## 2 Materials and Methods  
### Slab 3D-Printing, Assembly of Structures and Data Recording Setup  
Rectangular slabs were prepared as in [19]. Briefly, the printer (CTC i3, Zhuhai Electronic Ltd., Zhuhai City, Guangdong, China) operated in fused deposition modeling (FDM) mode, had a single nozzle with a diameter of 0.4 mm, and used a polymer filament of 1.75 mm. The filament was a short carbon fiber reinforced PET-G polymer (CFRP) by 20 wt% (NEEMA3D(tm) Carbon: plus) of NEEMA3D(tm), Petroupolis, Greece. According to the manufacturer's specifications, the filament's Young's modulus and Yield Strength were equal to E = 3800 MPa and 52.5 MPa, respectively. The printing temperature was set at 225 \({}^{\circ}\)C. Two slabs each measuring 170 \(\times\) 25 \(\times\) 3 mm\({}^{3}\) were printed, one of which incorporated a 2826 MB Metglas(tm) magnetoelastic (20 \(\upmu\)m) strip of 25 \(\times\) 5 mm\({}^{2}\), as sensing element. Another shorter slab (110 \(\times\) 25 \(\times\) 3 mm\({}^{3}\)) without sensing element was also printed. As described in [19], a sliced 3D design file was created for the considered slabs, and used in Pronterface(tm) software driving the 3D printer in Fused Deposition Mode (Figure 1a). The sensing element was easily incorporated during 3D-printing of slabs, by pausing the FDM printer once the desired layer height had been reached, so as to attach the strip with epoxy adhesive (Figure 1b). In the current case, 3D-printing of the slab in Figure 1b was temporarily halted when the slab's thickness was equal to 1.75 mm. Then, the strip was attached and subsequently covered by successive material layers until obtaining the final slab thickness of 3 mm (Figure 1c). Two structures were formed by either two or three slabs connected in series with 5 mm bolts and washers, as shown in Figure 2. Since this study investigates the feasibility of modifying the framework in [19] for detecting and identifying failing joints, it seemed evident not to consider structures of complex geometry and numerous joints at this (initial) stage. Therefore, two basic structures were investigated: The first (most basic) one was used to verify that in case of issues with failing joints, the framework proposed in [19] in a suitably modified form, could detect the problem. The second structure should additionally allow for using the proposed framework to identify the joint that is failing. As seen in Figure 2, each joint between two slabs is formed with 5 mm bolts. This allowed for easily switching between a normal (tight) joint condition to a failing (loose) one after each test run. Hence, failing or normal conditions for a given joint were easily replicated, without having to dismantle the structure. Note that, the aim was to assess the operational condition of joints (fit for purpose, or not), rather than to pick up particular defects (i.e., breakage of adhesive film between surfaces, delamination problems) leading to compromised operation. This macroscopic approach, in turn, means that the proposed monitoring framework could be applicable to various kinds of joints (adhesive, bolted, etc.). Figure 1: (**a**) Detail of the 3D design file used with Pronterface® software, which drives the 3D printing process in fused deposition modelling mode; (**b**) Fixing of the Metglas® strip (indicated by white arrow) via epoxy adhesive whilst 3D printing is temporarily halted (slab is 1.75 mm thick at that point); (**c**) continuation of material layering until a slab thickness of 3 mm is reached. Figure 2: (**a**) Structure-A formed by two slabs connected in-series and clamped at one end; (**b**) Structure-B formed by three slabs connected in-series and clamped at one end; (**c**) top view of structure-B showing locations of MetGlas® strip (marked with \(\Sigma\)) and the bolts (marked with I and II, respectively) of joints J1 and J2.