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Material Selection

Material selection is a step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. Systematic selection of the best material for a given application begins with properties and costs of candidate materials. For example, a thermal blanket must have poor thermal conductivity in order to minimize heat transfer for a given temperature difference. 

When considering what material you plan on using for a given project consider the following:

1. Strength/flexibility - is it strong enough to resist forces applied to the object/system? For example: If the goal of the object is to be a wing, we want it to be strong, yet flexible. A wing of cast iron is strong, yet the forces applied to it will ultimately cause it to fail. Aluminum on the other hand is strong and when assembled in the right way, flexible.

2. Weight - it the weight appropriate for the usage? In the case above, you could use steel or aluminum to make the plane wing - both of which have the right rigidity, yet steel is far more dense than aluminum, thus a poorer choice.

3. Cost - it's all well and fine to choose a material whose strength/weight are ideal for the object, but if the cost is so high that the product is untenable then it's not an ideal solution. E.g. Gold is both pliable as well as a fabulous conductor, but cost would prohibit it from being used as electrical cabling throughout a standard house.

In considering both points above designers need quantify the material choice so that the best material can be used for the job. Plotted against one another Young's Modulus (strength/elasticity) is a way of measuring stiffness for elastic materials (even metals can be considered elastic because they deform under pressure). When you plot Young's modulus (YM) against density you get the following result (figure above). Quickly designers can see ranges of what materials would be ideal for the job.

1) Composite materials - where the material is a mix of constituent materials. There are two types of constituent materials a) matrix and b) reinforcement. One of each is used in making the composite. Some examples of older composite materials are:

• concrete (lime mixed with aggregate)

• paper-mache (paper mixed with glue)

• wood (made of cellulose fibers in lignin matrix)

• wattle and daub (mix of straw and clay in bricks)

Newer composite materials started with the plastic bakelite in the early 1900's. Commonly used as matrices are resins such as polyester and epoxy although glass may be used as well. Modern reinforcements are often natural and artificial fiber. The life of a garage floor is extended 20+ years by the addition of fiber to the matrix.

2) Fasteners - used to reinforce the structure AFTER the structure has been built fasteners often amplify the structural strength of the object by a magnitude of 50x or more. Technically most fasteners are ties. Common fasteners are the: 

Ties:

In general the truss plate (or nail plate) is used as a type of gusset plate in wood construction. Ties in general are used in engineering to resist compression. There are MANY many more ties than listed here above. 

AutoCAD

The devil or your best friend?

AutoCAD (computer-aided-design) is a high-end program that is used by designers the world over to realize their ideas. AutoCAD not only allows you to design in 2D and 3D but it also has, as output options, the ability to send information from your designs to CnC routers, and 3D printers for modeling. We will explore the basics of AutoCAD as a tool in our design arsenal. We will follow this basic reference guide as we begin to head through the world of AutoCAD.

Here is how it works:

AutoCAD uses points to determine the location of objects, with the origin (0,0) as the starting reference. The X-axis extends to the right (positive) and the Y-axis extends upward (positive). For example, a point at (9,6) is 9 units to the right on the X-axis and 6 units up on the Y-axis, while a point at (-10,-4) is 10 units left on the X-axis and 4 units down on the Y-axis. A line in AutoCAD has two points: a start and an end, displayed using these coordinates. To draw relative lines from an existing endpoint, you use the "@" symbol. Typically, when using a mouse, you don't need to worry about exact coordinates, just the position of your endpoint.