# When to use what failure theory for MD?



## navyasw02 (Sep 9, 2010)

Today I worked a problem and found myself wondering which failure theory to use on the PE exam MD Depth. The problem I was working #530 in the 2001 exam using the Distortion Energy method from an almost identical example in Shigley. This gave an answer that wasn't an option, so I tried not using any failure theory and that answer was listed. I chose that one, but turns out the answer wanted to use Max Shear Stress instead.

In general, should I always plan on using the most conservative method?


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## Shaggy (Sep 10, 2010)

My recollection is that it is situational. This is paraphrased from Shigley:

_maximum shear stress or distortion energy is acceptable for design and analysis of materials that would fail in a ductile manner. The selection of these two is something that the engineer must decide. For design purposes the max shear stress theory is easy and quick to use. If the problem is to learn why a part failed, then the distortion energy theory may be best to use._

How close are the answers when you use MSS vs DE? They may be close enough to come to the same conclusion. The one that corresponded to the use of no failure theory was intentionally included. They include incorrect options based on common mistakes. The answers on the test probably won't match your calculation exactly. They typically ask you to mark the "most correct". Conservative is a good approach. IIRC the test may specify which failure theory to use.


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## Sschell (Sep 10, 2010)

navyasw02 said:


> In general, should I always plan on using the most conservative method?


I'm going to go with yes on this one...

The thing that I always had to remind myself on the exam was that they love to give too much information in the questions... don't over think it, and dont be afraid to NOT use some of the numbers provided.


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## navyasw02 (Sep 10, 2010)

Shaggy said:


> My recollection is that it is situational. This is paraphrased from Shigley:
> _maximum shear stress or distortion energy is acceptable for design and analysis of materials that would fail in a ductile manner. The selection of these two is something that the engineer must decide. For design purposes the max shear stress theory is easy and quick to use. If the problem is to learn why a part failed, then the distortion energy theory may be best to use._
> 
> How close are the answers when you use MSS vs DE? They may be close enough to come to the same conclusion. The one that corresponded to the use of no failure theory was intentionally included. They include incorrect options based on common mistakes. The answers on the test probably won't match your calculation exactly. They typically ask you to mark the "most correct". Conservative is a good approach. IIRC the test may specify which failure theory to use.


The problem I did (cant remember the number off the top of my head), the answer with DE was 75. The correct answer was 90 something with MSS and another choice was 50 something. I couldve guessed high to be safe. I reworked the question with no failure considerations, and got one of the answers listed. On the real exam, Id likely fall into the trap of picking the answer that looks like one of the given choices instead of one that is in between.

You're right, the failure criteria should be situational, but when a problem just says something like the material is steel or even worse, doesn't give a material, then it makes it difficult to decide which to use. I just used DE because Shigley had an example that was pretty similar. I guess it doesn't hurt to do the most conservative though, but it doesn't make much sense from a real life standpoint. Then again, a multiple choice exam isnt real life.


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