Fig. 12: Crack propagation process
for an unpatched under fatigue loading
Monitoring the metallic crack growth with passive IrT shows that the
longer the crack grows, the better it can be made out on the thermal
image. For small crack lengths on the other hand, crack tip detection
can be difficult. One reason might be, that the movement of the lower
piston heats up the specimen during testing. Temperature thus increases
from the bottom to the top and the crack acts as a heat barrier. The
bigger the temperature differences become over time, the easier cracks
can be made out. Moreover, a circular area around the crack tip can be
identified which builds up during loading and vanishes while unloading
the specimen. This area helps to identify the crack tip and thereby to
measure the crack length.
As for the quasi-static test, differences between the images from the
unpatched specimen and the ones with patch monitored from the metallic
side are neglectable. Fig. 13 shows the temperature evaluation for two
patched specimens, one monitored from the metallic side and the other
from the patched one.