The short answer is â€˜Yesâ€™.Â If you are referring to one of the standard strain energy density functions (See Section 4.6 of the MAPDL Material Reference Guide), you can enter the coefficients that define a specific function, via TB,HYPER,,, and TBDATA,,, commands.Â This is assuming you already have these coefficients.Â Often folks have test data, but not coefficients. Hence, curve fitting is necessary to derive the coefficients for the strain energy density function of interest.Â Â If you are interested in a strain energy density expression that is not one of these standard options, then, you need to write your own routine via USERHYPER or USERMAT.Â However, even with a custom routine, you are probably still going to define the coefficients via TB,hyper,,,,user or TB,user,,,,Â to complete the customized expression.