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• ### It is not possible to enter a nonlinear stress-strain curve. The diagram initially appears error-free, but then the error message comes with the note that only positive values have to be entered in ascending order. How can I avoid this message?

New FAQ 003559 EN

When using a diagram in the program, the first strain is always given. (Initial strain). It depends on the output modulus of elasticity and cannot be controlled directly. There is a trick in the program to nevertheless adjust the first strain to the desired value.  To do this, you have to calculate the initial modulus of elasticity and enter it in the material parameter. In this case, the following procedure would be used.

• ### How does the calculation of models with plastic materials work?

FAQ 002992 EN

Finite elements with plastic material are divided into 10 layers. First, a normal elastic analysis is performed in the first iteration. Then, the stress in each element is calculated according to the set strength hypothesis in each individual layer. If the limit stress in one of the layers is exceeded, the stiffness of this layer is reduced. Based on the reduced stiffnesses of the 10 layers, an overall stiffness is determined for each element. With this new stiffness, we start a new calculation iteration.

Iterates until the changes are only small.

The total stress is converted into the stresses of the individual layers by means of the laminate theory. This theory is also applied for the conversion between the stiffnesses of the layers and the total stiffness.

4 different strength models can be used as the limit stress:

• Shape Change Energy Hypothesis (von Mises-tension)
• Shear stress hypothesis (Tresca stress)
• Failure hypothesis according to Drucker-Prager
• Failure hypothesis after Mohr-Coloumb

The choice of the failure hypothesis is shown in Figure 1.

The von Mises hypothesis is preset because it is the most frequently used strength theory.

• ### When using nonlinear material, I get an error message, what should I do?

FAQ 002874 EN

With RF-MAT NL, we offer the use of various nonlinear material models. If members, surfaces, and/or solids are combined in RFEM, the nonlinear material models must be assigned individually for members (1D) and surfaces and solids (2D/3D).

Members as 1D elements can not be assigned a nonlinear material model (2D/3D).

• ### Is it possible to set a plastic calculation of the internal forces in RSTAB or are the internal forces exclusively calculated elastically?

FAQ 002740 EN

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?

FAQ 002703 EN

When calculating nonlinear material models (especially for plastic material models), the calculation is always performed in relation to the original constant internal forces. Therefore, the smoothing of FE elements should be set to "Constant" (see Figure 01).

Furthermore, the stresses are related to the principal stresses $\ sigma_1$ or $\ sigma_2$.

Even taking these conditions into account, deviations from the defined limit stress may occur. They usually result from the defined solidification modulus. This fact is explained in detail in the technical article, which you can access under Links below.

• ### Do I have to calculate according to the second-order or large deformation analysis when using plastic material?

FAQ 002615 EN

No, it is not absolutely necessary to calculate according to the second-order or large deformation analysis when using a nonlinear material model. The material nonlinearity is also considered in the case of the calculation according to the linear static analysis.

The calculation according to the second-order analysis or the large deformation analysis means that the equilibrium is set on a deformed structure. So it is geometric nonlinearity.

The difference between the second-order and large deformation is that large rotation may occur in the case of the large deformation analysis.

Thus, if there is no stability problem or if the stability problem is further analysed, the calculation according to the linear static analysis is sufficient.

• ### Is it possible to assign a nonlinear material to a contact solid? Which material is most suitable?

FAQ 002272 EN

Nonlinear materials are not allowed for contact solids. It is not possible to make a general statement regarding which material should be used. A contact solid can represent a rubber body or a steel plate. We recommend to select an isotropic material here which corresponds or is similar to the real material in its properties.

• ### When performing a plastic calculation displays, there are stresses displayed, exceeding the yield criterion. Have I done anything wrong?

FAQ 002005 EN

You are working in your file with plastic material behavior and have entered the yield criterion. Therefore, the stress in your model cannot be higher as the material starts to plastify when exceeding the stress. RF-STEEL provides an option to set the stresses to "constant in elements". For this, please click [Details] and [Options].