Engineering of Materials
To specify the required properties, a material engineer must work with other engineers to anticipate the functional and service requirements of the final product. The materials engineer must be familiar with many fields of technology; chemistry, physics, metallurgy, ceramics, and so on. The guiding principles of materials technology are that:
- Properties depend directly on the internal structure of the materials, and
- Any desired change in properties requires an appropriate change in the internal structure.
Properties of Materials
The engineer who design machines, electrical products, buildings, refineries, nuclear, reactors, and ships has to know whether they will resist failure in service. Key properties include mechanical behavior, electrical and magnetic responses, thermal characteristics, and chemical stability.
Mechanical properties involve the response of materials to applied forces, or loads. Stress is the amount of load per unit of area. Materials respond to a stress with a strain, which is deformation per unit length. Typically, the initial strain is elasticity; its value is high for a rigid material such as steel, and much lower for flexible materials.
Common mechanical properties include strength, ductility, and toughness. Strength is the stress required for failure, ductility is the strain prior to fracture, and toughness is the energy required for fracture. Two different strengths are of interest; ultimate strength and yield strength. The ultimate strength indicates the maximum load a material can support when the original cross sectional area is the basis for calculation. This strength is important in any design calculation, because the component proceeds to complete rupture when it is loaded in access of this rating. Yield strength is the stress required to initiate plastic (permanent) deformation. This constitutes failure of a part. If it is shape or dimensions must remain unchanged in service. Stress and the various strengths have units of newtons/meter2. Ductility is expressed as the percentage of elongation at fracture of foot-pounds. Testing for strength and ductility is conducted in a tensile machine that pulls a sample and records the amount of strain.
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