Once the critical crack sizes for the different hypotheses (and
for fracture–plastic collapse mechanisms) have been obtained,
it is necessary to obtain those initial cracks in the tower that
would propagate under fatigue processes during its lifespan.
Previous fracture–plastic collapse assessment is focussed on the
application of a certain load (bending moment) in the component
containing some specified defects. However, subcritical fatigue
crack propagation is produced when the stress intensity factor
variation, ∆KI , is higher than the material fatigue threshold, ∆Kth
(see Table 2). In such a case, the towers would not fail at the
initial stage, but critical conditions could be achieved during their
lifespan due to the fatigue crack propagation.