Mechanical Properties Flashcards
Def of mechanical properties
Group of physical properties that describe the behavior of the material under force or load
Importance of mechanical properties
Understanding & predicting the behavior of the material under load
Proper selection & designing of the restoration
Understanding reasons of failure
Force
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
Stress
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
Types of stress
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
Strain
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.
Types of strain
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)
Stress-Strain relationship test
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 (
Proportional Limit
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
Elastic Limit
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)
Significance of elastic limit
Dental restorations should be constructed from materials with a high PL to avoid permanent deformation as this represents a functional failure
Yield stress or proof stress
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
Significance of yield stress
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
Ultimate (Tensile) Strength
highest point in stress-strain Curve
✓ Maximum stress the material can withstand without fracture
Significance of ultimate strength
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
Fracture strength
It is the strength at which the material fractures (Point F
Flexibility
Large elastic strain with low stress.
✓ Maximum flexibility is defined as the strain occurring when the material is stressed to its PL. ↳ elastic limit
Significance of flexibility
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
Elastic Modulus (young’s modulus)
✓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
Stiffness or rigidity
The resistance of the material to elastic deformation
The steeper the slope, the greater the modulus & rigidity.
E=Stress/strain
Significance of stiffness and rigidity
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
Ductility
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.
Malleability
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
Significance of ductility and malleability
Burnishability of the margins of a casted restoration
Adjustments of clasps of RPD Removable partial denture
✔Preparation of orthodontic appliance
Brittleness
Inability of the material to show little or any plastic deformation.
✓ Fracture at or near to its P.L
Resilience
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
Significance of resilience
Resilience has particular importance in the evaluation of resilient- denture lining materials, tissue conditioners.
Evaluation of orthodontic wires
Toughness
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
Significance of toughness
Addition of zirconia, alumina and leucite to dental porcelain to resist crack propagation and increase fracture toughness
Tensile strength
The maximum stress the material can withstand before failure under tensile mode of loading
Significance of tensile strength
Used for testing bond strength between 2 materials
microtensile bond strength test.
Compressive strength
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.
Significance of compression strength
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)
Shear strength
It is the maximum stress that a material can withstand before failure in a shear mode of loading
Significance of shear strength
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
Diametral tensile strength-Indirect tensile test-Brazilian test-Diametral compression test
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.
Transverse strength =Flexural strength-Modulus of rupture= 3 point bending test
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
Significance of transverse strength
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.
Impact strength
The amount of energy required to induce fracture of such material under an impact or sudden force
Significance of impact strength
Dentures should have high impact strength to withstand sudden fall down of a denture on a hard floor.
two methods for measuring the impact strength
Charpy Impact Test
The specimen is supported horizontally at the two ends.
Izod Impact Test
The specimen is supported vertically at one end
Tear strength
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
Significance of tear strength
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
