General Tissue Mechanics

Question 1

Injury occurs when

Answer 1

an imposed load esceeds the tolerance (load carrying ability) of a tissue

Question 2

What can change tissue and its tolerance?

Answer 2

Training effects
Drug effects
Equipment design effect

Question 3

Structural properties

Answer 3

Load-deformation relationships of like tissues

Apply force on a structure and will experience some deformation

Question 4

Material properties

Answer 4

Stress-strain relationships of different tissues

Normalizing so that we can compare muscle to tendon or bone (different tissue comparisons)

Question 5

Stress

Answer 5

A force tries to rearrange the molecule in an object –> the internal resistance to an external load
Units = pascal
Push them closer (compressive)
Pull them apart (tensile)
Slide one across the other (shear stress)

Question 6

Strain

Answer 6

Change in shape or deformation - absolute and relative
comp, tensile, shear
Measured as a proportional change in dimension so no units - is a ratio

Question 7

Relationship with stress and strain

Answer 7

Stress is what is done to an object and strain is how the object responds
THey are proportional to one another

Question 8

Stress Strain ratio

Answer 8

Stiffness or compliance of the material

Question 9

Biologic material - linear?

Answer 9

no - due to viscoeleastic properties

Question 10

Extensibility and Elasticity

Answer 10

THe ability of a material to resume its original size and shape upon removal of applied loads
No known material is elastic at all stresses
Determination of elastic limit establishes elastic range of limit of elasticity

Question 11

Bending - long bones

Answer 11

Compressive stress - inner portion
Tensile stress - outer portion
Max stresses near the edges
Less near the neutral axis

Question 12

Three point bending

Answer 12

Failure at the middle

Question 13

Four point bending

Answer 13

Failure at the weakest point btw two inside forces

Question 14

Bending summary

Id a moment is applied to an object –

Answer 14

it will produce bending
The amount of bending depends on the magnitude of the bending moment and on the shape and properties of the object being bent

Question 15

When an object bends…

Answer 15

one side becomes concave (compress) and the other becomes convex (tensile)
Btw the two sides is an unstressed neutral axis

Question 16

Torsion

Answer 16

Twisting action applied to a structure
Larger radius of the shaft - greater resistance
Stiffer the material - the harder to deform

Question 17

Brittleness

Answer 17

absence of any plastic deformation prior to failure

Question 18

Resilience

Answer 18

Measure of energy absorbed by a material and returned when load is removed; material that quickly return to their original shape are called resilient

Question 19

Toughness

Answer 19

Property of a material enabling it to endure high impact shock loads; ability to absorb energy during plastic deformation; measure of the capacity of a material to sustain permanent deformation

Question 20

Unique properties of biologic materials

Answer 20

Anisotropic = not uniform throughout the material
Viscoelastic = time dependent, loading rate dependent
Organic = self repair, adaptation to changes in mechanical demand

Question 21

Viscoelasticity

Answer 21

Provided by the fluid component in biological tissue
Resistance to flow
Affects stress/strain
Inc strain rate produces inc stiff of the material

Question 22

Pure elastic material - viscoelasticity

Answer 22

Not viscoelastic

All energy is returned and no energy is lost

Question 23

Viscoelastic tissues

Answer 23

Loss energy due to heat, energy is not returened immediately

Question 24

Hysteresis

Answer 24

area reperesneting the energy that was lost

Question 25

Creep

Answer 25

slow, progressive deformation of a material under constant stress

Question 26

Stress relaxation

Answer 26

gradual dec of stress when the material is held at constant strain

Question 27

Stresses and strains are dep on

Answer 27

Time and rate of loading

Question 28

General rules

The longer it takes for a load to be applied

Answer 28

The higher the strain
The lower the stress
dec rate of loading

Question 29

The faster it takes for a load to be applied

Answer 29

inc rate of loading
Higher the stress
Lower the strain

Question 30

If a material completely recovers from strain it shows

Answer 30

elastic deformation

Question 31

If a materal shows permanent deforamtion following strain is shows

Answer 31

plastic deformation

Question 32

At higher strains

Answer 32

brittle material will continue to deform until it breaks

Malleable material will deform plastically before it breaks

Question 33

Yield stress or Elastic limit

Answer 33

The stress at which a materials deformation changes from elastic to plastic