  We investigate strong nonlinear damping effects which occur during high
amplitude oscillations of neutron stars, and the gravitational waves they
produce. For this, we use a general relativistic nonlinear hydrodynamics code
in conjunction with a fixed spacetime (Cowling approximation) and a polytropic
equation of state (EOS). Gravitational waves are estimated using the quadrupole
formula. Our main interest are l=m=2 f modes subject to the CFS (Chandrasekhar,
Friedman, Schutz) instability, but we also investigate axisymmetric and
quasiradial modes. We study various models to determine the influence of
rotation rate and EOS. We find that axisymmetric oscillations at high
amplitudes are predominantly damped by shock formation, while the
nonaxisymmetric f modes are mainly damped by wave breaking and, for rapidly
rotating models, coupling to nonaxisymmetric inertial modes. From the observed
nonlinear damping, we derive upper limits for the saturation amplitude of
CFS-unstable f modes. Finally, we estimate that the corresponding gravitational
waves for an oscillation amplitude at the upper limit should be detectable with
the advanced LIGO and VIRGO interferometers at distances above 10 MPc. This
strongly depends on the stellar model, in particular on the mode frequency.
