§1The four material classes
Engineering materials fall into four families, each with a characteristic bonding and therefore a characteristic behaviour: metals, ceramics, polymers (plastics) and natural materials such as wood.
Metals are stiff, strong, ductile and tough — they bend before they break, which is why they dominate load-bearing structure. Ceramics are stiffer and harder still, and hold that hardness when hot, but they are brittle, failing suddenly in tension. Polymers are light, cheap, corrosion-proof and easily formed, but comparatively soft and temperature-sensitive. Wood and other natural composites are light, renewable and, along the grain, remarkably efficient. The rest of this page compares them by the three properties that decide most designs — and then by those same properties divided by weight, which is where the real lessons hide.
Contents§2Stiffness, strength and density
Three numbers describe a material’s mechanical character: how much it deflects under load (stiffness, the elastic modulus E), how much load it survives (strength σ), and how heavy it is (density ρ).
| Material | E (GPa) | Strength (MPa) | ρ (kg/m³) |
|---|---|---|---|
| Steel | 200 | 250–1500 | 7850 |
| Aluminium | 69 | 100–500 | 2700 |
| Titanium | 116 | 800–1000 | 4500 |
| Alumina (ceramic) | 380 | 300 (comp. far higher) | 3900 |
| Nylon (plastic) | 3 | 70 | 1140 |
| Pine (along grain) | 11 | 40–90 | 500 |
| Stiffness and strength are different properties: stiffness (E) sets deflection and is fixed by the material’s bonding, essentially unchanged by heat treatment; strength is raised dramatically by alloying and treatment, which is why steel shows one modulus but a six-fold strength range. | |||
§3Specific stiffness — the surprise
Divide stiffness by density and you get specific stiffness (E/ρ) — stiffness per unit weight, the figure that matters when a part must be both rigid and light. The result astonishes newcomers.
Steel E/ρ = 200/7850 = 25.5 MJ/kg. Aluminium = 69/2700 = 25.6. Titanium = 116/4500 = 25.8. Magnesium = 45/1740 = 25.9. They are all but identical. This is the crucial lesson of materials selection: you cannot make a stiffness-limited part lighter by swapping one common metal for another — a lighter metal is exactly as much less stiff. To beat it you must leave metals entirely: carbon-fibre composite reaches about 94 MJ/kg, and wood along the grain, at 22, is nearly as good as steel. It is why aircraft moved to composites and why timber remains a serious structural material.
§4Specific strength — the difference
Do the same with strength — σ/ρ, strength per unit weight — and the metals now separate sharply, the opposite of stiffness.
| Material | Specific strength (kN·m/kg) |
|---|---|
| Mild steel | 32 |
| High-strength steel | 127 |
| 7075-T6 aluminium | 178 |
| Ti-6Al-4V titanium | 199 |
| Carbon-fibre composite | 938 |
| Because strength (unlike modulus) responds to alloying and treatment, a strength-limited part can be made lighter by choosing a better material: aluminium and titanium alloys beat mild steel decisively on strength-for-weight, which is exactly why they fill aerospace roles. The pairing to remember: switch metals to save strength-weight, but not to save stiffness-weight. | |
§5Reading a property for selection
Choosing a material is choosing which property, adjusted for what constraint, to maximise — and the two examples above show the method generalises.
The discipline is to identify what actually limits the part. If it must not deflect, stiffness governs and E/ρ ranks the candidates. If it must not yield, strength governs and σ/ρ ranks them. If cost or corrosion or temperature dominates, a different property leads. The mistake is to reach for a “better” material without asking which property is limiting — the specific-stiffness result shows how that intuition misfires, since the obvious move (a lighter metal) buys nothing when stiffness is the constraint. Good selection is property-led, weight-adjusted, and constraint-specific.
Contents§6Class by class
A one-line character sketch of each family, and where it wins.
Metals
Stiff, strong, tough and ductile; forgiving because they yield before fracture. The default for load-bearing structure.
Ceramics
Hardest and most heat-resistant, stiff, but brittle in tension. For wear surfaces, cutting tools and high-temperature parts.
Plastics
Light, cheap, corrosion-proof, easily moulded; soft and temperature-limited. For housings, low-load and low-friction parts.
Wood
Light and efficient along the grain, renewable, anisotropic. A genuine structural material, not merely a traditional one.
§7Quick reference
The working core of the page on one card rack.
Three properties
E (stiffness) · σ (strength) · ρ
Specific stiffness
E/ρ · metals ≈ 25–26 MJ/kg
cannot beat by swapping metal
Specific strength
σ/ρ · Al, Ti beat mild steel
E is fixed
heat treatment ↑ strength
not modulus
Select by
the limiting property
adjusted for weight