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  • Answer

    Line releases are defined by an additional line automatically generated by the software. The theoretical background of line releases is a also described in this article.

    For a detailed explanation of this alert a model is attached to this FAQ. 

    In this model for the line release 3 a definition node has to be defined. Because if the software would generate an additional virtuall node surface 2 would be destroyed. 
    Imagine the following sceanrio.
    You have 4 surfaces 1,2,10,14 (picture 2).
    If you define a line release to line 3 it`ll look like shown in picture 3.
    As the surface 2 would be destroyed by that as line 6 is the same for surface 2 and 14 the line 6 has to start at the node 3 and at the generated node (picture 4).
    This is where you need the definition node and in case it is not defined the software gives the alert.

  • Answer

    The differences can be explained with an example (see figure) that compares the results of a nonlinear calculation with those of a linear calculation. The model file can be found below Downloads .

    The left and the middle model have linear linear hinges. In the left model, a slip is defined in u x and u y , in the middle model only in the direction u x . In the right model, a slip in u x and u y is defined, but a pressure contact between the surfaces is still specified. Thus, these surfaces can not penetrate to the compression.

    When checking the results, it can be seen that the left model is too soft: Due to the flexibility, the surfaces penetrate into u y . The mean and right model, however, provides identical results.

    It often makes sense to remain as linear as possible when calculating results. Therefore, the center model is the most favorable for numerical analyzes, even if the right (nonlinear) model represents reality even better. However, the example is only a small comparison model. In reality, the spring in u y / u z would probably not be applied, but the force would be assigned to a tie rod. For this reason too, the middle model without u y is recommended.

    The consideration of the flexibility between surfaces is also discussed in an article of our Knowledge Base, which you can access under Links below.

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