GCSE Engineering and Manufacturing 3.3.1 Materials and Applications

FACTFILE: GCSE Engineering and Manufacturing 3.3.1 Materials and Applications – Mechanical Properties

Materials and Applications – Mechanical Properties

Learning outcomes Students should be able to: apply knowledge and understanding of the mechanical properties of materials: • brittleness; • ductility; • malleability; • hardness; • strength (tensile, compressive, bending, shear and torsion); • stiffness; • toughness; • plasticity; • elasticity; and • conduction (heat and electricity).

Mechanical Properties Mechanical properties determine how a material will react when forces are applied to it. In manufacturing, parts and components are made by using tools and equipment to apply forces to cause deformation in the material. The mechanical properties of a material are determined by the internal structure and the nature of the chemical bonds between the atoms of the material.

Mechanical properties can be modified by the use of alloying in metals and by the use of various heat treatments. Mechanical properties also vary according to temperature, for example malleability increases with temperature and thus many metals are heated prior to forging or bending.

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Strength Strength is defined as the ability of a material to withstand force without breaking or permanently changing shape. Different types of strength resist different types of forces.

Tensile Strength Tensile strength is required for steel cables, drive chains and for structural components in buildings, cranes and bridges.

Compressive Strength Compressive strength is required for pillars or columns that support the massive weight of a building or bridges. Press tools and hydraulic rams also need to resist high compressive forces.

Bending Strength Bending strength is important in many products such as structural steel beams in building construction. The action of bending will cause the outer faces to be compressed and stretched, often leading to a fracture on the outer surface of the bend.

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Shear Strength Shear force is apparent when looking at the cutting action of tin snips and scissors but also on the forces a rivet is subjected to in a ship’s hull or aircraft fuselage.

Torsional Strength Torsion is the name given to twisting forces such as those applied to drive shafts or when tightening a bolt. Torsional strength and bending strength can be improved by modifying the cross section of the material.

Toughness Toughness is the ability to withstand shock loading without fracture. Materials that have high toughness generally have high hardness and high ductility. © RobertCrum / iStock Thinkstockphotos © RobertCrum

Toughness measures the energy required to crack a material. It is important for things like hammers and cutting tools which suffer impact and dynamic loads. The toughness of mild steel is used to absorb the impact of a crash in a car crumple zone. Tough materials can absorb a lot of energy without cracking.

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Brittleness Brittleness is the opposite of toughness; brittle materials simply fracture without any plastic deformation and are not suitable for most forming processes without heat treatment. Brittleness is linked with hardness in that brittle materials often have high resistance to scratching and wear. In high carbon steel the hardest state the steel can attain is a very brittle state that is not useable; it must be tempered to introduce toughness. © frankoppermann / iStock Thinkstockphotos © frankoppermann

Glass and ceramic materials are characteristically brittle. Hardness Hardness is defined as the ability of a material to resist abrasive wear and indentation or deformation. Hardness is an important property required for cutting tools. Abrasive materials such as aluminium oxide and silicon carbide are very hard and are used in abrasive cutting tools. © rybaz / iStock Thinkstockphotos © rybaz

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Elasticity and Plasticity When a load is applied to a material there is a characteristic relationship between the amount of force applied and the deformation of the material.

Materials will deform in different ways. Under light loads a material may bend slightly but spring back to its original shape when the load is removed, this is known as elastic deformation. If a more substantial load is applied the material may exceed its elastic limit and be deformed permanently, this is known as plastic deformation.

Below is a stress/strain curve for a tensile test on a metal.

Elastic Deformation It can be seen that the initial phase between zero and the yield point is a straight line. If the load is removed before the material reaches its yield point it will return to its original shape. Elastic materials tend to have stress/strain curves that show a lot of elongation before fracture, but each group of materials have their own characteristic stress/strain curve.

Yield Point The yield point is the point at which the material begins to change its shape permanently with only a small recovery when the load is removed.

Plastic Deformation As the load increases past the yield point the material will deform permanently until it fractures. Materials that are considered to have high plasticity have low yield strength and high elongation before fracture.

In summary, elastic materials tend to go back to their original shape when a deforming force is removed, plastic materials do not and remain in the new deformed shape. Both of these properties can be useful in manufacturing.

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Ductility Ductility is defined as the ability of a material to be formed by processes that involve tensile forces such as drawing, stretching and bending. The material can be stretched into long lengths without fracture. Silver, iron and nickel are highly ductile metals that can be formed into wire and tubing by being pulled through a series of dies and formers to reduce their cross section without cracking. Ductility decreases as the temperature of the material increases so drawing processes are carried out at room temperature. © tevanovicigor / iStock Thinkstockphotos © tevanovicigor

Malleability Malleability is defined as the ability of a material to be formed by processes that involve compression forces such as hammering, rolling and extrusion. The material can be bent and shaped in a number of different directions without fracture occurring. Malleability increases as the temperature of the material increases so materials are often heated before being formed by rolling or extrusion. Copper and pure aluminium are highly malleable materials that can be shaped easily by compressive forces without cracking.

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Stiffness Stiffness is the extent to which a material resists deformation in response to an applied force and is related to the modulus of elasticity (Young’s modulus) of a material. The less a material deforms under a given load within its elastic limit, the stiffer the material is. Essentially stiffness is observed as a steep slope in the elastic portion in its stress/strain curve and means that a material can withstand a high load with low elastic deformation.

The graph below shows the stress/ strain curves of some common engineering materials. The stiffness of the material can be observed by looking at the angle of the elastic portion of each curve. The steeper curves are the stiffest materials.

Stainless Steel (304)

Mild Steel Force (stress) Force

Pure Aluminium

Rubber Elongation (strain)

From the graph above we see that for a given force stainless steel deforms by the smallest distance and is therefore the stiffest of the four materials.

Rubber shows a very large amount of elongation at low load and is therefore a flexible material. Flexibility can be said to be the opposite of stiffness.

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Conductivity Electrical conductivity is a physical property that describes the ability of a material to allow electricity to pass through it. The electrical resistance of a material is a measure of the capability of a material to conduct electricity. Generally metals have high electrical conductivity but some liquids and gases are also good conductors. A material that has high resistance is known as an insulator.

© hairygit / iStock / Thinkstockphotos © byggarn79 / iStock / Thinkstockphotos Thermal conductivity is a physical property that describes the ability of a material to allow heat to pass through it. Thermal conductivity is an important factor when choosing suitable materials for the handles of cooking utensils.

Revision questions

1. How can the mechanical properties of a material be changed or modified?

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2. Compare the mechanical properties of strength and stiffness; outline any similarities and differences.

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3. Which mechanical property enables a material to resist impact and shock loading? Identify one applications where this property is required.

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4. Identify parts of the bicycle shown that may experience the following forces:

• Twisting forces • Tension forces • Bending forces • Shock and impact • Wear and abrasion © hamurishi / iStock Thinkstockphotos

5. Compare and contrast the properties of ductility and malleability.

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6. Why are materials such as steel and iron heated before they are shaped by hammering and forging?

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7. Explain the difference between plasticity and elasticity.

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8. Why is copper chosen for the base of a saucepan and melamine formaldehyde chosen for the handle?

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9. What mechanical properties are required of the materials used to make cutting tools such as lathe tools and milling cutters?

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Additional resources

Tensile test https://www.youtube.com/watch?v=D8U4G5kcpcM

Ductility / Brittleness https://www.youtube.com/watch?v=4poX-NwRcAY https://www.youtube.com/watch?v=gGXHdgsFA9s

Material properties https://www.youtube.com/watch?v=BHZALtqAjeM

Materials Database http://www.matweb.com/search/search.aspx http://www.makeitfrom.com/property-search