The Concise Oxford English Dictionary defines ‘substitution’ as ‘a person or thing performing some function instead of another’, or alternatively ‘putting a person or thing in exchange for another’. In the context of tangible products, substitution of materials, therefore, implies that a change can be made to one or more materials previously selected for use in a product.

Materials are not totally interchangeable

A moment's thought will show that in no case can any one material fully and exactly substitute for another, since if all their properties were congruent in all respects, they would constitute the same material by all the normal definitions of materials science. Chapter 3.2 has already explored the concept of envelopes of properties, and Figure 11.1 illustrates how two materials may have envelopes of properties sufficiently similar that a product in which the substitution is made can essentially be the same as the original, with virtually the same performance parameters. As an envelope of properties is a mix of mechanical, physical, thermal, electrical, chemical and other properties, it is self-evident that in one of the materials of Figure 11.1, there might be a preponderance of, say, particularly good electrical or thermal properties, against which the second material would be somewhat deficient, but which may in turn offer better mechanical properties.