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.
@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.
We were using HDPE specified by the customer. I do not remember the specified density, but it was an HMW grade of the material. We would have preferred to use the series production chemical, but unfortunately, it was too flamable to keep in our testing lab.
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?
@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.
@Ann: If you are looking for a good introduction to concepts of stress and strain for the general reader, I would strongly recommend Structures, or why things don't fall down, by J.E. Gordon. Another excellent book by Gordon is The new science of strong materials, or why you don't fall through the floor. Both of these books present materials science and engineering concepts in a way anyone can understand, and they are also enjoyable reads. They were written in the 1960s or 1970s, so some of the material is a little dated, but for the most part they cover fundamentals, which haven't changed over time.
Thanks, Dave. I've heard of the first one, but it looks way too general. Is the second one more advanced? I guess I meant 110 not 101. And not for the general subjects. I was asking specifically for a typology/taxonomy of the different stress types.
@Ann: It's been a while since I've read them. The new science of strong materials might be more appropriate for you, since it's specifically focused on materials. If you just look at the table of contents, it may seem very basic, but that's at least partly because Professor Gordon had a talent for making fairly complex things seem simple. He actually covers some fairly advanced topics such as elasticity and dislocation theory.
When it comes to terminology, very briefly, stress is defined as force per unit area. Often, although not always, it is a response to an externally-applied load. For example, if I pull or push with a force of 1000 pounds on an area of 1 square inch, I am producing a stress of 1000 pounds per square inch. So we can classify stresses based on the kinds of forces which produce them: tensile, compressive, torsional, bending, shear, etc. If I am pulling on the material, the stress is tensile; if I am pushing on the material, the stress is compressive, etc. It's also possible to have internal stresses, which have been discussed briefly here in the comments.
The resistance of a material to stress is called strength. If the stress on a material exceeds its strength, it will break.
Some things can reduce a material's strength. We have mentioned two of those (ultraviolet light and chemical exposure) in this discussion. Light exposure or chemical exposure are not types of stress; instead, they change the way that a material responds to stress. Both of these things can cause a material to fail at a stress far below its ordinary strength.
Materials can also break as a result of repeated applications of stress below their ordinary strength. This is called fatigue. Fatigue doesn't actually reduce a material's strength, although that is what was originally thought (which is where the term "fatigue" comes from; the idea was that the material's properties were somehow becoming "exhausted"). Instead, very tiny cracks form in regions of the material where the local stress is high. These tiny cracks grow with repeated applications of stress.
This is a very brief introduction, and maybe I am just telling you things you already know, but hopefully this is helpful.
Awesome! I'll check out Gordon's second book. It looks quite useful. This is much appreciated, as I've been looking for a refresher text that would also get me into some more advanced subjects.
And thanks for the terminology definitions. I'm already familiar with stress and how it's defined generally, but the specific types based on different kinds of force makes total sense. My original questions arose because your article mentions environmental stress cracking, which sounded like a type of stress, and then your comments mentioned thermal stress, etc. Anyway, thanks for adding from your wealth of knowledge and expertise in failure analysis.
Those catagories you list are examples of what can degrade the mechanical strength of the plastics . . . sometimes to the point of failure without external loads. Additionally, during manufacturing of a plastic product, such as by injection molding, the material molecular weight can be reduced by degrading the plastic by too much heat history in processing, too much shear breaking up the polymer chains, too much regrind (heat history, reduced fiber length in fiber reinforcement if any, and fines with low polymer chain length), moisture in the material driving a reversal in the polymerization back to its' raw materials, poor pigment concentrate blending and distribution, etc.
Naturally a material can also just be overstressed mechanically, leading to a fracture failure.
Can you look at a part and distinguish chemical environmental stress-cracking from mechanical overload: Yes, at least with some plastics. At one time, I was the product engineer for an electronic connector made from a high-temperature amorphous PEI. This was a great tough plastic; except, when exposed to chlorinated solvents such as methylene chloride, and it could break-up and crumble sometimes from even the molded-in stresses. Most PCB soldering and flux cleaning processes could be designed away from these solvents, but occasionally a rework with Freon TMC (containing methylene chloride) would crop-up. The failed surface of the connetor was always the give-away. If it was from gross loading, mechanical abuse, and mechanical failure, the failed surface was rough and grainy. If the failure was from chemical exposure and stress-corrosion-cracking the failed surface was usually curved, but glassy smooth.
@David12345: Great comments. As far as visually identifying environmental stress cracks, you're absolutely right that they tend to have a smooth, glassy fracture surface. Other clues include multiple crack origins (overload cracks usually have a single origin), and, of course, the presence of cracks at stresses well below the strength of the material.
The most insidious material incompatibility I've encountered is ABS and methyl-cyanoacrylate based adhesives and thread locking agents. Several projects I've worked on have been vexed by mysterious environmental stress crack failures, even with ABS blends, that were ultimately traced to the choice of thread locker or adhesive. The most irritating aspect of these failures was that the delay between assembly and failure, in each instance, was variable from days to weeks. Ultimately, eliminating the bonding agent or switching to alkoxyethyl cyanoacrylate based agents addressed our specific issues, but only after significant schedule/budget penalties.
Unless the agent or the plastic specifically indicates compatibility, the application must be carefully scrutinized. I now make a point of contacting the sales engineers for both the plastic and agent (adhesive, lubricant, etc.) and independently verifying compatibility with both. If compatibility can't be confirmed, my default is to change the combination. I only resort to testing or redesign as a last resort, given the potential program risks.
@N. Christopher Perry: Thanks for your comments. You're absolutely right -- methyl cyanoacrylate adhesives and anaerobic threadlockers are two things which should never be used in combination with ABS. In fact, they should never be used anywhere near ABS. All it takes is a little bit of liquid or vapor coming into contact with the plastic to cause cracking. If you can get away with it, you might be better off with a dry threadlocking patch.
In my experience, polycarbonate, ABS, and PC-ABS are the worst plastics in terms of environmental stress cracking susceptibility. When it comes to these three plastics, I think your approach of assuming that a given chemical will cause cracking unless shown evidence to the contrary is probably a good one.
@rmp2345: The particular plastic in this article was PC-ABS. As I mentioned in an earlier comment, PC, ABS, and PC-ABS are among the most susceptible to environmental stress cracking. These plastics have their advantages, particularly in terms of toughness and impact strength, but they are extremely sensitive to chemicals and so I am very cautious about where I use them. But any plastic can potentially be affected by environmental stress cracking. When in doubt, test (or at least request test data from your supplier).
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