Mechanical Properties Flashcards
What are mechanical properties associated with and depend on?
- deal with internal structures
- associated with response to externally applied forces
Some physical properties
- density
- coefficient of thermal expansion
- optical
- solubility
- viscosity
- electrical/thermal conductivity
- melting/solidification point
remember, independent of mechanical manipulation
What does testing mechanical properties tell us?
a useful predictor of clinical success especially when comparing materials
What is a force/load?
- push/pull exerted on the material
- units: pound, newton
What is stress? And the different types?
- force per unit area
types:
- tension: 2 sets of forces directed away from each other in same straight line
- compression: 2 sets of forces directed toward each other in same straight line
- shear: 2 sets of forces directed toward each other but not in the same line
In the mouth, compression can be translated to:
tension
(especially in sharp line angles representing a wedge)
What are the biting forces of each tooth?
Biting Forces:
- molars: 130 lbs
- premolars: 70 lbs
- incisors: 40 lbs
Biting Stresses:
- molars: 130,000 psi
What is the tensile strength compared to compressive strength of amalgam?
- tensile strength is much less (1/9-1/10 of compressive strength)
EvoCeram/composite compressive strength is much less than amalgams compressive strength though. tensile strength is 1/5 less than its compressive strength
T/F: small flaws can cause local high stresses and unexpected fracture at low stress
True!
Even a notch or scratch
What is strain?
- change in length/original length
- amount of deformation per unit length
What is strength the measure of?
- measure of level of stress required to make a material fail
Elastic Deformation:
- initially= strain is proportional to stress
- reversible
What is a stress- strain diagram?
graphic representation of deformation of a material as stress is applied
- shows how far a material can stretch out before it breaks when stress is applied
What is modulus of elasticity?
- measures rigidity (ratio of stress to strain in straight line)
- property can not be changed because it’s the measure of the interatomic bonding
- higher number, more rigid
What is one of the most rigid materials?
Cobalt-chromium
The steeper the slope on diagram, the more ________ the material
rigid
What is plastic deformation?
- permanent deformation of atoms so strain is NOT reversible
- once it starts, strain will increase faster than stress (will fracture quickly after putting more stress on it)
What is proportional limit?
- maximum stress a material can withstand without deviation (if stretch beyond the limit, will not go back to original)
increasing the proportional limit will __________ the resistance of the material to permanent deformation
increase
- high proportional limit= strong
- low proportional limit= weak
What is elastic limit?
basically the same as proportional limit (same on the graph)
What is yield strength?
- stress at which a material exhibits a small amount of permanent deformation
- just a little higher than proportional/elastic limit on the graph
- arbitrarily selected by manufacture
Any restoration/appliance that deforms in service is…
usually a clinical failure
At what psi can a material’s yield strength be burnished?
below 30,000 psi
(over that can not be burnished)
What is ultimate stress?
maximum stress before failing
- highest point on curve
