Safety Factor, Stresses, Failure Criteria & Resolution

For each optimization and iteration, a Failure Criterion is calculated which represents the goal of the optimisation and based on which the design is generated.

A Failure Criterion value of 1 is always the objective of the optimisation. Two different types of Failure Criteria are available (von Mises Stress, FFF Thumb Rule).

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.

FFF Thumb Rule

With the FFF Thumb Rule, a simplified version of the directional dependent Tsai-Hill Failure Criterion can be used to take directional dependencies into account. The FFF Thumb Rule 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.

For each resolution level a different Safety Coefficient is defined. The Safety Coefficient is a value between 0-1 and is multiplied with the chosen Stress Goal / Failure Criterion. On the lower resolution levels the value is beneath 1 and on the highest resolution level exactly 1. Thus, on the lower resolution the optimisation target is scaled to a lower value. On the highest resolution level, the requested Stress Goal / Failure Criterion reaches 100% and thus satisfies the defined Stress Goal / Failure Criterion as the geometry allows it.

Open

In one optimisation several Events (load cases) can be considered. Each Event is calculated by itself and the stresses / Failure Criteria of all Events are enveloped and the highest value for each area is considered. For information regarding the Event Specific Safety Factor have a look here.

The Stress / Failure Criterion calculation is from an FE point of view an estimation. That’s why we recommend a FE-Reanalysis using Nastran to verify the results regarding all boundary conditions. For the reanalysis the Nominal-Geometry should be chosen which will be used after manufacturing and post manufacturing processes. The Reanalysis can also take dynamic loading & other boundary conditions into consideration that are not considered in the design process.