General Physical Properties

General Physical Properties
Density
Density is one of the most fundamental physical properties of any material. It is defined as the ratio of an objects mass to its volume. Because most designs are limited by either size and or weight density is an important consideration in many calculations.
Density is a function of the mass of the atoms making up the materials and the distance between them.Massive, closely packed atoms characterize high density materials such as Tungsten or Neptunium. In contrast light, relatively distant atoms compose low density materials such as Beryllium or Aluminum. Density on a macroscopic level is also a function of the microscopic structure of a material. A relatively dense material may be capable of forming a cellular structure such as a foam which can benearly as strong and much less dense than the bulk material. Composites including natural constituents such as wood and bone, for example, generally rely on microscopic structure to achieve densities far lower than common monolithic materials.
 
Availability/Manufacturability
Availability and manufacturability requirements are often unseen limiting factors in materials selection. The importanceof a material being available is obvious. Materials which are not available cannot be used. The importance of processibility is not always so obvious.
Any other desirable qualities are useless if a material cannot be processed into the shape required to perform its function. Most engineering materials in use today have well known substitutes which would perform better and often at lower cost butprocesses for forming, cutting, machining, joining, etc. are not available or commercially viable. There is often a period of time after a new material is introduced during which its application is severely limited while processing techniques are developed which facilitate its use.
 
Cost
A materials cost is also generally a limiting factor. While cost is universally recognized and perhaps theeasiest of all properties to understand there are specific cost considerations for materials selection. Just as materials and their processing go hand in hand so do material costs and processing costs. Understanding the entire processing sequence is critical to accurately evaluating the true cost of a material.
 
Appearance
Because the appearance of many mechanical components seems fairlytrivial it is also easy to overlook its importance in the marketing and commercial success of a product.

Mechanical Properties
The mechanical properties of a material describe how it will react to physical forces. Mechanical properties occur as a result of the physical properties inherent to each material, and are determined through a series of standardized mechanical tests.
 
Strength
Strengthhas several definitions depending on the material type and application. Before choosing a material based on its published or measured strength it is important to understand the manner in which strength is defined and how it is measured. When designing for strength, material class and mode of loading are important considerations.
For metals the most common measure of strength is the yieldstrength. For most polymers it is more convenient to measure the failure strength, the stress at the point where the stress strain curve becomes obviously non-linear. Strength, for ceramics however, is more difficult to define. Failure in ceramics is highly dependent on the mode of loading. The typical failure strength in compression is fifteen times the failure strength in tension. The more commonreported value is the compressive failure strength.
 
Elastic limit
The elastic limit is the highest stress at which all deformation strains are fully recoverable. For most materials and applications this can be considered the practical limit to the maximum stress a component can withstand and still function as designed. Beyond the elastic limit permanent strains are likely to deform the material...