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

Understanding Fatigue Failures

NO RATINGS
< Previous Page 2 / 2
View Comments: Oldest First|Newest First|Threaded View
<<  <  Page 2/3  >  >>
naperlou
User Rank
Blogger
Re: fatigue failure
naperlou   4/10/2012 9:57:05 AM
NO RATINGS
Many systems are over designed.  It is just difficult to predict what aspects will be critical in actual applications.  It is amazing the products and systems that last well beyond their design life (while others don't make it).

ChasChas
User Rank
Platinum
Re: fatigue failure
ChasChas   4/10/2012 11:02:06 AM
NO RATINGS
 

Over-design is a good thing if cost and weight is not a deciding factor. Things that are impossible to inspect after being built must be "over-designed" or designed for no post inspection.

Great article by Dave.

 

Dave Palmer
User Rank
Platinum
Re: fatigue failure
Dave Palmer   4/10/2012 12:15:20 PM
NO RATINGS
@vimalkumarp: Thanks for the reference to McDonough and Braungart's Cradle to Cradle.  It looks like an interesting book.  I will try to find it.

Just to illustrate the point about how the lack of information affects design, right now I'm working on a die cast aluminum part which has an electroless nickel coating and operates at high temperatures (400 - 500°F).

We know something about how the die cast aluminum alloy behaves in fatigue at room temperature, but we don't know much about how it behaves at elevated temperatures.  We know the fatigue strength will be lower, but by how much? We also don't know much about how the electroless nickel coating affects the fatigue strength.  Again, we expect that the coating will reduce the fatigue strength, but we're not sure by how much.

Because we don't have the capability to do high temperature fatigue testing in-house, we might not fully answer these questions in the course of this project.  Instead, we will have to make educated, conservative assumptions which will probably result in the component being somewhat overdesigned.  As resources (hopefully!) become available in the future, we will try to do further testing to fill in the gaps in our knowlege.

By the way, it's very important to consider the effect of coatings on the mechanical properties of a material.  In general, coatings which are more brittle than the substrate tend to reduce ductility, impact strength, and fatigue life.  This is particularly true if the coatings apply tensile residual stresses to the substrate.  These principles apply not only to plating of metals, but also painting of plastics. (For plastics, solvent attack on the substrate is another concern; in metals, the parallel to this is hydrogen embrittlement).  You should never assume that you can apply a coating to a material without affecting its mechanical behavior.

Ann R. Thryft
User Rank
Blogger
Re: Mastering the art of designing for fatigue
Ann R. Thryft   4/10/2012 1:20:32 PM
NO RATINGS

Dave, I understood what you meant about wanting to have your own internal database and why. What I was trying to find out was, at a broader view, isn't it more or less redundant with everyone else's internal database, and why can't all of this data be maintained in (one or more) centralized repositories, which might be accessible to the software tools? Perhaps the answer is there's too much data, or perhaps the answer is it's too product-specific to a manufacturer's/service provider's own products/services. Is it one of these or something else?


kenish
User Rank
Platinum
Re: Mastering the art of designing for fatigue
kenish   4/10/2012 3:45:28 PM
NO RATINGS
Dave, great article.  I'm a EE and this helped me visualize fatigue in a different way.  Even I understood it!  You mention design, manufacturing, and materials as sources of fatigue cracks.  Aren't they also important to prevent minor damage in-service from turning into a fatigue site (along with periodic inspection of critical areas...I'm thinking about aircraft skins)?

William K.
User Rank
Platinum
Understanding fatigue failure.
William K.   4/10/2012 3:52:32 PM
NO RATINGS
If "overdesign" is the term for a design that includes only just barely enough to last until the warranty expires, possibly, then it is a very good attribute. The reality is that many products are sometimes used beyond the "typical" levels, and so they do need to be stronger than only enough to handle "typical". Designing only to the lower boundry of typical is why such a large portion of consumer goods are trash at the very instant that they are made. 

Fatigue failure is indeed a whole lot more subtle than the other kinds, such as yield and wear failure, but it is avery important consideration in a lot of places. The article was both useful and needed.

Shelly
User Rank
Iron
Re: Mastering the art of designing for fatigue
Shelly   4/10/2012 4:50:37 PM
NO RATINGS
Ncode/Somat has a good fatigue calculator, designed by Daryll Socie from the University of Illinois, Champaign.  He also offers seminars/short courses for engineers who need to know more about designing for fatigue.

Using the NCode/Somat software to perform the calculations works fine, but you still have to know all of the properties of the materials in each situation to plug into the software in order to get accurate results.

Dave Palmer
User Rank
Platinum
Re: Mastering the art of designing for fatigue
Dave Palmer   4/10/2012 6:03:25 PM
NO RATINGS
@Ann: I think it would be great if there were more cooperation between companies with this type of data.  There are already some industry efforts in this direction.  The American Iron and Steel Institute's Bar Steel Fatigue Database, which I linked to before, is a good example.  The American Foundry Society has a database of fatigue properties for cast irons.  USCAR and the Department of Energy have developed, or are developing, a similar but much larger database for light metals (aluminum, magnesium, etc.) -- I don't know much about this database, but I'd like to.  Ultimately, assembling these databases is just one part of an much more ambitious project called integrated computational materials engineering.

However, in order for this to work, companies have to shed their old mentality of holding this kind of information closely.  Industry associations and government can help to facilitate this.

Dave Palmer
User Rank
Platinum
Re: Mastering the art of designing for fatigue
Dave Palmer   4/10/2012 6:08:12 PM
NO RATINGS
@kenish: You are absolutely right; any interruption to the surface is a potential site for initiation of a fatigue crack.  This might mean nicks or dings which occur either during manufacturing or during use.

Fatigue testing is usually done on highly polished samples.  Broadly speaking, the rougher the surface, the lower the effective fatigue strength.  Surface finish is a very important parameter, which unfortunately is very difficult to incorporate into a FEA model.

Beth Stackpole
User Rank
Blogger
Re: Understanding fatigue failure.
Beth Stackpole   4/11/2012 6:58:09 AM
NO RATINGS
If the Made for Monkeys column is any indication, there are a ton of folks out there that are well willing to juryrig products or trouble shoot issues just to get a longer product life span. Interesting, often those older products have failure points that don't necessary have to do with fatigue of parts, but rather quirky design choices that lead to issues.

<<  <  Page 2/3  >  >>
Partner Zone
More Blogs from Guest Blogs
Perhaps you didn't know that there are a variety of classes, both live and archived, offered via the Design News Continuing Education Center (CEC) sponsored by Digi-Key? The best part – they are free!
The complexity of diesel engines means optimizing their performance requires a large amount of experimentation. Computational fluid dynamics (CFD) is a very useful and intuitive tool in this, and cold flow analysis using CFD is an ideal approach to study the flow characteristics without going into the details of chemical reactions occurring during the combustion.
We Have FPGAs with On-chip MCUs, but How About MCUs with On-chip FPGAs?
Programmable logic has come a long way from the simple devices we started out with. Remember Programmable Array Logic, or PALs? But where will we be in the next five to 10 years?
As industrial applications increasingly use process control systems utilizing sensor feedbacks to monitor various operating parameters, energy sources and consumption are becoming major factors of a system.
Design News Webinar Series
12/11/2014 8:00 a.m. California / 11:00 a.m. New York
12/10/2014 8:00 a.m. California / 11:00 a.m. New York
11/19/2014 11:00 a.m. California / 2:00 p.m. New York
11/6/2014 11:00 a.m. California / 2:00 p.m. New York
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Jan 12 - 16, Programmable Logic - How do they do that?
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  67


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.
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