Von Mises stress and safety factor? 1

[SteveFinnan]

In FEMAP, i obtain the max stress of Bar EndA and Bar EndB. Then from there i calculate the von mises stress using the von mises yield equation, (σa)^2 - (σa)(σb) + (σb)^2 = (σv)^2

where σv = von mises stress
Let's say after my analysis, I get σv = 208MPa

Given the yield strength is 210MPa, and use the safety factor of 1.5

210MPa / 208 MPa = 1.01, which is less than 1.5
The structure is yielded.

Is it correct?

I forgot what I learn in 2nd year.

Thanks

 

[Drej]

Where to start?

Not sure why you would want to calculate VM stresses by hand, surely the code will do this for you. Also not sure what you mean by obtaining "max stress" and using these to calculate VM stress. You need to make sure you are using the correct stress components if you do want to obtain VM by hand.

I think your understanding of a safety factor is slightly misguided, and you are confusing this with the state of yield of the structure - although these are related in structural analysis, they are two completely separate entities. VM stress is used to predict yield. The safety factor gives you a margin against some state of loading (in the case above, the state of stress caused by the loading compared directly to the yield). May be worth digging out your books and refreshing your memory.


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[rb1957]

your structure hasn't yielded in reality, but you haven't clearly your required safety factor or it's failed by definition.

i'd look into your vM equation. i think you're calc'ing by hand cause your FEA doesn't give you vM for bars.

i'd suggest using max principal for a bar.

 

[jagad5]

Is von Mises the right stress to look at? Is the material ductile or brittle? Using the wrong failure criterion can negate everything else you do.

Doug

 

[BlasMolero]

Dear Steve,
FEMAP allows you to temporarily Show Stress data (8 different types, including vonMises stress) as a contour plot directly on the cross-sections of bar and beam elements. FEMAP calculates the stress data "dynamically" using a combination of element force data from Output Vectors which are typically imported from an analysis involving beam elements. Cross-section stress data can be plotted on a single element or multiple elements, then modified using a number of different Location, Output From Vectors, and Advanced options.

All options are specified using the Beam Cross Section Stress Control dialog box ("View, Advanced Post, Beam Cross Section..." command):

The Show Stress command allows you to specify which type of stress FEMAP should calculate from the vector data. The 8 options are von Mises Stress, Maximum Shear Stress, Maximum Principle Stress, Minimum Principle Stress, Axial Stress, Y Shear Stress, Z Shear Stress, and Combined Shear Stress. The type selected in Show Stress will persist until changed or FEMAP is closed. The default stress type for new models is Axial Stress. When the Show Stress option is set to Combined Shear Stress, check the Vector Plot option to view the stress as vectors instead of a contour plot. There are several options under Advanced which may be used to create the desired Vector Plot.

Here you are a plot of vonMises stress of your bike:

To learn more about how to use FEMAP & NX NASTRAN regarding plotting results in CBEAM elements, take a look to my blog in the following address:

http://iberisa.wordpress.com/2011/04/18/offset-en-elementos-cbarcbeam/

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

IBERISA
48011 BILBAO (SPAIN)
WEB:

http://www.iberisa.com

Blog de FEMAP & NX Nastran:

http://iberisa.wordpress.com/