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When Environmental Stress Cracking Strikes

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Dave Palmer
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Re: Very Relavent Artical.
Dave Palmer   2/20/2012 10:27:25 PM
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@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).

rmp2345
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Iron
Re: Very Relavent Artical.
rmp2345   2/20/2012 7:46:26 PM
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Hey Dave,

What plastic was used that failed?

vimalkumarp
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Gold
When Environmental Stress Cracking Strikes
vimalkumarp   2/7/2012 10:59:24 PM
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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

Dave Palmer
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Platinum
Re: Very Relavent Artical.
Dave Palmer   2/6/2012 6:02:00 PM
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@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. 

N. Christopher Perry
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Very Relavent Artical.
N. Christopher Perry   2/6/2012 5:30:54 PM
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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.

Ann R. Thryft
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Blogger
Re: labeling of stress types
Ann R. Thryft   2/6/2012 3:46:48 PM
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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.


Dave Palmer
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Platinum
Re: labeling of stress types
Dave Palmer   2/6/2012 3:31:59 PM
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@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.

Ann R. Thryft
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Blogger
Re: labeling of stress types
Ann R. Thryft   2/6/2012 2:18:25 PM
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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.


Dave Palmer
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Platinum
Re: labeling of stress types
Dave Palmer   2/6/2012 2:08:38 PM
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@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.

Tim
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Platinum
Re: Stress crack
Tim   2/3/2012 6:25:32 PM
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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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