Have you ever read Consumer Reports magazine? It contains ratings of products based on performance, reliability, and cost. I’m always fascinated by products that are rated poorly. I wonder - what led to that? Did the design teams start out knowing the products were going to be bad? Did problems come up along the way that required making sub-optimum decisions? Were the design teams not well-organized? Did the design teams not care?
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Part of producing a successful product is designing it so it meets its performance and reliability requirements and can be produced at low cost (enough to make a decent profit). Well, the performance, reliability, and cost of any product depend on the performance, reliability, and cost of its components and the joints between components. And the performance, reliability, and cost of components and joints depend on their form and their materials.
The trick is to optimize component form and materials to meet performance and reliability requirements at low-cost, preferably lowest possible cost. This requires engineering both form and materials, i.e. making choices about various aspects of the form and materials.
Aspects of form that can be engineered are the shape, dimensions, and features. Aspects of materials that can be engineered include metal alloy composition, mill condition and temper, post-fabrication heat treatment, and coating. For example, choosing a hot-rolled steel for ease of component fabrication and low cost, a post-fabrication through hardening heat treatment to obtain the desired strength, and a coating for corrosion protection.
Optimizing component form and materials requires using a methodical process that involves engineering considerations and data gathering that enable well-informed decisions. In addition to improving the likelihood of meeting performance and reliability requirements at low cost, such a process helps reduce unpleasant surprises that cause delays, extra work, and stress. Such a process is discussed next.
The process outline is:
Steps 1 through 4 are often iterative, as engineers consider different options and make trade-offs to optimize a component’s design. Sometimes, discussions to identify candidate materials take only 30 minutes. Other times, for components with complex design requirements, discussions might take longer.
The strength of the process lies in the ability to…
The rest of this article discusses each step of the process.
The process starts with a stab at a component’s form – size, shape, and features. Based on the form it is possible to identify the design requirements for
Performance requirements are the attributes a component must have to function as required. For example, mechanical loads that must be supported, electrical current that must be carried, and cosmetic appearance.
Reliability requirements refer to the conditions to which a component will be exposed that can cause degradation and the expected time before degradation. A component fails if its materials degrade to the point where the component no longer functions as required. Examples of conditions that cause degradation are cyclic stresses, rubbing between mating surfaces, high temperatures, and salt water (corrosion).
Sometimes there are requirements to use specific manufacturing or assembly processes to fabricate and join components. Perhaps a company has manufacturing capabilities that must be used or is familiar and comfortable with component or joints fabricated using certain processes. These requirements limit the materials that can be used because the materials must be compatible with the processes. For example, components to be joined by resistance welding must be made of metals that enable good joints to be formed using that process.
Finally, there may be other requirements such as industry standards and government regulations that specify component form and/or materials or sustainability requirements.
All these requirements are the basis for the materials selection criteria.
The materials selection criteria are specific materials properties derived from the component design requirements. For example, for a component that must support a certain load, the minimum yield stress required for the component’s material can be determined. This will be one of the material selection criteria. Resistance to corrosion might be another. And so on.
Identifying all the materials selection criteria early in the design process is critical to prevent problems later in the design process. Sometimes, no suitable materials can be identified for a component. Making modifications to a component’s form is not a problem if done early in the design process. However, learning too late in the design process that no materials meet all the design requirements is a problem - the costs and time required to redesign a component’s form at this point are high, and using select a sub-optimum material may be the only option.
Trying to select materials based on a subset of requirements is risky - it’s no fun finding late in the design process that a critical component fails during testing.
Use the materials selection criteria to rule out materials that will not satisfy all the materials selection criteria. When evaluating whether a material might be appropriate for the application, be sure to consider the materials’ range of values for the properties of interest. Do not rely upon nominal properties values.
There may be candidate materials for which there is not sufficient data available to determine whether the materials satisfy all the materials selection criteria. These materials must be evaluated to determine whether they do meet the selection criteria.
The evaluations may include metallurgical evaluation of metal stock or samples of custom-made components, weld joint strength, and amount of degradation of materials exposed to simulated use conditions. These evaluations can be performed without building the entire product, enabling data to be gathered in parallel with other engineering efforts. For example, placing samples of painted steel plate in contact with the corrosive liquid of concern and evaluating the time before the paint degrades.
Select the materials that satisfy all the materials selection criteria at the lowest cost. Cost includes the cost of the material and the cost to fabricate a component or form a joint between components.
This design process is straight forward, but there are many details. So, being successful requires focus and discipline on the part of individual engineers and entire engineering organizations.
The benefits of using this process is improved likelihood of developing good products, fewer headaches and surprises during product design, reduced risks of falling behind schedule or exceeding budget, and increased chances of innovation that provide a competitive advantage.
Interested in learning more? Check out this episode of the Metals Conversations podcast. Michael Pfeifer discusses the metals engineering perspective to component design. The concepts actually apply to all materials – metals, polymer, ceramics, and new classes of materials.