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

Understanding Overload Failures

NO RATINGS
View Comments: Newest First|Oldest First|Threaded View
<<  <  Page 2/2
Shelly
User Rank
Iron
Vibration big cause of fatigue/crack propagation
Shelly   3/26/2012 10:21:54 AM
NO RATINGS
As mentioned earlier, repeated stresses/cycles on an assembly are a major contributor to fatigue and crack propagation.  The biggest contributor to repeated cycles is vibration.  Sometimes it's difficult to observe, but even high frequency vibration (though very small displacements) can be a fatigue factor due to their high cycle rate.

Vibration can be an issue when attaching a component to a moving machine (frequency depends on the machine dynamics), when designed to handle siesmic vibrations near fault lines (relatively low frequency <10Hz), or just designing to handle transportation to the end user (between 2-500Hz).  The frequency and amplitudes vary, but the main goal is to design components with resonant frequencies well above what the sample will see while in use or transport, and when designing machinery, to avoid stacking resonant frequencies so the components aren't exciting each other's resonant frequencies while in use.

Dave Palmer
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Dave Palmer   3/26/2012 7:07:21 AM
NO RATINGS
@Mydesign: You're right that loads redistribute to a certain extent as a result of localized yielding, so that a linear FEA which predicts a stress greater than the yield strength in a small region doesn't necessarily indicate failure of the component. This is why designing to "get the red out" of a FEA model, without any insight into the physical situation, can result in overdesign. On the other hand, stresses below the yield strength can lead to fatigue failure if they are repeatedly applied. To get a handle on fatigue, it's important to know how the loads on a part vary over time. This is what I will discuss in the next installment.

Mydesign
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Mydesign   3/26/2012 6:05:08 AM
NO RATINGS
1 saves
Dave, I think the weight is eventually distributed across the area, and then it can bear more weight than concentrate to particular points. I think in most of the industrial wing, the stress tests are doing for a mass areas rather than stress test in cubic/cm sqd.

Dave Palmer
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Dave Palmer   3/23/2012 7:30:08 PM
NO RATINGS
@Chuck: Other engineers should feel free to weigh in on this, but in my experience, it's most common to design to the yield strength, with an appropriate factor of safety. Doing this should protect you against overload failures, provided that (as I pointed out in the article) the loads are what you think they are, and the yield strength is what you think it is.

Charles Murray
User Rank
Blogger
Re: Migrating stress cracks versus quick breaks
Charles Murray   3/23/2012 6:37:37 PM
NO RATINGS
Dave: In the kinds of parts that are mentioned here, such as the brake cam, are the parts typically designed in accordance with the yield strength of the material, or is there some "allowable stress" design method that's set forth that is not dependent on yield? If yield is not used as criteria, does it make any difference in terms of failure rates?

Dave Palmer
User Rank
Platinum
Re: Migrating stress cracks versus quick breaks
Dave Palmer   3/23/2012 10:07:35 AM
NO RATINGS
@Alex: Usually, a crack which grows over time is due to fatigue, which I'll cover in my next installment. (In plastic parts, cracks which grow over time could also be due to environmental stress cracking, which I've written about before. In metals, there is a phenomenon called stress corrosion cracking, which is analagous to environmental stress cracking; I might write about this later).

Based on what I've seen in my career, fatigue failures are actually far more common than overload failures.  However, overload failures are the easiest to understand, which is why I wanted to cover them first.

The mechanics of fatigue are a little more complicated.  As I'll discuss, a common mistake is to treat "fatigue strength" as though it's a property like yield strength or ultimate tensile strength.  It's not.  But the big picture is the same: you need to understand the forces that act on the part, and the properties of the material from which it is made -- as well as all of the variables which might cause either one to vary from its normal value.

Alexander Wolfe
User Rank
Blogger
Migrating stress cracks versus quick breaks
Alexander Wolfe   3/23/2012 9:08:17 AM
NO RATINGS
Could you do an explanation in a future post of the differences between a part (say, a bracket on a car) failing due to a migrating stress fracture versus a total, quick failure where it just breaks in two? Is that the same stress dynamic in play with different outcomes, or are they different processes entirely?

<<  <  Page 2/2
Partner Zone
More Blogs from Guest Blogs
It seems that gears have been around forever -- what could be new?
The use of ZNE technology buildings not only reduces cost and creates renewable energy, but it can give a boost to the economy. The implementation and execution will require a workforce, which means the result can be increased employment.
The use of CFD is more profound in automobile design because of the diverse physics involved. It can be used to optimize components for downsizing and reducing the environmental footprint while maintaining or improving operational efficiency.
A Detroit automaker was struggling with a semi-manual, labor-intensive parts receiving process. Workers had enter parts into a spreadsheet manually at a stationary workstation. This stopped the receiving process dead in its tracks. A better system was needed.
Pressure sensors can be used in a wide range of applications. It’s imperative that they’re suited for use in hazardous situations and are fully compliant with one of several protection standards.
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.
Jul 21 - 25, Design Products With Bluetooth Low Energy
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: August 12 - 14
Sponsored by igus
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