How does COSMOS define compressive strength

Discussion created by James Canney on May 16, 2007
Latest reply on Nov 4, 2015 by Hampus Johansson

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I have done an analysis using COSMOSWorks on a steel structure we are designing.

One of my colleuges has asked how COSMOS defines compressive strength of the material when it is not defined in the material properties.

The material we are using is AISI 304, from the COSMOSWorks material library. My VAR has been unable to help with this.

Any assistance or advise would be very much appreciated.

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Vince Adams May 16, 2007 6:53 PM
Hi James, a quick review of basic FEA might help here. COSMOSWorks doesn't define or even require "strength" information in a linear analysis and only requires Yield Strength in a nonlinear plasticity analysis. A Static analysis only requires Young's Modulus and Poisson's Ratio to complete the stress/deflection calculations. The failure strengths, Tensile & Compressive, are listed in the database only as a reference to you. The solver doesn't look at them.

So... in answer to your question, COSMOSWorks doesn't define Compressive Strength because it doesn't really care what it is to do its job. That said, 304 is an extremely ductile SS at room temps and ductile materials tend to have the same compressive properties as tensile properties (a characteristic called 'symmetry') While the data gnome who typed all these properties in many many years ago may simply have neglected to enter a compressive strength, it is equally likely that he assumed compressive strength was equal and opposite to tensile and left it blank intentionally. I opened 2 references I keep on my desk and neither list a compressive strength for 304.

Make sense?

Vince
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- Ranga Narasimhan @ Vince Adams on May 17, 2007 8:52 AM
  Vince,
  
  Thats was really helpful literature on COSMOSWorks.. Well.. where can i get docs on stuff which tells how COSMOSWorks correlates with Strength of materials.. Because, i thought CW makes use of SOM props.. but your
  last msg enlightened me..
  
  thanks
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  - Vince Adams @ Ranga Narasimhan on May 17, 2007 3:10 PM
    FEA is just another method of calculating response. If used and interpreted similarly, it correlates to other methods well. When presented with a case where FEA doesn't compare to hand calcs or test well, I can usually show that we weren't comparing "apples to apples."
    
    I've attached a couple of slides from a presentation on "Why Analyze?" comparing COSMOSWorks results to standard hand calcs where I made sure all the assumptions were identical. You can see the correlation was excellent. As problems become more real-world, identifying the variation between test and analysis starts to become more difficult. I guess that's where the"art" of analysis meets up with the science of engineering.
    
    Vince
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    Comparing FEA to HAnd Calcs 2.jpg68.6 KB
    Comparing FEA to HAnd Calcs 1.jpg60.0 KB
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Andrew Hallas May 18, 2007 12:25 AM
In any static linear analysis performed in a FEA program the program calculates deformation results first based on the applied loads and boundary conditions. Once knowing the deformed and undeformed shapes, strain is calculated and once knowing the strain results, stress results are calculated. In calculating the stress values based on the strain results only Young's Modulus (E) and Poisson's ratio are used. Yield stress values are only used in factor of safety's calculations when performing linear analysis problems.
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- 1-6IQYAK @ Andrew Hallas on May 18, 2007 3:49 AM
  In any static linear analysis young's modulus and poisons ratio information is enough to run the analysis.
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Hampus Johansson Nov 4, 2015 6:37 AM
Bump/borrow this thread :-)
Im currently facing a problem with high compressive loads on a plastic part between two metal sheets in a stone crusher machine. In order to get results that in some way close to reality I guess I can't go with only the E-modulus and poissons ratio since all loads will be compressive and for polymers the tensile modulus and compression modulus do differ quite a bit.
So wouldn't Cosmos / SW need a compression modulus to give a more accurate answer in this case?
Vince Adams
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How does COSMOS define compressive strength

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