Mechanical Properties

Question 1

what are the 2 objectives of measuring mechanical properties of a material

Answer 1

1. to obtain fundamental properties - strength, elasticity, etc.
2. to attempt to determine properties under conditions attempting to simulate service conditions of the material - impact, fatigue, abrasion, etc.

Question 2

How do you test for hardness

Answer 2

press an indenter of known shape into surface with a known force for a known time

Question 3

hardness

Answer 3

a surface property

the ability of a material to resist indentation/scratching

Question 4

stress relaxation

Answer 4

when a material is deformed under a constant level of strain, it may show a drop in stress with time

Question 5

define wear

Answer 5

result of abrasion

Question 6

Name the classifications of bulk properties

Answer 6

fundamental properties

applied properties

Question 7

proportional limit

Answer 7

highest value of stress where stress is proportional to strain
marks the end of the stress-strain graph following Hooke’s law
lesser than or equal to the elastic limit, depending on the material

Question 8

define mechanical properties

Answer 8

the behavior of materials under applied forces

Question 9

elongation

Answer 9

maximum deformation (strain) before a material breaks

Question 10

why is strain dimensionless

Answer 10

because it is a change in length per unit length

Question 11

plastic strain

Answer 11

permanent

if you remove the force, there is a permanent change in size and shape

Question 12

name the fundamental types of stress

Answer 12

tension
compression
shear

Question 13

define fundamental properties

Answer 13

classification of bulk properties

Obtained from test steadily increasing load (stress-strain)

Question 14

which law of physics does the first portion of the stress-strain graph follow?

Answer 14

Hooke’s law

Question 15

tension

Answer 15

a fundamental type of stress where the force is perpendicular to the resisting area
opposite of compression

Question 16

the slope of the first (linear) part of the stress-strain graph is _____ and called ______

Answer 16

constant

Young’s modulus/modulus of elasticity

Question 17

what is the only ‘pure’ elastic fundamental property?

Answer 17

Young’s modulus/modulus of elasticity

Question 18

ultimate tensile strength

Answer 18

stress at which the material breaks

last point in the stress-strain curve

Question 19

bulk properties

Answer 19

a classification of mechanical properties
consideration of the whole mass of material
strength, elasticity, etc.

Question 20

elastic limit

Answer 20

stress at which a material changes from elastic to plastic strain

Question 21

Hooke’s Law

Answer 21

law of elasticity
for relatively small deformations of an object, the displacement/size of deformation is directly proportional to the deforming force/load
The linear (first) portion of the stress-strain graph follows this law

Question 22

name the 2 consequences of viscoelasticity

Answer 22

creep

stress relaxation

Question 23

how is strain considered in restorations and prostheses?

Answer 23

restorations/prosthesis are subjected to stress

plastic deformation in service must be avoided by choosing the right material and design

Question 24

the design of prostheses depends on ___

Answer 24

mechanical properties

partial denture - clasps must be flexible, connectors must be rigid

Question 25

Young's moldulus

Answer 25

aka modulus of elasticity slope of linear portion of stress-strain graph stress/strain constant measure of rigidity/stiffness of material

Question 26

ductility is measured by

Answer 26

% of elongation

Question 27

viscoelasticity is ______ - viscous part is released/shown when constant stress/strain is applied for a certain period of time

Answer 27

time dependant

Question 28

wear

Answer 28

result of abrasion

Question 29

ductility

Answer 29

in TENSION the ability of a material to be drawn into the form of a wire measured by percentage of elongation

Question 30

the ultimate tensile strength and maximum strength correspond to...

Answer 30

the plastic region of the material

Question 31

a high percentage of elongation indicates a ______ material, and a low percentage of elongation indicates a _____ material

Answer 31

high - ductile | low - brittle

Question 32

define strain

Answer 32

a number that describes the relative deformation or change in shape and size of materials under applied forces (stresses)

Question 33

stress concentration

Answer 33

stress increases/concentrates at notches/grooves/cracks/surface irregularities Importance of smooth surfaces

Question 34

stresses equal to or above the elastic limit...

Answer 34

will yield plastic (permanent) strain/deformation

Question 35

What are the x and y axis of the graph showing fatigue strength

Answer 35

x - log N (N = # cycles to failure) | Y - stress

Question 36

name the 2 (first) classifications of mechanical properties

Answer 36

bulk properties | surface properties

Question 37

In the fatigue strength graph, what does N represent?

Answer 37

the number of repetitions of low stresses applied to the material

Question 38

abrasion

Answer 38

loss of anatomic form from surface of restoration (or tooth) surface property difficult to measure relevantly and with reproducibility UMN - artificial mouth results in WEAR

Question 39

define applied properties

Answer 39

classification of bulk properties impact - single sudden force fatigue - repeated low loads viscoelasticity - time dependant properties

Question 40

Modulus of resilience

Answer 40

amount of energy a material can absorb until it deforms permanently area below linear portion of stress-strain line (up until the elastic limit)

Question 41

a ______ Young's modulus indicates that the material is ____

Answer 41

higher modulus | more rigid

Question 42

define stress

Answer 42

the force per unit area within materials that arises from externally applied forces (solids)

Question 43

what is the major problem when testing mechanical properties of materials?

Answer 43

lab testing (in vitro) cannot adequately predict in vivo behavior

Question 44

name the combined types of stress

Answer 44

torsion | flexture/bending

Question 45

yeild point

Answer 45

when plastic strain becomes very pronounced Higher than elastic limit pretty much the same as the elastic limit

Question 46

strength/maximum strength

Answer 46

max stress the material undergoes during the stress-strain test highest point on the stress-strain graph

Question 47

_____ are very sensitive to surface roughness because of stress concentrations at the top and bottom of topographical irregularities

Answer 47

ceramics and glasses

Question 48

Generally, materials that are more rigid are less _____

Answer 48

flexible

Question 49

name the types of strain

Answer 49

elastic | plastic

Question 50

flexibility

Answer 50

refers to when a material breaks | generally, materials that are more rigid are less flexible

Question 51

discuss strain as it pertains to alginate impressions

Answer 51

when inserted, the strain on the alginate is plastic - permanent deformation upon setting, the alginate undergoes elastic strain as it is pulled out of the patient's mouth

Question 52

viscoelasticity

Answer 52

combination of viscous (plastic) and elastic behavior | TIME DEPENDENT - properties depend on rate of testing

Question 53

fatigue limit

Answer 53

lowest stress value on the fatigue strengh graph under this value of stress, the material will never break aka endurance limit

Question 54

the first part of a stress-strain graph is _____, representing ____

Answer 54

linear | elastic strains/deformation

Question 55

how is impact strength tested

Answer 55

notched specimen hit with a swinging pendulum (knife edge mounted in a weighted hammer) Energy to fracture material reduces amplitude of swing of pendulum

Question 56

shear

Answer 56

a fundamental type of stress where the force is parallel to the resisting area

Question 57

surface properties

Answer 57

a classification of mechanical properties | hardness, wear, etc.

Question 58

toughness

Answer 58

the amount of energy a material can absorb until it fractures area of all space below stress-strain graph

Question 59

define hardness

Answer 59

resistance to indentation

Question 60

in a stress-strain graph, stress is on the ____ axis

Answer 60

stress is the Y axis

Question 61

strength is a value of...

Answer 61

stress

Question 62

fatigue strength

Answer 62

process of degradation by repeated low forces

Question 63

elastic strain

Answer 63

recoverable | if you remove the force, the material reverts to its original size and shape

Question 64

areas under the stress-strain graph has units of...

Answer 64

energy | J/m^3

Question 65

malleability

Answer 65

in COMPRESSION | the abiligy of a material to be hammered into a sheet

Question 66

creep

Answer 66

consequence of viscoelasticity slow deformation with time of a material under a constant level of stress amalgam, braces make of plastic

Question 67

ductility and malleability relate to...

Answer 67

plastic strain

Question 68

toughness is the opposite of...

Answer 68

brittleness

Question 69

compression

Answer 69

a fundamental type of stress where the force is perpendicular to the resisting area opposite of tension

Question 70

impact strength

Answer 70

a measure of energy | sudden single force