I am trying to use the Cast Iron Material model to simulate different yield strengths in Tension and Compression. My Tensile strength is higher then my compressive strength. The stress-strain curve in /POST26 does not match the curve defined in the material law! It seems that the compression curve is used in the tension case? Is this a bug?

No. When you defined the tensile strength as higher then the compressive strength, you essentially shifted the composite (Rankine-Von Mises) yield surface (illustrated in Figure 4.10 of Theory Manual) to the right such that there now exists a subset of stress states in tension that will yield in accordance with Von Mises criteria. If the strain becomes large enough in tension, then max principle stress again becomes the yield criteria. Your material data does not represent gray cast iron.

This exercise underscores what is already documented in the Theory Manual regarding the Cast Iron model.
"The cast iron plasticity model is designed to model gray cast iron. The microstructure of gray cast iron can be looked at as a two-phase material, graphite flakes inserted into a steel matrix. This microstructure leads to a substantial difference in behavior in tension and compression. In tension, the material is more brittle with low strength and cracks form due to the graphite flakes. In compression, no cracks form, the graphite flakes behave as incompressible media that transmit stress and the steel matrix only governs the overall behavior."

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