OK, so I took a bit of a hiatus from the blog. I'll try to make this up somehow.

During this time away, 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).

I have a list of about 150 good to know analysis terms that I want to (hopefully) get through all of them eventually, so here we go to start 1-11:

**1. FEA (or finite element analysis)**- sometimes referred to as FEM (M meaning method). The method used by the structural analysis modules of SW SIM by breaking the CAD models into smaller pieces, called elements, on which the physical properties, loads and restraints are applied and finally solved collectively.

**2. Pre-processing**- what is done in setting up the analysis model prior to solving. This may include: creating the geometry; simplifying it for analysis purposes; applying material properties; applying loads, restraints, connectors and contacts; and meshing the model. This is basically formulating a question; if this is not done appropriately, people may refer to this as GARBAGE IN, which leads to GARBAGE OUT (in the post-processing phase).

**3. Solving**- takes the input by the user from the pre-processing phase, puts it into a form preferred by the solver, and calculates a solution for the question. The solution is typically very precise, but whether it is accurate is left for interpretation in the post-processing phase.

**4. Post-processing**- or viewing and evaluating the results from the solver. There are many methods available to view the results, such as contour plots, section/cut plots, probing, tables or listing of values, and charts. Evaluating results can be challenging and can be helped by experience and judgement, but I think that answering the question "Does this move or react to the loads as expected in reality?" Thus determining whether the output is good or not (GARBAGE OUT), we can say whether we need to go back to the pre-processing step or not. If accurate results are required then more than one solution for finer mesh sizes will be required. Accurate results may not be needed if one is comparing different design configurations where consistency is more important.

**5. Modulus of Elasticity**- also known as Young's Modulus. It is a material property relating the stress in the material to how much it is strained, and is typically obtained by pulling a sample of the material in a testing machine. It is a linear ratio of stress over strain, so it has the same units of stress (psi, ksi, Pa, MPa) since strain is without units, and is valid up to the point of yielding in the material. Materials which are linear-elastic follow Hooke's Law.

**6. Hooke's Law**- Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit. Materials for which Hooke's law is a useful approximation are known as linear-elastic or "Hookean" materials. Hooke's law in simple terms says that strain is directly proportional to stress. (Definition taken directly from Wikipedia.)

**7. von Mises stress**- or equivalent (tensile) stress. The von Mises stress is meant as a way to try to fully describe the multiaxial stress state as a positive scalar value, which also makes it nice to show as a contour plot. It has its downfalls in that: it doesn't tell the whole story in how a part is being stressed, thus one should not rely on this quantity alone to get the full picture, so show additional stress components such as principal, normal and shear stresses; and secondly it's based on what's called 'distortion energy' simply meaning that it's good for deformations that distort the geometry, like pushing a small area on the outside of a sphere, and ignores uniform deformation (or hydrostatic stress), like a uniform pressure on the entire outside of the sphere. Note that von Mises yield criterion surface circumscribes (fully envelops) the Tresca max shear stress criterion surface, thus von Mises is less conservative.

History buffs might like to know that while it primarily carries von Mises' name, it was formulated by Maxwell many years before and others, so the entire mouthful name for the stress criterion is

*Maxwell-Huber-Hencky-von Mises*theory (which I'm guessing was just shortened to 'von Mises.')**8. Shear stress**- is the stress applied tangential to a face of a material, as opposed to normal to the face.

**9. Poisson's ratio**- symbolized by the Greek letter nu, ν, is the ratio of how much a material contracts in the direction perpendicular to the direction pulled, or transverse direction. It describes similarly how it much it expands transversely when compressed. The Poisson's ratio of an isotropic, linear-elastic material must be -1<ν<0.5, but most materials are greater than 0. Orthotropic materials can have Poisson's ratios outside of these limits. Rubber materials have a Poisson's ratio very close to 0.5, such as 0.4999, and cork has a Poisson's ratio of nearly zero which is why it's used for sealing bottles so that it can be inserted and removed while still holding the internal pressure. Negative Poisson's ratio materials are called auxetic, and here is a cool animation of an example.

**10. CFD (or computational fluid dynamics)**- CFD is a general term applied to the approach to solving the fluid dynamics equations numerically with a computer, as opposed to experimental or analytical methods. The method that SW Flow Simulation uses is the finite volume (FV) method. Our SW Flow Simulation software is classified as a CFD program, although Flow Simulation (and FloWorks before it) helped pioneer a subset of CFD called EFD.

**11. EFD (or Engineering Fluid Dynamics)**- is an upfront approach to CFD that offers a straight-forward easy-to-use interface that speaks the language of the design engineer working with fluids. Key technologies include: direct use of SolidWorks CAD data; automatic detection of fluid volume; Wizard interface; automatic meshing; automatic laminar-transitional-turbulent modeling; automated control of analysis runs; robust convergence behavior; simulation of design variants; and results reporting and presentation in MS Office.

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Do not distribute or reproduce without the written consent of Dassault Systèmes SolidWorks Corp.

Do not distribute or reproduce without the written consent of Dassault Systèmes SolidWorks Corp.

Thanks for sharing the terminology on Basic Simulation Definition.

Very handing as a conceptual reminder.

Thanks Joe Galiera / Richard Dole for blog that one.

ROBERT CRUZ