专业英语 Proerties of materials.doc

Unit 7 Properties of materials The final strength of any material used in an engineering component depends on its mechanical and physical properties after it has been subjected to one or more different manufacturing processes. Also, there are several properties that determine the suitability of the material in its initial state for any particular manufacturing process. The initial strength of the virgin material is important, because that strength will affect the ease with which it can be deformed into its required shape and finally, its ability to resist loads during service. Factors which increase or decrease the strength of the starting material may be equally important. It may be desirable either to reduce its strength sufficiently to allow it to be formed into shape easily with the available machines, or alternatively to increase the final strength of the manufactured component and render it more serviceable . Strength is an imprecise term that may here be understood to indicate the ability of a material either to accept or to resist deformation. A similar argument applies to a rather more elusive property of any material, namely, its ductility, which is understood to mean the ability of a material to accept large amount of deformation (mainly tensile) without fracture. Again considering manufacturing processes, a large value of this parameter will obviously be beneficial. Many mental-working process are limited only by the available ductility of the material being working, so that the amount of deformation which can be imposed on the material has to be restricted to avoid fracture. There are, however, some manufacturing processes for which the opposite of ductility is beneficial. A suitable generic term for this property might be brittleness; for example, it is well-known that certain brittle materials are much easier to machine or shear than are ductile materials. It is mainly the interplay of the properties such as strength and ductility during fabrication