Abstract :
Whatever the external loading, a crack
front in a solid tries to reach mode I loading conditions
after propagation. In mode I+II, the crack kinks
to annihilate mode II, kinking angle being well predicted
by the principle of local symmetry (PLS) or by
themaximum tangential stress criterion (MTS). In presence
of mode III, the problem becomes three-dimensional
and the proposed propagation criterion are not
yet well proved and established. In particular in three
point bending experiments (3PB) with an initially
inclined crack, the crack twists around the direction of
propagation to finally reach a situation of pure mode I.
The aim of the paper is to compare the propagation
paths predicted by two different criteria for 3PB fatigue
experiments performed on PMMA. The first criterion
developed by Schollmann et al. (Int J Fract 117(2):129-
141, 2002), is a three-dimensional extension of the
MTS criterion and predicts the local angles that annihilates
mode II and III at each point of the front.
The second one developed by Lazarus et al. (J MechPhys Solids 49(7):1421-1443, 2001b), predicts an
abrupt and then progressive twisting of the front to annihilate
mode III. Due to presence of sign changing mode
II and almost uniform mode III in the experiments, both
criteria give good results. However, since mode III is
predominant over mode II in the case under consideration,
the global criterion gives better results. Nevertheless,
the local type criterion seems to be of greater
universality for practical engineering applications
