Mechanical Properties I and II

Question 1

1. Describe the importance of the mechanical properties of dental biomaterials

Answer 1

to properly select materials and design devices and appliances in dentistry

Question 2

2. List the objectives in measuring mechanical properties of materials

Answer 2

• Know the fundamental properties (strength, elasticity, etc.)
• Determine how those properties hold up in conditions the material will be used (impact, fatigue, abrasion…)

Question 3

State the 1st Classifications of mechanical properties:

Answer 3

o Bulk Properties

o Surface Properties

Question 4

State the 2nd Classification of mechanical properties:

Answer 4

sub-classifications of bulk properties

fundamental properties

applied properties

Question 5

1st Classification of Mechanical Properties DEFINE Bulk:

Answer 5

consider whole mass of material

(strength, elasticity)

Question 6

1st Classification of Mechanical Properties DEFINE Surface:

Answer 6

hardness (indentation) and wear (abrasion)

Question 7

2nd Classification of Mechanical Properties

How to obtain Fundamental Properties:

Answer 7

Fundamental: obtained from test applying steadily increasing load (stress-strain)

Question 8

2nd Classification of Mechanical Properties DEFINE applied properties, 3 examples

Answer 8

Applied properties: specific conditions in service, such single sudden force (impact), repeated low loads (fatigue), time dependent properties (viscoelasticity).

Question 9

Define Stress

Answer 9

force per unit area from externally applied forces, units: Pa/MPa.

Question 10

Units for Stress

Answer 10

Pa/MPa

Question 11

State the 3 Fundamental Types of Stress

Answer 11

Tension, Compression, Sheer

Question 12

Define Tension (a fundamental type of stress)

Answer 12

force perpendicular to resisting area

Question 13

Define Compression (a fundamental type of stress)

Answer 13

force perpendicular to resisting area, sense of load opposite to that of tension

Question 14

Define Shear (a fundamental type of stress)

Answer 14

force parallel to resisting area

Question 15

State the 2 combined types of stress: combination of fundamental stresses

Answer 15

Torsion and Flexure/Bending

Question 16

Do stress concentrations increase or decrease on notches, grooves, cracks, surface regularities?

Answer 16

The stress concentrations increase - that is why we avoid sharp corners in teeth cavity preparations.

Question 17

Ceramics and glasses (smooth surfaces) are sensitive to surface roughness because of _____ _______ at the top and bottom of topographical irregularities

Answer 17

stress concentrations

Question 18

Define Strain

Answer 18

describes relative deformation or change in shape and size with respect to initial length (dimensionless)

Question 19

State the 2 Types of Strain

Answer 19

Elastic and Plastic

Question 20

Elastic is ____, Plastic is ______.

Answer 20

Elastic is recoverable, Plastic is permanent

Question 21

Recording an impression - _____ on insertion (____ deformation). ______ on displacement from tissue (______ deformation)

Answer 21

Plastic on insertion (permanent deformation). Elastic on displacement from tissue (recoverable deformation

Question 22

Define Elastic Limit

Answer 22

Stress at which material changes from elastic to plastic strain. Stresses equal or higher than the elastic limit produce permanent/plastic deformation in the material.

Question 23

Ductility is measured by % of ______

Answer 23

elongation

Question 24

High % of elongation means

Answer 24

it’s a ductile material

Question 25

Low % of elongation means

Answer 25

it's a brittle material

Question 26

Define Ultimate Tensile Strength

Answer 26

stress at which the material breaks the last point in a stress-strain curve

Question 27

\*\* REVIEW THE STRESS STRAIN CURVE

Answer 27

First part of graph is linear (Elastic strains), slope of linear part of the graph is Modulus of Elasticity or Young’s modulus. Material follows Hook’s law.

Question 28

Remember 1: flexibility is a combination of the notion of less rigid but also more extend of deformation in the elastic region of a material.

Answer 28

High modulus = more rigid material, Low modulus: less rigid material

Question 29

Define Maximum Strength

Answer 29

- Maximum stress the material undergoes during stress-strain test - Also called generically ‘strength’.

Question 30

Define Resilience

Answer 30

- Amount of energy a material can absorb without permanently deforming.

Question 31

Resilience on the Stress Strain Graph

Answer 31

- Area under the stress-strain graph in the elastic portion of it.

Question 32

Define Toughness

Answer 32

- Total amount of energy a material can absorb until fracture.

Question 33

Toughness on the stress strain graph

Answer 33

- Area under the total stress-strain graph.

Question 34

Big ole bold reminder on the summary page:

Answer 34

Fundamental properties of a specific material can not be related between them, i.e. you need to know the individual values of each of them to have the total mechanical description of the material. Most importantly, you can not infer any mechanical property associated in the plastic region of the material by knowing the mechanical properties of that same material in the elastic region.

Question 35

5 Applied Properties

Answer 35

Impact, Fatigue Strength, Hardness, Abrasion, Viscoelasticity

Question 36

A material should be tested for its mechanical response while suffering an \_\_\_\_\_

Answer 36

impact. and the energy the material absorbs is the measured property in impact test.

Question 37

Define Impact

Answer 37

Measure of brittleness under application of forces at high speed.

Question 38

Fatigue Strength

Answer 38

Repeated low forces (bellow elastic limit) that bring the material to an unexpected fracture

Question 39

Number or repetitions/cycles of load recorded until fracture depending of the value of the repeated stress

Answer 39

Also Fatigue Strength

Question 40

Relating to Fatigue Strength, if material is cyclically loaded with stresses lower than this value the material will ____ \_\_\_\_\_.

Answer 40

never break.

Question 41

Define Hardness

Answer 41

ability for material to resist indentations, resistance to be scratched

Question 42

Define Abrasion

Answer 42

loss of anatomic form from surface or restoration

Question 43

Abrasion results in __ of the material

Answer 43

wear

Question 44

Wear is the ___ \_\_\_\_\_\_\_ property to be reliably measured. Depends so much on conditions in service.

Answer 44

most difficult

Question 45

Define Creep (in relation to Viscoelasticity)

Answer 45

slow deformation with time under constant stress

Question 46

Define Stress Relaxation (in relation to Viscoelasticity)

Answer 46

material constantly deformed, may show drop in stress to keep deformation with time.

Question 47

Viscoelasticity is ____ dependant.

Answer 47

TIME dependant. Viscous part is released/shown when constant stress or strain is applied for a certain period of time.

Question 48

Viscoelasticity is a combination of _____ and _____ behavior.

Answer 48

a combination of viscous and elastic behavior.

Question 49

List the Five Types of Stress

Answer 49

Tension, Compression, Sheer, Torsion Flexure/Bending

Question 50

21. List three standard methods of measuring hardness

Answer 50

?

Question 51

State Hooke’s Law

Answer 51

see pic

Question 52

Point out on a stress-strain graph the point of transition from elastic to plastic strain

Answer 52

see pic

Question 53

Equation to measure Stress (Pa)

Answer 53

Force/Area = N/m2 = Pa

Question 54

Strain Problem Example: A rod of 100mm (Li) stretched to 102mm (Lf) what is the strain

Answer 54

Change in L divided by Li 2/100 = 2%

Question 55

"what happens when a regular shaped pieve of material is placed on a testing machine and a gradually increasing load (either compressive or tensile) is applied to it?

Answer 55

Keep applying and it keeps deforming and displacing the material until the material breaks. (mechanical energy/mechanical work disforms until the response is to break). The machine measures load/force nad displacement/change in length.