  Soft materials may break irreversibly upon applying sufficiently large shear
oscillations, a process which physical mechanism remains largely elusive. In
this work, the rupture of protein gels made of sodium caseinate under an
oscillatory stress is shown to occur in an abrupt, brittle-like manner. Upon
increasing the stress amplitude, the build-up of harmonic modes in the strain
response can be rescaled for all gel concentrations. This rescaling yields an
empirical criterion to predict the rupture point way before the samples are
significantly damaged. "Fatigue" experiments under stress oscillations of
constant amplitude can be mapped onto the former results, which indicates that
rupture is independent of the temporal pathway in which strain and damage
accumulate. Finally, using ultrasonic imaging, we measure the local mechanical
properties of the gels before, during and after breakdown, showing that the
strain field remains perfectly homogeneous up to rupture but suddenly gives way
to a solid-fluid phase separation upon breakdown.
