Physics Definitions

Question 1

Stress

Answer 1

Normal stress or shear stress
Is the force per unit area in a structure
Force / area (N/m2)
Synonymous with pressure
Allows analysis of an object’s material behaviour rather than the behaviour of the structure
More useful quantity than force in materials science

Question 2

Strain

Answer 2

Is the ratio of two lengths
Change in length / original length
Has no units of measurement
Expressed as a percentage
Not a measure of effort, force, or tension

Question 3

Hooke’s Law

Answer 3

Hooke’s law - stress applied to stretch or compress is proportional to the strain

When a material is loaded in the elastic zone, the stress is proportional to the strain

Object will return to its original shape after the load is removed

Question 4

Stiffness

Answer 4

Rigidity of material/object in response to an applied force

Related to the slope of linear region = Young’s modulus measured as unit of pressure GPa

eg aluminium 69 GPa, wood 11 GPa, Diamond 1220 Gpa

Can compare materials with same cross sectional area
relates to the structure of the material

Stiffness = cross sectional area x young’s modulus /
length of change

Question 5

Youngs modulus

Answer 5

Measure of stiffness of material
Also called modulus of elasticity
Calculation: elasticity = stress/strain
Material: a high elastic modulus is stiff. Undergoes less change/strain under load

Question 6

Stress- Strain curve

Answer 6

Toe region – in ligament stress strain curve is where the elastin fibres straighten out ( point before collagen stretching )

Linear region – Young’s modulus =stiffness
(more stiff the more vertical the line)

Yield point – starting to give /tearing of the tendon

Elastic phase – if stop stress – it will return to its original shape (starting point to yield point)

Plastic phase – past yield point – damage occurs

Failure Point - Complete failure of material

Question 7

Elasticity

Answer 7

Is the property of a material to return to its original form following the removal of the deforming load

When material is stressed, strain occurs

If the strain in the material returns to zero after the stress is removed then the material is elastic

The material is not permanently changed

Question 8

Plasticity

Answer 8

Some degree of strain remains after stress has been removed

The material has been permanently deformed, termed “inelastic” or “plastic deformation”

Energy has been absorbed
if a tissue is “plastic” there has been a change

Question 9

Strength

Answer 9

The amount of stress required to rupture the material

Question 10

Toughness

Answer 10

The amount of energy the material absorbs before rupture

Question 11

Hysteresis

Answer 11

Tissues response to loading and unloading
The restoration to the final length occurs at a slower rate – as there has been energy absorbed /lost eg heat has been released
It describes the amount of lengthening a tissue will maintain after a cycle of stretching (deformation) and then relaxation

Question 12

Mechanical failure

Answer 12

Relationship between stress at failure to number of cycles of loading
endurance limit if the level of stress below which an infinite number of cycles will not cause failure

Question 13

Ductility

Answer 13

A material which is permanently deformed to a large extent before rupturing is said to be highly ductile (>6% elongation)
E.g chewing gum

A material which will break before it becomes permanently deformed is said to be brittle (

Question 14

Stress-relaxation

Answer 14

decrease in tensile stress over time that occurs when a material under tensile stress is held at a fixed length

Question 15

Creep

Answer 15

low load over long time material will deform, affected by increase in temperature

Question 16

Effect of temp on creep

Answer 16

With increased tissue temp the tissues reach the point of strain – so effective stretching of heated tissues doesn’t take as long when compared to cold tissues
Creep is in relation to strain vs time

Question 17

Viscoelasticity

Answer 17

Viscoelasticity – thickness of fluid eg honey more viscous flows slowly, water low viscosity flows fast ,
Tissues will respond differently due to their properties with constant stress over long period of time eg synovial fluid – it will have an ability to absorb stresses /loads more so that a joint filled with blood

Question 18

Elasticity

Answer 18

of tissues – is where it will return to its original form after a deforming load

Question 19

Torsion

Answer 19

A twisting force is applied as compared to bending

Generates shear stresses within the object

Question 20

Shear

Answer 20

Compression is the force applied along the object’s axis

Tension is defined by forces that tend to pull something apart

Shear strain or stress is the force applied in opposite directions

Important for ductile materials where failure occurs when the maximum shear stress is reached

Question 21

Thixotrophy

Answer 21

when a material is handled or heated – changes to property of the material eg plasticine – change the stress/strain curve

Question 22

Davis’ law

Answer 22

Davis’ Law of soft tissue adaption – if a tissue is placed under stress/strain for a long period the tissues will adapt and undergo change – length, shorten or change function

Question 23

Wolfs’ Law of Bone Adaptation

Answer 23

If bone is placed in/under stress/strain for long periods of time the bone undergoes change (remodeling).

It may lengthen or diminish in size and shape

Form follows function