3. Mechanical Properties

Question 1

Mechanical Properties of Dental Materials

Properties defined by the laws of ____.

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

Answer 1

mechanics
applied
physical

Question 2

Forces on Dental Materials

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

Answer 2

tensile
compressive
shear

Question 3

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 ____

Answer 3

stress
force
area
length
length
original length
strain

Question 4

Stress

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

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

Tensile
Compressive
Shear

Answer 4

resists
F/A
N/m^2
Mpa

Question 5

Strain

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

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

Answer 5

dimensions

(L1 - L0)/L0

Question 6

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

Answer 6

young’s modulus

elongated

Question 7

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

Answer 7

proportional
permanently
ultimate tensile strength

stiffness/rigidity
yield strength (YS)

Question 8

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

Answer 8

ductility

Question 9

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

Answer 9

disappears

permanently

Question 10

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 ____

Answer 10

permanent deformation

0. 1%
1. 2%

stress

Question 11

Proportional Limit (PL)

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

The useful ____ of a dental material

Answer 11

linear

strength

Question 12

Elastic Limit vs. Proportional limit vs. Yield Strength

All of these are essentially ____

Answer 12

equal

Question 13

Modulus of Elasticity

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

____ of a material

Modulus of elasticity = ____

Answer 13

slope
rigidity
stress (MPa) / Strain

Question 14

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

Answer 14

maximum
fracture strength
different

Question 15

Failure of Dental Restorations

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

Answer 15

occlusal relationship

fit

Question 16

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

Answer 16

tensile
fracture
burnishing of inlays
orthodontic
burnishing inlays

Question 17

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)

Answer 17

compressive
permanent deformation
ceramic

Question 18

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

Answer 18

recoverable
elastic
springiness
linear

Question 19

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

Answer 19

linear

toughness

Question 20

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

Answer 20

fracture
proportional limit
strain
resilience/toughness

Question 21

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

Answer 21

stiff
ductile
YS
UTS
large area

Question 22

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 ____)

Answer 22

stiffness
UTS
tough
ductile

Question 23

Other Mechanical Properties

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

Answer 23

stress

two

Question 24

Three Point Bend Test

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

Answer 24

transverse strength

modulus of elasticity

Question 25

Diametral Tensile Strength

Tensile properties of ____ materials are obtained by loading a ____ of the sample:

epsilon = 2P/piDT (psi, MPa)

Answer 25

brittle

disk

Question 26

Diametral Tensile Strength

Stresses are ____ to the force and it breaks right down the ____; calculate tensile strength using this test (used for ____ and ____)

Answer 26

perpendicular
middle
amalgam
composites

Question 27

Hardness

Resistance of a material to ____ or to being ____ by another material

Answer 27

permanent indentation

scratched

Question 28

Hardness Measurements

Brinell 
Rockwell 
\_\_\_\_
\_\_\_\_
\_\_\_\_

Answer 28

Vickers
Knoop
Barcol

Question 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

Answer 29

round-tip
steel-tip
dental
diamond-tipped

Question 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

Answer 30

cycles
UTS
surface cracks

Question 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)

Answer 31

unlimited

break/fail

Question 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

Answer 32

synonomous

break
endurance limit
fatigue limit
ideal load

Question 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

Answer 33

1500

300,000

Question 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

Answer 34

constant stress
proportional limit
0.5
fracture
dental amalgam
high

Question 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.

Answer 35

immediate instantaneous deformation

gradual deformation

Question 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

Answer 36

surface defects
cracks (flaws)
crack tip

Question 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

Answer 37

plastic deformation
tensile force

plastic deformation
longer

compressive

tensile
shear

Question 38

Stress Concentrations

Clinical Consequences

____ restorations to minimize flaws

Avoid ____ in shape in design of restorations

____ cusp tips to spread area of load application

Answer 38

polish
sharp changes
round