  Recent laboratory experiments have revealed that destructive collisions of
icy dust particles may occur at much lower velocities than previously believed.
These low fragmentation velocities push down the maximum grain size in
collisional growth models. Motivated by the smooth radial distribution of
pebble sizes inferred from ALMA/VLA multi-wavelength continuum analysis, we
propose a concise model to explain this feature and aim to constrain the
turbulence level at the midplane of protoplanetary disks. Our approach is built
on the assumption that the fragmentation threshold is the primary barrier
limiting pebble growth within pressure maxima. Consequently, the grain size at
the ring location can provide direct insights into the turbulent velocity
governing pebble collisions and, by extension, the turbulence level at the disk
midplane. We validate this method using the Dustpy code, which simulates dust
transport and coagulation. We apply our method to 7 disks, TW Hya, IM Lup, GM
Aur, AS 209, HL Tau, HD 163296, and MWC 480, for which grain sizes have been
measured from multi-wavelength continuum analysis. A common feature emerges
from our analysis, with an overall low turbulence coefficient of
$\alpha\sim10^{-4}$ observed in five out of seven disks when taking
fragmentation velocity $v_{\rm frag} = 1{\rm \,m\,s}^{-1}$. A higher
fragmentation velocity would imply a turbulence coefficient significantly
larger than the current observational constraints. IM Lup stands out with a
relatively higher coefficient of $10^{-3}$. Notably, HL Tau exhibits an
increasing trend in $\alpha$ with distance, which supports enhanced turbulence
at its outer disk region, possibly associated with the infalling streamer onto
HL~Tau. The current (sub)mm pebble size constrained in disks implies low levels
of turbulence, as well as fragile pebbles consistent with recent laboratory
measurements.
