Introduction
Engineering materials is a subject that covers all branches of engineering, as all branches of engineering involve some form of physical interaction, however small, and hence all must involve materials. One of the main areas in any engineering projects must be selecting the right materials for the job – a chocolate tea pot is no use, and nor is a waterproof teabag to use the old sayings. To select the right material, we need to ensure it has appropriate properties for the task, properties which we will look at now.
These notes have been adapted (very slightly) from materials kindly supplied by Dr Gareth Bradley of Perth College UHI. His hard work is acknowledged with thanks.
Engineering Materials – Groups
Engineering materials can be broadly defined as falling into one of the groups shown in the diagram below:
Gareth Bradley, Perth College UHI
Over the rest of this section, we will look at each of these groups in terms of their general properties. We will also define what each of these properties means.
Metallic Materials
Metallic materials are widely used in many applications and are familiar to all of us in one form or another.
Metallic Materials - Groups
Metals are often subdivided into Ferrous and non-ferrous. Ferrous metals contain iron and take their name from ferrum – the Latin name for iron. Conversely, non-ferrous metals are those that do not contain iron – as you may expect. Examples are shown in the diagram below.
Gareth Bradley, Perth College UHI
Metallic Materials – General Properties
Despite the fact that over 75% of known elements are metals, they tend to have several general properties in common. These general properties are outlined in the table below – do not worry too much if you do not know what these properties mean, we will define them later.
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Tensile strength - High |
High temperature strength - Moderate |
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Compressive strength – Medium to high |
Melting point – Low to high |
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Stiffness (elastic modulus) – Medium to high |
Electrical conductivity - High |
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Hardness – Medium |
Thermal conductivity – High |
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Toughness - High |
Appearance – Opaque |
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Ductility – High |
Density – Low to High |
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Corrosion resistance – Low to medium |
Thermal expansion – Medium to high |
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Thermal shock – medium to high |
Polymeric Materials
Unlike metallic materials, it is quite possible that you have never encountered the term polymeric materials before, however you will certainly have encountered polymeric materials in your day-to-day life. They encompass a wide range of materials, including all common plastics.
Polymeric Materials - Groups
As with metallic materials, polymeric materials are further divided into 3 main subgroups, as shown in the diagram below. Broadly, the difference between them is that thermoplastics will soften when heated, thermosetting plastics do not soften when heated, and elastomers as soft and stretchy at almost any temperature.
Gareth Bradley, Perth College UHI
Polymeric Materials – General Properties
Due to the diverse nature of polymeric materials, it may be expected that they are likely to have fewer general properties in common, however it is still possible to show some commonality across the group. These common properties are outlined in the table below:
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Tensile strength - Low |
High temperature strength - Low |
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Compressive strength – Low |
Melting point – Low |
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Stiffness (elastic modulus) – Low |
Electrical conductivity – Low (insulator) |
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Hardness – Low |
Thermal conductivity – Low (insulator) |
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Toughness - Low |
Appearance – Opaque, translucent, transparent. |
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Ductility – Low to high |
Density – Low |
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Corrosion resistance – High |
Thermal expansion – High |
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Thermal shock – High |
Ceramic Materials
As with metallic materials, it is likely that you have heard ceramic materials – or more likely just ceramics – referred to in the past; probably in the context of crockery or something similar. These are indeed ceramics, but these area contains many others, often used in surprising ways.
Ceramic Materials – Groups
Ceramic materials are probably the most widely found in nature, and are possibly the most widely used by humans when building materials are taken into account – though there would also be a strong argument that natural materials are even more widely used in this respect. Again, they are broken down into further subgroups as shown in the diagram, though the distinctions this time are perhaps not as well defined. One definition could be that stones are ceramics that are used in their natural state, glasses are ceramics that have been processed but are widely used, and engineering ceramics have been processed and are usually only found in specialist applications. These distinctions are by no means definite however, and the split could be defined in other ways.
Gareth Bradley, Perth College UHI
Ceramic Materials - Properties
As before, ceramics tend to have certain things in common, though there are definite differences across the piece. Again, their common properties are outlined in the table below:
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Tensile strength – Low (high) |
High temperature strength – Low (high) |
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Compressive strength – High |
Melting point – High |
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Stiffness (elastic modulus) – High |
Electrical conductivity – Low (insulator) |
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Hardness – High |
Thermal conductivity – Medium |
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Toughness - Low |
Appearance – Opaque, translucent, transparent |
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Ductility – Low |
Density – Low |
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Corrosion resistance – High |
Thermal expansion – Low |
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Thermal shock – Low |
Composite Materials
Composite materials could be seen in many ways to be the most interesting group, as they are materials that are created by combining two or more different materials to achieve specific properties that differ from the original. Perhaps the best known composite is CFRP – Carbon Fibre Reinforced Plastic, commonly known as carbon fibre – but the group is much broader than this.
Composite Materials – Groups
Unlike the material types already looked at, composites tend not to be divided into groups in quite the same way, but rather just referred to directly by their name. This is possibly because they are a comparatively small group of materials at present, having only been recognised as a separate group fairly recently:
Gareth Bradley, Perth College UHI
Composite Materials – General Properties
Because many composite materials can be engineered to have almost any property desired, it is more difficult to make statements about general properties for this group than some of the others such as the metals. However, as a generalisation, it is possible to say that the will have properties as outlined in the table below.
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Tensile strength* – Medium to high |
High temperature strength* – Low to medium |
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Compressive strength* – Medium to high |
Melting point – Low |
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Stiffness (elastic modulus)* – Low to high |
Electrical conductivity – Low (insulator) |
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Hardness – Low |
Thermal conductivity – Low to medium |
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Toughness* – Medium to high |
Appearance – Opaque, translucent. |
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Ductility* – Low |
Density – Low |
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Corrosion resistance – High |
Thermal expansion* – Low to high |
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Thermal shock – High |
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* The properties of many composite materials depends on the direction on which they are measured, they are anisotropic. |
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Natural Materials
Finally we come to natural materials – those that are found in the world around and can be used without any processing. This group cannot be divided up in the same way as the other groups, nor can we make statements about general properties as the group is just too diverse. Rather, we would tend to look at the specific material, and then see if it fits into one of the other groups discussed above. The diagram below outlines this more clearly.
Gareth Bradley, Perth College UHI
Definition of Material Properties
Over the course of this section we will look at how we define each of the properties above, and where appropriate how we test them. You will have the opportunity to carry out or observe some of these tests in outcome 4.
Click the items below to expand:
Description:
Yield stress (σy = force/area) is the stress at which the material’s behaviour changes from elastic to plastic behaviour.
Tensile stress (σTS = force/area) is the maximum stress a material can withstand before failure.
Test: Tensile test involves a tensile load being gradually applied to a specimen of material or component until it fails.
Description: The maximum impact load a material can withstand.
Fracture toughness is also used to measure the toughness. Although there is a correlation between impact toughness and fracture toughness you cannot convert between them.
Test: Charpy or Izod impact test where a pendulum swings and impacts a specimen.
Description: The ability of a material to be permanently stretched or deformed under a tensile stress.
Test:Tensile test
Deformation of Materials to fracture
(Gareth Bradley, Perth College UHI)
Description: The ability of a material to be deformed by a compressive stress, e.g. hammering or rolling.
Test: No definitive test.
Description: Ability of a material to withstand cyclic (repeated) loading.
Test: Fatigue test involving applying cyclic loads to a specimen or component.
If you have an internet connection, the following video is of interest.
Description: Ability of a material to resist permanently deforming under the application of a constant load.
Test: Creep test where a force is applied to a specimen, often at elevated temperature in the case of metals.
If you have an internet connection, the following video is of interest.
Description: Ability of a material to resist wearing when affected by an abrasive.
Test: The weight loss of a specimen can be measured when it has been exposed to an abrasive environment or the depth of penetration.
If you have an internet connection, the following video is of interest.
Other Properties
There are several other properties that a material will posses that we need to be aware of. Some of these you may well have encountered previously, some may be new to you.
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Property |
Description |
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Density |
Mass per unit volume of a material. |
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Stiffness (elastic modulus) |
Measure of how much a material deforms elastically for a given load. |
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Electrical & thermal conductivity/resistivity |
Measure of the resistance to the flow of electricity/heat. Resistivity is the inverse of conductivity. |
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Optical properties |
Whether a material is opaque, translucent or transparent to light or various other forms of radiation. |
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Surface finish and frictional properties. |
The surface finish may be important with respect to the aesthetics of a product or it may have an effect on other properties such as corrosion and/or friction. The surface roughness is important for materials used for bearing surfaces amongst others. |
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Corrosion resistance and stability.
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The ability of a material to resist reacting with its environment. Stability relates to how different environments affect a materials property. The properties of some materials may be affected by temperature, time, stress etc.
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Toxicity
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The degree to which a material can damage organisms.
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Permeabilty
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Permeability is the measure of how easy it is for a substance, often gases or liquids, to move through a material.
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Aesthetic |
The feel, shape and appearance of a component can frequently be limited by the properties of a material. The colour, or ability to colour, a component may also be limited by specific material properties.
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General Characteristics of Structural Materials
As can be seen, the properties of materials vary massively across and within groups, and the way in which properties can be defined is extensive. For this reason, there is no such thing as a perfect material, rather we need to select the material for the application – and even then, it is usually a case of best fit rather than being the ideal. The table below gives a brief summary of the main groups of materials and the properties of each.
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General Characteristics of Structural Materials |
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Characteristic |
Ceramics |
Metals |
Polymers |
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Density |
Low to High |
Low to High |
Low |
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Hardness |
High |
Medium |
Low |
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Tensile Strength |
Low to Medium |
High |
Low |
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Compressive Strength |
High |
Medium to High |
Low to Medium |
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Young’s Modulus |
Medium to High |
Low to High |
Low |
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Melting Point |
High |
Low to High |
Low |
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Dimensional Stability |
High |
Low to Medium |
Low |
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Thermal Expansion |
Low to Medium |
Medium to High |
High |
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Thermal Conductivity |
Medium |
Medium to High |
Low |
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Thermal Shock |
Low |
Medium to High |
High |
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Electrical Resistance |
High |
Low |
High |
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Chemical Resistance |
High |
Low to Medium |
Medium |
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Oxidation Resistance |
Medium to High |
Low |
Low |
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Machinability |
Medium |
Low |
Medium |
Summary
As you have seen, we tend to split engineering materials into one of four or five groups, namely metallic, polymeric, ceramic, composite and natural. We chose a material for an application based on its properties, and not all properties need to be considered for every application – for example the Young’s Modulus for a particular type of paper is somewhat irrelevant if we are looking to print a book. Because of this, there can be several materials to chose from for some applications but hardly any for others. For example, kitchen work surfaces can be made from marble, stainless steel, slate, laminate and so on, whereas a jet engine blade can – at present – only be made of certain exotic steels.
