Mechanical and physical properties

A material property describes how a material behaves under particular conditions. Properties are used to explain why a material is suitable for a particular product, component or manufacturing process.

Cost is not a material property. Cost, availability, aesthetics, manufacture and disposal can still affect material selection, but they are separate selection factors.

Property groups

Property group	What it describes	Important examples
Mechanical / working	How a material behaves when forces act on it or it is manipulated	tensile strength, compressive strength, shear strength, torsional strength, hardness, toughness, plasticity, malleability, elasticity, ductility
Physical	Inherent behaviour or condition of a material	density, fusibility, corrosion/degradation resistance, absorbency, water resistance, chemical resistance
Electrical	Behaviour when electricity is applied	electrical conductivity, electrical insulation
Thermal	Behaviour when exposed to heat or temperature change	thermal conductivity, thermal insulation, thermal expansion
Optical	How light passes through a material	transparent, translucent, opaque

Mechanical and working properties

Mechanical properties describe how a material reacts to external forces.

Strength

Strength is the ability of a material to resist an applied force without failing. The type of loading should be identified rather than describing a material only as “strong”.

Tensile strength: resistance to forces pulling the material apart.
Compressive strength: resistance to forces pushing or crushing the material together.
Torsional strength: resistance to twisting.
Shear strength: resistance to opposing forces acting across the material, similar to a cutting action.

Strength-to-weight ratio compares useful strength with mass. It is important where a product must be strong without becoming unnecessarily heavy, such as aircraft structures, bicycles or performance equipment.

Toughness

Toughness is the ability to absorb energy and resist cracking or breaking when subjected to impact or a sudden blow.

Toughness should not be confused with hardness. A material may be hard but brittle.

Hardness

Hardness is resistance to surface wear, scratching or indentation. It is important for cutting tools, work surfaces, bearings and components that experience repeated surface contact.

Brittleness

A brittle material is likely to crack or fracture with relatively little plastic deformation. Brittle materials can be hard but have low toughness.

Ductility

Ductility is the ability to undergo permanent deformation in tension, becoming longer and thinner without cracking. It is important when metals are drawn into wire or stretched during forming.

Malleability

Malleability is the ability to undergo permanent deformation under compression and be reshaped without cracking. Malleable metals can be rolled, pressed, hammered or formed into sheet and 3D shapes.

Elasticity

Elasticity is the ability to deform and then return to the original shape when the force is removed. This is useful for springs, flexible clips and elastomer products.

Plasticity

Plasticity is the ability to be permanently reshaped without returning to the original form. It is important in forming processes where the material must retain the manufactured shape.

Flexibility and stiffness

A flexible material bends relatively easily, while a stiff material resists bending and deformation. The required balance depends on the product: a flexible hinge and a structural beam have very different requirements.

Physical properties

Density

Density is mass per unit volume:

Density = mass ÷ volume

A low-density material can reduce product mass, while a higher-density material may be useful where weight, stability or compact mass is required.

Fusibility and melting behaviour

Fusibility describes the ability of a material to melt or change from solid to liquid when heated. Melting behaviour can determine whether a material is suitable for processes such as casting, extrusion or injection moulding.

Corrosion and degradation resistance

Corrosion resistance is the ability to resist deterioration caused by chemical or environmental attack. Metals may corrode through processes such as oxidation, while polymers and woods can degrade through UV exposure, moisture, chemicals, fungi or other environmental conditions.

Resistance to corrosion or degradation is especially important for outdoor products, food equipment, chemical containers and products expected to have a long service life.

Absorbency and water resistance

Absorbency describes how readily a material takes in water. Water-resistant materials resist this. Moisture absorption can affect hygiene, dimensional stability, outdoor suitability and durability.

Chemical resistance

Chemical resistance is the ability to withstand exposure to chemicals without significant degradation. This is important for laboratory equipment, cleaning-product containers, food products and chemical storage.

Magnetism

Magnetism describes whether a material is attracted to a magnet or can exhibit magnetic behaviour. It can influence material selection in motors, catches, sensors, tools and electrical products.

Electrical properties

Electrical conductivity

An electrical conductor allows electric current to pass through it relatively easily. Copper is widely used for electrical wiring because of its high electrical conductivity.

Electrical insulation

An electrical insulator resists the flow of electric current. Many polymers are used around wires, plugs and electrical housings because they provide electrical insulation.

Thermal properties

Thermal conductivity

A thermal conductor transfers heat readily. Metals generally conduct heat well, which can be useful for cookware, heat sinks and heat exchangers.

Thermal insulation

A thermal insulator slows the transfer of heat. Polymers, foams and some woods are useful where users need protection from hot or cold surfaces or where heat loss must be reduced.

Thermal expansion

Thermal expansion is the change in dimensions that occurs when temperature changes. Designers must allow for expansion and contraction where components experience large temperature ranges or where dimensional accuracy matters.

Optical properties

Transparent: allows light through so objects can be seen clearly through the material.
Translucent: allows some light through but scatters it, so objects are not seen clearly.
Opaque: does not allow light to pass through.

Properties and material selection

A property should be connected to a specific product requirement rather than simply listed. For example, PET is tough, so it resists cracking when a drinks bottle is knocked or dropped; this improves durability during transport and repeated handling.

Material choice may also depend on manufacturing method, aesthetics, availability, cost, maintenance and end-of-life. A material with excellent mechanical properties may still be unsuitable if it cannot be formed economically, gives the wrong appearance or creates unacceptable environmental impacts.