IntroductionAccording to the American Society for Non Destructive Testing (ASNT), NDT is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used. Modern NDT practices are used extensively in manufacturing, in-service inspections and construction. They are used to ensure product integrity and reliability and to ensure quality. For example, NDT is widely used to test railroad tracks. This is an example of in-service NDT inspection and is necessary to ensure structural integrity of tracks and public safety. There are many kinds of NDT - ultrasonic, acoustic, electro-magnetic, laser-methods etc. In this article, we will cover an important electro-magnetic based NDT method called Alternating Current Field Measurement (or ACFM). EMS is a 3D field simulation software that can be used to design and test the effectiveness of NDT probes.
What is ACFM?Alternating current field measurement (ACFM) is an electromagnetic technique that permits non-destructive detection and size estimation of cracks extending to the surface of ferrous and non-ferrous metals. It is important because it is a technique that is applicable to test both ferrous and non-ferrous materials. The advantages of this technique are - little or no surface cleaning requirements, no need for repeated calibration, evaluation is quantitative and there is no need for a magnetic medium. Hence it is considered more promising than conventional magnetic particles testing methods. The ACFM probe includes one or more coils that induce electric currents (or induced currents or eddy currents) on the surface of the material that needs to be tested. The induced currents create a magnetic field above the surface. In the presence of an exposed crack, the induced currents are disturbed (or perturbed) and also the magnetic field. This change in the magnetic field is measured and gives us a good idea about the presence of a crack as well as the size of the crack. When you design a probe to precisely do this function, there are many design parameters which control the effectiveness and performance of the probe. They are the size, location of the coils, current and excitation frequency. Trying to find the optimal values for each of these parameters using experimental techniques is time consuming, costly and often error-prone. A software like EMS (which employs Finite Element Method or FEM) can be used to effectively design and optimize NDT probes.
Example of a Twin Coil Inducer ACFM probeThe ACFM probe (Figure 1) contains two coils and a magnetic sensor packaged inside a container. The container is made of plastic (a non-magnetic material). The magnetic sensor can detect the value of the magnetic flux density when the probe is kept in contact with the test surface. The probe is typically scanned along the direction shown. The magnetic flux density in the Y direction (By) is measured by the sensor. When there is no crack, the value of By is more or less uniform but when a surface crack is detected, the value of By changes. The change in the value of By is detected and compared against pre-defined values to detect the size of the crack. Each coil consists of 36 turns of 0.5 mm dia copper wire. The RMS value of the current is 0.6 amps and the frequency is 6 KHz.
Figure 1 - A model of an ACFM probe
Results from EMSThe following figures show the results obtained from EMS. Figure 2 shows the current density vector distribution on the specimen and Figure 3 shows the magnetic flux density (By) on the surface of the specimen. Figure 4 shows the variation of magnetic flux density, By at the position of the magnetic sensor as the probe is moved along the length of the crack.
Figure 2 - Current density distribution around the crack
Figure 3 - By distribution on the plate
Figure 3-A - Magnetic Flux Density plot
Figure 4 - Variation of By with distance of the probe from the crack