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Hi, the are still some issues in my model for simulating the cyclic performance of a RC column,can you give me some advise?
（1）Why is the stress value of the concrete in the post-processor much greater than the peak strength of the input concrete constitutive? For example, in my model，the peak strength of concrete constitutive is 22.8Mpa, while the maximum stress of concrete in the post-processor reaches 114Mpa. Is it all because of the triaxial compression effect?
（2）When I used a 50mm mesh size, the model worked fine, but when I considered 25mm or a mixture of 25mm and 50mm, the model either didn't converge or the result was strange. Can you explain why that is?
（3）Why is the material damage greater than 1 in the post-processor, and the damage is also negative?
I would appreciate it if you could take a look at my model!my model is listed below.

（1）Why is the stress value of the concrete in the post-processor much greater than the peak strength of the input concrete constitutive? For example, in my model， the peak strength of concrete constitutive is 22.8Mpa, while the maximum stress of concrete in the post-processor reaches 114Mpa. Is it all because of the triaxial compression effect?

Yes, it's the triaxial compression effect, necessary to simulate the increased strength under lateral confinement

（2）When I used a 50mm mesh size, the model worked fine, but when I considered 25mm or a mixture of 25mm and 50mm, the model either didn't converge or the result was strange. Can you explain why that is?

Ideally the model should behave similarly because you are using automatic regularization. Note, however, that fracture energy regularization is only "scaling" the fracture energy to make the dissipated energy independent of the mesh size.
However, it is not perfect and the process of cracking will always be very sensitive to mesh size.
Can you try with the ASDConcrete3D (using default parameters for 1P). It also allows the use of crackPlanes for better capturing the cyclic effects. It should be more stable.

（3）Why is the material damage greater than 1 in the post-processor, and the damage is also negative?

It's just because you are visualizing damage with a surface plot. In a surface plot you see the "extrapolation" of the values from the gauss points to the nodes. Since the gauss points are in the interior of the element, doing this linear extrapolation may give damage lower than 0 or higher than 1.
If you want to see the real value, use the gauss plot.