According 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.
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.
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
The 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