You have glued the gauges in the ~45% and ~55% of the beam and not where stress from bending is the highest, why is that so?
I thought that the bending would be quite uniformly and therefore I've glued them approximately in the middle of the crankarm
I do not want to be picky but that is a compound stress state.
The beam is subjected to bending and at the same time to shearing (that is "beams 101"). And because the foot is not pushing in the plane of the rotation, there is also twisting.
Are you sure that in your arrangement:
Uout=0.98*bending+0.01*twisting+0.01*shearing
?
Did you do any calculations or was it "lets stick it in the middle and see the eevblog comments"?
And indeed in case of "pure bending 101" the stress in a beam of constant cross section the strain is uniform along the beam and the center is as good as any other place for sticking the gauge. But that is not in your case.
EDIT: Any thoughts on why this wrench does not have a "uniform" cross section:
wrench ?