Pulsed Eddy Current Testing

T.O. 33B-1-1 4-92 4.9.2.3 Pulsed Eddy Current Testing. Conventional multifrequency systems usually utilize two or three frequencies. Additional frequencies require very complex multiplex mixing systems to analyze the information from the test. A variety of experimental techniques have utilized the multifrequency characteristics of a short electrical pulse to achieve the same type of results as the multifrequency test technique. In principle, this technique is advantageous in that it requires simpler electronics to process the data. It can potentially generate higher frequencies than fixed frequency systems. This would allow testing of thinner materials, and materials with very low electrical conductivity (high resistivity). The eddy current pulse can also be a very short, high voltage pulse that can be used to momentarily produce magnetic saturation in a ferromagnetic part. This will allow detection of subsurface flaws in ferromagnetic materials. 4.9.2.4 Low Frequency Eddy Current Inspection. In the past most eddy current testing utilized test frequencies of 10 kHz to 1 MHz. Improved equipment and data processing techniques now allow the use of test frequencies as low as 55 Hz. Along with impedance plane equipment to measure signal phase, this has provided a means for testing multilayer materials and thick materials. Detection of deep subsurface cracks, cracking in intermediate layers of material, and corrosion on the backside of a material are possible. 4.9.2.5 Barkhausen Noise Testing Of Ferromagnetic Materials. Abnormal stresses induced by shot peening, other cold working processes, and grinding burns affect the structural properties of a material and can lead to flaw growth and part failure. In ferromagnetic materials, these processes affect the ease with which the magnetic domains in the surface of the material can be moved. In unmagnetized ferromagnetic material, the magnetic domains are randomly oriented. If the material is subjected to a magnetic field, the magnetic domains tend to align themselves in the direction of the magnetic field. When the domains move to align themselves, electrical pulses are generated during the domain movement. This is called Barkhausen noise. This electrical noise can be detected and measured by Hall effect sensors. If the material is free of abnormal stresses, the domains are relatively free to move and little Barkhausen noise is generated. Areas of tensile stress parallel to the applied magnetic field cause an increase in Barkhausen noise. Examples of applications of this test method are ferromagnetic engine components and landing gear. Barkhausen noise measurements are also used to detect the quality of drilling and reaming of holes in ferromagnetic material. 4.9.3 Developmental Techniques. 4.9.3.1 Metal Thickness Measurements. A wide range of thickness can be measured with low frequency eddy current test equipment. 4.9.3.2 Metal Spacing. The spacing of metal sheets separated by a nonconductive adhesive layer can be successfully measured by using an eddy current frequency for which the thickness of both metal sheets is less than or equal to three times the corresponding standard depth of penetration. 4.9.3.3 Alpha-Case on Titanium. Oxygen diffusion from the surface of titanium alloys, known as alpha-case, can lead to surface embrittlement and cracking. This condition can be detected using high frequency (frequencies above 500 kHz) eddy current testing. 4.9.3.4 Titanium Aluminide. Brazed honeycomb panels formed from titanium alloys and 3003 aluminum create a brittle intermetallic titanium aluminide at the braze interface. The thickness of this interface is critical to the integrity of the structure. While eddy current methods show promise of measuring the interface thickness, further testing is required to produce reliable measurements. 4.9.3.5 Magneto Optic Imaging (MOI). Corrosion in aircraft structures is difficult to detect with existing NDI techniques, particularly in combination with moisture entrapment. Magneto-optic imaging, also referred to as magneto-optic/eddy current imaging, has been identified as a potential candidate NDI technique which may be more reliable than the currently mandated eddy current