An injection-molded part with a pressed-in brass insert was found to crack during testing. There were multiple cracks, all originating from the insert. The cracked part was brought into my lab late one afternoon. I took some photographs of the cracks, then went home for the day. The next morning, I looked at the part again. Comparison with the photos I had taken the previous day confirmed that the cracks had grown overnight.
The presence of multiple cracks, and the fact that the cracks continued to grow even after the part was taken out of service, pointed toward environmental stress cracking. This is a common failure mode for plastics, in which cracks form at relatively low levels of stress as a result of chemical exposure. The stress may be much lower than the strength of the material. Most plastic parts have some level of molded-in residual stress, so cracking can occur even when there is no external stress -- if the part is exposed to the wrong chemical.
Some of the chemicals which can cause environmental stress cracking include fuel, oil, grease, solvents, adhesives, and cleaning products. It can often be difficult to find environmental stress cracking data for a given plastic with a given fluid. This is particularly true these days, since many resin manufacturers have closed or scaled back their laboratory facilities. You may need to do your own testing to make sure that the plastic you plan to use will not crack when exposed to any of the fluids it is likely to come into contact with.
I found some oil residue on the cracked part. Using Fourier transform infrared spectroscopy (FTIR), I was able to identify the specific oil type.
In order to test the environmental stress cracking resistance of the plastic to this oil, I drilled three holes in a sample of plastic, and inserted pins into the holes. The pins were oversized with respect to the holes. The first pin was oversized by 1 percent, the second pin was oversized by 5 percent, and the third pin was oversized by 10 percent. The interference fits between the pins, and the holes gave me three different stress levels. I then immersed the sample in oil and checked for cracks every 24 hours.
My testing showed that cracks formed in a short period of time, even at the lowest stress level. This indicated that the plastic was extremely sensitive to this oil, which explained why the failed part had continued to crack overnight in my lab.
I never realized the commonplace chemicals like oil and grease were precursors to stress cracking in plastics. Good to know. Also didn't realize there's some built-in prestress. It seems that, in consumer systems, the plastic always ends up cracking at some point. Is that because thin(ner) plastics are always prone to cracking (and on the other side of the design equation, making them thick enough to be more crack resistant doesn't comport with weight and cost requirements. Or are the thicker plastics just as stress-crack prone?)
@Alex: When it comes to environmental stress cracking, there is not necessarily any advantage to making the plastic thicker or thinner. The key variables are stress and chemical exposure. If by making the plastic thicker, you can reduce the stress below the threshold, then it might be a solution. But often the threshold stress is so low that this is impractical. And if the internal stresses in the material are high enough, it doesn't matter what you do with the external stress.
Here is a good introduction to residual stress in plastics. At some risk of oversimplification, thin-wall sections are more likely to have flow-induced residual stresses, while thick-wall sections are more likely to have thermal-induced residual stresses. But either way, molded-in stresses can be significant.
In a previous life, we used a low stress constantly applied to parts submerged in an Igepal solution. The purpose of the test was to act as an accelerated life test for the product. It worked pretty well, and if the part survived Igepal solution, it wouldn't fail over time.
Thanks for another informative post. I have a question about the nomenclature and taxonomy of stress types that lead to cracking. So environmental stress is caused entirely by chemical exposure? What about exposure to other environmental factors such as temperature, humidity and corrosion, e.g.? Are those also classified as environmental, or are they classified in a different category, with a different label?
@Tim: Were you working with polyethylene? There is a standard test for evaluating the stress cracking resistance of polyethylene which uses Igepal. It's a good screening test. But there is really no substitute for testing the specific plastic you are planning to use with the specific fluid you're concerned about.
When it comes to polyethylene, density is an important factor. A higher density means a higher degree of crystallinity, which results in higher molded-in stresses and an increased susceptibility to cracking. We were able to solve a stress cracking problem with polyethylene parts simply by specifying a somewhat lower density range. The difference between 0.96 grams per cubic centimeter and 0.95 grams per cubic centimeter was the difference between parts that cracked and parts that didn't.
@Ann: Yes, the term environmental stress cracking refers specifically to cracking which is caused by a chemical agent. This includes water or humidity, for some plastics. There is also such a thing as thermal stress cracking, which is considered to be a separate phenomenon. And of course there are all kinds of reasons why plastic or other parts might break, such as fatigue.
You mentioned corrosion. As I said in the article, nylon is generally very resistant to environmental stress cracking, but there are exceptions. One thing which will cause stress cracking in nylon is zinc chloride. Zinc chloride can form as a corrosion product on zinc. So if you are using zinc-plated inserts or fasteners with a nylon part, this is something you should definitely look out for. The same goes for brass inserts or fasteners, since brass is an alloy of copper and zinc.
So the categories are environmental stress, which is caused by chemicals, thermal stress, caused by temperature, fatigue, and...? Light, as TJ asked? That makes sense, since I know UV can cause cracks in many plastics. What other categories?
@Ann: I could easily fill up another blog post with all of the different possible failure modes which materials can experience. My goal here has just been to describe a few failure modes (such as galvanic corrosion in metals, or environmental stress cracking in plastics) which are commonly seen, but less often understood.
Light, especially ultraviolet light, can cause degradation of plastics. The ultraviolet light attacks and breaks down the polymer chains, making the plastic weaker. This is different from environmental stress cracking, which typically doesn't involve chemical bonds being broken.
Thanks, Dave. I think a blog post that gives a 101 description of the basic failure modes is a great idea. I was just asking for a simple list: name of stress type and what it covers, so we have a context for the discussion. Looks like there's also a difference between types of stress, i.e., whether bonds get broken or not. Anyway, an overall brief taxonomy would be helpful.
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