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.
@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).
This is indeed a very relevant and important article. I have faced some of the problems mentioned when I was designing some of the patient monitoring systems. Thanks a lot for this wonderful article
@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.
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.
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.
@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.
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: 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.
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.
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