# Rapid Characterization Method for SMC Materials for a Preliminary Selection
## 2 Problem Statement and Methods  
The primary magnetic test used for SMC materials is the toroidal magnetic test. Such a test is defined in the standards IEC 60404-6 and IEC 60404-4, and the characterization is performed with the well-known \"transformer approach\", where two windings are wound around the toroidal magnetic core. The first winding provides the magnetization to the magnetic core with a current proportional to the magnetic field H; the second winding, prepared with a very thin copper wire accurately wrapped very close to the magnetic core, is adopted to detect the induced voltage, proportional to the magnetic flux density B. This test method requires a lot of time to prepare the specimens, mainly due to the mechanical machining and winding process. From this viewpoint, with the laboratory facilities, the production of the single toroidal sample takes around 3-4 h. The preparation of SMC toroidal samples for the toroidal magnetic test is described in Figure 2, where all production steps are shown in chronological order. The toroidal magnetic test is a complete and accurate measured method; on the other hand, the lengthy procedure for its preparation is the main drawback for extensive research activity and quick verification of the magnets. Figure 1: Typical texture of SMC materials: ferromagnetic particles covered with an insulating layer. Figure 2: Different procedures of the SMC preparation specimens for magnetic characterization through a toroidal magnetic test. Furthermore, the use of toroidal-shaped specimens limits the types of analysis that may be conducted on the same specimen, such as mechanical tests. On the other hand, the magnetic characterization based on the single sheet tester (SST) method allows its application for laminated steel and SMC materials [37; 38; 39]. The parallelepiped samples (about 3 \(\times\) 10 \(\times\) 50 mm) can be easily prepared and arranged, as shown in Figure 3. Such samples do not require windings, and the measurements can be immediately performed, saving and optimizing the time of the specimen preparation process. In fact, parallelepiped samples are easily obtained after compression moulding without further machining processes. The single sheet tester is prepared with a secondary winding positioned around the middle of the sample and a split primary winding at both sides, as shown in Figure 4.  
The used self-constructed SST joke follows a different design rule compared to most commercial ones. The primary winding is split into two equal parts, similar to a Helmholtz coil, resulting in an upgraded uniformity in the middle zone. In that place, one can find the secondary winding, which is very limited in turn number and overall dimensions. The low number of turns allows measuring the higher frequency with the same hardware while preserving a good sensibility at the lower extreme of the spectrum. The short length helps select a volume portion where the field is more uniform, leaving the side artefacts and field deviations out. The adaptation of a single shorting c-core is considered to be sufficient to avoid saturation of the core itself. This is due to the lower maximum relative permeability of the SMCs (500-600) compared to the laminated sheets. The used c-core is specifically designed for frequencies until 20 kHz to assure a good shorting path throughout the entire expected frequency range. As mentioned before, the field distribution at the sides of the specimen lies well outside the measuring volume coupled to the secondary coil, thus causing a very  
Figure 4: SST method with the material under investigation with primary and secondary windings. Figure 3: The parallelepiped samples and their insertion in the SST device, not required to be wound and ready for the measurement. subtle or even no perturbation in the measured material properties, depending on the sample thickness.