3. Mechanical Properties Flashcards

1
Q

Mechanical Properties of Dental Materials

Properties defined by the laws of ____.

The response of a material to an
____ force. A sub-set of ____ properties.

A

mechanics
applied
physical

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2
Q

Forces on Dental Materials

Three types of forces or loads may be applied to a solid material.
____
____
____

A

tensile
compressive
shear

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3
Q

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 ____

A
stress
force
area
length
length
original length
strain
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4
Q

Stress

The internal force within a material which ____ an externally applied force or load.

stress = ____ (lb/inch^2), (psi), ____, ____)

Tensile
Compressive
Shear

A

resists
F/A
N/m^2
Mpa

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5
Q

Strain

Change in ____ of a material which results from an externally applied force.

e = ____ (inch/inch, mm/mm)

A

dimensions

(L1 - L0)/L0

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6
Q

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

A

young’s modulus

elongated

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7
Q

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

A

proportional
permanently
ultimate tensile strength

stiffness/rigidity
yield strength (YS)
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8
Q

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

A

ductility

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9
Q

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

A

disappears

permanently

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10
Q

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 ____

A

permanent deformation

  1. 1%
  2. 2%

stress

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11
Q

Proportional Limit (PL)

The value of stress at which the stress/strain diagram deviates from the initial ____ relation

The useful ____ of a dental material

A

linear

strength

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12
Q

Elastic Limit vs. Proportional limit vs. Yield Strength

All of these are essentially ____

A

equal

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13
Q

Modulus of Elasticity

____ of the initial linear portion of the stress/strain diagram

____ of a material

Modulus of elasticity = ____

A

slope
rigidity
stress (MPa) / Strain

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14
Q

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

A

maximum
fracture strength
different

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15
Q

Failure of Dental Restorations

A permanent deformation of a dental appliance under occlusal loading represents a functional failure - by changes in ____ and ____

A

occlusal relationship

fit

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16
Q

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

A
tensile
fracture
burnishing of inlays
orthodontic
burnishing inlays
17
Q

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)

A

compressive
permanent deformation
ceramic

18
Q

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

A

recoverable
elastic
springiness
linear

19
Q

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

A

linear

toughness

20
Q

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

A

fracture
proportional limit
strain
resilience/toughness

21
Q

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

A
stiff
ductile
YS
UTS
large area
22
Q

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 ____)

A

stiffness
UTS
tough
ductile

23
Q

Other Mechanical Properties

Transverse strength, bend test:
The maximum ____ which a material can support before failure when loaded as a beam supported at ____ ends.

A

stress

two

24
Q

Three Point Bend Test

Stress at where it breaks is the ____ strength (same as flexural); modulus of rupture: determining the ____ using the bend test

A

transverse strength

modulus of elasticity

25
Diametral Tensile Strength Tensile properties of ____ materials are obtained by loading a ____ of the sample: epsilon = 2P/piDT (psi, MPa)
brittle | disk
26
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
27
Hardness Resistance of a material to ____ or to being ____ by another material
permanent indentation | scratched
28
Hardness Measurements ``` Brinell Rockwell ____ ____ ____ ```
Vickers Knoop Barcol
29
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
30
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
31
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
32
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
33
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
34
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 ```
35
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
36
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
37
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
38
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