  We investigate the influence of microstructural traps on hydrogen diffusion
and embrittlement in the presence of cyclic loads. A mechanistic, multi-trap
model for hydrogen transport is developed, implemented into a finite element
framework, and used to capture the variation of crack tip lattice and trapped
hydrogen concentrations as a function of the loading frequency, the trap
binding energies and the trap densities. We show that the maximum value
attained by the lattice hydrogen concentration during the cyclic analysis
exhibits a notable sensitivity to the ratio between the loading frequency and
the effective diffusion coefficient. This is observed for both hydrogen
pre-charged samples (closed-systems) and samples exposed to a permanent source
of hydrogen (open-systems). Experiments are used to determine the critical
concentration for embrittlement, by mapping the range of frequencies where the
output is the same as testing in inert environments. We then quantitatively
investigate and discuss the implications of developing materials with higher
trap densities in mitigating embrittlement in the presence of cyclic loads. It
is shown that, unlike the static case, increasing the density of "beneficial
traps" is a viable strategy in designing alloys resistant to hydrogen assisted
fatigue for both closed- and open-systems.
