4. Discussion
Under quasi-static loading, the knowledge of local temperature changes
helps understanding the thermal examination through the bonded patch.
Monitoring a patched specimen, sudden local temperature increases at an
angle of around 45° to the loading direction, combined with noise
effects in the load/displacement curve can thus be allocated to the PLC
effect. An increase of the effect over time is measurable. Compared to
the specimens without patch the detectable temperature difference from
the patched side is reduced, because temperature differences are rather
small, and the resulting infrared radiation is weakened by the CFRP
patch. Additionally, temperature flashes run suddenly through the
specimen so that a clear identification of damage initiation through the
patch is difficult. Further, due to the local patch detachment the air
between the patch and the specimen additionally influences the radiation
intensity. For patches of one layer of the chosen CFRP material,
subsurface effects which are related to heat generation can generally be
identified on thermal images. But, temperature difference \(T\) has to
be high enough. The origin of heat generation cannot clearly be
identified without further information. Interpretation of thermal images
needs for additional knowledge resulting from the stress/strain
behaviour e.g.
For specimens loaded in fatigue, results are different. Crack length
measurement is possible from the metallic side and becomes even better,
the longer the crack growth. As the specimens are loaded by the lower
piston, they heat up from the bottom to the top. The temperature
difference between the upper and the lower part of the specimen
increases with increasing number of load cycles. As the heat circles the
crack the crack itself acts as a temperature barrier and becomes visible
through the temperature difference of the metallic part above and the
one below the crack. From the patched side it is not as clear, but a
good qualitative estimation is possible as well. Further, under cyclic
loading, crack growth additionally goes along with local patch
detachment, leading to a further reduction of crack detectability. A
comparison of the results monitored from the metallic and from the
patched side shows that crack length detection is possible from both
sides. As seen for the unpatched specimen, a discontinuous temperature
distribution appears around the crack tips with a maximum temperature
increase \(T\) at the peak stress of each loading/unloading cycle. Using
this top dead centre for test evaluation, crack tips can be identified
also from the patched side. Imaging frequency should therefore be higher
than test frequency so that it can be made sure that pictures at or
close to the top dead centre are available. Unlike for the quasi-static
loading, here, the measured temperature of the metallic and the patched
side do not differ strongly, as long as there is a proper bonding
between patch and specimen. Increasing local damages in the adhesive
layer have an impact on the thermal properties resulting in decreasing
visibility of the crack. Local patch delamination can be identified on
the thermal image as areas where the woven structure of the patch is
noticeable. But still, crack propagation process can be monitored.
Comparing crack length and detachment dimensions it seems that right in
front of the crack the patch has already delaminated from the metal.
Meaning that for the given specimen configuration and the given
conditions presumably delamination around the crack tip appears before
the crack grows. To further corroborate this hypothesis additional tests
using complementary monitoring techniques have to be made.