The testing of metals to determine tensile and shear strength, conductivity and the presence of defects can be achieved mainly in two ways. One involves testing the metal till it breaks or fails, for instance in a tensile strength test. The specimen is clamped at its ends, then pulled until it breaks cleanly or undergoes plastic deformation.
This first method is destructive, and while it still has its applications, it does not take a rocket scientist to figure out the limitations of such a method. When there are limitations, solutions are usually round the corner. The solution in this case is NDT. Tapping on various scientific principles, the same results can be derived, with less irreversible consequences. The benefits in terms of cost are, needless to say, substantial.
Eddy Current Testing (ECT) — Principle & Advantages
Eddy current testing, or ECT, works on the principle of electromagnetic induction. Alternating current is passed through a coil, producing a magnetic field. When the coil is placed near a conductive material, such as steel or copper, the changing magnetic field from the movement of the coil induces a current flow in the material.
This induced current flow, known commonly as an eddy current, travels in closed loops, producing a magnetic field of its own that can then be measured and used to find flaws as well as characterise conductivity, permeability and dimensional features. Usually, changes in eddy current flow are the result of the presence of cracks within the test material.
One huge advantage that ECT has over other techniques is its ability to inspect materials moving at fast speeds. This is especially helpful when used to check the quality of metal parts such as wires, bars, tubes and profiles being moved along on a conveyor system, such as in a production line. Other techniques like penetrant testing are more time-consuming, making it impossible to inspect all the produced parts.
Another advantage of ECT is its ability to inspect both ferromagnetic and non-ferromagnetic materials, instead of just ferromagnetic ones favoured in magnetic particle testing. The latter establishes a magnetic field in the test material itself, which exits and re-enters at the poles of the material. Defects such as cracks and voids cannot support too much flux, leading to leakage. This flux leakage is then detected to provide a more visible indication of the location of such defects.
A third advantage of ECT is its contactless inspection system. The ECT probe does not have to make any contact with the test material, reducing scratching or pitting of the metal surface. When scratched or pitted, metal surfaces can collect dust and bacteria in this micro-depressions, often invisible to the naked eye. Industries which require these metals to ultimately be used in clean rooms with negligible foreign matter and dust content, will therefore find ECT very helpful.
Probes & Sensors
While the principles behind ECT have mostly remained the same throughout the years, the technologies of probes and sensors used in ECT have certainly undergone some changes.
Some of these changes have been made to improve the reliability of ECT. For instance, it is widely known in the industry that one of the disadvantages of the contemporary ECT inductive coil probe is its inability to detect flaws that lie parallel to the inspection probe coil winding direction.
This usually means that small cracks that originate at the edges of a specimen are tough to find. An example would be the cracks that appear around the fastener or rivet holes in aircraft multi-layered structures. Often, the signal created by the edge or hole itself drowns out the smaller signal coming from the crack.
As such, instead of using these coil probes, solid-state magnetic sensors based on Giant Magneto-Resistance (GMR) and Spin-Dependent Tunnelling (SDT) are now preferred. These solid-state magnetic sensors, unlike the coil probes, can be oriented in such a way to cut off any signal coming from the edge, allowing for defects there to be properly detected.
Nevertheless, coil probes are still the most widely used in the industry, given their low manufacturing costs, and ease of usage. There are several coil probe types which are commonly used. They include encircling coil probes, pancake-type probes, spiral coil probes and horseshoe-shaped coil probes, depending on the application.
Absolute Vs Differential
Probes in ECT can also be divided into two groups depending on whether they are used to measure an absolute voltage signal or to compare two different parts of an inspected material. Probes used to achieve the former are known as absolute-mode probes. They consist of a single coil that generates eddy currents and senses changes from the eddy current field.
Absolute probes can detect long flaws or slow dimensional variations in tubes or bars. Besides crack detection, the absolute change in impedance of the coil probe provides much information on grain size, hardness and stress measurement. However, these probes are usually susceptible to variations in temperature, where they can then lose sensitivity.
Probes which compare two different parts of the same material are known as differential-mode probes and have detecting coils wound in the opposite directions to make equal the induced voltages originating from the primary magnetic field.
Differential-mode probes have the advantage of being able to detect very small discontinuities. However, differential coils do not detect gradual dimensional or composition variations, unlike absolute probes. This is mainly due to the close proximity of the coils.
SQUID
Another sensor that has been used in ECT since the 1980s is the Superconducting Quantum Interference Device or SQUID, a very sensitive magnetometer designed to measure extremely weak magnetic fields.
While they may often be thought of as expensive, not to mention hugely cumbersome given the need to cryogenically refrigerate them to lower noise levels, using SQUIDs are highly advantageous given their high sensitivity, even in an unshielded environment.
SQUIDs have been used to test for mechanical stress, cracks and corrosion on aeroplane fuselages. As mentioned earlier, such defects are often found in hidden layers near the rivets. With SQUID, the signal from the crack is distinct and separated from the underlying signals caused by rivet or fastener holes in the fuselage itself. This is usually not possible with other, less sensitive sensors.
The technology for sensors and probes in ECT is no doubt growing, fuelled by the need to make readings and detect flaws in a shorter period of time, without the ambiguity that arises when measuring at edges, or near rivets and fasteners where holes are already present.
While ECT technology is changing, its usefulness in this regard can be said to be a constant, and those who choose to utilise it will continue to benefit from this NDT technique in the future.
