I thought it might be a good idea to provide some basic definitions of terms that are used in analysis that are good to know to be able to communicate intelligently about Simulation. These are not meant to be definitive technical definitions but more fundamental knowledge of these terms (i.e. they are mostly coming off the top of my head as I'm typing). If you believe that I am off on any of these, please let me know.

Last time two of the terms that I gave were von Mises and shear stresses; there's many more components of stress than that. Here's 12 to 18:

**12. normal stress**- if you were to take a look at an infinitesimally small cube oriented in the global coordinate system, the normal stress is the stress that develops normal to the face. Normal stress is denoted by the Greek letter sigma, σ, and subscript is the face that it is normal to, eg. σx. From last time, shear stress is defined as the stress that is tangential to the face, usually denoted by a Greek tau, τ, and subscript is the face and then the direction of shear, for example τxz is on the x-face in the z-direction. Thus for a given cube face, there is a single normal stress and two shear stresses. Both normal and shear stresses are vectors, having direction and magnitude, and since the proposed cube we are looking at is infinitesimally small, the stresses on opposing faces are equal in magnitude but in the opposite direction. Thus for a given cube, there are a grand total of 3 normal stresses and 6 shear stresses. The below image can be helpful in describing the stress vectors:

**13. shear modulus**- primarily denoted as G, is the ratio of shear stress to shear strain. For a linear isotropic material, it is useful to know that any 2 moduli can determine any other elastic moduli (a full matrix can be found at the bottom of http://en.wikipedia.org/wiki/Elastic_modulus). G = (E/2)/(1+ν), where E is Young's modulus and ν is Poisson's ratio. In SW SIM, if you define E and ν for a linear isotropic material, I recommend keeping shear modulus blank and the software will compute it by itself (if you put in a value it will use that one instead so again best to keep it blank).

**14. principal stresses**- at any point in a stressed body (in this case let's take the same point as given above), there exists three planes where there are no shear stresses and the normal stress vectors to these planes are in "principal" directions, and are denoted σ1, σ2, and σ3, (or in SW SIM we name them P1, P2 and P3). The normal and shear stresses from the stress cube above go through a matrix transformation (resulting in a matrix where all off-axis terms are zero) to obtain the principal directions, which are orthogonal to one another meaning that they are completely independent directions. It is always true that P1 ≥ P2 ≥ P3. In an pure tensile load, all three principal stresses are positive; in a pure compressive load, all 3 are negative; and when the signs of the principal stresses are mixed (P1 is positive and P3 is negative), then this usually means the body is in bending. I typically recommend to set your Simulation options to always show P1 and P3 stress components in addition to von Mises stress.

**15. bending stress**- Thinking of shell elements, they tend to bend easily because they are thin. During bending, compressive and tensile stresses develop; the maximum value of these stresses are on opposite sides of the outermost thickness of the shell, and the stress varies linearly through its thickness. The bending stress is the slope of the line between compressive and tensile stresses through the thickness of the shell.

**16. membrane stress**- is the stress in the plane of the shell that develops if the compressive and tensile stresses have different magnitudes. If the tensile stress is greater, then the membrane is pulled along the direction of the shell plane; if the compressive stress is greater, then the material in the membrane is compressed.

So in conclusion for a shell, show P1 & P3 for both top and bottom of the shells and both the membrane and bending stresses.

**17. torsional stress**- is a stress due to a twisting of an object due to an applied torque. It is a type of shear stress with an specific equation Tr/J to calculate it by hand based on the applied torque T, the radius r from the axis of torque, and the object (typically a cylindrical shaft) with a polar moment of inertia J. The max torsional stress is on the outer max radius, or surface, of the shaft.

**18. Hertzian contact stress**- the compressive stress due to contact of two curved parts under load. The important thing to note for Hertzian stresses is that the max stress occurs close to the area of contact in the subsurface, or just below its surface, thus the mesh needs to be finest below the surface so creating a mesh control just inside the sphere near the contact is important. Another important thing to know is that it is very difficult to get an accurate Hertzian contact stress result.

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Great review on SIMULATION Definitions !

Thanks to Joe / Richad Doyle

ROBERT CRUZ