Materials Selection: Design Requirements

This is the second article in a series of posts on the materials selection process. The first article gave an overview of the entire process for selecting a material to use for a component or a joint between components. This article discusses the first step of the process - identify the design requirements for the component or joint. As a reminder, here are the steps for the materials selection process:

1. Identify the design requirements
2. Identify the materials selection criteria.
3. Identify candidate materials.
4. Evaluate candidate materials.
5. Select materials.

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The design requirements

Here is a list of the categories of the requirements to consider when selecting a material for a component or a joint between components:

1. Performance requirements
2. Reliability requirements
3. Size, shape, and mass requirements
4. Cost requirements
5. Manufacturing requirements
6. Industry standards
7. Government regulations
8. Intellectual property requirements
9. Sustainability requirements

Below is an explanation of each category of requirements

Performance Requirements
The performance requirements describe the attributes that the component or joint must have to function as required. The attributes can be described in terms of mechanical, electromagnetic, thermal, optical, physical, chemical, electrochemical, and cosmetic properties.

Reliability Requirements
The reliability of a component or joint refers to its ability to function as required over a specific use period when exposed to a specific set of use conditions. A component or joint fails once the material degrades to the point where the component or joint no longer performs as required. The reliability requirements describe the use conditions to which the materials will be exposed and the expected response of the materials to the use conditions. Examples of use conditions are exposure to high temperatures, salt water (corrosion), and vibration.

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Size, shape, and mass requirements
The size, shape, and mass requirements for a component or joint will have a huge influence on the materials that can be used. Consider a component that must carry five amperes of current without heating up by more than 15^o C above the ambient temperature. The electrical conductivity for a component with a 1 mm diameter must be about four times greater than the electrical conductivity for a component that can be 2 mm in diameter. A bicycle frame that must weight 10 pounds must have frame tubes made of a lower density material compared to a 20 pound frame. For a component that must support 200 pounds, the yield stress for the material in a component that must be 0.20 inches diameter must be much greater compared to the material in a component that can be 0.50 inches in diameter.

Cost requirements
The cost to form a component or joint or purchase a component depends on 1) the materials that comprise a component or joint, 2) the manufacturing processes used to form a component or joint, 3) whether a component is custom made or purchased “off-the-shelf supplier”, 4) the quantity of materials or components being purchased and 5) quality problems associated with a material or component. If you want to reduce costs, consider what will be required from the materials engineering perspective to make manufacturing process changes that address items 2 and 5.

Manufacturing requirements
Companies may require that specific processes be used for fabricating components and building assemblies or sub-assemblies. Perhaps a company has internal manufacturing capabilities that must be used or a company is familiar and comfortable with component or joints fabricated using a familiar manufacturing process.

Restrictions on the processes that can be used to build a product will restrict the materials that can be used to make components because the materials must be compatible with the processes and other materials used to make the product. For example, components to be joined using a specific welding, brazing, or soldering process must be made of materials that enable good joints to be formed using the specific joining process. This may exclude off-the-shelf components from one or more suppliers because their components are made of materials that are incompatible with the process. For a custom component, the restriction may require the use of certain materials in order to form a good joint.

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Restricting the manufacturing process to only familiar ones will restrict the options of materials that can be used to form a component or joint since many manufacturing processes are limited to processing certain materials. In some respects manufacturing constraints are acceptable, and may in fact be desirable, since the use of familiar processes and materials reduces the risk associated with a change or new product. However, in cases when a new product is significantly different than older products, the constraints of using specific manufacturing processes may seem to be a burden.

Industry standards
There are industry standards concerned with the performance and reliability of components and joints. In some cases, a specific standard will discuss component and joint requirements. For component specific standards, the standards discuss

• The size and shape of components used for specific applications.
• The materials that can and cannot be used for components used for specific applications.
• The tests required to verify the properties of the materials used to make a component.

Government regulations
Government regulations regarding the materials used in a product are typically related to requirements on the materials from which components and joints can and cannot be made. The requirements address the materials that can or cannot be used in a component or joint and the expected quality and reliability of the materials for specific applications. Every country has its own set of regulations.

Intellectual property requirements
There are many patents regarding the design and manufacture of component or joints. If a patent is found that is applicable to the component or joint being selected or designed, then the design team has to decide whether to license the patent or engineer the component or joint order to avoid conflict with the patent.

Sustainability requirements
These requirements restrict the materials that can be used in components and joints to materials that can be re-used or recycled. The requirements might also restrict the manufacturing processes than can be used to form components and joints to processes that do not harm the environment and do not use chemicals and materials that are manufactured using environmentally unfriendly processes. The sustainability requirements for a product become the sustainability requirements for its components and joints.

Time and money; Focus and discipline

This list of different types of requirements to consider might seem long. It might take a bit of time to come up with a complete description of all the requirements for your next project. However, it will speed up the process of evaluating materials and suppliers that are identified based on the complete set of requirements. Consider the time and money associated with evaluating materials and suppliers that are found to be unsuitable.

It takes a bit of focus and discipline to implement this process, but the rewards of fewer problems and faster implementation or design are well worth it.