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The von Mises Stress is the default failure criterion. If a Target Stress scaling in build direction is entered, the FFF Thumb Rule is used.
Safety Factor
The Safety Factor calculates the goal for the optimisation in regard to the material properties. This works for all Failure Criteria.
The objective of the optimisation is to load all material with the calculated values from the material properties and the Safety Factor to reach a Failure Criterion value of 1.
Failure Criteria
For each optimization and iteration a failure criterion is calculated and based on this value the design is formed. Which failure criterion can be used depends on the material properties, manufacturing method and the optimisation intention.
Von Mises Stress*
The equivalent von Mises Stress is the default Failure Criterion. It can be used for all material properties (Isotropic, 3D Transversely Isotropic and 3D Orthotropic). The Stress Goal is calculated with the chosen Safety Factor and the tension strength in axis direction (build direction) of the entered material. For directional dependent stress limits, the von Mises Stress is not suitable and one of the following Failure Criteria should be used.
Scale Target Stress in build direction
With the Scale Target Stress in build direction, a simplified version of the directional dependent Tsai-Hill Failure Criterion can be used to take directional dependencies into account. The Scale Target Stress in build direction can be used in combination with Isotropic material behaviour. Thus, the Tension Strength in build direction can be modified by scaling only this one value. The Shear Strength is calculated automatically with a Thumb Rule [0.6 * Tension Strength]. This is especially useful for manufacturing methods like FFF where the material limit in building direction differs from the transversal in-plane directions.
Stresses & Failure Criteria in Combination with the Resolution
The used Stress Goal / Failure Criterion is dependent on the resolution level during the optimisation. The optimisation runs on different resolution levels and switches between them to achieve the best results. The first iterations are calculated on a coarse resolution level. With rising number of iterations, the resolution gets finer. This means that the first iterations calculate much faster than the last iterations but aren’t as detailed as the last. With the increasing resolution, the size of the output data, the calculation time as well as the surface quality increase.
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