PSI - Issue 13

Tatiana Oršulová et al. / Procedia Structural Integrity 13 (2018) 1689–1694 Tatiana Oršulová et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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dimensions: 20 mm x 10 mm x 10 mm. The all specimens were previously annealed at the defined temperature for solution annealing (1050°C/35min.). This regime was defined as the initial state (IS). Further, the controlled plastic deformation was applied. The deformation was performed by mechanical pressing of the opposite sides of these samples. Exact values of the plastic deformations were inspected: T1 = 0%, T2 = 1%, T3 = 2%, T4 = 5%, T5 = 10%, T6 = 20%, T7 = 30% and T8 = 40%. This value represents the percentage shortening of the length of the specimen after the plastic deformation, in comparison with the reference sample. The commercially available sensor of GF708AKA (Sensitec GmbH) was used in first measuring of the magnetic field. This magnetic field sensor is based on the Giant-Magneto-Resistive (GMR) effect. Its functional magnetic layer is pinned within a synthetic spin-valve connected as a Wheatstone bridge. With its on-chip flux concentrators an extremely large sensitivity can be achieved, resulting in an almost step-like bipolar transfer curve. This way the sensor is predestined for the key application field: as a highly sensitive magnetic field sensor. Due to the spin valve technology the transfer curve within ±1 mT features an extremely high sensitivity of 130 mV/V/mT with very low coercitivity at the same time. The GMR sensor was positioned in each axis of the 3D coordinate system with respect to the investigated material, respectively. It means that its sensitive axis was oriented in that way to be able to sense all the individual components of the residual magnetic field. Thus, the magnetic field values were picked-up, to be the resulting graphs displayed. Measured magnetic field component in a given direction was represented as an output voltage signal of the sensor. This value was picked up from the diag onal of the Wheatstone’s bridge (Mach, 2012; Smetana, 2016; Stubendekova, 2015). The lift-off parameter was set to LO = 1mm. This value was kept at a constant level for all the specimens. A classical 2D raster scan was performed for all the surfaces of each specimen. Only the maximum value of the residual magnetic field was taken into account to be the graphical dependences visualized. The rectangular area shape of the 2D scan was defined as follows: number of scanning lines N = 120, step distance of S = 0.1 mm, scanned length per line of SP = 40 mm. The measured data were acquired using the data acquisition card (DAQ) with resolution of res = 16bits/channel, sampling frequency of f s = 10kS/sec. The user interface for data manipulation, controlling the stage and processing the data was designed using the LabVIEW software (virtual instrumentation). The second part of experimental measurements was realized on device Magnet Physik, which is used for measuring of hysteresis loops. On this device, it is possible to determine magnetic quantities (remanence, coercivity), make measurements with surrounding coils to determine the magnetic mean values and measure at temperatures up to 200 °C. The measurement was performed under normal conditions at room temperature. The temperature of specimens was 21 °C. The magnetic excitation fields that are necessary to record a hysteresis loop were generated by the electromagnet EP 3. The maximum current of the electromagnets power supply was set to ±10 A and time of increasing of current to maximum to 40 s. During all the measurements the demagnetization was on. Results of the experimental measurements are presented in this section. After each 2D raster scan of the whole biomaterial shell, the maximum of the gained values was extracted. This procedure was performed three times (individually for X, Y, Z axis). These values were used for construction of the following graphs. Further, the module value was computed as SQRT of summed squares of the three spatial values. As can be seen from the results, with increasing plastic deformation level, the output signal of the GMR sensor increased, as well. This means that the higher the deformation, the higher the magnetic response of the specimen. Of course, the great differences between individual materials were revealed: the strongest signals were gained for the AISI 304 biomaterial. This is in correlation with theoretical background (the highest amount of the ferromagnetic martensitic components was present here). Further, the responses valid for the AISI 316L and AISI 316Ti showed that the residual magnetic field was rapidly decreasing than for the previous one. Although, the AISI 316L material has better resolution among the individual field components, in comparison to the AISI 316Ti. Practically, it has to be concluded that deformation levels lower than 5% were not successfully detected by the sensor. On the other hand, change in magnetic biomaterial properties (caused by the mechanical deformation) was clearly revealed. The scanning procedure performed in all the three axis of the 3D coordinate system, showed that it is sufficient to sense only one component. Resulting value is approximately the same for each component. Fig. 3 displays the module value of the inspected 4. Results