Fig. 11: Patch detachment 1 and 2
, crack initiation and final failure
Again, the images show the region around the hole which is covered by
the CFRP patch and whose woven structure can be identified. For each
event three consecutive images are shown that are found to be taken
close to the characteristic events.
The two points of patch detachment ( and ) show a slight diagonal
increase in temperature around the central hole. In the first row two
diagonal lines can be seen and in the second one only one line. It seems
that these temperature increase can be related to metallic slip bands.
But it actually cannot clearly be said whether these lines of increased
temperature result from diagonal slip bands or from the diagonal
structure of the patch.
Considering crack initiation () and final failure (), an increase in
temperature can be seen through the patch in the specimen centre. As the
patch is already detached around the hole, outlines are blurred and only
a non-specific field of increased temperature can be made out. The
temperature difference here lies at around \(T=45\) at the patch
surface, compared to a \(T=7.1\)K for the temperature increase
measured at the metallic surface. The very last picture covers the
moment of patch detachment. The actual temperature increase around the
hole can be seen at the right-hand side where the metal sheet is not
covered by the patch. While the temperature difference is more than\(T=8\)K at the edge of the metallic part, on the patch surface only
difference of around \(T=2.59\)K is detectable.
3.2. Fatigue Loading
Compared to the quasi-static tests, under cyclic loading different
conditions apply. Fatigue crack growth differs from static failure. The
main difference is that cracks grow at lower stresses and, more
important here, lower strains. This means that also temperature
evaluation is less than for static damage. Further, due to the
continuous loading and unloading, temperature does not solely increase
because of plastic deformation at the stress peaks, but also globally
due to internal friction. Cyclic loading leads to molecular movements
inside the material. Thereby, the entire specimen heats up over testing
time. Therefore, temperature differences \(T\ \)are expected to be
small. On the other hand, due to smaller deformations, sudden patch
detachment is not expectable. A proper bonding leads to improved
visibility of the subsurface phenomenae.
Thermal images of one exemplarily chosen unpatched specimen, shown in
Fig. 12, depict the temperature development during crack propagation for
four exemplarily chosen numbers of load cycles N .