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• ### In an RFEM model with a plastic material model, how is it to be determined which plastic deformation remains after the relief?

New FAQ 003031 EN Calculation RFEM RF-LOAD-HISTORY

It is important that the material model "Plastic 2D / 3D" or "Plastic 1D" is used. How does it work ? Dlubal info tags shown.

• ### How can I define failure criterion s such as Tsai-Wu for orthotropic material?

If you want to define failure criterion s for orthotropic material you have to define a orthotropic plastic material in the Material Model (picture 1).

The yield criterion will be done according the Tsai-Wu criterion (picture 2).

By this Link you can find a full set of verification examples to this material model.

The material model itself is explained in several Knowledge base articles.
• ### I have purchased the add-on module RF-MAT NL, but I can not find it anywhere.

The add-on module RF-MAT NL is already integrated in RFEM 5. It is assumed that the following material models can be used:

Isotropic Plastic 1D
Isotropic Plastic 2D/3D
Isotropic Nonlinear Elastic 1D
Isotropic Nonlinear Elastic 2D/3D

Orthotropic plastic 2D
Orthotropic 3D plastic

Isotropic Masonry 2D

Isotropic damage 2D / 3D
• ### When trying to calculate a material from the "Aluminum" category in the RF- / EL-PL add-on module, I get an error message. What is the reason for this?

The RF- / EL-PL add-on module is designed only for the elastic-plastic design of steel cross-sections. As a result, you can only calculate materials from the "Steel" category in the add-on module.
• ### Is it possible to set a plastic calculation of the internal forces in RSTAB or are the internal forces exclusively calculated elastically?

In RSTAB, it is also possible to determine plastic internal forces. To do this, you have to select a plastic hinge as nonlinearity by using the member properties (see Figure 1). However, this does not include an interaction of the plastic internal forces. Another possibility is to set releases with nonlinear properties directly. However, you have to use this method to estimate beforehand where these joints form.

RFEM provides the option to select a corresponding material model (see Figure 2).
• ### Why do not the limit stresses when using a nonlinear material match the calculated stresses?

When calculating nonlinear material models, especially for plastic material models, the calculation is always performed in relation to the original constant internal forces.
Therefore, you should set the smoothing of the FE elements to "Constant."
Furthermore, the stresses are related to the principal stresses $\ sigma_1$ or $\ sigma_2$.
Even if all these factors have been taken into account, there may still be deviations from the defined limit stress. These usually result from the defined hardening modulus.

This fact is explained in detail for the technical articles listed below.
• ### In an older model in RFEM 4 or RSTAB 7, I designed a DuenQ cross-section or general cross-section in STEEL EC3. At that time, a cross-section class 1 or 2 was determined. Now I have opened the model in RFEM 5 and now the cross-section is only classified Class 3 or 4. What is the reason for this? Was the design more economical at the time?

The design of general cross-sections or DuenQ cross-sections is always performed elastically in STEEL EC3, which means according to Class 3 or 4. Since the conditions of interaction are restricted to some cross-sections, no plastic design is possible for any cross-section shapes. In RFEM 4 or RSTAB 7, however, it was possible to classify a cross-section as class 1 or 2, even though the subsequent design was carried out elastically.
To eliminate this ambiguity and to create consistency, the classification has also been changed to cross-section classes 3 and 4.
• ### The plastic stresses are not shown in DUENQ. What is the reason?

The plastic stresses are output exclusively in the representation type isosurfaces . The display type can be selected via the icon in the toolbar (see Figure 1) or in the Project Navigator - Show under Results -> Display type can be set.
• ### When using plastic material according to the second-order or large deformation analysis III. Expect Order?

FAQ 002615 EN General RFEM

No, when using a nonlinear material model, it is not absolutely necessary to determine according to Theorie II. Or III. To calculate order. The material nonlinearity is also considered in the calculation according to the linear static analysis.

The calculation according to Theorie II. Or III. Order means that the equilibrium is set on the deformed system. Thus, it is a geometric nonlinearity.

Difference between II. And III. Order is that at III. Also large rotations may occur.

Thus, if there is no stability problem or the stability problem is analyzed in another way, the calculation according to the linear deformation analysis is sufficient.

• ### What causes the option "Use selected method also for the calculation of all supported cross sections" in RF / STEEL plasticity?

For example, this check box enables the use of the Simplex method, also for I-sections, which would otherwise be designed automatically with the partial internal -angidivision process including the redistribution.

If the check box is clear, the simplex method or the partial internal local area method (depending on the selection) is only used if a general cross-section (for example of SHAPE-THIN) is available.

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