## 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= P
^{2}/2E - R = resilience
- P = proportional limit
- E = modulus of elasticity

- R= P

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

- First is gold- It is the most ductile and malleable
- Second is silver
- 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

- Enamel is stiffer and more brittle
- 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 = P
^{2}/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.