  Nanomechanical measurements of minimally twisted van der Waals materials
remained elusive despite their fundamental importance for device realisation.
Here, we use Ultrasonic Force Microscopy (UFM) to locally quantify the
variation of out-of-plane Young's modulus in minimally twisted double bilayer
graphene (TDBG). We reveal a softening of the Young's modulus by 7\% and 17\%
along single and double domain walls, respectively. Our experimental results
are confirmed by force-field relaxation models. This study highlights the
strong tunability of nanomechanical properties in engineered twisted materials,
and paves the way for future applications of designer 2D nanomechanical
systems.
