# Rapid Characterization Method for SMC Materials for a Preliminary Selection
## 2 Problem Statement and Methods
The SST method has a faster application with respect to the toroidal magnetic test; however, it requires a preliminary calibration, made only once through a toroidal-shaped specimen, before starting. The verified reliability of our SST method [20] allows performing quick tests for a primary selection of the considered materials, to be then subjected to an exhaustive characterization by the use of toroidal specimens. However, many parameters are involved in SMC research. In this sense, a screening of specimens is necessary and may be performed in a very quick way, employing resistivity and coercivity tests, especially if a large number of specimens whose coating or production processes differ from each other. The electrical resistivity is related to eddy current losses and depends on the layer typology and content. High resistivity values should correspond to low iron losses; simultaneously, the hysteresis losses can be increased for high layer thickness. Considering relations between iron losses, hysteresis and resistivity, coercivity and resistivity tests may be adopted before or instead of the SST, as the first step of SMCs analysis, to discard the non-compliant specimens. The accuracy required by the measurement of resistivity implies a dedicated system that has been arranged for the measurement of the same parallelepiped samples prepared for SST tests, as shown in Figure 5.  
The electrical resistivity values were found indirectly through the measurement of the electrical resistance, whose relationship is given by the following formula:  
\[\rho=\frac{(R\cdot S)}{\ell}\left[\mu\Omega\mathrm{m}\right] \tag{1}\]  
where \(R\) is the resistance, \(S\) is the surface and \(l\) in the denominator is the distance between the electrodes. A multimeter (Keithley) connected to a test clamp with a programming circuit has been used to obtain the resistance measurements. The method applied is based on DC current and four-terminal measurement. The clamp was used instead of the usual \"probes or test leads\" to obtain a more accurate result by limiting the variables due to the positioning and inclination of the tips on the samples. For each SMC material sample, five resistance measurements have been taken by slightly changing the position of the clamp with each measure and then the average value has been calculated. The coercivity values are associated with the hysteresis losses: by reducing such parameters, the iron losses decrease too. Additionally, the high coercivity can be linked with poor magnetic permeability. The measurement has been achieved using the coercimeter, which detects the coercivity value applying a static magnetic field. The method and measure, quick and very simple, are shown in Figure 6.  
Figure 5: The measurement system used to obtain the resistivity value: adopted devices (**a**), and prepared test clamp (**b**) and sample measurement (**c**). In summary, concerning the energetic aspects, the perfect SMC materials should have low coercivity (less than 500 A/m) and high electrical resistivity (up to 100 \(\upmu\Omega\)m). This combination is not always easy to achieve; therefore, compromising solutions are assumed. Only specimens that were compliant with the required values of coercivity and resistivity are considered suitable for further characterization in SST and, if particularly well-performing in terms of iron losses and permeability, further characterized through mechanical tests. For a clearer and more detailed understanding, the two procedures are compared in Figure 7, starting from two different types of samples. Mainly, parallelepiped specimens are suitable for several tests without any sample preparation or machining. Conversely, the toroid samples cannot provide mechanical results or coercivity values in a static magnetic field. However, after the material selection utilizing the parallelepiped specimens, the toroidal sample must be used to validate the magnetic measures. So, with the proposed procedure, a single specimen (i.e., a single moulding process) leads to performing different measures.