On January 28, 1986, the Challenger shuttle exploded 72 seconds into its launch. Besides organizational mismanagement and miscommunication involved in the infamous launch, a technical investigation homed in on the failure of the O-Ring seals within the rockets.
The O-rings were made of FKM, a common thermoset elastomer used in sealing applications across several industries. At first glance, it seems like an ideal choice—the thermal decomposition of the material occurs around 880°F according to NASA’s tests. This can withstand the immense heat during operation. However, FKM at a cold temperatures is another story.
The glass transition temperature of a polymer is an important design property. As polymers approach a certain cold temperature, they become less elastic and more brittle. This is what happened as temperatures hit a record-setting 20°F the night before. The O-rings—being the thermal insulators that they are—held a lower temperature even as the outside warmed up. FKM seals at this temperature didn’t readily conform to the necessary shape, and resulted in the chain reactions that led to an explosion. Since this incident, NASA has tested its parts harshly on both extremes of environments to make sure that these situations don’t happen again.
Materials selection based on the full possibilities of your product is an essential, and can end up making or breaking your product. Even something as simple as an O-Ring sealer or screws and fasteners can end up costing investments and product credibility.
Selecting the right material(s) for a system is always a daunting task, having a balance between not only properties but manufacturability and cost as well. Here are some steps that I use to break down a materials problem.
What’s the intention of your product? Who are you selling to? Why is it necessary?
Furthermore – what’s the function of each individual part? Although this a part of the engineering process that you used to design your product, it’s important to re-emphasize specific needs for the materials selection process.
Now that we have an understanding of what your products end-goal is, we can back-track and figure out what your needs are. What’s the range of strength that your material needs? Does it need to be thermal or electrically insulating? Can you trade-off some properties for another? Establishing a range is important, like the Challenger disaster, we need to understand ALL the possible environments a part needs to go through.
Bonus points if you can consider material interactions: If the materials system is hybrid, interactions between specific materials can result in furthered corrosion and property failure. We must consider issues such as thermal expansion (like in electronics packaging), galvanic potential, and dielectric breakdown in some instances. Make note of these as you set your material parameters.
Benchmark existing products and intellectual property in your market. Which materials fit the property ranges that you’re seeking? Don’t worry about price too much now, because you can trade-off prices of some parts for prices of others. What environmental and safety hazards do these materials exhibit?
This is where we start to look at business constraints. Make sure that not only the raw material fits within the cost, but the manufacturing processes to create the parts is also inexpensive. If you have to do additional treatments on some parts, be sure to take that into account. Additionally where are you getting this material from? Verify your raw material sources and ensure that you’re getting the exact type of product that you need before buying.
By no means is this the final step – a constant evaluation of the chosen materials is needed. You might have to adjust some design criteria, change manufacturing techniques, or work to decrease cost. That’s okay! Constant improvement is a part of any engineering process.
