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Choosing a material is one of the first real decisions in a design, and one of the hardest to change later. It sets the part’s weight, its strength, how long it lasts and how it can be made. A good choice follows a method: start from what the part must do, turn that into the properties it needs, rule out the materials that can’t do the job, then pick the best of the rest on cost and manufacturing.
The five questions
Before thinking about any particular material, answer five questions about the part.
- Function. What does the part do? Carry a load, locate another part, seal, conduct heat, insulate, slide, look good?
- Load. What forces act on it, and how? Steady or cyclic (fatigue), tension, bending, torsion, impact, wear? Is the limit strength (it must not break) or stiffness (it must not bend too much)?
- Environment. Temperature range, moisture, salt, chemicals, sunlight (UV), contact with food or skin, other metals it touches.
- Cost and quantity. One prototype, a batch of 200 or 100 000 a year? The cheapest material per kilogram isn’t always the cheapest part.
- Manufacturability. How will it be made: machined, sheet metal, cast, forged, moulded or printed? Does it need welding, heat treatment or a coating?
From requirements to properties
Each answer points to measurable properties.
| Requirement | Property to compare | Unit |
|---|---|---|
| Must not break or yield | Yield strength, tensile strength | MPa |
| Must not bend too much | Young’s modulus (stiffness) | GPa |
| Light and strong / light and stiff | Strength / density, modulus / density | — |
| Survives repeated loads | Fatigue (endurance) limit | MPa |
| Survives impact or cold | Toughness (impact energy) | J |
| Resists wear and indentation | Hardness | HB, HRC, HV |
| Works hot | Maximum service temperature, creep strength | °C |
| Doesn’t rust or degrade | Corrosion resistance, UV and chemical resistance | rating |
| Carries or blocks heat | Thermal conductivity | W/m·K |
| Fits the budget | Cost per kg × mass, plus processing cost | money |
The properties guide explains strength, hardness and toughness and how they are measured.
Comparing material families
Material selection charts plot one property against another for every material family. The one below compares strength with density. It shows at a glance why aircraft use aluminium, titanium and carbon fibre, and why most machine frames are still steel.
A five-step method
- Write the requirements from the five questions, with numbers wherever possible: “carries 2 kN with a factor of safety of 2”, “outdoors, −20 to +60 °C”, “200 per year”.
- Screen. Rule out every family that fails a hard limit. If the part sits in sea spray, plain carbon steel without a coating is out. If it runs at 150 °C, most plastics are out.
- Rank. Compare the survivors on what matters most. For a light, stiff part, compare modulus per unit density; for a cheap, strong part, compare strength per unit cost.
- Check manufacturing and cost. Can your process and suppliers handle it? Is the material available in the size you need? What does the finished part cost, including machining, welding and coating?
- Specify. Write the exact grade and condition on the drawing, with any heat treatment and finish. “Steel” or “aluminium” alone is not a specification.
Worked example
Part: a bracket that holds a 5 kg sensor on an outdoor machine. It is folded from sheet, 200 are needed per year, and it must not rust or bend visibly.
Requirements: stiffness more important than strength; outdoors with rain; bendable sheet; low cost at 200 per year.
Screen: uncoated mild steel would rust; plastics creep and weaken in sunlight at this size. Candidates: galvanised mild steel, 304 stainless steel, 5052 aluminium.
| Galvanised steel (DX51D + Z275) | Stainless steel 304 | Aluminium 5052-H32 | |
|---|---|---|---|
| Stiffness (E) | 210 GPa | 193 GPa | 70 GPa |
| Weight for the same stiffness | Heaviest | Heavy | About half the steel version |
| Corrosion outdoors | Good until the zinc is cut or worn | Very good | Very good |
| Bending | Easy | Springs back more | Easy |
| Material cost | Lowest | Highest | Medium |
Choice: 5052-H32 aluminium, 3 mm thick. It does not rust, bends cleanly, and is about half the weight of a steel bracket of equal stiffness (it needs to be thicker, but its density is a third of steel’s). Galvanised steel would be the choice if cost per part mattered more than weight.
Common material grades and where they are used
Typical properties; check the supplier’s certificate for the exact values of a given size and condition.
| Material (EN / US) | Yield strength | Tensile strength | Typical uses |
|---|---|---|---|
| S275JR / similar to A36 | 275 MPa | 410–560 MPa | Welded frames, plates, brackets |
| S355JR | 355 MPa | 470–630 MPa | Heavier structures, machine bases |
| C45 / AISI 1045 (normalised) | about 340 MPa | about 600 MPa | Shafts, pins, gears, general machined parts |
| 42CrMo4 / AISI 4140 (quenched and tempered) | about 750 MPa | 900–1100 MPa | Highly loaded shafts, axles, bolts |
| 1.4301 / 304 stainless | about 230 MPa | 520–720 MPa | Food equipment, covers, outdoor parts |
| 1.4401 / 316 stainless | about 240 MPa | 520–680 MPa | Marine and chemical environments |
| EN AW-6082-T6 / 6061-T6 | about 260 MPa | about 300 MPa | Machined aluminium parts, frames, profiles |
| EN AW-5052-H32 | about 190 MPa | about 230 MPa | Sheet-metal parts, enclosures, marine fittings |
| EN AW-7075-T6 | about 500 MPa | about 570 MPa | Highly stressed light parts, aerospace |
| EN-GJL-250 grey cast iron | — | 250 MPa | Machine beds, housings, brake discs |
| POM (acetal) | about 65 MPa | — | Gears, bushes, clips, sliding parts |
| PA66 (nylon), glass-filled | about 150 MPa | — | Housings, brackets, under-bonnet parts |
Common mistakes
- Designing for strength when stiffness is the limit. A stronger steel has the same Young’s modulus as a cheap one, so it bends just as much.
- Ignoring the joints. Welding softens heat-treated aluminium (6061-T6 near the weld drops to roughly half its yield strength), and dissimilar metals in contact corrode.
- Forgetting availability. An exotic grade with a 12-week lead time can stop a project.
- Leaving the specification vague. The drawing should state the grade, the condition (T6, H32, Q+T), any heat treatment and the finish.
Key points
- Start from function, load, environment, cost and manufacturing, not from a favourite material.
- Turn each requirement into a property you can compare with numbers.
- Screen out what fails, rank what’s left, then check cost and manufacturing.
- Specify the full grade and condition on the drawing.