Lecture 1: Introduction,Tensile Test

To understand and describe how materials deform(elongate, compress, twist) or break as a function of applied load, time, temperature, and other conditions we need first to discuss standard test methods and standard language for mechanical properties of materials.Why do we study materials? Many an applied scientist or engineer, whether mechanical,
civil, chemical, or electrical, will at one time or another be exposed to a
design problem involving materials. Examples might include a transmission gear,
the superstructure for a building, an oil refinery component, or an integrated circuit
chip. Of course, materials scientists and engineers are specialists who are totally
involved in the investigation and design of materials.
Many times, a materials problem is one of selecting the right material from the
many thousands that are available. There are several criteria on which the final
decision is normally based. First of all, the in-service conditions must be characterized,
for these will dictate the properties required of the material. On only rare
occasions does a material possess the maximum or ideal combination of properties.
Thus, it may be necessary to trade off one characteristic for another.The classic example
involves strength and ductility; normally, a material having a high strength
will have only a limited ductility. In such cases a reasonable compromise between
two or more properties may be necessary.
A second selection consideration is any deterioration of material properties that
may occur during service operation. For example, significant reductions in mechanical
strength may result from exposure to elevated temperatures or corrosive environments.
Finally, probably the overriding consideration is that of economics:What will
the finished product cost? A material may be found that has the ideal set of properties
but is prohibitively expensive. Here again, some compromise is inevitable.
The cost of a finished piece also includes any expense incurred during fabrication
to produce the desired shape.
The more familiar an engineer or scientist is with the various characteristics
and structure–property relationships, as well as processing techniques of materials,
the more proficient and confident he or she will be to make judicious materials
choices based on these criteria.