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Alexander Wolfe
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Blogger
Commonplace Chemicals
Alexander Wolfe   2/3/2012 11:16:41 AM
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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?)

Dave Palmer
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Platinum
Re: Commonplace Chemicals
Dave Palmer   2/3/2012 11:50:24 AM
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@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.

Tim
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Platinum
Stress crack
Tim   2/3/2012 12:14:36 PM
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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.

Ann R. Thryft
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Blogger
labeling of stress types
Ann R. Thryft   2/3/2012 12:30:04 PM
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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?

Dave Palmer
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Platinum
Re: Stress crack
Dave Palmer   2/3/2012 12:40:07 PM
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@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.

TJ McDermott
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Blogger
Re: labeling of stress types
TJ McDermott   2/3/2012 12:53:23 PM
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I'd add to Ann's list of environmental factors questions, does light count as environmental (in different portions of the spectrum)?

Dave Palmer
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Platinum
Re: labeling of stress types
Dave Palmer   2/3/2012 12:57:17 PM
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@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.

Ann R. Thryft
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Blogger
Re: labeling of stress types
Ann R. Thryft   2/3/2012 1:05:06 PM
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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? 


Dave Palmer
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Platinum
Re: labeling of stress types
Dave Palmer   2/3/2012 1:22:21 PM
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@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.

Ann R. Thryft
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Blogger
Re: labeling of stress types
Ann R. Thryft   2/3/2012 1:34:56 PM
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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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