Solved
The symbolic does not know anything about units and treats them as unknown variables. Thats the reason why the symbolic can't decide, which variable is bigger. If MPa would be negative, the max would be sigma1, if MPa is positive, the max is sigma2 and if MPa is zero, both are equal (0).
You may tell the symbolic that MPa is positive with the "assume" modifier and also add the "simplify" modifier to get the correct numeric result.
But its easier to achieve using a simple numeric evaluation.
But in case you expect the symbolic answer "sigma2" instead of "300 MPa" - there is no way to achieve this answer.
You could use the if-function or an if-statement with a text result as a workaround:
In Prime use "else" instead of "otherwise".
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