## 1 Introduction
### Effective Structural Health Monitoring System  
Structural health monitoring (SHM) is the integration of sensing and possibly also actuation devices to allow the damaging conditions of a structure to be recorded, analyzed, localized and predicted in a way that non-destructive testing becomes an integral part of the structure [6].  
Many structural health monitoring principles have been developed in laboratory conditions but suffered problems when introduced in real applications. Grease, environmental changes, electrical noise, vibrations and many other external influences have shown to have a severe impact on the performance of these structural health monitoring systems. Despite the clear benefits of an SHM system, but due to this lack of robustness, only a few systems have made the transition from the laboratory to a real application [7; 8]. A successful structural health monitoring system must be robust in the first place: very low false alarm rates, high detectability of the cracks and insensitive to external influences.  
Bearing this in mind, a new effective structural health monitoring philosophy (eSHM) was recently developed [7]. Small subsurface capillaries are embedded in the structure by means of additive manufacturing techniques, close to the region where fatigue cracks are expected to grow. The capillaries are then pressurized (vacuum or overpressure) and the capillary pressure is monitored continuously during the operation of the component. A crack, that initiated at the surface of the component, propagates through the material until it finally breaches the capillary. The resulting leak changes the internal capillary pressure. The presence of a fatigue crack is then derived from the moment that the capillary pressure bypasses a certain pre-set limit. In Figure 1, a possible application of the new effective SHM system is depicted. The embedded capillaries are indicated in red.  
The capillaries of the eSHM system must be embedded in the regions where fatigue cracks are expected to grow. In order not to reduce the strength of the component, one must take into account the presence of the capillaries during the design phase. It was concluded in the paper of [PERSON] _et al._[9] that--for the considered structure in the paper--the presence of the capillary did not influence the fatigue life negatively, although the cracks initiated from the capillary when positioned close to the specimen edge. Another paper of [PERSON] _et al._[10] reported on four point bending test results of test samples with a different layout of the embedded capillaries. It was concluded that the capillary had no crucial influence on the fatigue initiation location for the test specimen and test conditions under investigation. Future studies are being focused on robustness improvements of the eSHM to delay crack initiation at the capillary location.  
Embedding the capillary in a way that it connects all locations prone to fatigue allows monitoring the critical areas without the need for multiple sensing devices. In the example of the cogwheel of Figure 1, only one pressure measurement is required to inspect all teeth. This largely facilitates the installation, data storage and post-processing of the data of the eSHM system. After detection of a fatigue crack, further analysis has to be performed on the critical area. It is therefore interesting to know, on basis of the pressure measurement, where the cracked region is located (e.g., which tooth suffers from a crack).