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Non-Destructive Testing applications: TEAM problem 15 Application


Eddy Current Testing  

Non-destructive testing (NDT) is nowadays widely used for many applications in the aerospace, petroleum and civil engineering and many more manufacturing and service environments to ensure safety and production quality. Eddy current testing (ECT) is a NDT technique that helps to efficiently examin elarge surfaces of electrically conductive material using electromagnetic laws. It does not require the use of coupling liquids or any contact with the specimen. In addition to surface and subsurface flaw inspection, ECT can be used to determine a variety of material properties, identify corrosion, etc.

Computer aided-design and simulation help to enhance and implement ECT techniques. In this example, EMS results are validated on the TEAM problem 15 - Rectangular Slot in a Thick Plate.

Description and objective of the problem[1]

The experimental arrangement is shown schematically in Figure 1. Here, a circular air-cored coil is translated, parallel to the x-axis, along the rectangular slot in an aluminum alloy plate. The current frequency and the coil lift-off are fixed, while the change in the coil impedance is measured as a function of the coil position. The parameters for this test experiment are listed in Table 1.

The objective is to compute the variation of impedance of the coil (compared to its value over an unflawed portion of the plate) as a function of coil position. EMS results will be compared to benchmark results. 

Table 1- Parameters of simulation and test experiment

The coil
Inner radius (mm) 6.15
Outer radius (mm) 12.4
Length (mm) 6.15
Lift-off (mm) 0.88
The test specimen
Thickness (mm) 12.22
The defect
Length (mm) 12.60
Depth (mm) 5.00
Width (mm) 0.28
Other parameters
Excitation frequency 900 Hz
Skin depth 3.04 mm
Isolated coil inductance 221.8 mH

 Figure below shows a 3D CAD model of the simulated problem. 

CAD model of simulated NDT example
Figure 1 - CAD model of simulated NDT example
 

Study

The AC Magnetic module of EMS is used to compute results such as magnetic flux density,eddy current density and loss density for a sinusoidal excitation. Other results given by EMS are totaleddy current loss, Joule loss, impedance matrix,etc.These four steps should be followed to perform an AC Magnetic simulation in EMS:

  1. Create a new AC Magnetic study
  2. Apply suitable materials to the parts
  3. Apply a suitable coil (either voltage or current driven) with the correct excitation.
  4. Mesh and Run the simulation
Thanks to the symmetry, only half model is simulated to limit the solving time and give more accuracy to the results by using finer mesh.  

Materials

The Coil is made of copper with high conductivity up to 57e+7 (S/m) while the specimen material has a conductivity of 3.06 e+07 (S/m).

Load Restraint 

The sensor here is the coil. Table 2 contains the coil properties.

Table 2 - Coil properties

  Number of turns Current magnitude Current frequency
Wound Coil 3790 6.15 A 900 Hz

Since, the symmetry is exploited in this example, tangential flux on faces regarding the symmetry plane should be added as shown in the figure below.

Preview showing the applied tangential flux

Figure 2 - Preview showing the applied tangential flux 

Mesh

Mesh quality is critical to every FEM simulation. The results accuracy and the solving time are strongly dependent on the mesh size. EMS allows user to control the mesh size on solid bodies and faces through the Mesh control feature. In this example, a mesh control is applied on the air domain surrounding the coil. The air in the crack is also finely meshed, with the maximum element dimension of 0.1mm. The whole specimen is too thick to be meshed with such a small element size. By splitting it using SOLIDWORKS features, two bodies are obtained and a mesh control is applied on the top body (Figure 3).


 Meshed Model
Figure 3 - Meshed Model

EMS results 

A parametric analysis in EMS allows user to sweep either geometrical or simulation parameters (current, number of turns, frequency, mesh size…). in the actual case, the distance between crack and coil center has to be variable. After running the parameterized simulation with flawed and unflawed specimen. The results of all scenarios in each case are given in a single study.

The isolated coil inductance omputed by EMS is 214.98 mH. Figures 4 and 5 show a comparison of the eddy current distribution on the plate in case of presence and absence of the defect. 

Current density distribution in case of plate with a flaw
Figure 4 - Current density distribution in case of plate with a flaw

Current density distribution in case of plate without flaw
Figure 6 - Current density distribution in case of plate without a flaw


The absolute impedance is calculated as in the formula below:

open vertical bar increment Z close vertical bar equals square root of increment X squared plus increment R squared end root

Where X and R respectively the reactance and the resistance of the coil. 

increment X equals omega increment L

Figures 6 through 9 show comparison between EMS and the benchmark results for , and impedance and phase variation.EMS solution agrees very well with the TEAM 15 results.                                                                                                            


EMS and benchmark results of delta L variation.
Figure 6 - EMS and benchmark results of delta L variation. 

EMS and benchmark results of delta R variation
Figure 7 - EMS and benchmark results of delta R variation.

Absolute impedance variation
Figure 8 - Absolute impedance variation

Phase variation 
Figure 9 - Phase variation

Conclusion 

AC Magnetic module of EMS can be used to accurately capture the effect eddy currents have on the coil inductance and losses in the material, making it the simulation tool of choice for a variety of Eddy current testing applications. It helps to developefficiently more accurate and reliable surface and subsurface flaw detectors. EMS results are confirmed by comparison with TEAM problem 15 results. 

Reference

[1]: http://www.compumag.org/jsite/images/stories/TEAM/problem15.pdf