Physics Definitions Flashcards
Stress
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
Strain
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
Hooke’s Law
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
Stiffness
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
Youngs modulus
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
Stress- Strain curve
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
Elasticity
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
Plasticity
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
Strength
The amount of stress required to rupture the material
Toughness
The amount of energy the material absorbs before rupture
Hysteresis
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
Mechanical failure
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
Ductility
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 (
Stress-relaxation
decrease in tensile stress over time that occurs when a material under tensile stress is held at a fixed length
Creep
low load over long time material will deform, affected by increase in temperature