  We investigate the delayed rupture of biopolymer gels under a constant shear
load by simultaneous dynamic light scattering and rheology measurements. We
unveil the crucial role of normal stresses built up during gelation: all
samples that eventually fracture self-weaken during the gelation process, as
revealed by a partial relaxation of the normal stress concomitant to a burst of
microscopic plastic rearrangements. Upon applying a shear stress, weakened gels
exhibit in the creep regime distinctive signatures in their microscopic
dynamics, which anticipate macroscopic fracture by up to thousands of seconds.
The dynamics in fracturing gels are faster than those of non-fracturing gels
and exhibit large spatio-temporal fluctuations. A spatially localized region
with significant plasticity eventually nucleates, expands progressively, and
finally invades the whole sample triggering macroscopic failure.
