Mechanical properties of tissue

Question 1

What is stress?

Answer 1

A measure of a material’s ability to resist an applied force

-applied force / cross sectional area

Question 2

What does the ability to resist deformation depend on?

Answer 2

Material properties, size and shape, environmental factors, magnitude, direction and duration of applied forces

Question 3

What is strain?

Answer 3

The deformation of the material produced by the applied force

= change in length / original length

Question 4

What is elasticity?

Answer 4

Ability to resume original shape and size when load is removed

Question 5

Viscoelastic materials

Answer 5

Biological materials such as ligaments - the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation

have elements of both viscosity and elasticity properties

Non linear stress strain relationshop

Question 6

Viscoelastic properties under a constant applied load

Answer 6

As temperature of material increases over time with a constant stress, a slow continuous deformation is observed

At a constant deformation, the urial high stress decreases over time - stress relaxation

Question 7

What is hysteresis?

Answer 7

The difference between the loading and unloading curves

Question 8

What is resilience?

Answer 8

The energy that is absorbed by the material and then retuned when load is removed

Question 9

Anistropy

Answer 9

Biological materials are anisotropic - behaviour varies depending on the direction of loading

Question 10

Properties of bone

Answer 10

Fairly inelastic
Viscoelastic
Anisotropic

Question 11

Active vs repetitive loading

Answer 11

Single very high load may cause failure in material

Repeated cycles of loading and unloading may also cause failure e.g stress fractures in runners - chronic loading

Question 12

Properties of cartilage

Answer 12

high elasticity
Anistropic
Viscoelastic
Deforms instantly to low or moderate loads
If load if prolonged or greater, elasticity is impaired which may cause degeneration
Rate dependent - if rapidly loaded, becomes stiffer and deforms over longer period

Creep in intervertebral discs- tallest in the morning

Question 13

Properties of ligaments

Answer 13

Depends on construction
Can return to pre stretched length if in elastic portion of curve
Viscoelastic - usually damaged through shear and tensile loading at fast rates
Strength and stiffness increase over time and with force applied

Question 14

Properties of tendons

Answer 14

Large collagen content - very strong and stiff
Elasticity very important in role with muscle
little hysteresis - tendon very good at returning elastic energy

Question 15

What is stress

Answer 15

A measure of a materials ability to resist an applied force

= applied force x cross sectional area

Question 16

Stress strain relationship

Answer 16

Non linear
Strain rate sensitive. i.e deformation depends on rate at which load is applied or removed
A higher rate of loading results in greater stiffness

Question 17

Creep

Answer 17

As temperature of material increases over time with a constant stress, a slow continuous deformation is observed

Question 18

Stress relaxation

Answer 18

At a constant deformation, the initial high stress decreases over time

Question 19

Acute loading

Answer 19

Single very high load may cause failure in material

E.g bone fracture

Question 20

Chronic loading

Answer 20

Repeated cycles of loading and unloading may also cause failure
E.g stress fractures in runners