Coupled Failure Criteria

Technical Article

If a bending load of a brittle beam element (an unreinforced concrete beam) is increased by means of the bending capacity, the structure responds by breaking the cross-section, and the member is separated into two segments. At the time of the failure, the broken part suddenly loses its potential to transfer the bending moment. Due to the segmentation, the critical part also fails to transfer the other force types, such as axial forces.

Looking at the broken cross-section, this situation seems to be plausible and close to reality. However, there are still several challenges when simulating this failure matter in a FE analysis environment. RFEM is ready to handle such tasks, providing nonlinear release elements for the implementation of the failure location by using various load levels.

Release Elements

The program environment of RFEM provides nodal, line and surface releases under Model Data. A release in the model always describes the properties between two groups of structural components at the selected contact point. The properties between the component groups can be defined with regard to the setting of a members end release for each direction of degrees of freedom (x, y, z, φx, φy, φz). In addition to a complete release of the direction up to a nonlinear deformation behavior, the releases provide a wide range of options for defining the properties.

Figure 01 - Properties by Release Direction

In general, all options except for the nonlinearity setting "Coupled diagram - permanent release" only respond in the assigned direction. In the case of the coupled release, the degrees of freedom can be linked together. The linkage of the degrees of freedom refers to the "Tearing" failure condition of the respective work diagram. If the deformation reaches the failure point of the assigned diagram at the respective contact side in a specific direction, the connection in the actual direction as well as in all the other directions with the "Coupled diagram - permanent release" setting also fails at this location. In the case of the coupled failure, the degrees of freedom in the x- and y-direction apply the failure point in the first and third quadrants of the diagram, and the z-direction apply the failure point in the first quadrant.

Figure 02 - Coupled Failure

The release elements display no coupling forces. The force transfer results from the discontinuity between the adjacent elements.

Application

Open the release elements using the menu "Insert" -> "Model Data" -> "Nodal, Line, Surface release". After specifying the contact point (Node, Line, or Surface No.), contact property (Release Type), and which elements belong to the adjacent group (Released Objects), the elements can be transferred to the model. The program then copies the contact point in the background and assigns this generated definition to the selected adjacent elements (released elements). The elements on the other adjacent side remain as they are. Between the originally defined contact point and the copied contact point, the defined contact property is determined.

Figure 03 - Release Element

With this function, RFEM opens up a completely new field of application: In the midst of supporting structures consisting of members, shells, and solids, it is possible to simulate discrete connections such as cracks, pressure contact, brittle joints, etc.

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RFEM Main Program
RFEM 5.xx

Main Program

Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements