## 1 Introduction
Adhesive joints have also been considered for inspection in [13] using the Single-Leg Bending (SLB) test. Obviously, this is an \"off duty\" mixed-mode (tensile and shear stress) testing procedure, which evaluates the structural condition of joints based on the strain energy release rate in tension and shear. For metal plate-like structures, nondestructive assessment of single-lap adhesive joints was carried out via ultrasonic guided wave propagation [14], again during the time interval that the structure was \"off duty\" for inspection purposes. Other examples based on \"off duty\" inspection include [15] where a composite pick-up truck box involving adhesively bonded composite joints was diagnosed by means of pulsed thermography methodologies, and [16] with CFRP-epoxy adhesive single-lap joints inspected by means of eddy current pulse-compression thermography. These are two examples of NDT and E techniques traditionally used for detecting defects in parts/components which have been adapted for monitoring of joints in structures. A nice overview of such NDT and E techniques may be found in [17]. The current study aims at introducing a vibration-based framework for monitoring joints in structures with composite members, based on measurements from embedded (via 3D-printing) sensing elements in them. The sensing ability thereby induced aims at limiting the use of dedicated equipment (amplifiers, accelerometers, piezoelectric sensors,3D laser microscopes etc. ), as found in AE [10] and other NDT and E [17] or traditional vibration-testing [18] methods. Furthermore, the framework aims at using data potentially resulting from normal structure operation, in order to minimize the \"off duty\" inspection time of the monitored structure. Finally, with respect to test data evaluation/analysis, the framework aims at introducing techniques which do not require excessive time [19] or a significant level of expertise from the user [19; 20] to yield accurate results. To this end, the current study extends the use of the contactless damage diagnosis principle [19] from a single slab to more complex structures formed by in-series connected slabs and characterized by more challenging dynamics. Embedding sensing elements via 3D-printing in a structural member means that (even) contactless transmission and recording of signals can be obtained via the principle proposed in [19]. The added benefit is that the use of equipment such as amplifiers, accelerometers and piezoelectric sensors may be minimized. On the other hand, all joints should be monitored, even though only one of the slabs involved incorporates the 3D-printed sensing element. For this reason, the contactless diagnosis principle in [19] (presented therein for a single slab) is now upgraded with respect to data evaluation techniques. Specifically, a novel approach for estimating damping values for a given mode is postulated. It undertakes the principle of the empirical procedure found in [20], but achieves enhanced accuracy and applicability to noisy signals, while requiring less time and user-expertise than system identification techniques [19]. Several test runs are conducted with structures formed by two or three composite slabs connected in series with bolted joints. These structures have been tested with either firmly fastened or failing joints. It is shown that specific frequency shifting and/or damping alteration patterns for given modes may be distinguished and associated to specific failing joints. The rest of the paper is organized as follows: Section 2 presents the fabrication of composite slabs via 3D-printing, the method for incorporating sensing elements during 3D-printing of slabs, the test structures resulting from connecting slabs in-series, the testing protocol used for the experiments and the procedure for damping factor estimation. In Section 3, the experimental data along with results on detection and identification of failing joints are shown and commented upon. Finally, Section 4 presents some concluding remarks.