Rate-sensitive behavior of plastics: Lessons from daylight saving time and Silly Putty

This is part of an ongoing series of articles on materials selection authored by Eric Larson, a mechanical engineer with 30 years of experience in plastics design, for PlasticsToday. His most recent book is Plastics Materials Selection: A Practical Guide.

Image courtesy Winnond/freedigitalphotos.net.

A few weeks ago, I started having problems with my clocks. After months of being in sync, all of a sudden it seemed they were all out of whack. I'd check the time, go about my business, but a few minutes later when I checked the time again—boom—over an hour had gone by. I was puzzled. I am a bit embarrassed, but it took me almost a day to realize that we had just changed from daylight saving time to standard time.

For those of us who live in North America and Europe, the change to and from daylight saving time happens twice each year. And, for many of us, that change takes some getting used to. The sun rises at a different hour, and sets at a different hour. The shadows at noon are different, and the overall experience of the day is different—all because of the time change.

But what does that really mean—the time change? Time is a basic unit of measurement, along with mass, length and temperature. How can you possibly change time? With all due respect to Albert Einstein, for most us, you can't. An hour is an hour, a minute is a minute, and a second is a second. And for all of the expressions we have about time—time is on your side, time is a healer, all good things come in time, my how times have changed—the measurement of time doesn't change.

What changes is the comparison of other things in relation to time. Things are moving faster (velocity, a measurement of distance traveled per unit of time), things appear darker (lumens measured at a certain time instance), sounds are higher in pitch (the number of sound vibrations per unit of time).

In our July article, we talked about the toughness of plastic materials, and discussed some of the various test methods used to measure and quantify toughness. We also discussed the concept of toughness being a rate-sensitive behavior—basically a mechanical measurement that changes depending on how fast (or slow) forces are applied.

This concept of rate-sensitive behavior is not an annoying conversation to be had with Dr. Sheldon Cooper—it is a fundamental principle that affects the behavior of all plastic materials. Let me say this simply and clearly:

The measured mechanical properties of a plastic material—any plastic material—will depend on the rate at which loads are applied.

How can I explain this better? As an example, think of Silly Putty. Silly Putty is a children's toy, and consists of a small blob of material encased in a plastic egg. If you take the blob of Silly Putty in your fingers and pull it slowly, it will stretch and thin out, in much the same way as when you blow bubbles out of your bubble gum. But if you pull the blob of Silly Putty quickly, it will snap in half, with a clean break—just like how that bubble from your bubble gum will pop if you blow too quickly.

In both examples we are using the same material, but we are experiencing two different behaviors based on the speed at which the material is pulled apart. This is rate-sensitive behavior.

We can often account for this kind of behavior through proper design and/or proper material selection, but the funny thing is, most property data for thermoplastic materials is generated from data gathered under controlled conditions at very specific rates of loading. If the rate of loading in your application is different from the test scenario, what do you do? Chew gum? Blow bubbles? Or go back to playing with Silly Putty? No. You do what we all do. You take measurements under controlled conditions, and compare and extrapolate the results. Still, sound plastics engineering demands that we are aware of the rate of loading.

So, the next time you are having problems with time, instead of checking your clock, consider what you are trying to measure. You may find that it is time to change material.

Eric R. Larson is a mechanical engineer with over 30 years' experience in plastics design. He has helped develop products ranging from boogie boards, water basketball games and SCUBA diving equipment to disposable lighters, cell phones and handheld medical devices.

Larson is owner of the Art of Mass Production (AMP), an engineering consulting company based in San Diego, CA. AMP provides services to manufacturing companies in the consumer electronics, wireless, and medical device industries.

Larson is also moderator of the blog site, plasticsguy.com, where he writes about plastics technology and its effect on people and the planet. His newest book, Plastics Materials Selection: A Practical Guide, can be purchased through his website.