PL+Pb Limits, Linearization, and FEA 3

[cab1990]

Help,

I have been explaining to my colleagues that the primary membrane + bending stress calculation found in APPX 4, section VII, is designed to prevent collapse due to plastic hinge and that it is the highest value found at the reference section (see para 4-133). I have finished a linear FEA and rather that take the liearized membrane + bending from the reference line, I am suggesting that we use the maximum SI as viewed from the SI stress distribution. There are no peak stresses in way of the reference line. My rationale is that since they are combined and reference the same allowable, no separation is required. And really the only reason the separation is required is to compare the primary membrane to its lower allowable. I'm looking for arguments for and against.

Thanks,

CAB

 

[cab1990]

HolaPronto,

If I understood your description of the Pro/Mechanica output, it appears that the output is not directly comparable to the ASME code limits as are the outputs by Ansys and Patran. The ASME code does not use principal stress or von Mises stress as comparables. It uses Stress Intensity which is based upon the maximum shear stress failure theory. The max shear theory states that material failure occurs when the maximum shear stress reaches the yeild stress/2. The ASME code multiplies this by 2 and calls it Stress Intensity (SI) to which the factors of safety ( 2/3 1.5 and 3) are applied. The values are then compared to the design stress Sm. An FEA output will produce three principal shear stresses. The maximum absolute value of these three numbers should be used in the detrmination of stress intensity as opposed to the principal normal stresses or von mises stress.

cab

Hope this doesn't confuse anyone

cab

 

[prex]

cab1990,
Max Prin – Min Prin as called for by HolaPronto is the stress intensity defined by ASME.
However don't want to imply that the linearization procedure of that code is correct: personally found in the past quite odd results output by Ansys.

prex

http://www.xcalcs.com

Online tools for structural design

 

[cab1990]

Prex,

I am not an ANSYS user so I cannot really say much about the program in general. I have, however, compared the linearization routine output from anysy solutions and found that they match the Patran results. I agree with you about the max-min comment, but I supposed that Holaproto was trying to view a prinicpal stress distribution which would be either max, mid, or min and not directly comparable to stress intensity.

cab

 

[napoleonm]

hello Cab1990 and prex,

Have you got a simple result model from ansys or patran using the linearization routine output, (I mean the sketch, dimensions, loads and constrains conditions) for me run the same model in pro/mechanical with the goal to compare results and do a little benchmark.

I can be in Pdf, Doc... let me know..

thanks.