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Rectangular Slot in a Thick Plate: a Problem in Non-Destructive Evaluation (T.E.A.M. Problem 15)

Sensors and NDT with MagNet for SOLIDWORKS

This example demonstrates how MagNet for SOLIDWORKS can be used for non-destructive testing and sensor problems. A circular coil moves along an aluminum plate that contains a slot. The goal is to detect the slot by calculating the resistance and inductance of the driving coil at various positions.

In order to detect flaws, sensor problems require the accurate calculation of induced eddy currents. The AC Time Harmonic solver automatically calculates these eddy currents. The results presented compare published experimental data demonstrating the accuracy of the software.

The following problem is based on the Testing Electromagnetic Analysis Methods (T.E.A.M.) problem #15: Rectangular Slot in a Thick Plate: a Problem in Non-destructive Evaluation. The benchmark can be found on the International Compumag Society's website.

Half Model View of the Coil Over the Slot

METHODS and RESULTS

DEVICE MODELING: MESH

The rectangular defect is quite thin compared to the size of the coil therefore needs to be finely defined in terms of mesh. This will ensure accurate calculation of inductance and resistance of the coil. MagNet for SOLIDWORKS has easy to use meshing features allowing for maximum accuracy while keeping simulation times short. Surface mesh refinement on the plate was only applied on faces where the coil will travel over.

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ARROW PLOT of the FLUX DENSITY FIELD

The arrow plot shows the impact of a defect in the aluminum sheet on the magnetic field created by a driving coil.

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FLUX DENSITY FIELD DISTORTION due to the DEFECT

The image shows vector magnitude of the flux density field displayed concurrently. This enables extra information coming with arrow's direction to be displayed while keeping the clarity of vector magnitude plot. This image is with the coil shifted by 10 mm and therefore the flux density field in the plate is affected by the air gap created by the slot. It can be seen that the field inside the plate is not symmetric about the coil center.

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CURRENT DENSITY FIELD

The current density field line plot also gives insight of the effect of a defect in an aluminum plate. Here we can see the clockwise induced current in the plate created by upward sinusoidally varying magnetic created by the coil. MagNet for SOLIDWORKS' AC Time Harmonic accurately calculates these induced eddy currents. Since the coil is shifted by 10 mm in this picture, it can be observed how the current in the plate is altered by the defect. These eddy current in the plate affect the coil's inductance and resistance.

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COMPARISON OF THE CHANGES IN THE COIL'S INDUCTANCE

The inductance values were obtained using complex voltage and current. Here they are compared to experimental values that were included in the problem's definition. The experimenter reported 0.04 mH error associated with these values. The computed results are in agreement with experimental values.

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COMPARISON OF THE CHANGES IN THE COIL'S RESISTANCE

The resistance values were also obtained using complex voltage and current. The reported experimental error for the resistance values is 0.1 Ω. Computed values nearly all fall within this experimental error, this demonstrates the accuracy of the software.

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