Chapter 12: Engineering Materials and Workshop Technology
Properties of materials
Engineers choose a material for its properties: strength (resisting load), stiffness (resisting deformation, measured by Young’s modulus ), hardness, toughness (absorbing energy before breaking), ductility (stretching without breaking), malleability, brittleness, fatigue resistance, creep resistance, thermal and electrical conductivity, density, corrosion resistance, and cost (Callister & Rethwisch, 2018).
Stress and strain: and , with in the elastic region. A tensile test produces a stress–strain curve showing the elastic limit, yield point, ultimate tensile strength (UTS), and fracture. The factor of safety is ; a material with UTS 400 MPa used at a working stress of 160 MPa has a factor of safety of 2.5.
Example: a steel bar of diameter 20 mm carries a tensile force of 50 kN. , so MPa. With GPa, , and over a 2 m length the extension is mm.
Families of materials
| Family | Examples | Typical properties and uses |
|---|---|---|
| Ferrous metals | Mild steel, carbon steels, cast iron, stainless steel | Strong and tough; structures, machinery, tools; rust unless protected |
| Non-ferrous metals | Aluminium, copper, brass, bronze, zinc | Light, conductive, corrosion-resistant; wiring, castings, bearings |
| Polymers | Thermoplastics (PVC, polyethene), thermosets (epoxy), elastomers (rubber) | Light, insulating, easily formed; pipes, covers, seals |
| Ceramics | Brick, glass, cement, porcelain | Hard, heat-resistant, brittle; construction, insulation |
| Composites | Fibreglass, reinforced concrete, carbon fibre | Combine properties of the parts; boats, buildings, aircraft |
| Timber | Teak, mahogany, pine, plywood | Renewable, workable; furniture, framing |
Steel is an alloy of iron and a small percentage of carbon (low-carbon steel is tough and weldable; high-carbon steel is hard); cast iron has a higher carbon content and is brittle. Heat treatment changes properties: annealing (softening by slow cooling), quenching (hardening by fast cooling), tempering (reducing brittleness after hardening), and case hardening. Corrosion protection: painting, galvanizing, plating, and sacrificial anodes (Chapter 8).
Hand tools and measuring
Fundamental skills: marking out (scriber, punch, square, vernier height gauge), cutting (hacksaw, chisel), filing, drilling, threading (taps and dies), and measuring (steel rule, vernier caliper, micrometer, feeler gauges, dial gauge). Accuracy depends on method as much as tools.
Machining
On a lathe, the workpiece rotates and a fixed tool removes material for turning, facing, drilling, threading, and boring; on a milling machine a rotating cutter shapes a stationary workpiece; shapers, planers, grinders, and drill presses are other machines. The cutting speed (in m/min for in mm and in rpm), so . Example: drilling a 10 mm hole at a cutting speed of 30 m/min needs rpm. CNC (computer numerical control) machines follow programmed instructions for repeatable precision.
Joining and forming
Welding: arc welding (manual metal arc, MIG, TIG) and gas welding melt and fuse metals; protect the eyes and skin from radiation, fumes, and sparks.
Brazing and soldering: join metals with a lower-melting filler.
Mechanical fastening: bolts, nuts, screws, rivets; adhesives.
Casting: molten metal poured into a mould (sand, die).
Forging, rolling, extrusion, and sheet-metal forming shape metals in the solid state.
Additive manufacturing (3D printing) builds parts layer by layer.
Workshop organization, quality, and safety
A good workshop uses 5S (sort, set in order, shine, standardize, sustain), has clear walkways, machine guards, ventilation, fire extinguishers, first-aid kits, and good lighting. Rules: wear safety glasses and appropriate footwear; no loose clothing or rings near rotating machines; never leave a chuck key in the chuck; secure workpieces; keep guards in place; isolate machines before maintenance; and use the correct fire extinguisher (water for solids, CO₂ or dry powder for electrical fires). Quality control uses inspection, gauges, sampling, and standards (ISO 9001) to keep products within tolerance.
Common mistakes
Using a tool for a purpose it was not designed for.
Forgetting safety eyewear for grinding and welding.
Measuring without checking for zero error.
Practice questions
A steel wire of cross-section carries a load of 400 N. Find the stress. [200 MPa]
Find the lathe speed to turn a 50 mm diameter bar at a cutting speed of 60 m/min. [≈382 rpm]
Explain why the quenching of steel is often followed by tempering.