Chapter 11: Polymer Structures and Creation (HL)
1. Polymer Structures
Polymers are formed when monomer molecules react together to create very long chains, known as polymer molecules.
This process is called polymerisation.
The repeating units within the polymer chains are called mers. Mers differ slightly from monomers because monomers may lose atoms during the reaction, or the inter-mer bonding might be different from the monomer bonding.
2. Copolymers
A copolymer is formed by the chemical combination of two different types of monomer molecules.
Copolymers acquire useful properties from both constituent monomers.
Common examples include ABS and EVA.
A copolymer can be likened to a solid solution metal alloy, where two metals are mixed and chemically bonded.
3. Types of Polymer Structures
When monomers chemically combine, they can form three basic polymer structure types:
-
Linear Structures:
Form thermoplastics.
Examples: Polypropylene, nylon.
Linear chains can pack more closely, resulting in higher density, stiffness, tensile strength, and melting point. -
Branched Structures:
Form thermoplastics.
Example: Polythene (LDPE).
Branched chains do not pack as closely, leading to lower density, lighter weight, low tensile strength, and low melting point. -
Cross-Linked Structures:
Form thermosets.
Example: Phenol Formaldehyde.
Monomers can bond at multiple points, creating cross-links between different polymer chains.
This forms an enormous, inter-linked, strong lattice, resulting in very strong materials with high melting points.
4. Importance of Bonds in Thermosets and Thermoplastics
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Primary Bonds in Thermosets:
These are strong covalent chemical bonds formed by sharing electrons.
Primary bonds link atoms in all polymer chains, including the cross-links in thermoset polymers.
Strong primary bonds in cross-links make thermoset plastics strong, rigid, and give them high melting points.
Heat cannot break these strong primary bonds, so thermoset polymer chains do not separate or become molten when heated.
Thermosets can only be set/cured once, so polymerisation must be finalized in the product moulds. -
Secondary Bonds in Thermoplastics:
These are weak intermolecular bonds, also known as Van der Waals forces.
Secondary bonds loosely bind different thermoplastic polymer chains together.
Weak secondary bonds between polymer chains make thermoplastics softer, more flexible, and give them low melting points.
Heat can break these weak secondary bonds, allowing polymer chains to slide over each other, making the thermoplastic soft and pliable.
When heat is removed, the secondary bonds re-bind, holding the polymer chains in their new position.
5. Amorphous and Crystalline Thermoplastics
Polymer chains in thermoplastics can arrange themselves in amorphous or crystalline patterns, or a combination.
-
Amorphous Polymers:
Polymer chains are jumbled up and less densely packed.
This makes them lighter and more impact-resistant.
They soften slowly with heat.
Examples: Acrylic, ABS, PVC, polystyrene, elastomers. -
Crystalline Polymers:
Polymer chains are ordered in patterns and are more densely packed.
This makes them heavier, stiffer, and they have a higher, more defined melting point.
Examples: Nylon, Polypropylene, HDPE. -
Note: Thermosets are always amorphous because cross-links lock polymer chains in place during polymerisation, preventing them from settling into regular patterns.
6. Glass Transition Temperature
Amorphous thermoplastics do not have a single, clear melting point; they soften gradually over a range of temperatures.
Instead, they have a glass transition temperature.
This is the temperature above which an amorphous polymer begins to change from a hard, glass-like solid to a soft, rubbery state.
Elastomers have a glass transition temperature lower than room temperature, so they remain in a rubbery state.
Cooling elastomers will make them glassy and easily breakable.
Increasing the temperature of an amorphous thermoplastic above its glass transition temperature will gradually make it softer until it flows.
Adding plasticisers can lower the glass transition temperature. Plasticisers act as separators and lubricants between polymer chains, allowing them to flow more easily.
7. Polymer Shape Memory Properties
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Elastic Memory:
Elastomers exhibit elastic memory, meaning they return to their original shape after a stretching force is removed. -
Plastic Memory / Shape Memory:
This is a material's ability to remain in a different shape until an external stimulus (like heat) is received, at which point it returns to its original shape.
Acrylic demonstrates this property (a bent sheet will return to a flat sheet when heated).
8. Polymerisation Processes
-
Addition Polymerisation:
Used to create most thermoplastics (e.g., polythene, polypropylene, ABS).
Creates polymer chains without any by-products.
Polymers grow in long chains by adding mers to the end of the chain.Initiation: A free radical (a reactive molecule with an unpaired electron) or a catalyst (which speeds up the reaction without changing) is introduced to the monomer molecules.
Polymer Growth (Chain-Growth): The free radical or catalyst reacts with a monomer, creating a reactive mer. This reactive mer then bonds with another mer, creating another reactive mer, and the process continues, adding mers one-by-one.
Termination: Polymer chain growth stops when monomers are used up, the reactive mer bonds with an impurity, bonds with another polymer chain or part of itself, or bonds with a deliberately introduced inhibitor molecule.
Catalysts can control the amount of branching in polymer chains and the reaction speed.
Addition polymerisation reactions generate heat. -
Condensation Polymerisation:
Used to create most thermosets (e.g., phenol formaldehyde) and some thermoplastics (e.g., nylon, polyester, PET).
Creates polymers along with additional by-products, such as water.
Depending on the mer, polymers can grow in different directions and form cross-linked structures (thermosets).Initiation: No additional substances are needed; monomers react when brought into contact.
Polymer Growth (Step-Growth): Monomers react and bond in small groups, releasing a small molecule by-product (typically water or alcohol).
If the mer molecule has more than two bonding points, the polymer can branch and grow in multiple directions.
The presence of multi-bond-point mers also allows polymer chains to covalently bond with each other, forming a cross-linked structure (a thermoset).
Termination: If a thermoset is being created, polymerisation stops when the monomers are used up.
Reactions can be sped up using heat and catalysts.
9. Resins and 'Curing' Thermosets
Thermosets are typically manufactured in two stages.
The first stage creates a partially-formed thermoset called a resin.
A resin requires a second 'curing' stage to create the cross-links.
Curing can be achieved using heat and pressure, UV light, or a hardener (e.g., 2-part epoxy glue).
Manufacturing processes for thermoset products use resins to avoid waiting for long polymerisation processes, instead relying on the faster curing.
10. Altering Polymer Properties
Polymer properties like strength, elasticity, plasticity, melting point, and impact-resistance can be influenced by:
-
Choosing monomers with desired properties.
-
Creating copolymers using two different types of monomers.
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Using catalysts to control the amount of branching in polymer chains.
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Generating cross-links by choosing monomers that form cross-linked thermosets or by vulcanizing natural rubber with sulfur and heat.
-
Using additives, such as plasticisers to increase flexibility.
-
Creating composites of polymers with other materials.
11. Polymer Additives
-
Plasticisers: Make polymers softer, more flexible, impact-resistant, and durable.
They also make plastics easier to mould.
Plasticiser molecules get between polymer chains, reducing bond strength and allowing chains to slip past each other.
Almost all polymers contain plasticisers; otherwise, they are too rigid.
PVC is a major user of plasticisers; unplasticised PVC (uPVC) is very rigid, while heavily plasticised PVC is flexible for items like clothing and hoses. -
Pigment/Colourant: Changes the color of the polymer.
-
Filler: Cheap materials (like chalk, clay, paper, flour) added to polymers to reduce cost and increase strength.
-
Foaming Agent: Creates gas bubbles in the polymer for lighter products or increased volume (e.g., packing material, sponges).
-
Flame Retardant: Added to polymers to make them more difficult to catch fire, used in clothing, furniture, and building materials.
-
Stabiliser: Helps prevent the polymer from degrading due to exposure to UV light, heat, chemical reactions, or microbes.
-
Lubricant: Added during manufacturing processes to improve polymer flow and prevent sticking in machines.
12. Polymer Composites
Combining a polymer with materials like glass or carbon fibers creates strong yet light materials.
The polymer provides flexibility, impact-resistance, and mouldability, while the composite material provides strength.
Thermosets are typically chosen for composites due to their inherent strength and ability to be 'cured' during manufacturing.
Examples include GRP (Glass-Reinforced Plastic/Fibreglass) for boat hulls and storage tanks, and Carbon Fibre composites for aircraft, spacecraft, and supercars.
Chapter 12: Structure of Metals and Alloys (HL)
1. Basic Terms
Atoms are the fundamental building blocks of matter, consisting of a nucleus (with positively charged protons) surrounded by negatively charged electrons. Normally, atoms have an equal number of protons and electrons, resulting in no overall charge.
Ions are atoms with an unequal number of electrons and protons, giving them an overall positive or negative charge.
Atomic structure refers to how atoms are arranged and bonded, which largely determines a material's properties.
An element is a material made of only one type of atom (e.g., iron, copper, gold).
Metals are the largest subgroup of elements.
An alloy is a combination of metals or a metal with other elements mixed at the atomic level (e.g., steel is an alloy of iron and carbon; brass is an alloy of copper and zinc).
2. Chemical Bonds
Materials are held together by three main types of chemical bonds:
Ionic Bonds: One atom donates electrons to another, creating oppositely charged ions (anions and cations) that form a very strong bond, typically found in salts.
Covalent Bonds: Atoms share one or more electrons. These form the strong primary bonds in polymers.
Metallic Bonds: Atoms donate electrons to a shared "sea" of electrons, which are free to move. This sea holds the positive metal ions together in a crystal lattice structure.
3. Properties Explained by Metallic Bonds
Metallic bonds explain several key properties of metals:
-
High Strength and High Melting Points: A large amount of energy and force is needed to break metallic bonds.
-
Good Conductors of Electricity: The free-moving "sea" of electrons allows for easy electron movement.
-
Good Conductors of Heat: Free electrons transmit heat vibrations quickly throughout the metal, and closely packed ions also aid in heat transmission.
-
Malleability and Ductility: An external force can cause metal ions to "slip" into new positions within the crystal lattice without breaking the metallic bond, allowing plastic deformation without fracture.
4. Crystalline and Amorphous Structures
Crystalline Solids: Atoms are organized in repeating patterns in a lattice structure (e.g., metals, diamond, salt, sugar, snow).
Amorphous Solids: There is no pattern to the arrangement of atoms (e.g., glass, most plastics, rubbers).
5. Metal Crystal Structures
When metals cool from liquid to solid, they form crystalline structures, typically as polycrystalline structures composed of multiple individual crystals called grains. Two common metal crystal structures are:
Body Centred Cubic (BCC): Atoms are at the 8 corners of a cube plus one in the center (e.g., iron (ferrite), chromium, vanadium).
Properties: BCC metals are generally stronger, less ductile, and more brittle because their less closely packed atoms require a larger force to slip over each other.
Face Centred Cubic (FCC): Atoms are at the 8 corners of a cube plus one in the middle of each face (e.g., aluminum, copper, lead, silver, gold, iron (austenite) at high temperatures).
Properties: FCC metals are generally more ductile and malleable, and less strong, because their more closely packed and "in-line" atoms require a smaller force for slip.
6. Metal Grains and Their Properties
Grains are individual crystals that form within a metal, each with its own random orientation, separated by grain boundaries.
Effect of Grain Size:
-
Fine (Small) Grains: Result in stronger and less ductile metals because grain boundaries act as barriers to slip and fractures, requiring more force for them to cross or change direction.
-
Coarse (Large) Grains: Result in less strong and more ductile metals.
7. Crystal Defects and Metal Properties
Metals are never 100% pure and always contain imperfections or defects in their crystal structure, which significantly influence their mechanical properties (strength, hardness, ductility). These defects lead to plastic deformation and fracture at forces well below the theoretical strength of a perfect crystal structure.
Types of Defects:
-
Dislocation (Line Defect): An incomplete line in the lattice. These are the primary reason metals are ductile, as they create weak points where small forces can cause slip. However, a very large amount of dislocations can harden and strengthen the metal.
-
Substitutional Defect: An atom of a different type fills a lattice position. If the "foreign" atom is a different size, it distorts the lattice, making it harder for dislocations to move and strengthening the metal.
-
Interstitial Defect: An impurity or alloying atom sits between lattice atoms. This distorts the lattice, creates internal stresses, and prevents dislocations from moving, thus strengthening the metal.
-
Vacant Site Defect: A missing atom in the lattice. This distorts the lattice, creating stresses that hinder dislocation movement and strengthen the metal.
Chapter 13: Thermal Equilibrium Diagrams (HL)
1. What is a Thermal Equilibrium Diagram (T.E.D.)?
A T.E.D. illustrates the structure of an alloy at various temperatures and alloy compositions.
It provides crucial information for creating alloys and performing heat treatment processes (as detailed in Chapter 14).
T.E.D.s are also known as phase diagrams because they show the number, composition, and ratio of phases present at each temperature for a given alloy composition.
2. What is a Phase?
A phase is a state of matter where physical properties are uniform throughout a specific region.
Examples of phases include the liquid and solid forms of a substance.
Within a solid, different crystal structures or compositions of the same substance can also constitute distinct phases (e.g., ferrite, austenite, cementite in steel).
3. Types of Alloys Covered in T.E.D.s:
The chapter presents T.E.D.s for the following types of binary (two-component) alloys, ordered by increasing complexity:
-
Solid Solution Alloy: Components fully dissolve into each other as solids (e.g., copper-nickel).
-
Eutectic Alloy: Components do not dissolve in each other (e.g., cadmium-bismuth).
-
Partial Solubility Alloy: Components partially dissolve in each other as solids (e.g., lead-tin).
-
Iron-Carbon Alloys: A more complex case of partial solubility that also includes a eutectoid point.
4. Iron-Carbon Phase Diagram (Simplified)
The chapter provides a simplified Iron-Carbon Thermal Equilibrium Diagram, focusing on the region up to 2% carbon (relevant for steels).
Key Phases:
-
Liquid: Above the liquidus line, the iron-carbon mixture is entirely liquid.
-
Austenite (γ-iron): A solid solution of carbon in FCC iron. This phase exists at high temperatures.
-
Ferrite (α-iron): A solid solution of carbon in BCC iron. It has very low solubility for carbon (<0.008%).
-
Cementite (Fe₃C): A very hard and brittle intermetallic compound of iron and carbon.
Critical Temperatures:
-
Liquidus Line: Above this temperature, the alloy is entirely liquid.
-
Solidus Line: Below this temperature, the alloy is entirely solid.
-
Upper Critical Temperature: The temperature above which only austenite exists (for steel compositions).
-
Lower Critical Temperature (Eutectoid Temperature): The temperature at which the eutectoid reaction occurs (723°C for iron-carbon system). Below this temperature, no austenite exists.
Eutectoid Point:
At 723°C and 0.83% carbon, austenite transforms entirely into pearlite.
Pearlite: A lamellar (layered) microstructure composed of alternate layers of ferrite and cementite.
Steel Classifications based on Carbon Content (and their microstructures after slow cooling):
-
Hypoeutectoid Steels (carbon < 0.83%):
Microstructure: Grains of ferrite surrounded by grains of pearlite.
During cooling, ferrite grains form first as the steel cools through the upper critical temperature. Then, at the lower critical temperature, pearlite forms around the ferrite. -
Hypereutectoid Steels (carbon > 0.83%):
Microstructure: Grains of cementite surrounded by grains of pearlite.
During cooling, cementite grains form first as the steel cools through the upper critical temperature. Then, at the lower critical temperature, pearlite forms around the cementite. -
Pure Pearlite Steel (0.83% carbon):
All grains are pearlite.
1. What is a Thermal Equilibrium Diagram (T.E.D.)?
2. What is a Phase?
3. Types of Binary Alloys in T.E.D.s
- Solid solution alloys: The two components completely dissolve into each other as solids. Example: copper-nickel.
- Eutectic alloys: The components do not dissolve in each other and form a layered structure at a specific temperature. Example: cadmium-bismuth.
- Partial solubility alloys: The components dissolve partly in each other, forming two solid phases in some regions. Example: lead-tin.
- Iron-carbon alloys: A special type of partial solubility alloy that includes a eutectoid reaction. More complex than the others.
4. Iron-Carbon Thermal Equilibrium Diagram (Simplified):
- Liquid: Exists at high temperatures above the liquidus line.
- Austenite (gamma iron): A solid solution of carbon in face-centered cubic (FCC) iron. Exists at high temperatures.
- Ferrite (alpha iron): A solid solution of carbon in body-centered cubic (BCC) iron. Has very low carbon solubility.
- Cementite (Fe3C): A compound of iron and carbon. Very hard and brittle.
- Pearlite: A layered structure of ferrite and cementite. Formed during the eutectoid reaction.
- Liquidus line: Above this line, the alloy is completely liquid.
- Solidus line: Below this line, the alloy is completely solid.
- Upper critical temperature: The temperature above which only austenite exists.
- Lower critical temperature: 723 degrees Celsius, where the eutectoid reaction occurs.
- Eutectoid point: occurs at 0.83 percent carbon and 723 degrees Celsius.
5. Steel Types Based on Carbon Content
- Ferrite forms first as the alloy cools.
- At 723 degrees Celsius, remaining austenite turns into pearlite.
- Final structure: ferrite grains surrounded by pearlite.
- Eutectoid steel (exactly 0.83 percent carbon): At 723 degrees Celsius, all austenite turns into pearlite.
- Hypereutectoid steels (more than 0.83 percent carbon): Cementite forms first as the alloy cools.
- At 723 degrees Celsius, remaining austenite turns into pearlite.
Chapter 14: Heat Treatment of Metals and Alloys
1. What is Heat Treatment?
It is a process of heating and cooling metals and alloys to modify their properties (e.g., tensile strength, hardness, ductility, brittleness, corrosion-resistance).
It does not involve melting the metals or changing their shape.
It is widely applied to steels and some non-ferrous alloys.
2. How Does Heat Treatment Work?
It works by changing the internal mechanical structure (microstructure) of the material.
This involves altering the number and distribution of internal defects and stresses, and potentially growing or creating new crystal structures.
3. Uses of Heat Treatment:
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To harden and strengthen an entire workpiece.
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To create a hard-wearing surface while keeping the core ductile and impact-resistant.
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To create a hard cutting edge on a tool while maintaining a tough shank.
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To restore ductility, toughness, machinability, and weldability to metals that have become hard and brittle due to work hardening.
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To remove internal stresses caused by cold-working, machining, or welding.
4. Stages of Heat Treatment (General Process):
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Heating: The metal is heated to a specific temperature above its critical temperature, transforming its microstructure into austenite (for steels).
-
Soaking: The metal is held at this temperature for a period to ensure uniform temperature distribution and a consistent austenitic microstructure.
-
Quenching: The hot metal is cooled rapidly (e.g., in water, oil, or air) to "lock in" the new microstructure. The speed of quenching is critical to the final properties.
-
Tempering (if required): A subsequent heating to a lower temperature to improve ductility and toughness, reducing brittleness that might result from rapid quenching.
5. Common Heat Treatment Processes:
Hardening (or Quench Hardening):
Aim: To increase hardness and strength, usually at the expense of ductility.
Materials: Medium- and high-carbon steels, some cast irons. Pure irons and low-carbon steels cannot be hardened because they don't form enough austenite to transform into hard martensite.
Process for Steels:
-
Heated to 40°C above the Upper Critical Temperature (UCT) to achieve full austenitisation.
-
Soaked to ensure uniform temperature and grain structure.
-
Quenched rapidly in water, oil, or forced air. This rapid cooling transforms the austenite into martensite.
Martensite: A very hard, strong, and brittle body-centered tetragonal (BCT) crystal structure. It is a supersaturated solution of carbon in iron, formed by rapid quenching that prevents carbon atoms from diffusing out of the crystal lattice. The trapped carbon distorts the lattice, causing internal stresses and high hardness.
Tempering:
Aim: To reduce the brittleness of hardened steel while retaining most of its hardness, increasing ductility and toughness.
Materials: Hardened steels.
Process:
-
Hardened steel is reheated to a temperature below the Lower Critical Temperature (LCT) (typically 200°C - 600°C).
-
Soaked at this temperature.
-
Cooled slowly (usually in air).
Outcome: The controlled heating allows some carbon atoms to diffuse out of the martensite, relieving internal stresses and forming fine carbides, which results in a tougher, less brittle microstructure. Higher tempering temperatures reduce hardness but increase toughness significantly.
Annealing:
Aim: To increase ductility, toughness, and machinability; relieve internal stresses from cold-working; refine grain structure; and prepare steel for hardening.
Materials: Primarily medium- and high-carbon steels, cast iron, and non-ferrous metals/alloys. Little effect on low-carbon steel.
Process for Steels (Full Annealing):
-
Heated to 40°C above the UCT (for hypoeutectoid steels) or 40°C above the LCT (for hypereutectoid steels) to achieve austenite.
-
Soaked to ensure desired grain structure and even heating.
-
Cooled very slowly in the furnace (slow cooling allows for the growth of coarse, ductile pearlite grains).
Recrystallisation: During annealing, new, strain-free grains form, replacing deformed ones.
Normalising:
Aim: To refine grain structure, improve strength and toughness, and relieve internal stresses. It provides a more uniform and finer grain structure than annealing, resulting in higher strength.
Materials: Medium- and high-carbon steels, some cast irons, some non-ferrous alloys.
Process:
-
Heated to 40°C above the UCT (for hypoeutectoid steels) or 40°C above the LCT (for hypereutectoid steels).
-
Soaked.
-
Cooled in still air (faster than annealing, leading to finer pearlite and better strength/toughness).
Work Hardening (Strain Hardening):
Aim: To increase hardness and strength by deforming the metal at room temperature (cold working).
Mechanism: Cold working (e.g., rolling, drawing, bending) increases the number of dislocations in the crystal lattice. These dislocations tangle and impede further movement, making the metal harder and stronger but less ductile.
Applications: Cold drawn wire, cold rolled sheets.
Case Hardening:
Aim: To create a hard, wear-resistant surface (case) on a component while maintaining a tough, ductile core. This is for low-carbon steels that cannot be hardened through quench hardening.
Materials: Low-carbon steels (<0.4% carbon).
Processes:
-
Carburising: Carbon is diffused into the surface of the steel (e.g., pack carburising, gas carburising, liquid carburising), increasing its carbon content to allow hardening.
-
Nitriding: Nitrogen is diffused into the surface, forming hard nitrides.
-
Cyaniding: Both carbon and nitrogen are diffused.
-
Flame Hardening/Induction Hardening: Surface is rapidly heated and quenched using a flame or electromagnetic induction.
Chapter 15: Mechanical Testing (HL)
1. What is Mechanical Testing?
It is a destructive testing method that involves deforming or fracturing a material to measure its properties.
It is used to measure properties such as:
-
Tensile Strength (Ultimate Tensile Strength)
-
Ductility
-
Yield Point / Proof Stress
-
Stiffness (Young's Modulus)
-
Hardness
-
Toughness / Impact-resistance
Applications: Primarily used for quality assurance in design and manufacturing, and for safety testing of products.
Disadvantage: The test piece is destroyed, making it unsuitable for testing components that need to remain operational.
2. Types of Mechanical Testing:
Tensile Testing:
Process: A test specimen is stretched in a tensile testing machine. The applied forces and the corresponding increases in material length are measured and plotted on a load-extension or stress-strain graph.
Properties Calculated from Graphs:
-
Ultimate Tensile Strength (UTS): The maximum stress a material can withstand before fracturing. It is the peak stress on the stress-strain graph.
-
Yield Point (or Proof Stress): The stress at which the material begins to deform permanently (plastically). This is typically seen as a bend in the stress-strain curve where it deviates from its initial straight elastic region.
-
0.1% Proof Stress: Used when a clear yield point is not visible. A line is drawn parallel to the initial elastic region of the stress-strain graph, starting at a strain of 0.001 (0.1%). The stress value where this proof line intersects the stress-strain graph is the 0.1% proof stress.
-
Ductility: How much the material can stretch (plastically deform) before fracturing. It is typically measured as elongation or reduction in area after fracture.
-
Young's Modulus of Elasticity (Stiffness): A measure of a material's stiffness or its resistance to elastic deformation. It is calculated by dividing the stress value by the strain value at any point within the initial straight (elastic) section of the stress-strain graph.
Hardness Testing:
Process: An indenter of a specific shape is used to create an indent in the material. The size of the indentation and the force used are then used to calculate the hardness value.
Common Indenters and Tests:
-
Brinell: Uses a steel ball indenter.
-
Vickers: Uses a diamond pyramid indenter.
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Rockwell: Uses either a steel ball or a diamond cone indenter. Rockwell is noted for being a completely automated test, suitable for automated production lines.
Impact Testing:
Aim: To test a material's toughness, which is its ability to absorb energy and withstand impacts without breaking.
Process: A pendulum weight is raised to a certain height and then released to strike and break a small, notched bar of the test material. The distance the pendulum travels after fracturing the bar indicates the energy absorbed by the material (toughness).
Types of Impact Tests:
-
Izod Test: The test bar is held vertically at the bottom end, and the notch faces the hammer.
-
Charpy Test: The test bar is held horizontally at both ends, and the notch faces away from the hammer.
Chapter 16: Non-Destructive Testing (HL)
Non-Destructive Testing (NDT) consists of techniques for finding flaws in a material without causing damage to it
Advantages and Applications of NDT:
NDT methods do not damage the parts being tested, which means the parts can be sold or continue to be used
. NDT is used in manufacturing for finding defects in welds or castings
. It is also used for operational safety, such as checking aircraft parts, pipelines, and machines for defects caused by corrosion, wear, fatigue, or impacts
.
Disadvantages of NDT:
Some NDT equipment can be costly
.
Types of Non-Destructive Tests:
Visual Testing:
This method involves visually inspecting surface cracks and flaws
. It can be done with the naked eye or under magnification
. Macroscopic visual testing involves inspecting the part by eye or with low magnification in strong light
. Microscopic visual testing involves viewing the part under high magnification, for example, with a microscope
.
Liquid Penetrant and UV Light:
This technique uses a liquid dye to highlight surface cracks and flaws
. The process involves cleaning the surface, which must be smooth, and then applying the liquid dye to allow it to soak in
. Excess dye is cleaned off, and a developer is applied to draw out the dye
. The flaws are then viewed under normal or UV light, depending on the type of dye used
. This test only works on the surface of materials
.
Magnetic Particle Flaw Testing:
A magnetic powder is used to highlight surface flaws and cracks
. The surface is cleaned, magnetic powder is applied, and then a magnetic field is applied
. Excess powder is blown off, and the remaining powder is attracted to the magnetic field in the cracks
. This method only works on magnetic materials
. It can detect surface or internal flaws .
Ultrasonic Testing:
This method detects flaws using ultrasound reflections
. Ultrasound is emitted into the sample, and reflections from the front and back surfaces are recorded
. A flaw is indicated by an unexpected reflection.
This method can be used to find internal flaws
.
Eddy Current Testing:
A magnetic coil is used to induce electric currents in metal objects
. The coil creates a magnetic field that induces electric "eddy" currents in the test material
. Flaws are detected from disturbances in these currents and magnetic fields, which are shown on a monitor
. This test works only on electrical conductors, such as metals
. It can find surface or internal flaws .
X-Ray/Radiographic Testing:
An X-ray image is taken of the test material, and internal flaws appear on the image
. This is the only dangerous NDT method, as radiation exposure can cause serious sickness or even death
. Safety Precautions:
Operators must stand a safe distance away and limit the number of X-ray tests they perform in a given time
. The duration of the X-ray must be kept short
. Lead shielding can be used
. Operators should be tested periodically for radiation exposure
.
This method is used for detecting internal flaws
.