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Basic Biomaterials > Mechanical Properties > Flashcards

Mechanical Properties Flashcards

1
Q

Def of mechanical properties

A

Group of physical properties that describe the behavior of the material under force or load

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2
Q

Importance of mechanical properties

A

Understanding & predicting the behavior of the material under load

Proper selection & designing of the restoration

Understanding reasons of failure

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3
Q

Force

A

External action.

One body interacting with another generates force.

Def:

✓ The action which cause one of the following reactions or all of them:

o Displacement.

o Acceleration.

o Deformation.

Units:

Newton (international system)

Occlusal forces:

Range from 200-3500 N

Post > ant teeth

Adult > children

Male > female

Natural > restorations

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4
Q

Stress

A

Def:

The internal reaction to the external applied force which is equal in magnitude & opposite in direction to the applied force.

Force per unit area arising from applied load.

Stress Force/area G=F/A

Or

Units:

✓ Pascal (pa) = N/m²

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5
Q

Types of stress

A

Axial tension

Elongation -
Away from each other

Axial, compression

Compression
Toward each other

Shear

Shear
Parallel مش على نفس الخط

Twisting moment.
Torsion

Bending moment
Bending

Complex stresses:

Combination of various types of stresses.

Most occlusal forces are complex stresses. Compression

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6
Q

Strain

A

Deformation or distortion produced as a result of stress produced within the material.

✔ It’s the change in length per unit length.

Strain deformation/original length.

Units:

AL (L final- L original )/L original

Strain has no units.

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7
Q

Types of strain

A

Elastic deformation

✔Reversible: when the stress is removed, the material returns to the dimension it had before the loading.

Mechanism:

o Stretching of interatomic bonds on stress & on removal of stress recovery occurs

2- Plastic deformation Permanent

✔ Irreversible: when the stress is removed, the material does not return to its previous dimension.

Mechanism:

o Deformation occurs by breaking and re-arrangement of atomic bonds (in crystalline materials primarily by motion of dislocations)

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8
Q

Stress-Strain relationship test

A

This test can be done with tensile, compressive or shear loading of samples using universal testing machine.

Stress strain curve consists of two portions:

1- Elastic portion (linear)

2- Plastic portion (non linear)
قيمة ثابتة

Stress

X Ultimate tensile strength

Elastic region Linear

Elastic region
stress and strain.

علاقة طردية منتظمة

direct linear relation (

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9
Q

Proportional Limit

A

It is the maximum stress up to which, the stress is linearly proportional to strain (Point A).

end of linear relation between stress and strain

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10
Q

Elastic Limit

A

Maximum stress a material can withstand without undergoing permanent deformation (point B).

Elastic limit & proportional limit have nearly the same value for most materials but differ in the fundamental concept: t

The first deal with the proportionalality of stress & strain where the elastic limit describes the elastic behaviour of the material (except for superelastic materials)

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11
Q

Significance of elastic limit

A

Dental restorations should be constructed from materials with a high PL to avoid permanent deformation as this represents a functional failure

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12
Q

Yield stress or proof stress

A

Beginning of permanent formation.

✔ It is the stress at which materials begin to behave in a plastic manner and there is a defined amount of permanent strain

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13
Q

Significance of yield stress

A

The yield strength of restorations should be higher than masticatory forces to avoid permanent deformation

✔Permanent deformation & stresses in excess of EL are needed when shaping an orthodontic wires or clasps of a removable partial denture

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14
Q

Ultimate (Tensile) Strength

A

highest point in stress-strain Curve

✓ Maximum stress the material can withstand without fracture

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15
Q

Significance of ultimate strength

A

The yield strength is often of greater importance than ultimate strength as it is an estimate of when a material will start to deform permanently

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16
Q

Fracture strength

A

It is the strength at which the material fractures (Point F

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17
Q

Flexibility

A

Large elastic strain with low stress.

✓ Maximum flexibility is defined as the strain occurring when the material is stressed to its PL. ↳ elastic limit

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18
Q

Significance of flexibility

A

A larger strain or deformation with slight stresses is an important consideration in orthodontic appliances.

✔ Impression materials should have large flexibility or elastic deformation to be withdrawn from severe undercuts without permanent deformation

19
Q

Elastic Modulus (young’s modulus)

A

✓Constant of proportionality between stress & stain

✔The slope of the linear portion of the stress-strain curve

✓ Represents stiffness or rigidity of the material within the elastic limit

20
Q

Stiffness or rigidity

A

The resistance of the material to elastic deformation

The steeper the slope, the greater the modulus & rigidity.

E=Stress/strain

21
Q

Significance of stiffness and rigidity

A

High elastic modulus of denture bases & long span bridges is important to avoid bending during function and for uniform load distribution.

✔ In orthodontic:

Stiff wire
high, rapid force

Flexible wire
low, slow force
bending

22
Q

Ductility

A

The ability of the material to be plastically deformed under tension without fracture. tensile.

The ability of the material to be drawn into wire by means of tension.

23
Q

Malleability

A

The ability of the material to be plastically deformed under compression without fracture.

Or

The ability of the material to be hammered into thin sheets without fracture

24
Q

Significance of ductility and malleability

A

Burnishability of the margins of a casted restoration

Adjustments of clasps of RPD Removable partial denture

✔Preparation of orthodontic appliance

25
Q

Brittleness

A

Inability of the material to show little or any plastic deformation.

✓ Fracture at or near to its P.L

26
Q

Resilience

A

مقاومة ✔ Resilience is the resistance of a material to permanent deformation.

✔ It indicates the amount of energy necessary to deform the material to PL.

✔Measured by the area under the elastic portion of the stress-strain curve

27
Q

Significance of resilience

A

Resilience has particular importance in the evaluation of resilient- denture lining materials, tissue conditioners.

Evaluation of orthodontic wires

28
Q

Toughness

A

Toughness is the resistance of a material to fracture.

An indication of the amount of energy necessary to cause fracture.

✔Measured by the area under the whole stress-strain curve.

✔The toughest materials have high P.L & good ductility

29
Q

Significance of toughness

A

Addition of zirconia, alumina and leucite to dental porcelain to resist crack propagation and increase fracture toughness

30
Q

Tensile strength

A

The maximum stress the material can withstand before failure under tensile mode of loading

31
Q

Significance of tensile strength

A

Used for testing bond strength between 2 materials

microtensile bond strength test.

32
Q

Compressive strength

A

The maximum stress the material can withstand before failure under compressive mode of loading

✓Cylindrical specimen

complex stress formations in the body:

-o Forces of shear along a cone-shaped area at each end.

  • Tensile forces in the central portion of the mass.
33
Q

Significance of compression strength

A

Useful for comparing brittle materials that are brittle & generally weak in tension as amalgam, composite resin and dental cements.

Restorative materials should have high compressive strength to

withstand forces of mastication (Mostly compressive type)

34
Q

Shear strength

A

It is the maximum stress that a material can withstand before failure in a shear mode of loading

35
Q

Significance of shear strength

A

It is important in the study of interfaces between two materials, such as porcelain fused to metal or an implant tissue interface

Tensil and shear strength testing bond strength between 2 materials

36
Q

Diametral tensile strength-Indirect tensile test-Brazilian test-Diametral compression test

A

Alternative method for testing brittle materials that are week in tension

The ultimate tensile strength of a brittle material is determined through compressive testing.

Used for brittle materials only

A disc of a brittle material is compressed diametrically in a testing machine till fracture

✔The compressive stress applied to the specimen introduce tensile stress in the material in the plan perpendicular to the force application.

37
Q

Transverse strength =Flexural strength-Modulus of rupture= 3 point bending test

A

Obtained when a sample bar supported at each end is loaded with a load in the middle.

It expresses a material ability to bend before it breaks.

✓ This test determines not only the strength of the material indicated, but also the amount of deformation expected

38
Q

Significance of transverse strength

A

Denture base materials & long span bridges should have sufficient stiff more than 3 units transverse strength to avoid bending during function and to allow uniform load distribution.

stiffness rigidity + Elastic modulus.

39
Q

Impact strength

A

The amount of energy required to induce fracture of such material under an impact or sudden force

40
Q

Significance of impact strength

A

Dentures should have high impact strength to withstand sudden fall down of a denture on a hard floor.

41
Q

two methods for measuring the impact strength

A

Charpy Impact Test

The specimen is supported horizontally at the two ends.

Izod Impact Test

The specimen is supported vertically at one end

42
Q

Tear strength

A

It is the resistance of the material to tearing forces

Specimens for tear strength are usually crescent or trouser shaped

Tear strength is dependent on the rate of loading because of the viscoelastic nature of materials tested (rapid loading rates higher tear strength

43
Q

Significance of tear strength

A

Important for dental polymer as: flexible impression materials, maxillofacial materials, and resilient denture liners. soft

Snap removal of impression higher tear strength.

Tear strength of rubber impression materials is higher than that of hydrocolloids

Basic Biomaterials (6 decks)

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