Engineering / Mechanical Engineering
Properties of Wood, Ceramics, Plastics and Metals
Every material class trades stiffness, strength and weight differently. Reading those trade-offs — and especially the weight-adjusted versions — is what turns a materials list into a design decision. One result surprises everyone: the common metals are equally stiff for their weight.
- Reading time · 4 min
- 7 sections
- Specific stiffness, charted
- Metals cluster — worked
§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
Handbook application: from concept to controlled practice
Purpose. This expanded section turns the original page into a practical handbook. It preserves the supplied material and adds a repeatable way to apply, check and review Properties of Wood, Ceramics, Plastics and Metals. It does not replace a contract, legislation, a controlled standard, competent engineering judgement or specialist advice.
The operating aim is to carry the subject from function and assumptions through design evidence, verification and controlled release. Read the original explanation first, then use the workflow and checks below to convert knowledge into evidence.
Apply Properties of Wood, Ceramics, Plastics and Metals by beginning with the duty, not the component or software command. Convert the key ideas—stiffness, strength, specific, metals, material—into measurable requirements and interfaces. Record operating and non-operating environments, duty cycle, expected life, loads, energy sources, human interaction and reasonably foreseeable abnormal conditions. When a value is not a project requirement or verified supplier datum, identify it as an assumption or illustrative value.
Create a calculation and evidence trail that another competent person can audit. Every input should carry a source, unit, revision and uncertainty or tolerance where relevant. Every model should state its boundary conditions and limitations. Keep nominal capacity separate from design capacity, and keep verification margin separate from an arbitrary safety factor. If a code or standard governs the work, confirm the applicable edition and contractual status rather than copying a number from a secondary summary.
Design for manufacture, assembly, inspection, operation and maintenance at the same time. A technically valid geometry can still fail because it cannot be fixtured, measured, cleaned, guarded, reached or replaced. Review process capability, datum or reference strategy, tolerance accumulation, access, error-proofing and changeover. Where people interact with plant, apply the hierarchy of controls and consult those who will operate, clean, maintain and recover the equipment.
Plan verification before release. Define the characteristic, method, equipment, sample or test condition, acceptance criterion, record and responsible person. Validation then asks a different question: whether the resulting system is effective and suitable in the intended use context. A passed drawing check or analysis does not by itself validate usability, maintainability or production performance.
Step-by-step operating method
- Define the duty. Capture the required function, interfaces, operating environment, life, loads and unacceptable outcomes.
- Establish the model. Identify governing principles, units, material or process data, assumptions and uncertainty.
- Develop alternatives. Compare feasible concepts against performance, manufacturability, safety, maintainability and cost.
- Verify the design. Use analysis, test, inspection or demonstration with acceptance criteria defined before execution.
- Release and learn. Baseline the design, control changes, retain evidence and feed operating results into the next revision.
Illustrative design review record
Illustrative values only. Build a one-page record with the required function, input sources, assumptions, governing load or process condition, failure consequences, selected concept, verification method and acceptance criterion. Mark every numerical input as project requirement, verified supplier data, measured value, calculation output or assumption. Review the weakest evidence first. If an assumption can change safety, compliance, interchangeability or capacity, it must be resolved before release rather than buried in a calculation note.
| Evidence class | Question | Release expectation |
|---|---|---|
| Requirement | What must the design do and under which conditions? | Approved and traceable |
| Input | Where did the load, property, tolerance or process limit come from? | Source, unit and revision recorded |
| Analysis | Which model and assumptions connect input to result? | Checkable calculation or simulation |
| Verification | How will conformity be demonstrated? | Method and acceptance criterion agreed |
| Validation | Will the solution work for intended users and conditions? | Representative use evidence |
Common failure modes and recovery actions
1. Watch for
Starting detailed design before interfaces and operating limits are agreed.
Recovery: Return to the governing definition or requirement and restate the decision in one sentence.
2. Watch for
Using catalogue or typical values as though they were certified project inputs.
Recovery: Separate evidence from assumption, assign an owner and set a date for validation.
3. Watch for
Checking nominal performance while ignoring tolerances, degradation and foreseeable misuse.
Recovery: Run a small counterexample, boundary test, pilot or independent check before proceeding.
4. Watch for
Confusing verification of requirements with validation of user need.
Recovery: Record the consequence, decision and rationale, then update the controlled baseline.
5. Watch for
Releasing drawings or procedures without configuration, inspection and change controls.
Recovery: Escalate when the issue affects safety, compliance, acceptance, material value or an agreed tolerance.
Review checklist
- What function and failure consequence govern this decision?
- Which inputs are measured, specified, assumed or illustrative?
- How will conformity be demonstrated and recorded?
- What change would invalidate the current evidence?
- Are mandatory requirements distinguished from recommendations and illustrative values?
- Are sources, assumptions, units, dates and versions recorded closely enough to reproduce the decision?
- Have safety, legal, ethical, stakeholder and operational consequences been considered at the appropriate level?
- Is there a named owner and a trigger for review, escalation, change or retirement?
Questions for deeper application
What is the most important distinction a practitioner must preserve when applying Properties of Wood, Ceramics, Plastics and Metals?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Which assumption about stiffness would change the result most if it proved false?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
What evidence would allow an independent reviewer to reproduce or challenge the conclusion?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Which boundary, exception or failure case has not yet been tested?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
What must be handed over, monitored or reviewed after the immediate work is complete?
Answer with a fact or cited source where available. Where evidence is incomplete, record the assumption, consequence, responsible owner and next validation action.
Authoritative references and use notes
The sources below were selected as institutional or primary guidance for the broader practice. They support the handbook method; they do not imply that every statement or clause in a source applies to every project. Confirm the current edition, jurisdiction, contract and application before treating any requirement as mandatory.
- NASA Systems Engineering Handbook — NASA. Used for requirements, design, verification, validation and technical management. Accessed 2026-08-13.
- Identify, assess and control hazards — Safe Work Australia. Used for hazard identification, risk assessment, controls and review. Accessed 2026-08-13.