HOME  |  NEWS  |  BLOGS  |  MESSAGES  |  FEATURES  |  VIDEOS  |  WEBINARS  |  INDUSTRIES  |  FOCUS ON FUNDAMENTALS
  |  REGISTER  |  LOGIN  |  HELP
Blogs
Guest Blogs

When Environmental Stress Cracking Strikes

NO RATINGS
Page 1 / 2 Next >
View Comments: Oldest First|Newest First|Threaded View
Page 1/3  >  >>
Alexander Wolfe
User Rank
Blogger
Commonplace Chemicals
Alexander Wolfe   2/3/2012 11:16:41 AM
NO RATINGS
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
User Rank
Platinum
Re: Commonplace Chemicals
Dave Palmer   2/3/2012 11:50:24 AM
NO RATINGS
@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
User Rank
Platinum
Stress crack
Tim   2/3/2012 12:14:36 PM
NO RATINGS
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
User Rank
Blogger
labeling of stress types
Ann R. Thryft   2/3/2012 12:30:04 PM
NO RATINGS
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
User Rank
Platinum
Re: Stress crack
Dave Palmer   2/3/2012 12:40:07 PM
NO RATINGS
@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
User Rank
Blogger
Re: labeling of stress types
TJ McDermott   2/3/2012 12:53:23 PM
NO RATINGS
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: labeling of stress types
Dave Palmer   2/3/2012 12:57:17 PM
NO RATINGS
@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
User Rank
Blogger
Re: labeling of stress types
Ann R. Thryft   2/3/2012 1:05:06 PM
NO RATINGS

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 1:22:21 PM
NO RATINGS
@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
User Rank
Blogger
Re: labeling of stress types
Ann R. Thryft   2/3/2012 1:34:56 PM
NO RATINGS

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.


Page 1/3  >  >>
Partner Zone
More Blogs from Guest Blogs
The Smart Emergency Response System capitalizes on the latest advancements in cyber-physical systems to connect autonomous aircraft and ground vehicles, rescue dogs, robots, and a high-performance computing mission control center into a realistic vision.
Businesses cutting across industries are increasingly making use of portable display stands in the UK for marketing.
Igus retrofitted a car with 56 of its plastic iglide bearings to celebrate the brand's 30th anniversary. The car is currently being driven across the US and Canada.
The first and most obvious lesson of the following story is to remember to consider creep, along with all other potential failure modes.
Medical devices will look and feel different in the next 20 years, because, as design and product development people, our criteria are changing.
Design News Webinar Series
7/23/2014 11:00 a.m. California / 2:00 p.m. New York
7/17/2014 11:00 a.m. California / 2:00 p.m. New York
6/25/2014 11:00 a.m. California / 2:00 p.m. New York
5/13/2014 10:00 a.m. California / 1:00 p.m. New York / 6:00 p.m. London
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Sep 8 - 12, Get Ready for the New Internet: IPv6
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6


Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Next Class: September 30 - October 2
Sponsored by Altera
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Sponsored Content

Technology Marketplace

Copyright © 2014 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service