Thursday, April 22, 2010
Innovation in engineering often means the clever use of a new material - new to a particular application, but not necessarily (although sometimes) new in the sense of ‘recently developed’. Plastic paper clips and ceramic turbine-blades both represent attempts to do better with polymers and ceramics what had previously been done well with metals. And engineering disasters are frequently caused by the misuse of materials. When the plastic tea-spoon buckles as you stir your tea, and when a fleet of aircraft is grounded because cracks have appeared in the tailplane, it is because the engineer who designed them used the wrong materials or did not understand the properties of those used.
So it is vital that the professional engineer should know how to select materials which best fit the demands of the design - economic and aesthetic demands, as well as demands of strength and durability. The designer must understand the properties of materials, and their limitations.
This book gives a broad introduction to these properties and limitations. It cannot make you a materials expert, but it can teach you how to make a sensible choice of material, how to avoid the mistakes that have led to embarrassment or tragedy in the past, and where to turn for further, more detailed, help.
You will notice from the Contents list that the chapters are arranged in groups, each group describing a particular class of properties: the elastic modulus; the fracture toughness; resistance to corrosion and so forth. Each such group of chapters starts by defining the property, describing how it is measured, and giving a table of data that we use to solve problems involving the selection and use of materials. We then move on to the basic science that underlies each property, and show how we can use this fundamental knowledge to design materials with better properties. Each group ends with a chapter
of case studies in which the basic understanding and the data for each property are applied to practical engineering problems involving materials. Each chapter has a list of books for further reuding, ranked so that the more elementary come first.
At the end of the book you will find sets of examples; each example is meant to consolidate or develop a particular point covered in the text. Try to do the examples that derive from a particular chapter whilesthis is still fresh in your mind. In this way you will gain confidence that you are on top of the subject.No engineer attempts to learn or remember tables or lists of data for material
properties. But you should try to remember the broad orders-of-magnitude of these quantities. All grocers know that ’a kg of apples is about 10 apples’ - they still weigh them, but their knowledge prevents them making silly mistakes which might cost them money. In the same way, an engineer should know that ’most elastic moduli lie between 1 and lo3 GN m-2; and are around 102GN mW2 for metals’ - in any real design you need an accurate value, which you can get from suppliers’ specifications; but an order-of- magnitude knowledge prevents you getting the units wrong, or making other silly, and possibly expensive, mistakes. To help you in this, we have added at the end of the book a list of the important definitions and formulae that you should know, or should be able to derive, and a summary of the orders-of-magnitude of materials properties.
To the lecturer
This book is a course in Engineering Materials for engineering students with no previous background in the subject. It is designed to link up with the teaching of Design, Mechanics and Structures, and to meet the needs of engineering students in the 1990s for a first materials course, emphasising applications.
The text is deliberately concise. Each chapter is designed to cover the content of one 50-minute lecture, twenty-seven in all, and allows time for demonstrations and illustrative slides. A list of the slides, and a description of the demonstrations that we have found appropriate to each lecture, are given in Appendix 2. The text contains sets of worked case studies (Chapters 7, 12, 16, 20, 22, 24, 26 and 27) which apply the material of the preceding block of lectures. There are examples for the student at the end of the book; worked solutions are available separately from the publisher.
We have made every effort to keep the mathematical analysis as simple as possible while still retaining the essential physical understanding, and still arriving at results which, although approximate, are useful. But we have avoided mere description: most of the case studies and examples involve analysis, and the use of data, to arrive at numerical solutions to real or postulated problems. This level of analysis, and these data, are of the type that would be used in a preliminary study for the selection of a material or the analysis of a design (or design-failure). It is worth emphasising to students that the next step would be a detailed analysis, using more precise mechanics (from the texts given as 'further reading') and data from the supplier of the material or from in-house testing.
Materials data are notoriously variable. Approximate tabulations like those given here, though useful, should never be used for final designs.