Properties of Engineering Materials
Why Properties Matter
Choosing the right material means matching its properties to the product's demands. Properties divide into mechanical (how the material behaves under force) and other manufacturing properties. Precise definitions are essential — each must name the type of force or condition involved to score full marks in the exam.
Strength
Strength is the ability to withstand an applied force without permanently deforming or fracturing. There are two sub-types. Yield/tensile strength is the maximum tensile (pulling) stress a material can withstand — the yield point is where deformation becomes permanent; tensile strength is the maximum stress before fracture. Compressive strength is the ability to resist squashing without crushing or buckling. Cast iron has high compressive but low tensile strength, making it suitable for engine blocks loaded in compression but unsuitable for impact-loaded parts.
Strength has two sub-types: yield/tensile strength (resistance to pulling forces) and compressive strength (resistance to squashing forces). A material can have high compressive strength but low tensile strength — cast iron is the classic example.
Elasticity
Elasticity is the ability to return to the original shape after a force is removed, provided the elastic limit has not been exceeded. Below the elastic limit deformation is temporary and recoverable; beyond it deformation is permanent (plastic). Spring steel and rubber have high elasticity — essential for springs, snap-fit clips and flexible connectors.
Ductility
Ductility is the ability to be permanently deformed under tension — drawn into wire or elongated — without fracturing. Copper and aluminium are highly ductile; cast iron is virtually non-ductile and fractures suddenly. Ductility is needed for wire drawing, extrusion, and deep drawing, and is measured as percentage elongation at fracture.
Hardness
Hardness is the ability to resist indentation, scratching or surface wear. Tungsten carbide and high carbon steel are very hard — used for cutting tools and drill tips. Hardness is measured by indentation tests (Vickers, Brinell) in which a probe is pressed into the surface; a smaller indent means greater hardness. Hard materials are not necessarily tough — hardness and toughness are often inversely related.
Hardness resists surface indentation or scratching. Toughness is the ability to absorb energy and resist fracture under impact — a tough material bends before it breaks. High carbon steel is hard but brittle: it holds a chisel edge but can shatter if struck sideways. Low carbon steel is much less hard but far tougher, bending plastically before fracturing. When an exam question asks for the property needed to 'resist sudden impact loads', the answer is toughness, not hardness.
Malleability
Malleability is the ability to be permanently deformed by compressive forces — hammered, rolled, or pressed — without fracturing. Gold is the most malleable element; lead, copper and mild steel are also malleable. Malleability is the key property for forging, rolling and press forming. Remember: ductility = deformation under tension; malleability = deformation under compression. A material can be malleable but not ductile — lead is easily hammered into sheet but too weak to draw into fine wire.
Ductility is deformation under tension (drawing into wire); malleability is deformation under compression (rolling or forging into sheet or complex shapes). Both describe permanent plastic deformation without fracture, but the direction of force is different.
Machinability
Machinability describes how easily a material can be cut, drilled, turned, or milled. High machinability means fast cutting speeds, low cutting forces, manageable swarf and good tool life — brass and aluminium are examples. Stainless steel has low machinability because it work-hardens during cutting, dulling tools rapidly and requiring low speeds; this increases cycle time and tool costs. Machinability is influenced by hardness, thermal conductivity and work-hardening tendency.
Cost and Sustainability
The specification also lists cost and sustainability as manufacturing properties. Material cost: price per kilogram or standard stock form. Manufacturing cost: tooling, energy and labour needed to process it. Sustainability considers the environmental impact across the full lifecycle — recycling potential, embodied energy, renewable sourcing. PLA is more sustainable than ABS because it is plant-derived and biodegradable; recycled aluminium uses only ~5% of the energy of primary smelting.
Answer properties questions in three steps: (1) name the property using the exact specification term; (2) give its definition applied to the scenario; (3) explain the consequence if the material lacked that property. For a spanner: 'The spanner requires high yield strength so it can withstand tensile and compressive stresses when tightening a bolt without permanently deforming. A low yield-strength material would bend after first use.' This structure consistently scores full marks.