Selecting the right material makes a good design. Just as selecting the wrong material will yield bad results for your design. Design is not just about geometry. Think of it this way, design consists of three things:
Geometry
Material
Manufacturing process
To put to the limit. Imagine if you have designed a gearbox for a new sports car, the geometry, and the tolerances (Yes, tolerances are part of the geometry) just looks to be a superb design. You have selected a good manufacturing process. But you have selected chewing gum as a material. Without knowing the requirements, the capabilities of the manufacturing process, how great the geometry is I can tell you that selecting chewing gum is a bad choice as a material.
Now the example above might be putting it a bit too much to the limit. No sane engineer would choose that as a material, but it helps to clarify the point that material selection is in fact part of the design.
When should you think about material selection?
Material selection is not just something you want to do as the final thing before you send out your drawings to the manufacturer. In fact, material selection is something you must think about early on in your design.
If you are experienced in the industry, you are working in, you might already have some go-to materials that you in most cases end up selecting. Here experience is important, because there is no need to spend too much time on challenging what material to use for one part if you have already put in all the work for material selection of a similar part.
In some industries you might have materials that are approved for that industry, and you will need to apply for approval if you want to use another material. In most cases, this would take a long time and would probably not be worth the effort. Therefor in those cases you have already narrowed your material selection to maybe a few options.
If you are not restricted to few industry approved materials. You have a “clean sheet”, and you are early in the design process. It might not be helpful at this point to think about whether you should choose between 4130 steel or 4140 steel. This would be too specific too early. But in the early part of the design process, it would be beneficial to look at what kind of material category would work with different design concepts.
In the concept phase it will be beneficial to reflect on the requirements, because in the end the material you choose will need to support your design to fulfill this requirement.
What material categories should I consider?
There are basically four categories of materials:
Metals
Polymers
Ceramics
Composites
Each category has some general attributes that simplify the material selection process.
Metals are in general strong, stiff, and heavy. Metals are good heat and electrical conductors. And they are in general stable at higher temperatures. Metals can often be molded, machined, welded, and forged.
Polymers often referred to as plastics have the great advantage of being in general cheap to manufacture in large quantities. They can be very elastic and are more elastic than metals. Polymers are lightweight. Metals are in general stronger than polymers, you can still find some strong and stiff polymers, think of Kevlar. Common manufacturing processes for polymers are plastic injection molding, extrusion, machining, and gluing (Glue is usually a polymer).
Ceramics are hard and brittle, they are heat and corrosion resistant. Ceramics is maybe a less used engineering material than metals and polymers, but it is definitely not a worse material. For example, disc brakes for high performance cars are often made of ceramics as they don’t soften at higher temperatures as much as steel does. Ceramics are also often found as electrical insulators or used in high-temperature furnaces. The weight of ceramics is as a rule of thumb lighter than metals and heavier than polymers.
Composites are a mixture of two of above categories. Often the material consists of a fiber of ceramics in a polymer matrix. An important thing to know is that composites are not a homogenous material, and they have anisotropic mechanical properties. This means that a material can be stronger in one direction of load and weaker in another direction. The big advantage of composite materials is that that you can make very strong and lightweight parts. It can be tricky to design parts made of composite materials as you must think about in which and how many directions the fibers should be.
Later stage of design process: Getting into specifics.
A good general knowledge of material science is good to have in the early stage of the design process as this will help you go in the right direction. In the later stage though we need to get into the specifics. We need to consider many of the materials properties and at the same time have in mind the possible manufacturing methods. Material properties we might consider comparing:
Density (Weight or Mass).
Yield strength / Tensile strength (Material strength).
Young modulus (Stiffness).
Chemical resistance (Will it corrode in the environment we will put it in?).
Melting point / Yield strength at higher temperatures (Strength at elevated temperatures).
Heat conductivity.
Price per kg.
Etc.
As you can see, there are many properties that are worth considering, and it might be overwhelming with the amount of materials. Luckily there are many great software products out there that consist of a database of materials and material properties, these products can be very beneficial as we can select our materials in an analytical way by filtering on different max and min limits for the material properties we desire.
Conclusion
Material selection is one of three cornerstones of a successful design. It is important to think about what materials to use early in the design phase, and not just wait till the end, assigning material to the parts you designed. The world of materials is big, we navigate in this world of materials with analytics, we cannot remember all the materials and their properties in our head. The later you are in the design phase the more specific you should be about what material to use. It can be a great advantage to use specialized software to help you select the right materials.