  We study with molecular dynamics simulations of a generic bead-spring model
the cyclic crazing behaviour of glassy polymers. The aim is to elucidate the
mechanical response of sole fibrillated craze matter as well as its interaction
with bulk material. The macroscopic stress response exhibits a hysteresis,
which is quasi stationary after the first cycle and largely independent of
deformation rate and temperature. It results from a complex interplay between
constraints imposed by the entanglement network, pore space and pore space
closure. Once the craze fibrils are oriented, stretching of the covalent
backbone bonds leads to a rapid stress increase. In the initial stages of
unloading, a loss in entanglement contact yields a quick stress relaxation in
the backbone. During unloading, the craze fibrils undergo a rigid body (i.e.\
stress-free) folding motion due to the surrounding pore space, so that the
structural behaviour of craze fibrils during unloading is most accurately
described as string-like. The reloading response depends significantly on the
degree of pore space closure and the enforced intermolecular interaction during
unloading. It ranges from a linear stress increase to a re-cavitation with a
re-drawing response. Compared to the bulk stiffness, the craze stiffness is two
orders of magnitude lower and as a result, the macro response of coexisting
craze and bulk matter is governed by the sole fibrillated craze matter.
