By the end of this topic, you should be able to...
explain how and why materials are classified and discuss the advantages of classifying materials in terms of physical, chemical and mechanical properties.
Guiding Question
How do material properties and classifications aid material selection for a specified manufacturing process?
💡 Did You Know? Your AirPods case, your bike frame, the glass on your phone screen, and the grip on your sports shoes are all made from completely different materials. But designers didn't just grab whatever was nearby. Every material on the planet belongs to a family, and every material has a set of measurable characteristics — its properties — that describe exactly what it can and cannot do. Before a designer can select the right material, they need two things: a system for grouping materials into families, and a shared language for describing how those materials behave.
Why Do We Classify Materials?
Imagine you are a designer working on a new product. There are over 160,000 engineered materials currently available. Nobody can evaluate 160,000 options individually.
Classification solves this. By organising materials into groups with shared characteristics, a designer can:
Predict behaviour — if you know a material belongs to the thermoset family, you already know it cannot be re-melted, recycled through conventional streams, or reshaped after curing — before testing a single sample
Narrow the field rapidly — instead of evaluating 160,000 materials, a classification system might reduce that to 12 relevant candidates for a specific application
Use a shared language — when a designer specifies "thermoplastic elastomer" on a technical drawing, every engineer and manufacturer understands exactly what family of behaviour to expect
Guide manufacturing decisions — material families are directly linked to appropriate manufacturing processes; knowing the class gets you most of the way to knowing the process
Identify root causes when products fail — understanding classification helps engineers diagnose why a material failed and what category of replacement to investigate
The Three Property Groups Used in Classification
Beyond grouping materials into families, designers describe and compare materials using three main property groups. Understanding these three groups is one of the most powerful conceptual tools in material selection.
Physical Properties
Physical properties are inherent, measurable characteristics of a material that can be observed without applying mechanical force or triggering a chemical reaction. They describe what the material fundamentally is.
Physical Property | What It Describes | Example |
Density | Mass per unit volume (kg/m³) | Aluminium (2,700 kg/m³) vs. steel (7,850 kg/m³) |
Electrical conductivity | Ease of electric current flow | Copper (conductor) vs. rubber (insulator) |
Thermal conductivity | Rate of heat transfer through material | Aluminium (205 W/m·K) vs. wood (~0.15 W/m·K) |
Optical properties | Transparency, translucency, opacity | Phone screen glass vs. metal chassis |
Magnetic properties | Response to a magnetic field | Steel (ferromagnetic) vs. aluminium (paramagnetic) |
Advantage of classifying by physical properties:
Physical properties act as the first filter in material selection. A designer working on a wearable device can immediately use density to eliminate all dense metals before any other evaluation takes place. Knowing a material family's typical density range is instantly useful — metals are generally dense, polymers are not, carbon fibre composites sit in between. This narrows the candidate list before a single data sheet is opened.
Chemical Properties
Chemical properties describe how a material reacts or changes when it interacts with its environment — water, oxygen, acids, UV light, solvents, or biological organisms. Unlike physical properties, observing a chemical property involves the material changing at a molecular or atomic level.
Chemical Property | What It Describes | Example |
Corrosion resistance | Resistance to degradation by moisture and oxygen | Stainless steel vs. mild steel exposed to rain |
Oxidation | Rate of reaction with oxygen in the atmosphere | Iron rusts; gold does not oxidise under normal conditions |
Chemical resistance | Resistance to acids, alkalis, and solvents | PTFE (Teflon) resists almost all chemical attack |
Flammability | Tendency to ignite and sustain burning | Most polymers burn; ceramics do not |
UV degradation | Breakdown of material structure under ultraviolet light | Unprotected polypropylene becomes brittle outdoors |
Biocompatibility | Safety in prolonged contact with living tissue | Medical-grade silicone (safe); some plasticised PVCs (potentially unsafe) |
Biodegradability | Ability to be broken down by microbial activity | Cotton biodegrades within years; PET does not |
Corrosion and classification:
Mild steel and stainless steel are both ferrous metals — same family, broadly similar mechanical properties. But their chemical properties are radically different. Mild steel corrodes visibly within weeks in a damp outdoor environment. Grade 316 stainless steel lasts decades in the same conditions because its chromium content creates a self-repairing passive oxide layer. Classification tells you the possibility of corrosion; specific chemical properties tell you how much to worry about it and whether it has been engineered out.
UV degradation:
Cheap garden furniture made from unmodified polypropylene becomes chalky and brittle after a few summers because UV light breaks down the polymer chains. High-quality outdoor furniture uses UV-stabilised polypropylene — same material family, but with additives that block UV degradation. Classification identifies the risk; the chemical property specification tells you whether it has been managed.
Advantage of classifying by chemical properties:
Chemical properties determine whether a material will survive its operating environment over its intended lifespan. A mechanically excellent material that corrodes, burns, or UV-degrades in service is not fit for purpose — regardless of how strong it is. Classifying by material family gives a strong initial prediction of chemical behaviour: ceramics are generally chemically stable; ferrous metals generally need corrosion protection; most polymers are chemically resistant to water but vulnerable to UV and specific solvents.
Mechanical Properties
Mechanical properties describe how a material responds when forces are applied to it. They are the most frequently referenced property group in structural and product design because products are almost always under some form of mechanical load — even when stationary.
Mechanical Property | What It Describes | Example |
|---|---|---|
Tensile strength | Resistance to being pulled apart | Climbing rope must not snap under body weight |
Compressive strength | Resistance to being crushed | Concrete column supporting a building |
Hardness | Resistance to surface scratching or indentation | Gorilla Glass on a phone screen |
Toughness | Ability to absorb impact without fracturing | Skateboard deck; car bumper |
Elasticity | Ability to deform under load and fully recover shape | Running shoe sole; trampoline mat |
Stiffness | Resistance to elastic deformation (flexing) | Laptop frame must not flex when typing |
Ductility | Ability to be drawn into wire | Copper electrical wiring |
Malleability | Ability to be rolled or hammered flat | Aluminium foil; gold leaf |
Fatigue resistance | Resistance to crack growth under repeated loading | Bicycle crank arm over thousands of kilometres |
(These properties are explored in full technical detail in sub-topic A3.1.4)
Advantage of classifying by mechanical properties:
Material families predict mechanical behaviour strongly and reliably. Metals are generally strong and tough, and can be both ductile and malleable. Ceramics are hard and stiff but brittle — high compressive strength, low toughness. Elastomers have exceptional elasticity but low tensile strength. Composites can be engineered to combine mechanical properties unavailable in any single-family material. Classification gives a designer an immediate mechanical profile to work from before a single data sheet is consulted.
The Advantage of Using All Three Property Groups Together
Classification by a single property group alone is insufficient and can lead to poor material decisions. The full advantage of a classification system comes from applying all three groups simultaneously.
Worked example — choosing a material for a bicycle helmet outer shell:
Material Candidate | Physical Properties | Chemical Properties | Mechanical Properties | Overall Verdict |
|---|---|---|---|---|
ABS polymer | Low density ✅ | UV-stabilised grades available ✅ | Tough and impact resistant ✅ | ✅ Strong candidate |
Mild steel | High density ❌ | Corrodes without protection ⚠️ | Strong and tough ✅ | ❌ Eliminated by density and corrosion |
Polycarbonate (PC) | Low density ✅ | Good UV resistance ✅ | Very high toughness ✅ | ✅ Strong candidate |
Ceramic | Moderate density ⚠️ | Excellent chemical stability ✅ | Brittle, poor toughness ❌ | ❌ Eliminated by toughness failure |
Classifying by physical properties alone would leave mild steel as a possibility. Only when chemical properties (corrosion in rain) and mechanical properties (toughness on impact) are applied together does it become clear that the polymer family — specifically ABS and polycarbonate — is the correct family to focus on. The three-group framework eliminates wrong answers faster and more reliably than any single group could.
Table 1: The Three Property Groups and Their Classification Advantage
Property Group | Key Questions Answered | Advantage to the Designer | Product Example |
|---|---|---|---|
Physical | How heavy is it? Does it conduct heat or electricity? Is it transparent? | Instantly eliminates materials that are too heavy, too conductive, or wrong optically | Low-density aluminium chassis for wearable device |
Chemical | Will it rust? Degrade in UV? React with skin? Burn? | Eliminates materials that won't survive their operating environment over product lifespan | Stainless vs. mild steel for outdoor bike lock body |
Mechanical | Can it handle forces? Will it snap, bend, or absorb impact correctly? | Confirms the material family can perform the structural function required | Polycarbonate helmet shell — tough, not brittle |
Table 2: Material Family Classification Summary
Family | Sub-type | Specific Example | Product Example |
|---|---|---|---|
Metals | Ferrous | Mild steel | Bike frame, gym weights |
Metals | Non-ferrous | Aluminium | iPhone chassis |
Metals | Non-ferrous | Copper | Phone charger wiring |
Polymers | Thermoplastic | ABS | LEGO bricks |
Polymers | Thermoplastic | Polypropylene | Phone case |
Polymers | Thermoset | Epoxy resin | Skateboard deck lamination |
Polymers | Elastomer | Silicone | Apple Watch band |
Ceramics | — | Chemically strengthened glass-ceramic | Phone screen (Gorilla Glass) |
Composites | — | CFRP | F1 car bodywork, premium bike frame |
Smart | Thermochromic | Liquid crystal dye | Colour-changing novelty mug |
Smart | Shape memory alloy | Nitinol | Orthodontic braces wire |
Key Vocabulary
Term | Definition |
|---|---|
Physical property | A property describing how a material responds to non-mechanical stimuli — heat, electricity, light, magnetic fields — without changing the material's chemical composition |
Chemical property | A property describing how a material reacts or interacts with its chemical environment — including corrosion, oxidation, biodegradation, and flammability |
Mechanical property | A property describing how a material responds to applied forces and loads |
Density | Mass per unit volume of a material (kg/m³) |
Thermal conductivity | The rate at which a material transfers heat energy through itself per unit thickness per unit temperature difference (W/m·K) |
Coefficient of thermal expansion (CTE) | The fractional change in length of a material per degree Celsius of temperature change (µε/°C) |
Limiting Oxygen Index (LOI) | The minimum oxygen concentration (%) in an atmosphere required to sustain combustion of a material |
Young's modulus (E) | The ratio of stress to strain in the elastic region of a material — a measure of stiffness, not strength (GPa) |
Tensile strength (UTS) | The maximum stress a material can withstand in tension before fracture (MPa) |
Yield strength | The stress at which a material begins to deform permanently (plastically) — the design limit for most structural applications (MPa) |
Hardness | A material's resistance to localised surface indentation or scratching — measured on Vickers (HV), Rockwell (HRC) or Brinell (HB) scales |
Toughness | The total energy absorbed per unit volume by a material up to the point of fracture — quantified as the area under the stress-strain curve (J/m³ or MPa√m) |
Ductility | The ability of a material to undergo significant plastic (permanent) deformation in tension before fracture, expressed as percentage elongation (%) |
Malleability | The ability of a material to undergo plastic deformation in compression (rolling, hammering) without fracture |
Fatigue strength | The maximum cyclic stress amplitude below which a material can endure an essentially unlimited number of load cycles without fracturing — the endurance limit (MPa) |
Creep | The slow, permanent deformation of a material under sustained load, particularly at elevated temperatures |
Elastic deformation | Temporary, fully reversible deformation — the material returns to its original shape when the load is removed |
Plastic deformation | Permanent, irreversible deformation — the material does not return to its original shape when the load is removed |
Hooke's Law | The linear relationship σ=E⋅ε\sigma = E \cdot \varepsilonσ=E⋅ε between stress and strain in the elastic region of a material's behaviour |
Biodegradability | The capacity of a material to be broken down by biological organisms (bacteria, fungi) into natural compounds such as water, CO₂, and biomass |
Corrosion resistance | The ability of a material to resist electrochemical attack by moisture and ionic species in the environment |
Specific stiffness | Young's modulus divided by density (E/ρ)(E/\rho)(E/ρ) — a measure of stiffness per unit mass, used for lightweight structural design |
Specific strength | Strength divided by density (σ/ρ)(\sigma / \rho)(σ/ρ) — a measure of strength per unit mass |
Practice Questions
Question 1 (2 marks)
Explain why it is useful for designers to classify material properties into physical, chemical, and mechanical categories.
Examiner's hint: Two distinct reasons required — one mark each. Valid answers include: enables systematic comparison using quantified data; creates a universal communication framework; allows failure modes to be predicted; supports material selection tools such as Ashby charts; separates different types of material behaviour to prevent confusion between properties. Do not list properties — the question asks why classification is useful, not what the properties are.
Question 2 (4 marks)
Discuss the advantages and limitations of classifying materials by their physical, chemical, and mechanical properties for a product designer selecting a material for a children's outdoor climbing frame.
Examiner's hint: Advantages — physical properties (density critical for structural weight, thermal conductivity important as metal surfaces can heat dangerously in sun or cool in cold — burns/discomfort risk), chemical properties (corrosion resistance essential for outdoor weathering; UV resistance; toxicity critical for child safety), mechanical properties (toughness/impact resistance — children impact components; fatigue strength for repeated loading; hardness for scratch/wear resistance on high-traffic surfaces). Classification enables systematic evaluation against all these criteria. Limitations — cost is not captured (HDPE and galvanised steel may both meet property criteria but have different costs); processability not captured (welding, moulding constraints); aesthetic properties (colour, texture, appearance) are separate from the three categories; the classification framework must be supplemented with regulatory compliance checks (EN 1176 playground equipment standard) and lifecycle/sustainability analysis. A good answer presents both advantages and limitations with specific reference to the climbing frame context — not generic statements.
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.
Ulrich, K.T. and Eppinger, S.D. (2015). Product Design and Development. 6th edn. McGraw-Hill Education, New York.
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)