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
What is elongation/ductility?
- how much a material will stretch out/tensile force before it fractures
- indicates how much you can burnish a material
- the length of the curve shows the ductility
Total % elongation includes:
both elastic and plastic deformation
- an alloy with high value can be bent permanently with less risk of fracture
- value should be at least 10% elongation to be useful
Difference between malleability and ductility
- malleability: ability to be hammered or rolled into thin sheets
- ductility: ability to be drawn into wire/stretching. related to burnish ability
Wrought wires are….
flexibile
Cast alloys are…
ductile
What is resilience?
- how much energy you can put into material until it permanently deforms (until it reaches its elastic/proportional limit)
- measured by area under elastic portion of a stress-strain diagram
What is toughness?
- amount of energy necessary to fracture a material
- area under the total curve
What is the hardness of material?
- resistance to permanent indentation
- harder=harder to polish
What is the Brinell Hardness test method?
-small steel ball that makes indentation in materal
- measure indentation… smaller indentation=higher hardness
- used for metals and alloys, not brittle materials like porcelain
What is a diamond pyramid hardness test?
basically same as brine hardness just diamond shape
What is knoop hardness?
- diamond tooth with one long diagonal and one short diagonal that indents and cuts the material and then measure
- used in all dentistry materials
What does knoop hardness tell us clinically?
- when opposing two materials, the weaker hardness will wear and the harder material will be fine (enamel vs porcelain… porcelain will win and enamel will wear)
What is Rockwell hardness used for?
- elastic materials
What does yield strength tell us?
burnishability
What does tensile strength tell us?
load-bearing ability
What does elongation tell us?
similar to yield strength
- burnishability
- margin-finishing properties
What does hardness tell us?
- wear resistance
- finishing/polishing
