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Dave Palmer
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Platinum
Re: labeling of stress types
Dave Palmer   2/3/2012 3:51:29 PM
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@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.

David12345
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Platinum
Re: labeling of stress types
David12345   2/3/2012 1:41:32 PM
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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.

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.


Dave Palmer
User Rank
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 1:05:06 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
User Rank
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.

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
User Rank
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.

Ann R. Thryft
User Rank
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?

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.

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