Mechanical Engineering Design Notes

Materials Contents

Materials Selection

1. Introduction
Materials can conveniently be classified as:

  • Metals
  • Ceramics
  • Polymers
  • Composites

There are about 70 metals but only about 20 of these are of interest to engineers. They are not normally used when 'pure' but alloyed with other metals to give the required properties. There are many hundreds of alloys which are in use in various branches of engineering.

Organic materials, based on carbon, are very widespread - over a million organic compounds are known, some of them naturally occurring. However only a small proportion of organic materials are of interest from an engineering viewpoint. This still leaves a lot of plastics, but with a some knowledge many possibilities can quickly be eliminated.

A lot of recent developments in the materials field have concerned composite materials. Here two or more materials are combined to give quite specific properties, for example in fibre reinforced plastics where the reinforcement may be highly directional to provide high strength in a particular direction.

Because of the need to provide a reliable product, a great deal of material selection is based on past experience - a designer feels confident with a material that has been tried and tested in a previous or similar design. This approach risks products becoming out of date because new advances in materials are not incorporated - hence there is a need for an engineering designer to have a good understanding of a broad range of existing materials as well as being aware of developments in new materials.

As both the detail design and the manufacturing processes are dependent upon the choice of material, it is important that these three areas are considered at the same time - and this is one of the features of 'simultaneous' or 'concurrent' engineering. Some preliminary selection of material type may be made at the concept stage, or early on in the design, but most detail decisions, including processing, will need to be decided while the design is being detailed.

2. Material Requirements.
Some of the main materials properties that are of interest to engineers are shown in the table below.

Mechanical Properties Thermal Properties Resistance to Hostile Environments
Strength, UTS, Yield Coefficient of thermal expansion Moisture
Hardness Thermal conductivity Temperature extremes
Young's Modulus Thermal shock resistance Acids / alkalis
Ductility Specific heat Salt solution
Fatigue Strength hydrogen attack
Fracture Toughness Nuclear particles

For Hardness and other property conversion tables link to Gordon England site.

As well as the properties listed above, there are a number of factors which will influence the choice of material:

1. Availability - sizes, minimum quantities.

2. Ease of manufacture - machinability, weldability.

3. Compatibility - electrochemical compatibility with other parts of the system.

4. Reliability - how consistent are the material properties.

5. Cost - although 5th in this list, this factor may well be used first to eliminate a large number of possible options.

6. Recycleability - increasing environmental concern (and resulting legislation) worldwide is driving manufacturers to use materials that can be recycled with minimum difficulty.

3. Initial Material Selection.
A component can be considered to have a functional (eg. load carrying) requirment, some geometry requirement (eg position of supports) and to be optimum with respect to some aspect (eg minimum mass, minimum cost, maximum stiffness). See link on left to 'Performance Indices'. Depending upon these requirements, it is possible to find a ratio of properties which should be maximised with respect (for example) to cost per kg. Charts are published plotting a wide range of ratios against characteristics such as cost per kg. This type of chart will indicate types of materials which may meet the requirments. In many cases the groups are cover quite a wide range of properties and it will be necessary to carry out detailed assessment of some specific materials in groups of materials to enable an optimum (or near optimum) selection to be made.

Once you have an idea of the required ranges of some of the material properties that are required, you could try an internet search using MatWeb.

Often some significant requirement will dominate the choice of material type and limit the viable options to a narrow range. For example if one wished to manufacture very large numbers of car body shells at low cost in the immediate future, then the only feasible option is fine grain low carbon steel sheet. The exact grade, sheet thickness and processing will depend upon the strength required and how important weight saving is considered to be.

4. Detailed Evaluation of Materials.
Like all aspects of design, materials selection involves making compromises. It is desirable that materials selection is based on some sort of quantative assessment. Once the range of possible materials has been narrowed down, the final decision can be made with the aid of an evaluation matrix, where the relative importance of conflicting requirments can be quantified.

Once the detail of material specification is decided, then the detail of the production process must be finalised - this should include details of finishing and any inspection procedures or statistical process control to be applied.

For safety critical items, the inspection methodology, techniques and frequency of inspection of completed parts, will need to be specified, as well as a workable methodology for parts in service.

For information about compatibility and corrosion resistance see web sites:

Nero Pipeline Connections, Click on: Technical Information


For information about laminated plastics, link to Tufnol Ltd.

For information about heat treatment, suitable for the non-specialist, link to the Contract Heat Treatment Association (CHTA) web pages.

David J Grieve.

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