Mechanical Properties Of Dental Materials Question And Answers

Mechanical Properties Important Notes

1. Stress

• Stress is force per unit area perpendicular to the force direction
• Stress Types:
  • Tensile stress
  • Shear stress
  • Compressive stress

2. Strain

• It is the change in length per unit length of an object subjected to stress
• Strain Types:
  • Elastic strain
  • Plastic strain

Read And Learn More: Dental Materials Question and Answers

3. Hardness

• Hardness is resistance to abrasion and indentation
• Hardness tests
  • Macro hardness test – Brinell and Rockwell
  • Micro hardness tests – Knoop and Vickers test

4. Compressive, Tensile, and shear stress

5. Ductility

• It is the ability of a metal to be drawn into wire under tensile load without rupture
• It can be measured by percent elongation and cold blend test

6. Malleability

It is the ability of a material to sustain permanent deformation without fracture under compression.

7. Modulus of elasticity

• The stress/ strain ratio within the proportional limit is called the elastic modulus or Young’s modulus
• It measures the relative rigidity or stiffness of the material

8. Resilience

• It is the amount of energy absorbed by a structure when it is stressed to the proportional limit
• The elastic area of the stress-strain graph gives resilience
• It is measured by R
  • R= P²/2E
  • R = resilience
  • P = proportional limit
  • E = modulus of elasticity

9. Proportional limit

• It is the maximum stress at which stress is proportional to strain and above which plastic deformation occurs
• It represents the maximum stress above which stress is no longer proportional to strain

10. Poisson’s ratio

Poisson’s ratio = Lateral strain/ Axial strain.

Mechanical Properties Short Essays

Question 1. Define stress and strain. Name types of stresses and strains. How are these two properties important in the study of dental materials?
Answer:

Stress and strain

Importance of Stress and Strain:

• Stress and strain are related to the force applied
• Stress is the displacing force in a given area while strain is the change in dimension
• In the oral cavity, many complex forces exist which tend to deform the material of the restorations
• Thus, it is important to know about the stress and strain of the material to interpret how the material will behave under the effect of such forces

Question 2. Stress and strain relation.
Answer:

Stress and strain relation

For each material, there is a stress-strain proportional relationship establishing a stress-strain curve

• As shown in a curve, stress is directly proportional to strain up to point P called the proportional limit, and elastic deformation occurs
• At this point, the object will withstand the forces applied without permanent deformation
• It determines the elasticity of a material
• When the applied load exceeds this point, irreversible deformation occurs resulting in plastic deformation
• After this point, stress is no longer proportional to strain
• The area under the elastic portion of the curve represents resilience
• This indicates the amount of energy required to deform the material to its proportional limit
• The area under the elastic and plastic portion of the curve indicates the toughness of a material.

Question 3. Types of stresses
Answer:

Types of stresses are:

1. Tensile stress:

It is the ratio of tensile force to the original cross-sectional area perpendicular to the direction of the applied force

Tensile stress Direction offerees:

The two forces acting are directed away from each other in the same straight line

Tensile stress Effect:

The load tends to stretch or elongated a body

2. Shear stress:

It is the ratio of force to the original cross-sectional area parallel to the direction of the force applied

Shear stress Direction of forces:

The two forces are directed parallel to each other

Shear stress Effect:

It resists sliding one portion of a body over another

3. Compressive stress:

It is the ratio of compressive force to cross-sectional area perpendicular to the axis of applied force

Compressive stress Direction of forces:

The forces are directed toward each other in the same line

Compressive stress Effect:

The load tends to shorten a body

Question 4. Hardness (or) Hardness tests
Answer:

Hardness Definition:

Hardness is the resistance of a material to plastic deformation measured under an indentation load

Factors affecting hardness:

• The following factors influence the hardness of a material:
• Strength
• Proportional limit
• Ductility
• Malleability

Hardness tests:

• Microhardness tests:
  • Knoop hardness test
  • Vickers hardness test
• Macro hardness tests:
  • Brinell Hardness test
  • Rockwell hardness test

Question 5. Ductility and malleability
Answer:

1. Ductility:

Ductility Definition:

It is the ability of a material to withstand large permanent deformation under a tensile load before it fractures

Ductility Factors affecting it

• It is dependent on tensile strength
• It decreases as the temperature raises

Ductility Measurements:

Ductility is measured by three methods

• By measuring the percentage elongation after fracture
  • It is a common method
  • The initial length of the wire is measured before the fracture
  • This is compared with that of an increase in length after fracture
• Measuring reduction in cross-sectional area of fractured ends

The percentage of decrease in the cross-sectional area of the fractured end is calculated and compared to that of the cross-sectional area before the fracture

•  Cold bend test
  • The material is clamped in a vise and bent around a mandrel of a specified radius
  • The number of bends to fracture is counted

2. Malleability

Malleability Definition:

It is the ability of a material to sustain permanent deformation without fracture under compression

Malleability Importance:

Metals according to their ductility and malleability are

1. First is gold- It is the most ductile and malleable
2. Second is silver
3. Third in ductility is platinum and in malleability is copper

• The ductility is assessed by the stress-strain curve
• After fracture, the stress reduces to zero.

Mechanical Properties Short Question And Answers

Question 1. Name types of strength. Discuss the importance of strength in the study of dental materials
Answer:

Strength:

Definition:

It is the stress necessary to cause either fracture or a specified amount of plastic deformation.

Strength Types:

1. Tensile strength or ultimate tensile strength

It is the maximal stress the structure will withstand before the rupture

2. Shear strength

It is the maximal shear stress at the point of fracture of a test specimen

3. Compressive strength

It is the compressive stress within a compression test specimen at the point of fracture

4. Flexural strength

It is the force per unit area at the point of fracture of a test specimen subjected to flexural loading

5. Impact strength

It is the energy required to fracture a material under an impact force

Importance of strength:

• Strength describes the types of deformation that can occur
• It measures the interatomic forces over the stressed structure
• Shear strength is used to study the interface between two materials
• Transverse strength is used to test dentures base resins and long-span bridges

Question 2. Modulus of elasticity
Answer:

Modulus of elasticity Synonyms:

• Young’s modulus
• Elastic modulus

Modulus of elasticity Definition:

It is the relative stiffness or rigidity of material within the elastic range.

Modulus of elasticity Measurement:

• It is the ratio of stress to strain and is described as E.
• E= Stress / Strain

Modulus of elasticity Importance:

• It indicates that the less the strain, the greater will be the stiffness
• Elastic modulus has a constant value
• It is not affected by the amount of plastic and elastic stress that is induced in the material
• It is independent of the ductility of the material
• By determining Young’s modulus of enamel and dentin, it describes that

1. Enamel is stiffer and more brittle
2. Dentin is more flexible and tougher

Modulus of elasticity Unit:

Ciiganewtons per square meter ( GN/ m2)

Question 3. Proportional limit
Answer:

Proportional limit Definition:

It is the maximum stress at which stress is proportional to strain and above which plastic deformation occurs

Proportional limit Importance:

• It is the greatest elastic stress possible in accordance with Hooke’s law
• It represents the maximum stress above which stress is no longer proportional to strain

Question 4. Resilience
Answer:

Resilience Definition:

It is a relative amount of elastic energy’ absorbed within a unit volume of a structure when it is stressed to its proportional limit.

Resilience Measurement:

It is measured in terms of the modulus of resilience, R.

• R = P²/2E where,
• R- Modulus of resilience
• P- Proportional limit
• E- Modulus of elasticity
• Resilience can be illustrated by the stress-strain curve
• The area bounded by the elastic region is a measure of resilience

Question 5. Elastic limit
Answer:

Elastic limit Definition:

It is defined as the greatest stress to which a material can be subjected such that it returns to its original dimensions when the force is released

Elastic limit Importance:

• It describes the elastic behavior of the material
• It can measure any type of stress

Question 6. Poisson’s ratio
Answer:

Poisson’s ratio

• When a tensile force is applied along one axis to produce elongation, compressive strain is produced at right angles proportionately.
• It is related to the nature and symmetry of the interatomic bonding forces
• For an ideal isotropic material of constant volume, the ratio is 0.5
• Poisson’s ratio = Lateral strain / Axial strain

Question 6. Strain hardening
Answer:

Strain hardening Synonyms:

• Cold working
• Work hardening

Strain hardening Features:

• The process of plastically deforming a metal usually at room temperature is called cold working
• When a metal is stressed beyond its proportional limit, the hardness, and strength of the metal increase but the ductility decreases
• Repeated plastic deformation of the metal leads to brittleness of the wire and it will fracture
• In the stress-strain curve, as the elastic modulus remains constant, the plastic deformation part of the curve would be decreased.

Question 7. Define: Elasticity and stiffness
Answer:

Elasticity:

It is the tendency of solid materials to return to their original shape after being deformed.

Stiffness:

• It is the ratio of the force required to create a specified deflection
• It is the ability of a material to resist bending or stretching
• A stiff material has a high Young’s modulus.

Question 8. Yield strength
Answer:

Yield strength

• It is the stress at which a test specimen exhibits a specific amount of plastic strain.
• Yield strength is greater than the proportional limit
• If it exceeds the masticatory stresses, the restorations may not function any longer.

Question 9. Toughness
Answer:

Toughness

• It is the ability of a material to absorb elastic energy and to deform plastically before fracture
• It increases with increases in strength and ductility
• The greater the strength and the higher the ductility, the greater the toughness
• Tough material is generally strong.

Question 10. Ductility and malleability
Answer:

Ductility:

• It is the ability of a material to withstand large permanent deformation under a tensile load before it fractures
• Factors affecting it are:
  • It is dependent on tensile strength
  • It decreases as the temperature raises

Malleability:

It is the ability of a material to sustain permanent deformation without fracture under compression.