Shear

1
Shear

• Definitions, Relationships and Measurements
• Geoff Taylor VP Research and Development, Vista
  Medical Inc. Winnipeg Canada

2
What is Shear?

• Shear happens when tissue tries to go in two
  directions.
• Shear happens when there is relative motion
  between surfaces.
• Shear happens when there is fiction.

3
Causes of Shear

• No Compression Tangential
  Pinch Forces No Shear
  Shear Shear

4
Stress

• Stress is a force
• Shear forces are a Stresses
• Parallel Shear is a Stress
• Pinch Shear is a Stress

5
Strain

• Strain is distortion (change of shape)
• Shear Strain is distortion due to shear
• Shear Strain is a change is shape in two
  directions

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Displacement/Friction

• Displacement means movement occurred
• Friction plus displacement cause shear
• Friction comes in two types
  Static and Dynamic
• Coefficients of Friction are a measure of how
  sticky the surfaces are

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Friction

• The ratio of the static or dynamic friction
  force, to the normal force
• Csf Fsf / N and Cdf Fdf / N

• C coefficient of friction
• Sf static friction
• Df dynamic friction
• N normal force

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Static ltgt DynamicFriction

• Static Friction is always greater than Dynamic
  (Sliding) Friction
• This means that friction forces can change even
  if there is no movement
• Friction is the cause for Parallel shear
• Pinch forces also cause shear

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Parallel Shear Stress Tangential Force
Induced Shear

• Parallel Shear Stress is a force that exists
  whenever there is sliding or the potential for
  sliding between two surfaces.

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Pinch StressNormal Force Gradient Induced Shear

• The spatial changes in the forces normal to the
  support surface give rise to internal shear
  forces that are perpendicular to the support
  surface.

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Pinch Stress

• the shear stress component acting perpendicular
  to the skin generated by the non-uniformity of
  the pressure distribution

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Causes of Shear

• None Compression Tangential
  Pinch No Shear
  Shear Shear
• Strains combine

13
Internal Shear
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Vista Medical ISS

• Integrated Shear Sensor
  Combined Normal Force
  Shear Force Display

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Summary

• Shear Stress is the force that tries to distort
  things in two directions.
• Shear Strain is the distortion that results
• Friction tries to prevent sliding.
• Friction/Parallel, Pinch are sources of shear.

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Suggestions

• Pressure, Parallel and Pinch stresses can
  combine. Do what you can to minimize them or keep
  them apart.
• Low friction, Low shear.
• Low pressure, Low shear.
• Even Pressure Low Shear.

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More Suggestions

• Where displacements are high, the potential for
  parallel shear is high. gt Make sure the pressure
  is low.
• Where pressures are high, the potential for
  parallel shear is high. gt Make sure that friction
  is low, that the spot is slippery.
• Where pressures are high, the potential for pinch
  shear is high. gt Make sure the pressure changes
  are gradual.