Dear Dr Dhondt,
this type of element can be very useful, I often give it a try in various tests and comparisons. Here’s one such example: Elasto-plastic bending - elements, accuracy
I’ve also seen several other threads on this forum where people used C3D8I elements in their studies.
It would be great to have an improved version of these elements implemented in CalculiX.
Best regards,
Calc_em