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Old 12-22-2012, 10:01 PM   #1
The_Blur
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Materials: A Very Technical Discussion

The materials we use fundamentally impact their usefulness in their given application. Failure to use the proper materials for a given application can result in unexpected failure and thus expensive maintenance. You're going to drive your vehicle for a long time, so you need to plan by using the appropriate materials for a given job.

In order to plan for what materials you're going to need for a job, you'll need to determine your goals. For instance, if your goal is to produce in your garage a cold-air intake, then you'll want materials that help keep the air temperature down and have a low enough cost that you can do the work yourself. In this case, you need materials that do not transfer heat very well from the hot engine bay to the air you intend to push through your engine.

Let's discuss and define the properties that would be important for an automotive application.
  • Heat Transfer葉he ability of a material to transfer heat (energy due strictly to a difference in temperature) from one point of contact to a point of contact on the other side of the material.
  • Hardness葉he ability of a material to resist permanent changes in shape when a force is applied to it.
  • Strength葉he ability of a material to resist force.
  • Brittleness葉he ability of a material to fracture (break) when force is applied, as opposed to strength.
  • Ductility葉he ability of a material to deform under stress, as opposed to strength.
  • Toughness葉he ability of a material to deform without fracturing (breaking).
  • Availability葉he ability to acquire a material at a reasonable cost and without excessive search.

Many of these terms are closely related to the concept of stress. Unlike pressure, stress is a measure of force per unit area away from the point of measure. While pressure can be imagined as a large amount of push on a small point, stress can be imagined as a pull against the same point. As such, stress is used differently in determining which materials to use for a given application. Different materials, be they plastics or metals, have different breaking points. These can be measured graphically. The graph would look something like the image below for most materials.



I could discuss this for a while, and I could go into some equations and other graphs regarding stress, but you're not going to do the math on this. You're probably going to rely on someone else to tell you what the best materials are for your application. You're not likely producing metal alloys or plastics in your garage for your new intake system or whatever else you have planned. Instead, I'll discuss some other characteristics that make a significant difference when building parts.

Let's talk about heat transfer. There are various types of heat transfer. I like the below image as an explanation for how heat works. It applies nicely to your car.



Going with our intake example, the air in your hot engine bay will be radiating heat energy to the intake. This is not the most effective form of heat transfer, meaning that your intake's internal air is likely to remain significantly cooler than the air just outside of the intake. As a performance enthusiast, however, you want to maximize the gains of your intake, so you will want to choose something that poorly conducts heat and therefore poorly transfers that same heat to anything inside of it. Consider, for instance, how metals are known to get hot, like the pot in the image. If that pot were plastic, it would take longer to boil the water because less heat energy would make its way to the water in the same period of time.

This is an ongoing discussion that may be updated in the future. Feel free to add your own experiences using different materials for DIY projects and what benefits you had.
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