Wideband Micromagnetic Multi-parameter Materials Characterization with 3MA

The frequency-dependent penetration depth ("skin effect") of the electromagnetic field in electrically conductive materials is a well-known phenomenon. When applying this to micromagnetic testing, it becomes possible to control the depth of the characterized volume through variation of the excitation and analysis frequencies. In the case of IZFP's Micromagnetic Multi-parameter Microstructure and Stress Analysis method (3MA), the magnetizing frequency, the Barkhausen noise analysis frequency range and the eddy current testing frequencies may be varied. Applications such as the nondestructive detection and quantitative description of grinding damage via depth gradients of the hardness, residual stress and retained austenite percentage, have already been demonstrated lately. In order to further explore the frequency-domain information, raw wideband signals of all magnetic measured quantities are required. IZFP has developed a new generation of 3MA testing devices which allows us to excite the material with arbitrary waveforms in the frequency range of 0 Hz to 10 MHz and to record all relevant magnetic quantities with a 40 MSamples/s, multichannel, 14-bit A/D converter attached to an FPGA which performs most of the digital signal processing in real-time. This paper discusses the results obtained with this system on the example of samples with surface-near gradients. New analysis methods are proposed which are expected to increase the interpretability and repeatability of the measurements significantly.