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ELECTRO-MAGNETIC (EM) TECHNOLOGY

Electro-Magnetic (EM) technology is related to the EM induction of fields in a conducting material to generate an eddy current or an acoustic emission (AE) for nondestructive testing. While the EM induced eddy current is a mature technology, the EM induced AE is a fairly newer concept developed by our research group to solve the problem of detection and characterization of metal casting with complex structures, weldments, and weld repair defects.

Electromagnetics Diagram

It is known that an electrical current passing through a plate of material containing a defect (such as porosity, cracks, or inclusions) is concentrated at the tips of the defect. This effect of electric field distortion by a defect or flaw can be used for Nondestructive Evaluation (NDE) of the structure.

The induced high current density in conducting materials containing flaws (electric field concentrators) will load the defect faces with electromagnetic forces, which will tend to open them (due to current flowing in opposite directions along the defect faces). In addition, the tips of the defect act like single turn solenoid coils, which concentrate the magnetic flux and produce localized Lorentz forces along with local thermal stresses from Joule heating. The effective current density at the defect tips may be higher than in all of the surrounding material by one order of magnitude (depending on stress or field intensity factor). If the current density is high enough, the generated heat may be sufficient to melt the material at the defect tips, a phenomenon that has also been used to arrest fast-growing cracks. However, if the effective current density has the right value, the effect will be the generation of AE without an increase on the flaw size. Finite element analysis can predict the values of the electric current density around a defect embedded in a metallic sample.

Preliminary experimental work has demonstrated that AE can be generated by electric current redistribution around the tip of a defect in a metallic sample. The EM induced AE may also be enhanced by the application of an external magnetic field normal to the defect plane. The presence of this field increases the Lorentz stresses at the defect tips by the creation of an additional interaction between the dynamic electric current and a static magnetic field. This mechanism of coupling the electrical and acoustic energy is similar to the one used in to electromagnetic-acoustic transducers (EMAT).

By using the EM stimulation method to produce AE, specific areas may be inspected without loading the whole structure since EM induction produces local loading. Structures may be tested in-situ and there is no need for disassembly and fabrication of special test fixtures used to apply mechanical loading. EM induction loading may also permit AE "inspection on demand", which could reduce the need for long term AE monitoring.

As we know, short, high-intensity pulses of electric current are capable of causing flaws to generate AE. This method has the potential for advancing the identification and quantification of structural damage in aerospace structures. The potential exists for the detection and quantification of cracking and other structural defects in aging aircraft, large complex castings, and multilayered splices.
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