02. Classification of Materials

Question 1

What are the three basic classifications of solid materials?

Answer 1

Metals, ceramics, and polymers

Additionally, there are composites, which are combinations of two or more of the basic material classes.

Question 2

What are the distinguishing characteristics of metals?

Answer 2

• Stiff
• Strong
• Ductile
• Resistant to fracture

Metals have large numbers of localized electrons and some exhibit desirable magnetic properties.

Question 3

What are the common types of ceramics?

Answer 3

• Traditional ceramics (e.g., porcelain, cement, glass)
• Non-traditional ceramics (e.g., alumina, silica, silicon carbide, silicon nitride)

Ceramics are compounds between metallic and nonmetallic elements, typically oxides, nitrides, and carbides.

Question 4

What is a key mechanical characteristic of ceramics?

Answer 4

They are very hard but brittle

Ceramics are relatively stiff and strong, comparable to metals.

Question 5

What are polymers primarily composed of?

Answer 5

Large molecules made of repeating units typically connected by covalent chemical bonds

Many are organic compounds based on carbon, hydrogen, and other nonmetallic elements.

Question 6

Fill in the blank: Polymers have _______ densities.

Answer 6

low

This is one of the defining characteristics of polymers.

Question 7

What is a composite material?

Answer 7

Engineered materials made from two or more constituent materials with significantly different physical or chemical properties

Composites remain separate and distinct on a macroscopic level within the finished structure.

Question 8

What is the design goal of a composite material?

Answer 8

To achieve a combination of properties not displayed by any single material

Examples include fiberglass and naturally occurring composites like bone and wood.

Question 9

What does CFRP stand for?

Answer 9

Carbon fiber reinforced polymer

CFRP is stiffer and stronger than glass fiber-reinforced materials and is used in high-tech applications.

Question 10

What are advanced materials?

Answer 10

Materials whose properties have been enhanced or newly developed high-performance materials

Examples include semiconductors, biomaterials, and smart materials.

Question 11

What are semiconductors?

Answer 11

Materials with electrical properties intermediate between conductors and insulators

Common examples include crystalline solids and hydrogenated amorphous silicon.

Question 12

What defines a biomaterial?

Answer 12

Any material that comprises whole or part of a living structure or biomedical device, which performs, augments, or replaces a natural function

Biomaterials must be compatible with body tissues and not produce toxic substances.

Question 13

What are smart materials?

Answer 13

Materials that can significantly change one or more properties in a controlled manner by external stimuli

They include sensors and actuators.

Question 14

What are shape-memory alloys?

Answer 14

Alloys that ‘remember’ their original shape and return to it upon heating

They are a type of actuator material.

Question 15

What are piezoelectric ceramics?

Answer 15

Materials that expand and contract in response to an applied electric field

They can also generate an electric field when their dimensions are altered.

Question 16

What are magnetostrictive materials?

Answer 16

Materials that respond to magnetic fields, analogous to piezoelectrics

They exhibit changes in shape or dimensions when subjected to magnetic fields.

Question 17

What are nanomaterials?

Answer 17

Materials possessing at least one external dimension measuring 1-100nm

They can have different physical and chemical properties compared to their bulk form counterparts.

Question 19

What are the main types of properties of materials?

Answer 19

Mechanical Properties, Electrical Properties, Thermal Properties, Magnetic Properties, Optical Properties, Deteriorative Properties

Mechanical Properties include stress, strain, elasticity, and hardness.

Question 20

Define Elastic Deformation.

Answer 20

Deformation that is reversible when the load is removed.

In elastic deformation, bonds stretch and return to the initial state upon unloading.

Question 21

Define Plastic Deformation.

Answer 21

Deformation that is permanent and does not return to the original shape when the load is removed.

In plastic deformation, bonds stretch and planes shear, resulting in a change in shape.

Question 22

What is Engineering Stress?

Answer 22

The force applied divided by the original area before loading.

It can be expressed as tensile stress (σ) or shear stress (τ).

Question 23

What is the formula for tensile stress (σ)?

Answer 23

σ = Ft / Ao

Ft is the tensile force and Ao is the original cross-sectional area.

Question 24

What does Engineering Strain measure?

Answer 24

The deformation or elongation of a solid body due to tensile force or stress.

It is calculated as ε = ΔL / Lo, where ΔL is the change in length and Lo is the original length.

Question 25

What is Poisson’s Ratio?

Answer 25

The ratio of lateral strain to axial strain. ## Footnote It is defined as ν = -εLateral / εAxial.

Question 26

What does the Modulus of Elasticity (E) represent?

Answer 26

The constant of proportionality between stress and strain. ## Footnote It is defined by Hooke's Law: σ = Eε.

Question 27

What is Yield Strength (σy)?

Answer 27

The stress at which noticeable plastic deformation occurs. ## Footnote It is typically defined at an offset strain of 0.002.

Question 28

What is Tensile Strength (TS)?

Answer 28

The maximum stress on the engineering stress-strain curve before fracture occurs. ## Footnote It is the point where noticeable necking begins in metals.

Question 29

What is Ductility?

Answer 29

The ability of a material to undergo large permanent deformations without rupture. ## Footnote It is often measured as percent elongation or percent area reduction.

Question 30

Fill in the blank: True stress is defined as _______.

Answer 30

The instantaneous applied load divided by the instantaneous cross-sectional area.

Question 31

What does Toughness measure?

Answer 31

The strength with which a material opposes rupture and the energy to break a unit volume of material. ## Footnote It is approximately measured by the area under the stress-strain curve.

Question 32

Define Hardness.

Answer 32

Resistance to permanently indenting the surface of a material. ## Footnote Hardness indicates resistance to plastic deformation and better wear properties.

Question 33

What is the relationship between hardness and wear properties?

Answer 33

Large hardness means better wear properties and resistance to plastic deformation. ## Footnote Hardness tests often involve applying a known force and measuring the size of the indent.

Question 34

What is Elastic Bulk modulus (K)?

Answer 34

The ratio of the infinitesimal increase in pressure to the resulting relative decrease of volume. ## Footnote It is defined as P = -K (ΔV/Vo).

Question 35

What is the formula for shear strain (γ)?

Answer 35

γ = Δx / y ## Footnote Where Δx is the change in deformation and y is the original length perpendicular to the axes.

Question 36

True or False: Elastic deformation occurs permanently.

Answer 36

False. ## Footnote Elastic deformation is reversible.

Question 37

What is the significance of the area under the stress-strain curve?

Answer 37

It approximates the toughness of the material. ## Footnote Toughness indicates the energy a material can absorb before failing.

Question 38

What is temperature?

Answer 38

Degree of hotness or coldness measured on a definite scale ## Footnote Units: Celsius scale (⁰C), Fahrenheit (⁰F), Kelvin scale (K) – absolute temperature scale

Question 39

Define heat.

Answer 39

Energy that is transferred from one body to another as the result of a difference in temperature

Question 40

What is heat capacity (C)?

Answer 40

Indicates a material’s ability to absorb heat from external surroundings ## Footnote Amount of energy required to produce a unit temperature rise; unit: J/mol•K

Question 41

What is the formula for heat capacity?

Answer 41

𝐂 = 𝐝𝐐 / 𝐝𝐓

Question 42

Define specific heat (c).

Answer 42

Heat capacity per unit mass ## Footnote Units: J/kg ∙ K, cal/g ∙ K, Btu/lbm ∙ ⁰F

Question 43

What are the two ways to measure specific heat?

Answer 43

* Cp – specific heat at constant external pressure * Cv – specific heat at constant external volume

Question 44

What is the relationship between Cp and Cv?

Answer 44

The magnitude of Cp is always greater than or equal to Cv

Question 45

What is thermal expansion?

Answer 45

Change in length with Temperature

Question 46

What is the linear coefficient of thermal expansion (𝜶𝟏)?

Answer 46

Material property that is indicative of the extent to which a material expands upon heating ## Footnote Units: (℃)−1 or (℉)−1

Question 47

What is the volume coefficient of thermal expansion (𝜶𝒗)?

Answer 47

Change in volume with Temperature

Question 48

What is the range of linear coefficients of thermal expansion for common metals?

Answer 48

Range between about 5 × 10−6 and 25 × 10−6 (°C)−1

Question 49

How do ceramics compare in thermal expansion to metals?

Answer 49

Ceramics have comparatively low coefficients of thermal expansion, typically ranging between about 0.5 × 10−6 and 15 × 10−6 (°C)−1

Question 50

Which type of materials has very large thermal expansions?

Answer 50

Polymers

Question 51

Which polymers typically have the highest and lowest thermal expansion coefficients?

Answer 51

* Highest: linear and branched polymers * Lowest: thermosets

Question 52

What is thermal conductivity?

Answer 52

Property that characterizes the ability of a material to transfer heat

Question 53

What is the formula for heat flux (q)?

Answer 53

Heat flow, per unit area, per unit time [units: W/m2 (Btu/ft2·h)]

Question 54

What is the unit for thermal conductivity (k)?

Answer 54

W/m·K (Btu/ft·h·°F)

Question 55

What is the driving force behind thermal conductivity?

Answer 55

Temperature gradient (dT/dx)

Question 56

Why are metals considered good conductors of heat?

Answer 56

Because of a large number of free electrons

Question 57

What effect does alloying with impurities have on thermal conductivity?

Answer 57

Results in a reduction in thermal conductivity

Question 58

How do ceramics behave in terms of thermal conductivity?

Answer 58

Ceramics are thermal insulators

Question 59

What effect does porosity have on thermal conductivity in ceramics?

Answer 59

Increasing pore volume results in reduction of the thermal conductivity

Question 60

What is the relationship between thermal stresses and temperature changes?

Answer 60

Thermal stresses are induced in a body as a result of changes in temperature

Question 61

What does the magnitude of thermal stress (𝝈) depend on?

Answer 61

Change in temperature from To to Tf and modulus of elasticity (E)

Question 62

What happens to thermal stress when Tf > To?

Answer 62

Stress is compressive (𝝈 < 0)

Question 63

What happens to thermal stress when Tf < To?

Answer 63

Tensile stress is imposed (𝝈 > 0)