## 1 Introduction
In general, 3D printing procedures have been used for the sensor's construction [17] or the integration of the necessary circuitry/devices in order to facilitate operation of laser displacement sensors [20] or MFC sensors/transducers [23]. In other words, the 3D printing process has very often served as a means of facilitating the sensor's construction process, or it's mounting onto a given structure and its effective operation, but it has rarely been used for manufacturing a structure with a sensing element integrated within. The aim of the current study is to use the additive manufacturing principle for integrating sensing elements in a structure during its 3D printing process, and to diagnose structural damage based on data from these built-in sensing elements and conventional, affordable equipment. The considered structures are carbon fiber reinforced polymer (CFRP) composite slabs of different dimensions, which incorporate magnetoelastic strips acting as sensing elements under external slab loading. Then, due to the magnetoelastic phenomenon, such loading lead to changes in the strip's magnetization, which causes the induction of electrical signals in conventional reception coil circuits placed at a distance from the slab. Since these signals quantify the vibrating slab's response (itself depending on the slab's structural integrity) damage is detectable when changes of specific signal characteristics (selected eigenfrequencies) occur. Even if, at times, this detection principle may prove not very practical to use, it is a traditional approach for structural health monitoring purposes, as it can be sensitive to even small damages [25]. This detection principle has also been used in a recent study [26], which features a similar operational and testing concept to that proposed in the current work, but for the important detail of the magnetoelastic sensing material being attached onto (rather than integrated within) the structure. Due to this critical difference, two main issues must be dealt with, namely the possibility of recording meaningful (with respect to noise) electrical signals from the built-in strips despite their contact-less reception, and the potential of diagnosing damage using such signals. Therefore, slabs of various dimensions and damage levels have been vibrated at different frequencies and amplitudes. Via a dedicated test data processing framework along with finite element method (FEM) analysis, it is shown that magnetoelastic strips integrated within the material structure may successfully act as passive embedded sensors, transmitting meaningful signals during mechanical loading and providing qualitative and quantitative damage diagnosis results. The rest of the paper is organized as follows: Section 2 presents the 3D printing process of CFRP slabs with the integration of sensing elements, and the testing protocol used for the experiments. In Section 3, the experimental data along with the relevant framework for data analysis and validation via FEM are shown. Based on this framework, damage detection and assessment results are obtained and commented upon. Finally, Section 4 presents some concluding remarks.