**1. Introduction**

The history of aviation is sadly enough quite abundant with examples of propagating fatigue cracks that lead to a structural failure. Actually, the firsts phenomenological understandings of the fatigue phenomenon in metals came after the series of crashes experienced by the first jetliner to enter commercial service, namely the Comet I aircraft [1–3]. In the time when it was designed, the deterioration of mechanical properties due to the presence of cracks was largely underestimated. Nowadays, even if our knowledge concerning the fatigue of materials has dramatically progressed, fatigue failures continue to happen and in almost all sectors of the industry. An insight of this can be given through the review of failure causes in aircraft components done by Findlay and Harrisson in 2002 [4,5], which reveals that fatigue is the dominant failure mode in aeronautics (55% of the failures), well ahead of corrosion (16% of failures) and overload (14%; the rest being wear and stress/temperature corrosion). This illustrates very well that due to the complexity of the fatigue phenomenon in metals, design offices are still struggling with issues related to fatigue. Therefore, in the author's view, this calls for a new paradigm in the approach followed to deal with fatigue cracks. One of the innovative approaches that could be followed in that context and which is thought to be very promising is known under the generic terminology of "Structural Health Monitoring".

Formally, Structural Health Monitoring (SHM) can be defined as the process of acquiring and analyzing the data from on-board sensors to evaluate the health of a structure [6]. Up to now, multiple SHM systems have been developed, with different target applications. Most of these systems used to be vibrations based [7,8]. However, structural health monitoring systems have been a thriving research domain for several years now, implying the emergence of innovative approaches for performing SHM. It would be impossible to be complete here, but technologies such as carbon nanotubes based structural health monitoring or comparative vacuum monitoring sensors illustrate very well this diversity [9–11]. In all cases, robustness remains a crucial attention point in the design of structural health monitoring systems [7,12].

Aside from this, the advent of additive manufacturing opens the door to the development of innovative concepts, such as smart metals. Therefore, the authors of this work have been active on an alternative implementation of the SHM principle where the geometric freedom provided by additive manufacturing could be exploited. The previously conducted research efforts resulted in a technology that is now patented (patent number EP2801809A1, [13]) under the name "effective Structural Health Monitoring (eSHM)". The system consists in integrating over- or under-pressurized capillaries (the capillaries are thus internal channels) into the to-be-monitored component, so that when a fatigue crack breaches the capillary network, a leak flow is created, and the pressure equilibration between the capillary and the open atmosphere is detected by a pressure sensor. Figure 1 illustrates the working principle on a four point bending test sample equipped with an under-pressurized capillary. The pressure is initially set to 0.5 atm in the capillary before the experiment starts, but as soon as the propagating fatigue crack reaches it (clearly seen on the micro-XCT image), air from the open atmosphere enters the capillary, and internal capillary pressure rises sharply. This pressure rise is detected by a sensor, which triggers an alarm when a pre-defined value has been reached, thus revealing the presence of the crack [14]. In that sense, a parallel can be drawn between the proposed structural health monitoring approach and the previously developed Helicopter Blade Crack Detection System (patent number US5205710A, [15]). However, the proposed methodology is on purpose mainly targeted at additively manufactured components [6]. The geometric freedom offered by additive manufacturing implies that the capillaries can be placed according to the designer's needs. In other words, the proposed SHM methodology enables close and robust monitoring of the regions which were identified by design studies as prone to crack initiation. For illustration, it can be mentioned that it would be impossible to monitor the entire lug hole region of the lug studied below should it be manufactured with conventional methods. Moreover, the proposed technology requires no significant signal processing and electronic hardware, as only the capillary pressure should be measured and monitored (refer to the experimental investigations in [16]). Detailed information regarding the working principle of this novel SHM methodology can be found in the patent [13].

**Figure 1.** Illustration of the eSHM working principle on a four point bending test sample.

While additive manufacturing has shown significant potential for widespread usage in production of complex and/or customized parts, this potential has been dampened by, notably, the variation of mechanical properties, which includes the fatigue behavior of these parts [17]. Indeed, it is well-known in the literature that printing parameters such as technology used (powder bed fusion, laser metal deposition, etc.), scanning speed, powder flow rate (non-exhaustive list) have notably a strong influence on the microstructure and residual stresses present in the component, and hence, on the mechanical properties. Moreover, post-treatments applied (if any) on the part also result in a mechanical behavior different compared to the as-built conditions. As a matter of illustration, it has been reported that the Hot Isostatic Pressing process (hipping) contributes to a significant improvement of fatigue properties, and possibly to properties that are similar to their conventional counterparts [18–21]. The dispersal of mechanical properties of AM parts inevitably reduces their reliability and thus impinges the adoption of additive manufacturing in engineering applications [17]. In that context, it is thought that besides offering high level of robustness, the proposed SHM philosophy could also contribute in revealing the full potential of additive manufacturing.

However, the introduction of capillaries in regions which are potentially subject to high stresses might raise concerns on the influence the eSHM could have on the fatigue life of the structure. Günther et al. [22] have shown that test samples with internal channels have reduced fatigue lives compared to samples without internal channels, the reduction being dependent on the topology of the channels. However, still according to Günther et al., this is primarily due to rapid crack initiation at the primary roughness of the internal channels, and secondly to the modified topology of the specimens. In the present research, the effect of capillary roughness on the reduction of fatigue life has not been taken into account. Indeed, technologies such as hybrid manufacturing (combination of additive and subtractive manufacturing) or chemical etching could potentially enable a drastic reduction in capillary roughness levels in the near future [23–26]. Moreover, in the context of the eSHM system, the roughness influence has already been experimentally investigated (see [27]). Therefore, one focuses here on the consequences of the presence and topology of virtually smooth capillaries on fatigue life. This is particularly important for the further development of the system, as the eSHM system has to offer a quick detection of growing fatigue cracks (by being placed "as close as possible" to the most probable initiation site), while not affecting the component's function (the capillary should not trigger initiation nor reduce fatigue life). As a matter of fact, this study can be seen as the completion of our initial work [14], as both initiation and propagation are addressed, and as additional capillary topologies and initial flaws are studied. This should enable to draw a more general conclusion on the eSHM influence on fatigue behavior of equipped lugs.

### **2. Crack Initiation Considerations**
