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.
@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.
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?)
On April 21, NASA launched a novel project, putting into orbit three satellites that employ an off-the-shelf commercial smartphone as the control system.
The legacy endpoint devices that control our critical infrastructure (utility systems, water treatment plants, military networks, industrial control systems, etc.) are some of the most vulnerable devices on the Internet.
In a switched-capacitor filter, capacitors and switches take the place of resistors and accurately reproduce the characteristics of continuous-time Bessel, Butterworth, and elliptical filters.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 5
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
For industrial control applications, or even a simple assembly line, that machine can go almost 24/7 without a break. But what happens when the task is a little more complex? That’s where the “smart” machine would come in. The smart machine is one that has some simple (or complex in some cases) processing capability to be able to adapt to changing conditions. Such machines are suited for a host of applications, including automotive, aerospace, defense, medical, computers and electronics, telecommunications, consumer goods, and so on. This radio show will show what’s possible with smart machines, and what tradeoffs need to be made to implement such a solution.
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
2 
