## 2 Materials and Methods
### Experimental Procedures  
Successful M-sensor operation depends on achieving two objectives:  
* O1: That meaningful (with respect to noise) electrical signals passively emitted from M-sensors could be generated/recorded, and
* O2: That the analysis of recorded signals could provide damage detection and assessment (diagnosis) results, especially at high amplitude/low frequency vibrating operation. The reason for focusing on high amplitude/low frequency operation in O2 is because both objectives were achieved in a high frequency/low amplitude vibrating context [21], even though the magnetoelastic strip was attached on the surface of the vibrating slab. Nonetheless, objective O1 was checked under all frequency--amplitude combinations, simply to ensure that the slabs with built-in strips do operate as sensors under such conditions. Two testing procedures have been used for checking whether O1 and O2 were achievable:  
* P1: The procedure simulated high frequency and low amplitude operational conditions, typical of systems with fast dynamics and involved type-A slabs both in M-sensor and blanc (type-A blanc) forms. Testing was performed on a Dynamic Mechanical Analyzer (DMA Q800) of TA--Instruments (Figure 3), which led to choosing the specific dimensions of type-A slabs. Slabs were subjected to three-point dynamic bending under a linear frequency scan of 1-100 Hz @ RT and at v = 25 \(\mu\)m constant static deflection. Electrical signals created by induction to the remote  
Figure 2: The three main slab types tested with built-in M-sensor. Type-A, -A blanc: slabs for DMA testing, Type-B: slabs for high-amplitude vibration, Type-C1, -C2: notched slabs for damage diagnosis with one (-C1), or two (-C2) notches at points a, b. Arrows on the left sides of slabs denote neutral axis in bending levels. interrogation coil (placed at 20 mm from the slab) were recorded by PioneerHill-Spectraplus(c) Software for Data Acquisition at 4096 Hz via a PC-Soundcard 3.5 mm jack. Comparing signals from type-A and -A blanc slabs allows for validating whether the hidden magnetoelastic material could provide sensing capabilities to the (otherwise passive) slab. * P2: The procedure involved series of type-B and -C1, -C2 slabs, fixed as cantilevers (Figure 4). The free end suffered a load of quasi-sinusoidal form with frequency f = 4 Hz, mean value equal to 12.5 mm and amplitude d = 12.5 mm. In other words, the free end featured deflections between 0 and 25 mm, applied via a custom-built mechanical exciter, which allowed for using a (larger than type-A) size of type-B/-C1/-C2 slabs. Hence, low frequency and high amplitude operational conditions typical of systems (structures) with slow dynamics were simulated. Electrical signals created by induction to a low-cost interrogation coil (Vishay IWAS) placed at 20 mm from the vibrating slab's end, were recorded via a conventional digital oscilloscope at 200 KHz. C comparing signals from pristine (type-B) or damaged (-C1, -C2) slabs should conclude on the existence of frequency shifts and ensure that damage diagnosis results could be obtained using the M-sensor concept. Figure 4: (**a**): Single cantilever arrangement for testing type-B, -C1 and -C2 slabs. (**b**): Response of type-B and -C2 (the most damaged) slabs (in V) versus time. Figure 3: (**a**): DMA oscillation of smart type-A and -A blanc slabs. (**b**): Response of type-A, and A blanc slabs (in V) versus time. Note that the coil distance of 20 mm from the slab was defined by a trial-and-error process, and corresponds to the maximum distance for recording a meaningful signal with the given set up. Obviously, placing the coil very close (or onto) the surface should provide better signal recording capabilities, but it would undermine the practical aspects of the concept and could induce sensitivity to the \"lift-off\" effects noted, for instance, in applications based on Eddy currents.