# Failure Theories



## mechgirl (Mar 21, 2010)

I am reworking problems in the Lindeburg Sample Exam, and am struggling to understand the concepts for questions #19 and 20, and failure theories (MERM chapter 50) in general. Is there something in the problem wording that is a hint? For infinite life do I always use the Goodman equations? When would I use Soderberg? I'm having trouble grasping when to use yield strength vs. ultimate strength.

Thanks in advance for any help.


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## Matt-NM (Mar 22, 2010)

I can't remember the specifics as I took the test back in April 2008. But I remember having some of the same questions. MERM, I thought, didn't do that good of a job on failure theories. I figured there couldn't possibly be that many questions on this subject on the test. WRONG! There were several and when I saw them I thought it was going to cost me. Fortunately I passed but was stressed out during the waiting period because of these problems. I would suggest finding alternate resources to MERM for failure theories. Good luck.


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## gaidox (Mar 22, 2010)

I don't have my practice prob with me but I know that Goodman uses ultimate strength while

Soderberg uses yield strength. It means that Soderberg is a conservative approach

(soderberg line completely below yield point line).

Use Goodman (which is commonly used) unless stated otherwise for alternating stress/fluctuating loading analysis. Though check materials since it is not applicable to all.

Ultimate strength is max. stress that matl can withstand/supt without failure.

While Yield strength is stress that matl. begins to deform plastically (deform permanently).

Brittle matls fail by fracturing (uses Ultimate) and Ductile matls fails by yielding (uses Yield strength).

You could check other references for better understanding since MERM did'nt tackle it much.


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## Shaggy (Mar 22, 2010)

agree with gaidox. Investigate Shigley and Machinery Handbook for further info.


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