter category, fluids that decrease in viscosity as shearing rate increases are called shear thinning (examples include toothpaste and house paint), and those that increase in viscosity as shearing rate increases are called shear thickening.
  • This figure shows the relative relationships between shear stress and shear strain for a variety of materials.

• Bacterial Differentiation

  • For instance, rod shapes may allow bacteria to attach more readily in environments with shear stress (e.g., in flowing water).
  • Cocci may have access to small pores, creating more attachment sites per cell and hiding themselves from external shear forces.
  • Oxidative stress, nutrient limitation, DNA damage and antibiotics exposure are some stress conditions to which bacteria respond, altering their DNA replication and cell division.
  • Filamentous bacteria have been considered to be over-stressed, sick and dying members of the population.
  • Nutritional stress can change bacterial morphology.

• Water Waves

  • Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind (making the water to go into the shear stress), contribute to the growth of the waves.

• Patellofemoral Stress Syndrome

  • The cause of pain and dysfunction often results from either abnormal forces or prolonged repetitive compressive or shearing forces (running or jumping) on the PF joint .

• Fracture

  • Fracture strength, also known as breaking strength, is the stress at which a specimen fails via fracture.
  • This is usually determined for a given specimen by a tensile test, which charts the stress-strain curve .
  • Rather they generally fracture due to sideways impact or bending, resulting in the bone shearing or snapping.
  • The bones in different parts of the body serve different structural functions and are prone to different stresses.

• Desmosomes

  • Desmosomes help tissues resist shearing forces.
  • The disease often presents in young athletes because of the added stress on the heart caused by physical exertion.