I've tried everything, It seems I can't obtain a perfect exponent of 1.5. If you think about it, it's reasonable since I'm simulating a real system with intrinsic computational limitations (finite mesh size, solving algorythms, etc) and I can't expect to obtain a perfect agreement with theoretical models. For my purposes, at the end, I obtain a reasonable value for the Young's Modulus within error of 2%.
One question: is it possible to use the Adaptive Convergence tool with probes? I want to track the mash-based convergence of my reaction force, but It seems I can't add this tool under Force Reaction in Solution section.