Unit IV: Non-Metallic Materials - Review and Summary

 In this unit, the non-metallic materials—polymers, ceramics, and composites have been discussed in detail.

REVIEW AND SUMMARY

✔ In this unit, the non-metallic materials—polymers, ceramics, and composites have been discussed in detail.

✔ Polymers are large, high-molecular-weight molecules produced by joining smaller molecules called monomers.

✔ Polymerisation is the process of forming a polymer by linking together of monomers.

✔ Two polymerisation mechanisms used are:

1. Addition polymerisation, and

2. Condensation polymerisation.

✔ Addition polymerisation, also known as chain reaction poly- merisation, is a process by which two or more chemically similar monomers are polymerised to form long chain molecules.

✔ Condensation polymerisation, also known as step-growth poly- merisation, is the formation of polymers by stepwise inter- molecular chemical reactions that normally involve at least two different monomers.

✔ Polymers use various additives to improve their properties and performance. Some of the polymer additives are: filler materials, plasticizers, stabilizers, colorants, flame retardants, reinforcements, and lubricants.

✔ A plastic may be defined as an organic polymer, which can be moulded into any desired shape and size with the help of heat, pressure, or both.

✔ The two broad groups of plastics are:

1. Thermoplastics, and 2. Thermosetting plastics.

✔ Thermoplastics, also known as thermoplasts, are the plastics whose plasticity increases with the increase in temperature. Thermoplasts can be repeatedly moulded and remoulded to the desired shape and hence they have a good resale/scrap value. 

✔ Thermosetting plastics, also known as thermosets, are plastics which become permanently hard when heat is applied and do not soften upon subsequent heating. They cannot be remoulded/ reshaped again and again.

✔ It may be noted that most of the thermoplastics are formed by addition polymerisations whereas most of the thermosetting plastics are formed by condensation polymerisations.

✔ Based on the usage, commercial value, and cost, plastics can be classified as commodity plastics and engineering plastics. 

✔ The characteristics and typical applications of some important thermoplastics such as PE, PP, PS, PVC, PTFE, PMMA, PAN, PA, POM, PC, PET, PEEK, PPO, PPS, PI, and PAI have been discussed in this unit.

✔ Also the characteristics and typical applications of some important thermosetting plastics such PF, UF, MF, polyesters, EP and PUR have been elaborated.

✔ Ceramics are non-metallic and inorganic solids that are processed and/or used at high temperatures. Typical ceramics are electrical and thermal insulators with good chemical stability and good strength in compression.

✔ Engineering ceramics, also known as industrial ceramics or advanced ceramics, are those ceramics that are specially used in engineering applications or in industries.

✔ Engineering ceramics are mainly oxides, carbides, sulphides, and nitrides of metals.

✔ In this unit, the properties and typical applications of some important engineering ceramics such as Al2O3, SiC, Si3N4, PSZ, and sialon have been presented.

✔ Composites are produced when two or more materials are joined to give a combination of properties that cannot be attained in the original materials.

✔ Composites are composed of two phases: matrix phase and dispersed phase.

✔ This unit gives a summary of the particulate-reinforced and fibre-reinforced composites.

KEY TERMS YOU SHOULD REMEMBER

Non-metallic materials

Polymers

Polymerisation

Addition

polymerisation 

Condensation 

polymerisation 

Polymer additives

Fillers 

Plasticizers 

Stabilizers

Colorants

Flame retardants

Reinforcements

Lubricants

Natural plastics

Synthetic plastics 

Thermoplastics

PE, PP, PS, PVC, PTFE, PMMA,

PAN, PA, POM, PC,

PET, PEEK, PPO, PPS, PI, PAI 

Thermosetting plastics

PF, UF, MF, 

Polyesters, EP, PUR

Commodity plastics

Engineering plastics,

Ceramics

Engineering ceramics 

Al2O3, SiC, Si3N4 

PSZ, Sialon

Composites

Particulate-reinforced

composites 

Dispersion- strengthened 

composites

Large-particle

composites 

Fibre-reinforced

Composites

Unit V: Mechanical Properties And Testing - Review and Summary

 A number of the important mechanical properties of materials, predominantly metals, have been discussed in this unit.

REVIEW AND SUMMARY

✔ A number of the important mechanical properties of materials, predominantly metals, have been discussed in this unit. 

✔ Mechanical properties are those characteristics of material that describe its behaviour under the action of external forces. 

✔ Some of the important mechanical properties are elasticity, plasticity, ductility, malleability, brittleness, hardness, tough- ness, stiffness, resilience, creep, endurance, strength, impact strength and fatigue.

✔ Technological properties are those which have a bearing on the processing and/or application of materials.

✔ Some of the important technological properties are machina- bility, castability, weldability, and formability/workability. 

✔ Factors that highly influence the mechanical properties are 1. Grain size; 2. Heat treatment; 3. Atmospheric exposure; and 4. Low and high temperatures.

✔ When force is applied on a metal piece, then the size and/or shape will be altered. Any changes in the size and/or shape of the metal is called a deformation of the metal.

✔ Deformation of metals can be classified into: 1. Elastic deformation, and 2. Plastic deformation.

✔ Elastic deformation is the deformation of a body which completely disappears as soon as the external load is removed from the body.

✔ Plastic deformation is the deformation of a body which remains even after removing the external load from the body.

✔ Mechanisms/modes of plastic deformation: 1. Slip, and 2. Twinning.

✔ These mechanisms of plastic deformation are discussed in detail in this unit.

✔ Slip may be defined as the sliding of blocks of the crystal over another along definite crystallographic planes called slip planes.

✔ Slip begins when the shearing stress on the slip plane in the slip direction reaches a threshold value called the critical resolved shear stress (τcr).

τcr = σ cos φ cos λ

where the terms τcr, σ, φ and λ have usual meanings.

✔ In the above equation, the term 'cos φ cos λ' is known as the Schmid's factor.

✔ Schmid's law states that the stress required at a given temperature to initiate slip in a pure and perfect single crystal for a material is constant.

✔ Twinning is the process in which the atoms in a part of a crystal subjected to stress, rearrange themselves so that one part of the crystal becomes a mirror image of the other part. 

✔ Two types/causes of twins are: 1. Mechanical twins, and 2. Annealing twins.

✔ Fracture is the mechanical failure of the material which will produce the separation or fragmentation of a solid into two or more parts under the action of stresses.

✔ Types of fractures: 1. Brittle fracture, 2. Ductile fracture, 3. Fatigue fracture, and 4. Creep fracture.

✔ The mechanisms of these fractures are also presented in this unit.

✔ Brittle fracture may be defined as the fracture which takes place by the rapid propagation of crack with a negligible deformation.

✔ Griffith's fracture equation:

where notations have usual meanings.

✔ Ductile fracture may be defined as the fracture which takes place by a slow propagation of crack with appreciable plastic deformation.

✔ Fatigue fracture may be defined as the fracture which takes place under repeatedly applied stresses.

✔ The S-N diagram can be obtained by plotting the number of cycles of stress reversals (N) required to cause fracture against the applied stress levels (S).

✔ Creep may be defined as the property of a material by virtue of which it deforms continuously under a steady load.

✔ A creep curve shows the variation of the extension of a metal with time under different stresses.

✔ There are many different types of test are used for determining the various mechanical properties.

✔ Various mechanical tests are: 1. Tensile test; 2. Compression test; 3. Shear test; 4. Hardness test; 5. Impact test; 6. Fati- gue test and 7. Creep test.

✔ The tensile test describes the resistance of a material to a slowly applied stress. Important tensile properties include yield strength, tensile strength, modulus of elasticity, % elongation, and % reduction in area.

✔ The compression test is used to determine the tensile properties of brittle materials.

✔ The shear test describes the resistance of a material subjected to parallel and opposite tangential forces.

✔ The hardness test measures the resistance of a material to penetration and provides a measure of the wear and abrasion resistance of the material.

✔ The three most commonly used hardness tests are : 1. Brinell hardness test; 2. Vickers hardness test; and 3. Rockwell hardness test.

✔ The impact test describes the response of a material to a rapidly applied load. The energy required to fracture the specimen is measured and can be used as the basis for compa- rison of various materials tested under the same conditions. 

✔ Types of impact tests: 1. Izod tests, and 2. Charpy tests. 

✔ The fatigue test determine the resistance of a material to repeated pulsating or fluctuating loads. Important fatigue properties include fatigue or endurance limit, fatigue strength, and fatigue life.

✔ The creep test provides information on the load-carrying ability of a material at high temperatures. Creep rate and rupture time are important properties obtained from these tests.

KEY TERMS YOU SHOULD REMEMBER

Mechanical Properties

Elasticity

Plasticity

Ductility

Malleability

Brittleness

Hardness

Toug'iness

Stiffness

Resilience 

Creep

Endurance

Strength

Impact strength

Fatigue

Technological

Properties

Castability

Weldability

Formability/Workability

Deformation

Elastic deformation

Plastic deformation

Slip

Critical resolved shear

stress

Schmid's law

Twinning Fracture

Brittle Fracture

Cleavage planes 

Griffith crack theory

Elastic strain energy 

Surface energy 

Fracture strength 

Ductile fracture

Fatigue fracture

Stress cycles

S-N diagram

Endurance limit

Fatigue-limit

Tensile test

Shear test

Fatigue life 

Creep fracture

Creep curve

Creep strength 

Creep limit 

Creep life

Creep resistance 

Dislocation climb

Compression ¡est

Hardness test

Brinell hardness test

Vickers hardness test  

Rockwell hardness test Impact test

Izod test

Vacancy diffusion

Charpy test

Grain boundary 

sliding 

Fatigue test

Mechanical testing

Creep test

Fatigue strength

Destructive tests

Non-destructive tests