Contrarily to the behaviour found in the material characterization
tests conducted over plain round specimens, V-notched specimens
exhibited a clear maximum in the torque versus twist angle
curve prior to the failure of the specimen. This result can be surprising
since because of the stress concentration induced by the
notches, the response of these V-notched specimens was expected
to be more brittle than the one found on the plain round specimens.
In opinion of the authors, the reason behind this behaviour
is the progressive release of energy resulting from the combination
of the V-shaped notch plus the torsional loading. During the loading
process, a first crack is initiated once the weakest point of the
notch tip exceeds the strength of the material. However, since this
crack forms 45 with the notch plane, crack propagation requires to
create new crack surfaces on the V sides, moving away from the
crack tip and increasing the amount of energy necessary for this
crack to propagate. Therefore the propagation of this initial crack
is only possible if the torque applied is increased what, in turn,
induces the initiation of a new crack in a second weak point at
the notch tip, which also can be propagated if the applied torque
is increased. As a result, the applied torque increases while many
cracks are initiated and partially propagated from the notch tip.
The accumulation of cracks at the notch tip weakens the ligament
of the specimen until it finally fails. Fig. 9 shows several pictures of
the notch tip during and after one of the tests: the propagation of
multiple cracks can be observed. In the case of the plain round
specimens subjected to torsion for the mechanical characterization
of the material, given the absence of the V-shaped notch, one single
crack is propagated and therefore these specimens behave more
brittle