3. Mechanical Properties Flashcards
Mechanical Properties of Dental Materials
Properties defined by the laws of ____.
The response of a material to an
____ force. A sub-set of ____ properties.
mechanics
applied
physical
Forces on Dental Materials
Three types of forces or loads may be applied to a solid material.
____
____
____
tensile
compressive
shear
Stress and Strain
We can define force intensity because if
we have the same force on two materials of different sizes, we need to be able to describe the effects of the force on the two different sizes of material. We use what I’m calling a force intensity, which is referred to as ____. Stress is ____ divided by the ____ of the object, so if we are putting a tensile force on this material, we are pulling it apart, there will be an increase in ____. We can describe this increase in ____ divided by the ____, that is the fractional change in length of the material, which is referred to as ____
stress force area length length original length strain
Stress
The internal force within a material which ____ an externally applied force or load.
stress = ____ (lb/inch^2), (psi), ____, ____)
Tensile
Compressive
Shear
resists
F/A
N/m^2
Mpa
Strain
Change in ____ of a material which results from an externally applied force.
e = ____ (inch/inch, mm/mm)
dimensions
(L1 - L0)/L0
Modulus of Elasticity
(1) Two springs, difference in the stiffness (same force applied to both)
Dividing stress by strain > ____ (measure of stiffness of material)
Continue pulling on spring: becomes permanently ____, where you exceed its elastic limit; plot the stress vs strain
young’s modulus
elongated
S/S curve for stainless steel
(2) Initially, force is ____ to elongation (straight line) up to proportional limit; once you exceed this point it begins to deform ____ > continue elongation until the spring breaks (____)
Use it to describe mechanical properties of materials > the slope of the initial portion: measure of the ____ of the spring; next property of importance is the ____ > want to deform 0.2% of starting length > and can use comparatively between two materials
proportional
permanently
ultimate tensile strength
stiffness/rigidity yield strength (YS)
Stress/Strain Curves
(3) Degree to which a material deforms permanently is a measure of its ____, so when we measure we are looking at the full range of the material
ductility
Stress/strain Curves
Elastic Strain
Strain which ____ completely when the applied force is removed
Permanent (Plastic) Strain
Strain which remains ____ after the applied force is removed
disappears
permanently
Elastic Limit
Stress corresponding to the first measurable ____.
- Yield Strength
Stress corresponding to a designated amount of permanent strain - ____ - ____
Units: Psi, Mpa
Units for YS are the units for ____
permanent deformation
- 1%
- 2%
stress
Proportional Limit (PL)
The value of stress at which the stress/strain diagram deviates from the initial ____ relation
The useful ____ of a dental material
linear
strength
Elastic Limit vs. Proportional limit vs. Yield Strength
All of these are essentially ____
equal
Modulus of Elasticity
____ of the initial linear portion of the stress/strain diagram
____ of a material
Modulus of elasticity = ____
slope
rigidity
stress (MPa) / Strain
Ultimate Strength
The stress corresponding to the ____ value of applied load or stress a material can withstand without rupturing
Units: psi, MPa
Some materials fracture at the maximum stress, so the UTS is the ____; some materials you exceed the UTS, so you will have a ____ fracture strength from the UTS
maximum
fracture strength
different
Failure of Dental Restorations
A permanent deformation of a dental appliance under occlusal loading represents a functional failure - by changes in ____ and ____
occlusal relationship
fit
Ductility/Elongation
The ability of a material to undergo
permanent ____ deformation without ____.
Clasp adjustment, ____ and crowns, preparation of ____ appliances
The material shown on graph has some ductility to it; a good example of ductility is ____ to improve the marginal fit on the tooth
tensile fracture burnishing of inlays orthodontic burnishing inlays
Malleability
The ability of a material to undergo permanent ____ deformation without fracturing.
Brittleness:
Material behavior characterized by fracture with little or no prior ____
Opposite of the two is brittleness (i.e. ceramic plates, drinking glasses > not able to deform permanently when a stress is placed upon them) (i.e. all dental ____ materials, porcelain)
compressive
permanent deformation
ceramic
Modulus of Resilience
Resilience:
Amount of ____ energy stored in a material during ____ deformation
Indicates ____ of material
Area under ____ portion of SS
curve
R=(1/2) P^2/E
recoverable
elastic
springiness
linear
Resilience and Toughness
Resilience is the ____ portion of the stress-strain curve; ____ of material is the energy that is stored throughout the curve before it fractures
linear
toughness
Toughness
Total energy absorbed
by a unit volume of material prior to ____ of the material specimen.
Tough materials have high ____, ultimate strength and ____ at rupture
Ceramic plate on one hand, and a metallic plate on the other; if you drop both, the ceramic plate will crack and the metal plate will recoil > metallic plate only deformed up to the elastic limit, ceramic plate has little ____ so it cracks
fracture
proportional limit
strain
resilience/toughness
S/S Curves for Materials with Different Properties
Materials with high slope of linear portion > ____; the non-linear part, has a significant amount of plastic deformation (top left, ____, the one next is brittle); strong material has a high ____ and high ____; tough because of the ____ located under the curve
stiff ductile YS UTS large area
S/S curve for stainless steel
Curve is different; look at S/S Curves for Enamel/Dentin; enamel has higher ____ and it is stronger because the ____ is higher, however it is not as ____ as dentin (dentin is also more ____)
stiffness
UTS
tough
ductile
Other Mechanical Properties
Transverse strength, bend test:
The maximum ____ which a material can support before failure when loaded as a beam supported at ____ ends.
stress
two
Three Point Bend Test
Stress at where it breaks is the ____ strength (same as flexural); modulus of rupture: determining the ____ using the bend test
transverse strength
modulus of elasticity
Diametral Tensile Strength
Tensile properties of ____ materials are obtained by loading a ____ of the sample:
epsilon = 2P/piDT (psi, MPa)
brittle
disk
Diametral Tensile Strength
Stresses are ____ to the force and it breaks right down the ____; calculate tensile strength using this test (used for ____ and ____)
perpendicular
middle
amalgam
composites
Hardness
Resistance of a material to ____ or to being ____ by another material
permanent indentation
scratched
Hardness Measurements
Brinell Rockwell \_\_\_\_ \_\_\_\_ \_\_\_\_
Vickers
Knoop
Barcol
Hardness Measurements
Bronell uses a hardened ____ and loaded, and the projected area of indention (load/projected area of indentation); rockwell has a ____, similar principle; vickers, knoop and barcol are more useful for ____ measurements; vickers and knoop are ____ indentations
round-tip
steel-tip
dental
diamond-tipped
Cyclic Mechanical Properties (Fatigue)
Failure of materials due to ____ of loading and unloading
Failure occurs at stresses lower than the ____
Growth of small ____ which become larger upon cycling until failure
cycles
UTS
surface cracks
Fatigue Limit
Stress at which material can withstand an ____ number of cycles is the fatigue limit
Unlimited numbers of cycle at load where it doesn’t ____ (test it sequentially from higher loads)
unlimited
break/fail
Fatigue Curves
Endurance limit/fatigue limit are ____
We can do this by plotting fatigue curve where we show stress on the y axis and the number of cycles loading on the x axis, so for example this is an example of steel and if you place this level of stress on steel and you keep cycling the load, at the point the number of cycles will fail.
You lower the load/stress and you keep cycling and it will fail at this number of cycles. If you keep lowering the load, you get to a point where no matter how many cycles you load it with, the material doesn’t ____ and we say you have reached the ____ or ____ of the material. So this is the ____ we want our material to face while in function
synonomous
break
endurance limit
fatigue limit
ideal load
Fatigue Loading of Dental Appliances
Forcing P/D clasp arms over bulbous teeth into undercuts
____ cycles per year
Masticatory stresses amount to approximately ____ flexures of partial denture clasps per year
1500
300,000
Creep Test
Permanent deformation of a material under a ____ over time
Stress is below ____
Important above ____ Tm of material
Leads to ____
Important for ____
Tested under ____ temperatures; amalgam has a mp of 100 degrees C
constant stress proportional limit 0.5 fracture dental amalgam high
Creep Curve
This shows the typical creep curve where notice Y axis is strain and X axis is time, we put a fixed load on the
material and there is an ____, and then there is a ____ that occurs over time. This is creep and there are various sections that we will not go into for this course.
immediate instantaneous deformation
gradual deformation
Stress Concentrations
Restorations can fail under low stresses (below reported strength) due to presence of ____ or ____
If the material does not deform plastically high stresses result at the ____
Stresses at crack tip > fracture > failure of material
surface defects
cracks (flaws)
crack tip
Effect of Stresses on Flaws in Brittle & Ductile Materials
Brittle material: no mechanisms for ____ > the crack grows quickly because the stress concentration is high at the tip of the crack due to ____
ductile material: ____ leads to a rounding of the flaw or crack, and will take much ____ for it to fail when applying a tensile force
if either are subjected to ____ force > it is forced to close and will not fail
____ and ____ stresses are most damaging to dental material
plastic deformation
tensile force
plastic deformation
longer
compressive
tensile
shear
Stress Concentrations
Clinical Consequences
____ restorations to minimize flaws
Avoid ____ in shape in design of restorations
____ cusp tips to spread area of load application
polish
sharp changes
round
