Welcome to Materials and Their Applications
Ever wondered why an aeroplane wing is made from carbon composites while a kitchen saucepan uses stainless steel or copper? In engineering, choosing the right material for the job is one of the most important decisions you will ever make. Pick the wrong material, and a product might break, cost too much, corrode, or fail to keep users safe.
Don't worry if this seems like a lot of information at first! We will break down each material group, explore how materials behave under forces, practice the key calculations, and see how smart materials are changing the modern world. Let's dive in!
1. How Materials Behave: Stress, Strain, and Graphs
When an engineer applies a pulling force (tension) to a component, the material stretches. To understand how strong and stiff a material is, engineers test samples and plot their behaviour on graphs.
Load/Extension vs. Stress/Strain
Load/Extension Graphs:
These graphs show how much a specific test piece stretches (extension) when a pulling force (load) is applied to it. Load is measured in newtons (\(\text{N}\)) and extension is measured in millimetres (\(\text{mm}\)).
Stress/Strain Graphs:
Because a thick bar can hold more load than a thin wire of the same material, engineers convert load and extension into stress and strain. This allows us to compare different materials fairly, regardless of their size or shape.
• Stress (\(\sigma\)): The force applied per unit of cross-sectional area.
• Strain (\(\varepsilon\)): The ratio of extension (change in length) compared to the original length.
\(\text{Strain} = \frac{\text{Change in length}}{\text{Original length}}\)
\(\text{Young's Modulus} = \frac{\text{Stress}}{\text{Strain}}\)
Analogy: Think of Young's Modulus as a measure of a material's "stiffness". A material with a high Young's Modulus (like steel) does not stretch easily when pulled. A material with a low Young's Modulus (like rubber) stretches very easily.
The Elastic Limit
When you pull an ordinary rubber band gently and let go, it snaps back to its original shape. This is called elastic behaviour.
However, if you pull a material past its elastic limit, it will no longer return to its original length or shape when the force is removed. It has suffered permanent (plastic) deformation.
• Before the Elastic Limit: The material behaves elastically (stretches and returns fully).
• Beyond the Elastic Limit: The material deforms permanently. If pulled far enough, it will eventually reach its breaking point.
Factor of Safety (FoS)
Engineers never design structures to operate right at the edge of their maximum strength. Instead, they build in a safety buffer known as the Factor of Safety.
What is Factor of Safety?
It is the ratio between the maximum load a structure can withstand and the actual expected working load it will experience during normal use.
Issues Associated with Factor of Safety:
• If the FoS is too low: The product runs the risk of sudden, catastrophic failure if an unexpected load occurs, putting human lives in danger.
• If the FoS is too high: The product will require much more material, making it excessively heavy, bulky, and expensive to manufacture and transport.
Key Takeaway: Load/extension graphs show test data for a specific sample, whereas stress/strain graphs allow standard comparisons between materials. The elastic limit marks the point where deformation becomes permanent, and a suitable Factor of Safety balances structural safety against cost and weight.
2. Material Classifications and Applications
Materials in engineering are divided into distinct categories. You need to know the specific materials listed below and their common uses.
A. Ferrous Metals and Alloys
Ferrous metals are metals that contain mainly iron. Most (though not all) are magnetic and prone to rusting if not protected.
• Cast Iron: Very hard and brittle with excellent compressive strength and vibration damping. Applications: Engine blocks, heavy machine tool beds, vice bases.
• Low Carbon Steel (Mild Steel): Tough, ductile, easy to weld and shape, but rusts easily. Applications: Car body panels, construction girders, general nuts and bolts.
• Medium Carbon Steel: Stronger and harder than mild steel, with balanced ductility. Applications: Gears, axles, structural shafts.
• High Carbon Steel: Very hard, can be heat-treated, but is less ductile and more brittle. Applications: Cutting tools, saw blades, chisels, drills.
• Stainless Steel (Alloy): Steel alloyed with chromium and nickel to resist corrosion and staining. Applications: Kitchen sinks, surgical instruments, cutlery, food processing equipment.
B. Non-Ferrous Metals and Alloys
Non-ferrous metals do not contain iron. They do not rust (though they may tarnish or oxidise) and are non-magnetic.
• Aluminium: Lightweight, excellent strength-to-weight ratio, good thermal and electrical conductor, corrosion resistant. Applications: Aircraft frames, drinks cans, window frames.
• Copper: Excellent electrical and thermal conductor, ductile, malleable, corrosion resistant. Applications: Electrical wiring, domestic water pipes, central heating components.
• Brass (Alloy of copper and zinc): Corrosion resistant, good acoustic properties, low friction, attractive golden appearance. Applications: Musical instruments, decorative hardware, plumbing fittings, locks.
• Bronze (Alloy of copper and tin): Hard, tough, highly resistant to wear and salt-water corrosion. Applications: Ship propellers, marine bearings, statues.
• Aluminium Alloy: Pure aluminium combined with elements like copper or magnesium to significantly increase strength while keeping weight low. Applications: High-performance bicycle frames, automotive alloy wheels, aerospace components.
C. Polymers (Plastics)
Polymers are synthetic or semi-synthetic materials divided into two main categories based on how they react to heat.
1. Thermosetting Polymers
Thermosetting polymers undergo a permanent chemical change when heated and cured. Once set, they cannot be reheated and reshaped. If reheated, they will char or burn rather than melt.
• Melamine Formaldehyde: Hard, scratch-resistant, heat-resistant, and chemically inert. Applications: Heat-resistant kitchen worktop laminates, picnic tableware, electrical plug casings.
• Epoxy Resins: High strength, exceptional adhesive properties, excellent chemical and electrical resistance. Applications: High-performance adhesives (e.g. Araldite), printed circuit board coatings, bonding matrix in composites.
2. Thermoplastic Polymers
Thermoplastics can be repeatedly heated, melted, reshaped, and cooled without undergoing permanent chemical breakdown. This makes them ideal for recycling.
• Polythene (Polyethylene): Flexible, chemically resistant, tough, available in low and high densities. Applications: Plastic carrier bags, squeezy bottles, piping.
• Polypropylene: Tough, flexible, excellent fatigue resistance (can bend repeatedly without snapping). Applications: Integral hinges on bottle caps, storage containers, car bumpers.
• Polyvinyl Chloride (PVC): Rigid or flexible (with plasticisers), water-resistant, durable, good electrical insulator. Applications: Window frames, guttering, electrical wire insulation, drainage pipes.
• Acrylic (PMMA): Stiff, hard, brittle under high impact, excellent optical clarity and light transmission. Applications: Car light covers, display cases, protective safety screens, replacement glass.
• Nylon: Tough, self-lubricating, high tensile strength, highly resistant to abrasion. Applications: Gears, mechanical bearings, rope, power tool housings.
• Polyester: Chemically resistant, stable, durable polymer. Applications: Synthetic textile fibres, resin used in composite manufacturing, packaging films.
D. Woods
1. Hardwoods
Hardwoods come from broad-leaved (deciduous) trees that usually lose their leaves in winter. They grow slowly, making their timber generally dense and durable (with balsa being a unique exception!).
• Balsa: Exceptionally lightweight, very soft, fast-growing hardwood with an open cellular structure. Applications: Model aircraft, architectural prototyping.
• Beech: Hard, tough, close-grained, resists denting, easy to steam-bend. Applications: Children's toys, workshop tool handles, school furniture.
• Mahogany: Rich reddish-brown colour, durable, easy to work and carve, stable. Applications: High-end furniture, decorative veneers, musical instruments.
• Oak: Hard, heavy, strong, open-grained, highly durable and resistant to decay. Applications: Structural timber frames, high-quality flooring, outdoor furniture.
2. Softwoods
Softwoods come from cone-bearing (coniferous) evergreen trees with needle-like leaves. They grow much faster than hardwoods, making them cheaper and more sustainable.
• Pine: Lightweight to medium weight, easy to work, contains natural resin knots. Applications: Construction framing, interior joinery, budget flat-pack furniture.
• Redwood: Straight-grained, lightweight, naturally resistant to moisture and rot. Applications: Decking, outdoor fencing, cladding.
• Cedar: Contains natural oils that make it highly resistant to rot, insect attack, and weather. Applications: Outdoor cladding, roof shingles, garden sheds.
3. Manufactured Boards
Manufactured boards are timber sheets made by bonding wood veneers, strips, chips, or fibres together using heat and adhesives. They come in large, flat sheets with uniform properties and no natural grain defects.
• Plywood: Made by gluing multiple thin wood veneers together with the grain of alternate layers running at \(90^\circ\) angles. Very strong in all directions. Applications: Structural flooring, boat building, furniture construction.
• Blockboard: Made of a central core of solid softwood strips sandwiched between outer hardwood veneers. Applications: Heavy-duty shelving, worktops, solid doors.
• Chipboard: Made by compressing wood chips bonded with resin. Weak and vulnerable to moisture unless veneered or coated. Applications: Kitchen worktop cores, budget flat-pack furniture.
• Medium Density Fibreboard (MDF): Made from fine wood fibres compressed with resin. Completely smooth, easily machined, with no grain. Applications: Flat-pack furniture, skirting boards, interior decorative panels.
• Hardboard: Dense, compressed pulped wood fibres with one smooth face and one textured mesh face. Applications: Back panels of wardrobes, bottom of furniture drawers.
E. Composite Materials
A composite material is made by combining two or more distinct materials with different properties. The individual components do not dissolve into each other; they work together to create a material with superior properties (e.g. high strength-to-weight ratio).
• Glass Reinforced Plastic (GRP): Glass fibres embedded inside a thermosetting polymer matrix (such as polyester resin). Strong, lightweight, waterproof, and chemically resistant. Applications: Boat hulls, kayak shells, wind turbine blades.
• Carbon Composites: Carbon fibres woven into mats and bonded within an epoxy resin matrix. Extremely high tensile strength, exceptionally rigid, and ultra-lightweight. Applications: Formula 1 racing car chassis, aerospace airframes, high-end sporting equipment (e.g. tennis rackets, cycling frames).
Key Takeaway: Choosing a material depends on matching the mechanical and physical properties of the category (ferrous/non-ferrous metals, thermoset/thermoplastic polymers, hardwoods/softwoods/boards, composites) to the working environment of the product.
3. Smart Materials and Changing Material Properties
Smart Materials
What is a smart material?
A smart material is a material that reacts reversibly to an external change in its environment (such as a change in temperature, light, or stress), changing one or more of its properties automatically.
• Shape-Memory Alloys (SMAs): Metals (such as Nitinol) that can be deformed when cool, but return to their original pre-set shape when heated above a specific transformation temperature. Applications: Dental braces, self-expanding medical stents, spectacle frames that bend and recover.
• Polymorph: A biodegradable polymer that becomes mouldable and malleable like clay at a low temperature (around \(62^\circ\text{C}\) in hot water) and solidifies into a tough plastic when it cools. Applications: Prototyping, personalised ergonomic handles, repair work.
• Reactive Glass: Glass that automatically changes its transparency or tint in response to an external stimulus (such as light intensity or an electrical current). Applications: Privacy glass in modern offices, smart windows that reduce solar heat gain.
• Thermochromic Materials: Materials or pigments that change colour when their temperature changes. Applications: Forehead thermometers, baby feeding spoons that warn if food is too hot, novelty heat-sensitive mugs.
Benefits of Smart Materials:
• Provide automated responses without needing complex electronic sensors, computers, or power supplies.
• Improve safety and convenience for the user.
• Enable innovative design solutions and multifunctionality in everyday products.
Changing the Properties of Materials
Engineers often need to modify the natural properties of a material to make it harder, tougher, stronger, or more resistant to wear.
1. Alloying:
Alloying involves melting a base metal and mixing it with one or more other elements. When the alloy cools and solidifies, the atoms of the added element disrupt the regular crystal lattice structure of the base metal. This makes it harder for layers of atoms to slide over each other, increasing the strength and hardness, or improving corrosion resistance.
Example: Adding carbon to iron creates steel; adding chromium to steel creates stainless steel.
2. Cold Working:
Cold working (such as rolling, bending, hammering, or drawing) deforms a metal at room temperature without applying heat. This distorts the crystal grain structure and creates internal dislocations in the metal lattice, making the metal significantly harder and stronger (work hardening), though less ductile.
Reasons for Changing Material Properties:
• To increase tensile, compressive, or impact strength.
• To improve resistance to wear, scratching, and fatigue.
• To increase corrosion resistance and lifespan.
• To achieve specific functional performance requirements.
Key Takeaway: Smart materials sense changes in their environment and respond automatically. Engineers can also permanently alter material properties using chemical methods (alloying) or mechanical methods (cold working) to meet specific design needs.
4. Finishing Techniques and Methods
Finishing is the final stage of manufacturing. Finishes are applied to protect materials from environmental degradation (like corrosion or decay), improve aesthetics, or provide functional surface properties.
• Painting: Applying liquid paint (by brush or spray) to create an impermeable barrier coat on metals, plastics, or woods. Improves appearance and prevents corrosion.
• Dip Coating: Heating a metal component and dipping it into a tank of fluidised polymer powder (such as polythene). The powder melts onto the metal, creating a thick, smooth, protective plastic skin. Common on tool handles and wire dish racks.
• Powder Coating: Positively charged dry powder paint is sprayed onto an electrically grounded (negative) metal object, ensuring an even coat. It is then baked in an oven to cure into a tough, durable, uniform finish on metal frames and appliances.
• Lacquering: Applying a clear or tinted protective resin coat over wood or polished metal (such as brass) to protect the surface from oxidising or tarnishing while allowing the natural appearance underneath to show through.
• Electroplating: Using an electric current (electrolysis) to deposit a very thin layer of another metal (such as chrome, gold, or nickel) onto a conductive object. Provides corrosion protection and a high-shine decorative finish.
• Galvanising: Dipping clean steel or iron into a bath of molten zinc. The zinc forms a metallurgical bond with the steel, providing both a physical barrier and galvanic (sacrificial) corrosion protection. Widely used on outdoor gates, steel barriers, and corrugated roofing sheets.
• Polishing: Using mechanical abrasives or buffing wheels with polishing compounds to smooth the surface of metals, plastics, or wood to produce a high-gloss, reflective finish.
Key Takeaway: Finishes serve two vital roles: protection (stopping rust, moisture damage, and wear) and aesthetics (improving colour, texture, and shine).
5. Material Selection, Forms, and Engineering Calculations
When designing an engineered product, you must assess several practical aspects before choosing a material:
• Cost: Raw material price and processing cost.
• Form: How the material is supplied (e.g. sheet, tube, round bar, square rod).
• Size and Shape: Available dimensions and standard cross-sections.
• Density: Mass per unit volume (governs product weight).
• Availability: Ease of sourcing standard stock sizes reliably from suppliers.
Required Engineering Formulae
You must be able to use the following mathematical formulae in exam calculations:
1. Area of a Cuboid Face (Rectangle):
\(A = L \times W\)
Where \(L = \text{length}\) and \(W = \text{width}\).
2. Volume of a Cuboid:
\(V = L \times W \times H\)
Where \(L = \text{length}\), \(W = \text{width}\), and \(H = \text{height}\).
3. Area of a Circle:
\(A = \pi r^2\)
Where \(r = \text{radius}\) (remember: \(\text{radius} = \frac{\text{diameter}}{2}\)).
4. Circumference of a Circle:
\(\text{Circumference} = \pi D\)
Where \(D = \text{diameter}\).
5. Volume of a Cylinder:
\(V = A \times L = \pi r^2 \times L\)
Where \(A = \text{cross-sectional area}\) and \(L = \text{length (or height)}\).
6. Area of a Triangle:
\(A = \frac{1}{2} (B \times H)\)
Where \(B = \text{base}\) and \(H = \text{perpendicular height}\).
7. Density:
\(\rho = \frac{m}{V}\)
Where \(\rho = \text{density}\), \(m = \text{mass}\), and \(V = \text{volume}\).
8. Percentage:
\(\text{Percentage} = \text{Ratio} \times 100\%\)
Worked Example: Material Calculation
Problem: A solid aluminium round bar has a diameter of \(40\text{ mm}\) and a length of \(500\text{ mm}\). If the density of aluminium is \(0.0027\text{ g/mm}^3\), calculate the mass of the bar.
Step 1: Find the radius
\(r = \frac{40}{2} = 20\text{ mm}\)
Step 2: Calculate the cross-sectional area of the round bar
\(A = \pi r^2 = \pi \times 20^2 = \pi \times 400 \approx 1256.64\text{ mm}^2\)
Step 3: Calculate the volume of the cylindrical bar
\(V = A \times L = 1256.64 \times 500 = 628,320\text{ mm}^3\)
Step 4: Calculate the mass using the density formula
\(\rho = \frac{m}{V} \implies m = \rho \times V\)
\(m = 0.0027 \times 628,320 \approx 1696.46\text{ g}\) (or approx \(1.70\text{ kg}\))
Common Mistake to Avoid: Always double check whether the question gives you the diameter or the radius of a circular bar or tube! If given the diameter, always divide by \(2\) before calculating \(\pi r^2\).
Key Takeaway: Standard geometric formulae combined with the density formula (\(\rho = \frac{m}{V}\)) allow engineers to calculate material requirements, product weight, and component costs accurately before manufacturing begins.
6. Sustainability: Waste, Disposal, and Recycling
In modern engineering and manufacturing, material efficiency is vital for both economic profitability and environmental protection.
Waste Produced as a Cost
Whenever raw material is wasted during production (for example, offcuts of sheet metal, plastic swarf from milling, or sawdust from timber cutting), it represents a direct financial loss:
• Direct Purchase Loss: The manufacturer paid for the original raw material that ended up as scrap.
• Processing Cost: Energy and machine time were spent handling material that is discarded.
• Disposal Charges: Companies must often pay commercial waste disposal fees or landfill taxes to remove manufacturing waste.
Disposal and Recycling Issues
• Metals: Highly recyclable. Ferrous and non-ferrous metals can be melted down and reprocessed repeatedly with minimal loss of mechanical properties.
• Thermoplastics: Can be remelted and reformed into new products, saving crude oil and reducing landfill waste.
• Thermosetting Polymers: Cannot be remelted. They must be mechanically ground down as fillers or incinerated for energy recovery, posing disposal challenges.
• Composites: Difficult to separate into their individual component fibres and resin matrices, creating significant long-term recycling challenges.
• Treated Woods & Manufactured Boards: Contain synthetic resins and bonding adhesives that make them unsuitable for standard composting and difficult to recycle safely.
Key Takeaway: Waste represents lost money. Choosing recyclable materials, minimising offcuts through efficient layout planning, and properly managing material disposal helps reduce both environmental footprint and overall manufacturing costs.
Quick Summary Checklist
Before moving on, make sure you can:
• Read and interpret load/extension and stress/strain graphs and define the elastic limit.
• Explain what Factor of Safety means and why finding the right balance matters.
• Name key ferrous metals, non-ferrous metals, thermosets, thermoplastics, hardwoods, softwoods, manufactured boards, and composites.
• Identify smart materials (SMAs, polymorph, reactive glass, thermochromic) and give an application for each.
• Explain how alloying and cold working change material properties.
• Describe surface finishes: painting, dip coating, powder coating, lacquering, electroplating, galvanising, and polishing.
• Apply the standard area, volume, and density formulae (\(\rho = \frac{m}{V}\)) to solve engineering problems.
• Discuss the economic and environmental impacts of manufacturing waste and material recycling.