Maximising the benefits of NDT using Simulation

Eddy Current Non-Destructive Testing (Eddy Current NDT) is widely used to inspect conducting materials both during manufacture and in service. The phenomenon of Electromagnetic Induction is exploited by placing a current-carrying conductor in the vicinity of a test specimen and observing the effect of the changing flux on either secondary “receiving” coils, or back on the primary coil (Figure 1).

Figure 1: Coil set over crack showing eddy currents flowing in surface

Figure 1: Coil set over crack showing eddy currents flowing in surface

Variations in the electrical conductivity or magnetic permeability due to flaws in the test specimen will cause associated changes in eddy current distributions which leads to a corresponding difference in the induced current (and hence measured reactance and impedance – Figure 2) in the receiving coil.

 

Figure 2: Comparison of measured vs simulated data of real and imaginary flux linked by a pickup coil as the probe moves perpendicular to a defect.

Figure 2: Comparison of measured vs simulated data of real and imaginary flux linked by a pickup coil as the probe moves perpendicular to a defect.

Eddy Current NDT is a powerful tool for measurement of flaws in test specimens in the lab; in the proper circumstances it can be used for:

  1. Crack/flaw detection
  2. Material thickness measurement
  3. Conductivity measurements

However it is sensitive to a number of usage issues in the field, and hence the device itself and the way it is deployed is critical in the success of any potential NDT exercise. For this reason, if you are either an Eddy Current NDT probe designer, or user, you could benefit from simulation to help you to maximise the understanding and value to be gained from your equipment. Using simulation you can investigate real-world issues such as the effects of:

  1. stray fields
  2. residual magnetization
  3. operating frequency

and minimise the number of coupon tests you need to routinely carry out to fully understand your testing hardware, sensitivities and procedures.

 

Relevant Capabilities of Opera

 

Field Accuracy

 

The Opera Simulation Software Suite offers a number of techniques to ensure that a simulation minimises discretisation errors that can occur with finite element analysis. These include mesh control and mesh quality, the shape functions of elements used, advanced boundary conditions as well as novel coil methodologies that may be used to drive the electromagnetics. Similar to techniques used in MRI/NMR simulation to obtain field accuracy to 1 part in 1e6, the accepted NDT technique of performing the with/without crack analyses using exactly the same mesh (to minimise the errors introduced by the mesh) is very easy to apply in Opera thanks to the option to manually control mesh densities and positions.

 

Mesh control, quality & shape functions – Whilst overall control of the mesh is provided to the user the auto-meshing algorithms do all the hard work. The option to mesh three-dimensional problems with hexahedral and prismatic elements is vitally important to surface current flow problems where resolution of proximity/skin effects are paramount (as hexahedral/prismatic elements can withstand higher aspect ratios than tetrahedral elements without loss of solution accuracy). Functional mesh layering (which can be tied into skin-depths) provides an easy route to ensuring the right results are gained first time.

 

Boundary Conditions – As well as the usual external boundary conditions, Opera has several advanced boundary condition options. The Surface Impedance Boundary Condition (SIBC) provides an analytical description of a surface current flow that is mapped onto the body without the need to resolve the skin depth of the current flow with a mesh. This is especially useful for high frequency high conductivity evaluations (in which NDT often resides) where resolving surface current flow is impractical. The Electrically Insulating Boundary Condtion (EIBC) acts as a discontinuity in the conductivity of the material. Attach this to a face between two bodies and you have an instant flaw without any of the possible issues around meshing small gaps, which can introduce artificial discretisation errors and/or make the problem significantly larger.

 

Mesh–independent Conductors – the facility to have conductors that are not meshed, nor are attached to the mesh means that they are very easy to move around the problem (without re-meshing sections or the whole problem) whilst high field accuracy can be maintained. This makes movement studies simple to perform.

 

Process Automation

 

All of Opera’s menu-driven actions are recorded and are available for replay in macro files. The macros can be extended with conditional controls and used to automate pre-processing, solution and post-processing. There’s no need to repetitively manual drive Opera to study many coupon tests. Use something as simple as an Excel spreadsheet to configure the testing process.

 

Parametric Studies

 

Opera’s three dimensional modelling system has built-in parametric capabilities, allowing rapid parameter sweeps to be carried out. This is applicable to geometrical parameters, material properties and any other user defined quantity.

 

Design Optimization

 

All of Opera’s parameters can be used in an optimization study by the built-in Optimizer to determine the optimum parameters for the required response – the Optimizer can handle multiple competing objectives and constraints. A powerful design tool.

 

Validation Examples (Reports):

TEAM 8: Coil Above a Crack: A Problem in Non-Destructive Testing TEAM 8: Coil Above a Crack: A Problem in Non-Destructive Testing

TEAM 15: Rectangular Slot in a Thick Plate: A Problem in Non-Destructive Evaluation TEAM 15: Rectangular Slot in a Thick Plate: A Problem in Non-Destructive Evaluation

TEAM 27: Eddy Current NDT and Deep Flaws TEAM 27: Eddy Current NDT and Deep Flaws