## 4 Validation Tests
### Magnetorheological Elastomer  
When the sample was cured, some electrical-resistance tests were carried out on the MRE sample to verify if MWCNT-Fe particles increased the electrical conductivity. Electrical-resistance values within a range from 500 k\(\Omega\) to 10 M\(\Omega\) were measured during the experiment; the results supported the use of multiwall carbon nanotubes with iron (MWCNT-Fe) instead of iron (II and III) oxide nanopowder. This validation work considered some tension-compression tests with MRE reported in the literature [47; 48; 49].  
Figure 14: Energy-harvesting subsystem. (**a**) CAD drawing; (**b**) manufactured subsystem with protective case.  
Figure 15: Complete manufactured smart-sensor prototype.  
An open-force-tester machine was used to measure the strain for MRE. The LYNX testing software from the machine measured the total applied force and the length by which the MRE could be stretched before becoming loose. For the setup, the machine was set at an initial position; then, the MRE was fixed into the open force tester--into the bottom and top sections of the center base of the tester--having been attached to measure resistance. The center base was where the test subjects were placed, with detachable lower and upper bases in order to adjust to the size of what was being tested. The bottom base was static, while the upper base was the one that moved upward and downward; for tension, it moved up. At the start, the LYNX software captured the force that was being applied to the MRE while it was being stretched. At the same time, the program measured the elongating distance, and the multimeter read the electrical resistance. Twelve MRE samples were tested under tension considering three different designs for each MRE (each with a different distribution of nanoparticles) to find which design exhibited the highest resistance measurement. With the gathered data, a graph of applied force vs electrical resistance was obtained. Tensile testing efforts included two replicas of the same testing conditions to obtain more reliable results. The experimental setup for the tensile tests is shown in Figure 16.  
Compression tests were also performed to validate the behavior of the electrical resistance of the MRE samples. In the laboratory, a hydraulic jack was used to apply force to a steel surface where the MRE samples were placed, resulting in their compression. The force applied by the hydraulic jack was measured with a dynamometer, and to register the measurements of electrical resistance. National Instruments (NI) Educational Laboratory Virtual Instrumentation Suite (NI ELVIS) II was used with its multimeter function to measure electrical resistance. Measurements were taken and displayed in real time through a human-machine interface (HMI) in NI LabView. As performed with the tensile testing, two replicas for the same testing conditions were registered to obtain more reliable results.  
The gathered data were stored in an Excel file for additional processing. For the experiments, the three types of MRE samples in Figure 3 were tested. Each sample was fixed on a steel surface, and two crocodile clips connected the MRE sample to the data-acquisition (DAQ) system. With this setup, a hydraulic jack was employed to apply the force to compress the MRE sample. The NI ELVIS II took measurements of electrical resistance, and the HMI displayed the data in a waveform chart. When the test was finished, data were stored in an Excel spreadsheet. For the experiments, the applied force was in the range of 0 to 2250 N in steps of 250 N, with a total test duration of approximately 80 s. The compression-test equipment, including data acquisition, is shown in Figure 17.  
Figure 16: Laboratory setup for MRE tensile tests.