Magnetics Business & Technology - November/December 2019 - 10

FEATURE ARTICLE: TECH TIPS

Solving Eddy Current Non-Destructive Testing Benchmark Problem with
Integrated Engineering Software's Program Faraday
By Amandeep Bal, Ph.D., Integrated Engineering Software
Market need
Eddy current testing (ECT) is a non-destructive (NDT) testing
technique widely used in magnetic sensors to check the integrity of electrically conducting parts and notably to detect flaws.
This sensitive technique can locate tight cracks of various types
of surface on both non-ferromagnetic and ferromagnetic materials. The factors that are driving this market are advancements
in eddy testing technologies and this has encouraged customers to adopt it. The constant demand from power generation,
manufacturing, and aerospace sectors have resulted in high
adoption of this technique across the world. Stringent regulations related to quality control and periodic inspections laid
down by various governments, along with user-friendly software
and continuous advancements in automation sectors, are key
drivers for the growth of the eddy current and inspection market.

Scenario and objective

approach is to set the solver in "fields mode", second approach
is to set the solver in "circuit parameters mode", and third approach is to use "coils and windings" feature to simulate coils.
In "fields" mode, the "Analysis" menu will display bulk results
such as energy, total charge, etc. including mechanical calculations such as force and torque while in circuit parameters mode,
menu will display Capacitance/ Inductance, etc. The "coils and
windings" feature in the software is useful to design coils and
windings in electric motors and can easily overcome challenges
like analyzing resistive and inductive requirements, eddy currents and proximity effects.

Geometry

The input parameters for the model are taken from TEAM Workshop Problem #15-1. The description does not mention how big
the plate used in the measurement is. Therefore, in order to see
the effect of edge of the model on results, two different sizes of
plates with thickness 12.22 mm are studied. Both sizes are big
enough so that the coil does not reach the edge. The length
of defect parameters in plates is 12.60 mm, depth is 5.00 mm,
and width is 0.28 mm. The inner radius of coil is 6.15 mm, outer
radius is 12.4 mm, length is 6.15 mm, number of turns in coil
are 3790 and lift off is 0.88 mm. Only a half model is produced
for first two approaches (fields and circuit parameters mode)
because geometry is symmetric about the center for all coil
locations and the number of unknowns required to solve the
model could be reduced (hence reducing the solution time).

Setting up Physics in the software

In Faraday's Physics Global Setup menu, circuit parameters/
fields mode is setup with frequency as 900 Hz. In the Material
Table, a conductor with conductivity 3.06x107 S/m is created
and then applied to the plate. Symmetry of the model is set up
in Symmetry and Periodicity Setup. Volume Current of 3790 A
is introduced in the coil. Coil is assigned as Volume Conductor
#1 and volume Conductor #1 is assigned 1 A (per turn - together with the total current makes a 3790-turn coil).

Fig. 1

Integrated Engineering Software (IES) is an industry leader to
provide user-friendly software programs for various electromagnetic applications. IES's software program Faraday is a 3D
time-harmonic eddy current field solver that gives the advantage to meet ECT challenges head on.
ECT technology uses a time-varying magnetic field, generated by an excitation coil with a sinusoidal or transient current
regime, to induce eddy currents in the material. In this article,
most popular eddy current nondestructive testing Benchmark
problem TEAM Workshop Problem #15-1 * is validated in
version 10.1 of Faraday. The scenario involves a circular aircored coil scanned parallel to the x-axis, along the length of a
rectangular slot in an aluminum alloy plate. The image can be
seen in Fig. 1. The objective is to compute the change in the
inductance and resistance of the driving-point impedance of the
coil (compared to its value over an unflawed portion of the
plate) as a function of coil position, and compare the computed results to the experimental results. Both the frequency
and the coil lift-off are fixed. Three different approaches in the
Faraday v 10.1 program are followed to solve this problem. First

Setting up Solvers in the software

Faraday has both finite element method (FEM) and a Boundary
Element Method (BEM) solver available. FEM provides sufficient accuracy for engineering purposes, and many problems
are by their nature inherently closed region. BEM not only provides the most accurate numerical field solutions but is also the
method of choice for problems involving the modeling of space
around the device: that is what we call "large open regions".
Since this is an open region problem, and furthermore there are
no nonlinear materials, the BEM solver is expected to be superior and is applied to solve this problem in fields/circuit parameters mode. Since the "coils and windings" feature of software
works with FEM solver, so FEM is applied for the solution while
using this feature.

Generating an element distribution with the Automatic commands

The "Automatic All" commands in the software automatically
generate the specific type of 2D or 3D elements in all surfaces
(or volumes as appropriate) that require them based on physical
assignments to the geometry. In this problem, the plate requires

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