There is nothing in Mechanical for this. Even in Excel, curve fitting is not entirely automatic. You have to choose curve type, such as logarithmic, exponential, or polynomial, etc... For polynomial, you then must choose an order (2-6). We do have some curve fitting in the Engineering Data for material models to fit test data. But you wrote that this is for displacement. If this is a displacement load you can define by equation with a variable of "time":
If you are trying to curve-fit displacement or deformation output data, you could do it by script. You could do it in the "Automation > Scripting" console in Mechanical or insert a "Python Code" object under Solution. Your code would have to get the displacement data either from a native result in Mechanical or get the raw data, and your code would have to handle the curve fitting. Or you could just use a native deformation result in Mechanical, and right click on it to "Export > Export Text File" and then do the curve fitting in Excel.
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