5. Conclusions
When analyzing the damage processes of crack patched structures under
quasi-static and cyclic loading, passive IrT is a good opportunity to
help understanding the complex failure behaviour. From the unpatched
side, of course, the metallic crack growth can be monitored easily,
while it is more difficult from the patched one. For quasi-static
loading big strain differences lead to extensive patch delamination
which strongly detracts the thermal analysis. Fatigue loading on the
other hand goes usually along with only local patch detachment. And, as
long as the patch is bonded to the specimen for the most part, and
temperature changes can be made out on the patch surface, also the
subsurface crack growth process can be monitored. Due to the overall
increase in temperature, crack detectability improves over time. On the
other hand, with increasing local delamination image interpretation
becomes more difficult. Here, at maximum load, crack length can even be
measured for locally detached patches. In total, the method of IrT
provides the opportunity to gain a better understanding of the fatigue
behaviour of crack patched structures.
Additional tests are needed to proof whether patches made of several
layers still lead to utilizable thermal images and how an additional
adhesive layer influences the results. Provided that the temperature
increase must be strong enough to be detectable on the patch surface,
passive IrT can help understanding the degradation process of a crack
patched structure under long-term loading. The design assumption
commonly used that a proper bonding is present, is usually not given
under service conditions. Even for patch application under laboratory
conditions long-term loading and environmental influences degrade the
mechanical properties of the adhesive. Thus, designing the patch using
ideal bonding conditions can lead to false assessments in the
performance prediction. In forthcoming studies, the method of passive
IrT can be used to describe the damage behaviour of crack patched
components more realistically. The method of passive IrT comprises the
possibility to identify the component (metallic sheet, adhesive zone, or
patch) where the first damage onset occurs and how the different damage
mechanisms influence one another. Thereby it provides great potential
for improving the patch design process and to make failure prediction
more reliable.