Dan Faulkner, AC Principal Mechanical Engineer
In Part 1 of our series on Material Creep & Stress, we examined the mechanisms of stress-strain. Going beyond a typical FEA, we recommend testing for the effects of creep and stress to predict long-term material performance to prevent premature failure over a product’s expected service life.
In Part 2, we now explore how to work with various types of creep and stress data and use them to improve our product engineering and material design choices.
Designing with Isochronous Curve Data
If you can find isochronous curves for your material, consider yourself lucky! These curves are often hard to find, but they do occasionally appear in material datasheets. CAMPUS Plastics provides a free database of isochronous curves for many common polymers.
Designing with these curves is straightforward:
- Select your initial design point on the short-term stress-strain curve and plot it on the graph.
- Then move from that point to the appropriate isochronous curve.
- If your tested device is experiencing creep, move horizontally (constant stress) until you hit the isochronous curve.
- For stress relaxation move vertically (constant strain) until you hit the curve.
Sometimes, real-world loads combine creep and stress relaxation. In those cases, you may not know exactly where your end-of-life point will be, but at least you’ll know that it’s somewhere between the two extremes.
If your device will be subject to elevated temperatures, it’s important to use isochronous curves gathered at the appropriate temperature. You may need to interpolate or extrapolate the data to get the desired temperature or at a different time duration using the principles that are described in the next section.
Designing with Creep Modulus Data
Within material datasheets, finding the creep modulus is much more common than finding the entire isochronous curve. However, since E_creep is just the slope of the curve, we can reconstruct an approximation of the isochronous curves, as shown with the values below.
We can then use these reconstructed curves like before:
- Find the start-of-life point on the short-term curve.
- Then move horizontally or vertically in the graph to find the end-of-life point.
- Add some extra design margin because the real isochronous curves are not nice straight lines.
When interpolating or extrapolating the datasheet values, remember a simple rule of thumb illustrated in the chart below: When plotted on log-log axes, a graph of E_creep versus time will usually be close to a straight line.
Designing Without Creep Data
Sometimes, creep data is simply not available for your material. What can you do? Here are a few tips:
Leverage similar material data cautiously
You could try using data for some other, similar material as a very rough approximation. For example, maybe you can’t find data for PC-ABS, but you have data for plain PC.
As you can imagine, this is fraught with all kinds of concerns. However, working cautiously with flawed data is better than ignoring creep entirely.
Set a safe boundary for your design
Another option is to use a rough rule of thumb to put a boundary on the problem.
For example, after looking at a bunch of polymer creep data, we’ve noticed that for most of them, E_creep rarely drops below 20% of the short-term modulus, after a 10-year load at room temperature. So, you could just use E for your design and then apply a 5X safety factor to account for possible creep or stress relaxation.
Again, this is a very poor approximation of a complex behavior, but it’s better than nothing.
Creating your own test is best
When faced with a lack of creep data, running a test is best.
If the actual parts exist, or if you can build a representative prototype, you can apply the actual loads and measure the real behavior. (3D-printed parts are probably not representative!)
You may want to use an accelerated aging method called “time-temperature superposition.” This method uses elevated temperatures to simulate long durations of time.
Alternatively, we often need to make design decisions for parts that don’t exist yet. This could be because they require expensive injection molds that will be built later in the project. In this case, the best option is to request test results from the material supplier or procure some flat samples and run your own tensile tests.
Selecting Creep-Resistant Materials
When creep and stress relaxation are concerns, choosing creep-resistant materials can improve the odds of success. Because these phenomena are complex and application-specific, we can only give some general guidance.
The three groupings of creep-resistant materials are very approximate but will give you a rough starting point for your designs:
Most Resistant
- PET
- PC
- PC-ABS
- PMMA
- SAN
Less Resistant
- ABS
- PA6
- PBT
- PS
Least Resistant
- HDPE
- PA12
- PE-UHMW
- PP
- POM
Tip: Adding glass-fiber reinforcement generally improves creep resistance.
The Effect of Creep on Ultimate Strength
In some cases, plastic parts can rupture due to creep. The polymer chains disentangle so much that they separate entirely. It’s important to note that this can happen even if the stresses are well below the yield point. Higher temperatures tend to accelerate this behavior.
Similar to the creep modulus, it can be difficult to get high-quality rupture data, so you may need to conduct tests or make some crude assumptions.
RESOURCE CREDIT: The Madison Group | Lifetime Prediction of Plastic Parts, Fig. 1
The Creep & Stress of "Compression Set" with Elastomers
Keep in mind that elastomers have similar creep and stress relaxation behaviors, typically referred to as “compression set.”
As illustrated below, if you squeeze an elastomeric material, such as an o-ring, for an extended time, it won’t return to its original shape when the load is removed. The material will permanently deform, and the contact force between the seal and the housing will be reduced, which can result in leaks.
Thankfully, most major o-ring manufacturers publish extensive data on compression set. For example, the plot below, taken from the Parker O-ring Handbook, is typical of the information they supply.
Sometimes Creep & Stress Effects are Useful!
So far, we’ve treated creep and stress as universally bad effects, but exceptions exist.
We once worked on a design with two plastic parts, one sliding over the other.
We needed to minimize the sliding friction and maintain a seal between the two parts. The sliding part had a thin, flexible section that created a wiper seal.
We quickly discovered that we had a tolerance problem.
In the tightest scenario, there was too much friction. In the loosest scenario, it tended to leak. Plus, there was no way to improve the tolerances of the molded parts.
We realized, though, that the stresses were fairly high in the seal’s thin wall, and we knew this would result in a rapid rate of stress relaxation. As a result, we designed the parts for a tight fit, confident that as the stresses relaxed, the friction would drop.
By the time the product had been assembled, tested, warehoused, shipped, etc., the friction was reduced to an acceptable level. So, actually, because of stress relaxation, we got a good fit between the parts every time.
Key Takeaway? Don't Ignore Creep & Stress Relaxation.
Hopefully, this series on material creep and stress has reinforced the importance of estimating the end-of-life behavior of parts that bear continuous loads.
Because creep and stress phenomena are complicated, some engineers like to pretend they don’t exist. But as we’ve shown here, simple approximations can be applied to almost any design. In those cases, we won’t get decimal-perfect answers, but we can try to predict whether there will be a durability problem.
This extra effort and analysis are invaluable. By using insights from creep and stress testing and data, we can compensate for material performance issues ahead of time through better design solutions. For instance, we reduce stress concentration areas like sharp corners and thin walls, minimize temperature effects, and consider glass-filled materials for better creep resistance when constant stresses are inevitable.
Avoid premature product failure from material creep & stress. We can help!
In today’s competitive landscape, product durability and reliability are paramount. Does your team need engineering support with material testing and creep & stress analysis? Let AC optimize your designs, mitigate potential issues, and ensure your products deliver exceptional performance.
Consult with our material and product development team to take the first step towards superior product design today.
