## 2 Related Work  
Reported efforts about monitoring integrity in automotive and aircraft structures have been reported. One applied method is to track the inner vehicle's chassis, looking for normal deformation of the car's life cycle due to accidents, where a structure of smart sensors detects possible failures on the chassis [15]. Moreover, a low-power-consumption smart sensor for SHM with four piezoelectric transducers was tested to identify impact and damage in automotive and aircraft structures [16]. Another reported outcome of SHM is a virtual test model to detect problems in airfoils [17]. Recently, a magnetoresistive actuator was tested to detect small amounts of damage in aircraft [18]; moreover, a review was carried out on sensing technologies for structural-health monitoring (SHM), and included a case study for a commercial aircraft. Findings showed that the use of smart sensors reduced the time required for aircraft maintenance, and therefore, the cost of maintenance. Contrarily, by increasing the number of sensors, the total weight of the aircraft increased as well as the lifetime cost. Thus, an aim is to create smart sensors that require fewer sensing points to perform structural monitoring [19], such as magnetorheological elastomers (MREs). Among the different types of MRE, some contain nanoparticles. Due to their suitable characteristics to develop as transducers, this research explored MREs with iron oxide nanopowder and multiwall carbon nanotubes with iron. MR elastomers with iron particles have better mechanical properties and magnetic characteristics compared to those of MREs without the iron microparticles [20]. Furthermore, including carbon nanotubes in the elastomer improves tensile strength, field-dependent storage, loss moduli, and magnetorheological efficiency [21]. A variety of reported results on energy harvesting with piezoelectric elements can be found in the literature. The use of cantilever beams in energy-harvesting systems was widely explored and reviewed [22; 23; 24; 25; 26]. Studies focused on the different geometries that the cantilever could have, and, for example, experimental results recorded the generation of a greater voltage with a cantilever of triangular geometry against that of others of the rectangular and trapezoidal type [27]. Another research work obtained higher levels of collected energy with a T-shape cantilever [28], whereas another result applied different configurations of the cantilever beam and its impact mechanism to generate electricity [29]. Additionally, there are reported experimental results where a cavity was added to the cantilever beam to increase strain and generated voltage by approximately 75% compared with those of the option without the cavity [30]. Additional experimental results indicated that the use of multilayered piezoelectric sensors generated more voltage than that of a single-layered structure [31], and showed the use of a cantilever with a coupled pendulum to gather energy from displacements in two degrees of freedom (DOF) [32]. In automotive applications, vibration frequencies are generally very low (\(<\)100 Hz), and for a vehicle chassis and vertical dynamics of the vehicle structure, they are approximately \(<\)15 Hz [33]. It can be seen from previous works that these frequency bandwidths have not been widely addressed with this kind of mechanism. Recent smart-sensor applications include the IoT paradigm and taking advantage of cloud computing. Reported SHM results with IoT capabilities were proposed with a tendency to deploy wireless communication as an option to solve wired-sensor technologies [34]. Some approaches have developed remote sensing and a reconfigurable environment such as a wireless-sensor network (WSN) [35], whereas other achievements focused on technology to transfer information among heterogeneous platforms (middleware) within the context of IoT [36]. Moreover, new IoT remote-sensing technologies have been developed. One example is the Xnode wireless platform that has been tested for SHM.