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Principal Stress/Strain direction vector

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Guest0527211403

Guest0527211403

Mechanical
Apr 24, 2004
1,125
CA
Hi Folks,

I'm working on a plan to validate our FEA of a particular prototype with some strain gauge measurements.

In choosing the placement of one or more uniaxial gauges I'm trying to first find the principal stresses and their orientations as predicted by Solidworks Simulation.

I'm able to plot my principal strains as a vector plot, but for the life of me I can't make it tell me which direction the principal stresses are directed. Ideally I'd like the orientations of these vectors at a particular location, or the average direction over a small patch on the surface.

The fact that I can generate the vector plot leads me to believe that SW Simulation has this information somewhere, but all I can get using a list or probe is the magnitude of the principal stresses.

I can pull out the global csys normal and shear stresses and use a stress transformation to determine the orientation of the principal axes, but before I get into that I'd like to see if there's an easier way to pull this info out of Solidworks Simulation directly.

So, is anyone aware of how I might be able to list or probe the orientations of my principal stresses?
Thanks in advance
 
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Sorry, I used principal stresses/strains rather interchangeably in the above post. Primarily, I'm interested in the principal strains.
 
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Hopefully you are placing your strain gauges in areas of low stress gradient. I suppose a strategically placed pair of intersecting split lines will force the mesher to place a node at the intersection. That you should be able to probe.

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CSWP, BSSE
Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."
 
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Hi kellnerp,

That's a good point re: stress gradients, thanks.

I'm able to probe a specific location as you suggested, but my issue is that I'm only able to get the magnitude of principal strains, and not the direction in which they point (with respect to the global csys, for example).

My concern is that if I place the strain gauge in an orientation off of the principal axis (P1) at that particular point, I'll be introducing error into my measurement due to the resultant combinations of normal/shear stresses acting on the surface under the gauge.

If I'm wrong here, or I'm way off in my methodology, I'm very open to ideas. I'm doing a bit of "forensic engineering" on a prototype sitting on the shop floor for the purposes of improving the way we use our FEA tools to design our machines.

I can sort of "eyeball" the directions by looking at the vector plot, and come up with a fairly good guess as to which orientation I should place my strain gauge, but I'd like some numbers to back up my decisions regarding placement & orientation.

Thanks again

 
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Forgive me if this is a silly question, but is it possible to run Cosmos/M using a "regular" installation of Solidworks + SW Simulation? Looking through some past posts/FAQs on the subject, it looks like I need to set up a virtual machine and XP32. I can already hear our IT guy grumbling over that one.

I'm on a Win7 64 bit machine, running Solidworks 2011 with one floating license of Simulation Professional.

Thanks for the help
 
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I'm working on a project pretty much doing exactly the same thing as the original poster and have the same question: how to get Solidworks Sim to tell me the principal stress or strain orientation. It easily shows magnitudes for all nodes, but not direction.
justkeepgiviner: did you ever figure this out?
 
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justkeepgiviner said:
My concern is that if I place the strain gauge in an orientation off of the principal axis (P1) at that particular point, I'll be introducing error into my measurement due to the resultant combinations of normal/shear stresses acting on the surface under the gauge.
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Why not use a rosette gauge to determine the actual orientation of the stress and a strip gauge to confirm the gradient.

Rosettes

TOP
CSWP, BSSE
Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."
 
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Hi Folks,

An update on this project - I believe I've found a workaround, although I'm working on correlating back to the gauge measurements.

To get a measure of the strain in a particular direction I've created coordinate systems at the centre of a surface patch used to describe where I placed the Strain Gauge, with the X axis aligned with my gauge direction.

I then generated a plot of strains in the "X" direction using the coordinate system as a reference, and then created a probe at my csys location so that I can extract the value of the principal stresses in my known direction.

I also aligned the "Z" axis 90 degrees to my strain normal direction so that I can get an idea of the transverse strain impacting my measurements. In some locations (not all due to I/O limitations) I've applied biaxial gauges to directly check these transverse as well.

Vishay Tech Note TN-511 provides some useful information on the impacts of misaligning the straing gauges.
 
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On my project, I used a number of strain gages on the physical test sample (mostly rosettes since I didn't know the orientation of the strain).

But, I never did figure out how to get SW to tell me the orientation of the strain vectors. The above method sounds like it will work, but I would sure hope there's an easier way.
 
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In Cosmos/M you may use the command language to query particular nodes and then do computations on their results. Unfortunately, the API for simulation is rather limited and I don't know of a way to do it there.

TOP
CSWP, BSSE
Phenom IIx6 1100T = 8GB = FX1400 = XP64SP2 = SW2009SP3
"Node news is good news."
 
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Somewhere in my solid mechanics textbook (which is itself acting as a structural support for something on my desk at the moment so I can't open it) there's an equation for determining the principal stresses using the normal & shear stresses.

So if you stick a csys at the point in question, you can extract this information by probing the approprate normal/shear stresses, and feeding this info into excel to calculate the direction with respect to your csys.

My VAR suggested a visual basic macro if its something that has to be done repeatedly.

It ain't pretty, I know... I've submitted my enhancement request to let us probe this information directly.
 
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