That is, hydrogen is driven by three forces to
an area of lower concentration, colder temperature,
and higher tensile stresses. Because DHC
may occur at low temperatures in a matrix of
uniformly distributed hydrogen, its mechanism
is believed to be as follows: Stress gradients at
stress concentration sites attract hydrogen,
resulting in local hydride precipitation, growth,
and reorientation. When the growing hydride
reaches some critical size, the hydride either
cleaves, or the hydride-matrix interface opens
up to nucleate a crack. Once a crack has nucleated,
propagation occurs by repeating the same
process at the crack tip and, as such, is a discontinuous
process. It should be noted that the formation
of hydrides is not a necessary requirement
for this mechanism to operate, as is the
case in delayed hydrogen embrittlement in
high-strength steels.