PSI - Issue 13

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Marc Moonens et al. / Procedia Structural Integrity 13 (2018) 1708–1713 M. Moonens et al. / Structural Integrity Procedia 00 (2018) 000–000

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Fig. 1: Working principle of the eSHM system

components where the propagating crack remained undetected, and which lead to possibly dramatic consequences. Aircraft industry has proven that propagating fatigue crack can lead to the crash or emergency landing of an airplane, the Comet aircraft’s in the years 1950 or the Aloha Airlines flight 243 in 1988 being the most famous examples. The latter two examples might however give the impression that fatigue failures in aircrafts in operation were a concern in the past, but is now resolved. One should note that this is a faulty impression, as fatigue failures still occur nowadays. To the purpose of illustration, one should not dig very far in the history of aircraft accidents involving passenger fatalities in the United States to find the trace of a deadly fatigue crack. Indeed, the third most recent deadly crash took place on the 19 th of December 2005, and concerned a Chalks Ocean Airways Grumman G-73T that ditched into the water due to an in-flight wing / fuselage separation, which had been triggered by a fatigue crack in the wing structure (NTSB AAR0704 [2007]). Thanks to a very comprehensive study performed by the National Bureau of Standards (now, NIST) on 230 failed aircraft components, one can estimate that about 60% of all failures of aeronautical components are due to fatigue (see also Manson and Halford [2006]). Aviation is thus highly concerned, still today, by the consequences of undetected fatigue cracks, and other industries are concerned as well. Another famous - and tragic! - example is the accident of an ICE German high speed train near the village of Eschede on the 5 th of June 1998. The train derailed while driving at 250 km h , leading to 100 deaths and more than 100 injured passengers. After investigations, it was found that the derailment was due to the rupture of one wheel, which had been provoked by a propagating fatigue crack (detailed information can be found in Esslinger et al. [2004]). All this illustrates very well the need for new approaches to deal with the detection of propagating fatigue cracks in structural components, and Structural Health Monitoring is precisely one of the alternatives, thought to be very promising. Formally, Structural Health Monitoring (SHM) can be defined as the process of acquiring an analyzing the data from on-board sensors to evaluate the health of a structure (De Baere et al. [2014]). Up to now, research around SHM has primarily focused on vibration-based damage identification systems (Farrar and Worden [2007]). However, the advent of the additive manufacturing technology and inherent smart metals concepts lead the researchers of the VUB to consider another implementation of the SHM philosophy, which lead to the now patented e ffi cient Structural Health Monitoring, or eSHM system (De Baere et al. [2014], Strantza et al. [2015]). The system consists in integrating 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 sinusoidal capillary: the pressure is normally 0.5bar in the capillary, 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. The system presents the intrinsic advantage of robustness (since the capillaries are integrated in the component, there are no concerns of surface contamination by oil or dirt; only one sensor is necessary to inspect the entire component; etc.), but it remains to be proven that the integration of capillaries inside the real life component does not a ff ect the component’s quality of fulfilling its function with all related specifications. Previous researches have already shown that the presence of the capillaries had no influence on the crack initiation behavior of the component, provided it was located at an