By the end of this topic, you should be able to...
evaluate the physical, chemical and mechanical properties to ensure the selection of the most appropriate material for a specific purpose.
Guiding Question
How do material properties and classifications aid material selection for a specified manufacturing process?
💡 Did You Know? Nike's React foam midsole. Apple's aerospace-grade aluminium unibody. A surgical scalpel blade. What do these have in common? Behind every one of them is a designer who looked at hundreds of possible materials and made a decision — this one, not that one. Get it right and you have a product that performs, sells, and lasts. Get it wrong and you have the Samsung Galaxy Note 7: a phone with a battery whose casing material could not handle the thermal and chemical environment inside the device — resulting in fires on aircraft worldwide and a $5 billion recall. Material selection is not a footnote in the design process. It IS the design process.
The Core Idea
Designers don't just pick materials they like the look of. Every material selection decision is a balancing act — weighing up physical, chemical, and mechanical properties against the real demands of the product's function, manufacturing process, user, and environment.
The framework used by professional engineers — developed by Professor Mike Ashby at Cambridge — puts it simply: define what the material needs to do, then find the material that does it best (Ashby, 2011).
The Three Property Categories
1. Physical Properties
What the material IS — its intrinsic characteristics, independent of loading
Property | What it means | Why it matters in design |
Density | Mass per unit volume (kg/m³) | Lightweight products → low density. Ballast, counterweights → high density |
Melting point | Temperature at which solid → liquid | Determines manufacturing process and maximum service temperature |
Thermal conductivity | Rate of heat transfer through the material | Cookware needs high conductivity. Insulation needs low conductivity |
Electrical conductivity | Ability to carry electric current | Wiring → high conductivity (copper). Casing → low conductivity (ABS polymer) |
Optical properties | Transparency, reflectivity, colour | Lenses → optical clarity. Solar panels → maximum light absorption |
Thermal expansion | How much a material expands when heated | Gaps in railway tracks, expansion joints in bridges — thermal expansion is non-negotiable in design |
Real example: The Mars Rover Curiosity uses aluminium alloy wheels — not steel, not titanium. Why? In the Martian environment, the mass penalty of carrying steel across 560 million km of space and across Martian terrain is unjustifiable. Density wins the argument (NASA, 2012).
2. Chemical Properties
How the material reacts with its environment
Property | What it means | Why it matters in design |
Corrosion resistance | Resistance to electrochemical degradation (rusting, oxidation) | Outdoor structures, marine environments, food contact surfaces |
Chemical resistance | Resistance to acids, alkalis, solvents, oils | Laboratory equipment, fuel tanks, chemical pipework |
Oxidation resistance | Resistance to reacting with oxygen at high temperatures | Jet engine turbine blades, exhaust systems |
Biocompatibility | Ability to exist inside the human body without toxic reaction | Medical implants, surgical instruments, drug packaging |
Flammability | Tendency to ignite and sustain combustion | Aircraft interiors, children's products, building materials — regulated by law |
Real example: 316 stainless steel contains 2–3% molybdenum — which does almost nothing for its mechanical strength but dramatically improves its resistance to chloride corrosion. That's why it's specified for marine hardware, surgical implants, and food processing equipment where 304 stainless steel would corrode. One alloying element. A completely different chemical behaviour. (Callister & Rethwisch, 2018).
3. Mechanical Properties
How the material responds to forces — see A3.1.6 for full detail
Property | One-line definition | Key design application |
Tensile strength | Max stress under pulling load before failure | Cables, structural members in tension |
Compressive strength | Max stress under pushing load before failure | Columns, arches, pavements |
Stiffness (Young's Modulus) | Resistance to elastic deformation | Frames, shelving, precision components |
Toughness | Energy absorbed before fracture | Crash structures, helmets, safety-critical components |
Hardness | Resistance to surface indentation | Cutting tools, gear teeth, bearing surfaces |
Ductility | Extent of plastic deformation under tension before fracture | Wire drawing, sheet metal forming |
Malleability | Extent of plastic deformation under compression before fracture | Forging, rolling, coinage |
Elasticity | Ability to recover original shape after loading | Springs, seals, flexible packaging |
Ashby's Material Selection Strategy
Ashby formalised material selection into a systematic method using material property charts — sometimes called Ashby Maps (Ashby, 2011). These charts plot two material properties against each other (e.g. stiffness vs. density, strength vs. toughness) and reveal which materials offer the best combination of both.
The process follows four steps:
Step 1 — Translate the design requirementsConvert the product's function and constraints into required material properties.Example: A bicycle frame must be light (low density) and stiff (high Young's Modulus).
Step 2 — Screen candidatesEliminate all materials that fail to meet minimum threshold requirements — e.g. exclude all materials with density > 3000 kg/m³ for a lightweight frame.
Step 3 — Rank remaining candidatesUse a performance index — a ratio of material properties that captures the combined requirement — to rank survivors.Example: For a stiffness-limited beam minimising mass, the performance index is E1/2ρ\frac{E^{1/2}}{\rho}ρE1/2 (Ashby, 2011).
Step 4 — Evaluate and decideCross-check shortlisted materials against properties that were not captured in the performance index — chemical compatibility, cost, availability, recyclability, processability. The "best" material on paper may be impossible to manufacture into the required shape, or impossibly expensive.
The crucial insight from Ashby: There is almost never a single "best" material. There are always trade-offs. The designer's job is not to find perfection — it is to make a justified, evidence-based decision about which compromise is most acceptable for the specific context (Ashby, 2011).
Evaluating Trade-Offs — The Real Design Challenge
Material selection is where evaluation — weighing up strengths against limitations — becomes the central skill.
Scenario | Competing demands | The trade-off |
Sports bicycle frame | Low density AND high stiffness AND high toughness | Carbon fibre composite: exceptional stiffness-to-density ratio, low toughness — catastrophic brittle failure in crashes. Titanium: good toughness, excellent density-to-strength, expensive, harder to manufacture. Aluminium: cheap, light, decent stiffness, work-hardens, good enough. The choice depends on budget, performance level, and crash risk context. |
Climbing carabiner | High tensile strength AND light AND corrosion resistant | 7075 aluminium alloy: strong, light, resists corrosion adequately, affordable — selected for non-critical recreational hardware. Steel: stronger, heavier, corrodes. Titanium: strongest-to-weight, fully corrosion resistant, very expensive — selected where weight is critical (mountaineering rack). |
Disposable coffee cup lid | Cheap AND chemical resistant AND processable | Polypropylene (PP): chemical resistance to hot liquids, cheap, injection moulded, food-safe. PLA bioplastic: similar processability, compostable, but lower heat resistance — risks softening with very hot beverages. The sustainability benefit of PLA must be evaluated against the functional limitation. |
Key Vocabulary
Learn these. Use them precisely. The examiner rewards accurate technical language.
Term | Definition |
Physical property | An intrinsic characteristic of a material independent of applied force — density, melting point, thermal and electrical conductivity, optical properties |
Chemical property | A material's behaviour in response to its chemical environment — corrosion, oxidation, chemical resistance, biocompatibility, flammability |
Mechanical property | A material's response to applied forces — strength, stiffness, toughness, hardness, ductility, elasticity |
Performance index | A ratio of material properties derived from design requirements — used to rank candidate materials for a specific application (Ashby, 2011) |
Material screening | The elimination of candidate materials that fail to meet minimum threshold requirements for one or more properties |
Trade-off | The design situation in which improving one property necessarily compromises another — the central challenge of material selection |
Density | Mass per unit volume, measured in kg/m³ — the primary physical property driver in lightweight design |
Biocompatibility | The ability of a material to perform with an appropriate host response when in contact with biological tissue |
Corrosion | Electrochemical degradation of a material — most commonly oxidation of metals in the presence of moisture and oxygen |
Specific strength | Tensile strength divided by density — the key performance index for lightweight structural applications (Ashby, 2011) |
Ashby Map | A material property chart plotting two properties against each other across the full range of engineering materials — used to identify optimal material candidates for a given design requirement |
Practice Questions
The learning objective for this topic uses the command term EVALUATE — "make an appraisal by weighing up the strengths and limitations." Every practice question here mirrors that demand. Half-marks are what you get for describing one side. Full marks require you to weigh both sides and reach a justified conclusion.
Question 1 (4 marks)
Evaluate the use of carbon fibre reinforced polymer (CFRP) versus aluminium alloy for the frame of a high-performance road bicycle. Consider at least two material properties from different categories in your response.
Examiner's hint: CFRP strengths — specific stiffness, specific strength, vibration damping. CFRP limitations — brittle fracture behaviour, cost, difficulty of repair, recyclability. Aluminium strengths — toughness, cost, repairability, recyclability. Aluminium limitations — higher density, lower specific stiffness. Your conclusion must state which is more appropriate AND why — for this specific context.
Question 2 (4 marks)
Evaluate the selection of polypropylene (PP) for a reusable food storage container. Identify the properties that make it suitable and the properties that may limit its performance.
Examiner's hint: Consider chemical resistance to food acids and oils, thermal resistance (microwave/dishwasher), density, cost, processability, and chemical properties such as leaching at high temperatures. Weigh strengths against limitations and conclude.
Question 3 (6 marks)
A designer is selecting a material for a marine boat cleat — a fitting used to secure ropes on a sailing vessel. The cleat must resist corrosion in a saltwater environment, sustain high tensile loads from ropes, and be low maintenance for the user. Evaluate the suitability of the following three candidate materials: 316 stainless steel, nylon 6,6 (polyamide), and bronze (Cu-Sn alloy). Justify which material you would select for this application.
Examiner's hint: This is a full evaluation. Address physical, chemical, AND mechanical properties for each candidate. 316 stainless — corrosion resistance, high strength, dense, cold to touch. Nylon — lightweight, low corrosion, lower strength, UV degradation risk, flexible. Bronze — excellent marine corrosion resistance, high strength, traditional/aesthetic, heavy, expensive. Justify selection with explicit reference to the design requirements stated in the question.
Question 4 (6 marks)
Evaluate the statement: "The material with the highest tensile strength is always the best choice for a structural application." Use at least two named material examples and reference both mechanical and physical properties in your response.
Examiner's hint: Acknowledge contexts where high UTS is correctly prioritised — suspension bridge cables, aircraft structural members. Challenge the statement with counter-examples: a diamond has extremely high hardness but is brittle and impractical; a concrete column needs compressive strength, not tensile strength; a crash structure needs toughness not just strength. A spacecraft structure needs high specific strength (UTS/density), not raw UTS. Conclude with a nuanced position.
Question 5 (2 marks)
Evaluate whether density or tensile strength is the more important material property when selecting a material for a commercial passenger aircraft wing spar.
Examiner's hint: Brief but balanced — the spar must carry tensile loads (strength matters) but every kilogram of structural mass reduces payload capacity and increases fuel burn over millions of flight cycles (density matters equally or more). The performance index specific strength (UTS/ρ) captures both simultaneously — neither alone is sufficient.
Sources
Ashby, M.F. (2011). Materials Selection in Mechanical Design. 4th edn. Butterworth-Heinemann, Oxford.
Callister, W.D. and Rethwisch, D.G. (2018). Materials Science and Engineering: An Introduction. 10th edn. Wiley, New York.
International Baccalaureate Organization (2024). Design Technology Guide: Diploma Programme. First Assessment 2027. IBO, Geneva.
NASA (2012). Mars Science Laboratory — Curiosity Rover: Mission Overview. Jet Propulsion Laboratory, California Institute of Technology.
Linking Questions
Why is a good understanding of material properties important when designing structural systems? (A3.2)
When do the physical properties of materials restrict the ability to use certain prototyping techniques? (A2.2)
How do the properties of a material influence the choice of manufacturing techniques for a product? (A4.1)
How can the characteristics of a material limit the effectiveness of modelling and prototyping as designs are developed? (B2.2)
How important is an understanding of the mechanical properties of a material when considering structural and mechanical systems, and their applications? (A3.2, A3.3, B3.2, B3.3)
Which classifications of properties are important when developing electronic systems and their applications? (A3.4, B3.4)
How could the continued development of biodegradable materials influence designers’ ability to address aspects of design for sustainability and design for a circular economy? (C2.1, C2.2)
Why is a thorough understanding of materials key for effective product analysis and evaluation of products? (C3.1)
How do design decisions related to the properties of materials and components impact a product’s life-cycle analysis? (C3.2)