NDT is widely used to check for defects
Keith Syvinski, Franklin, United States
Design&Measurement
Recent developments in probe and sensor technology have made it easier to detect flaws and defects in aircraft fuselages
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 multilayered 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 MagnetoResistance (GMR) and SpinDependent 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 horseshoeshaped coil probes, depending on the application.
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.
Absolute Vs Differential Probes in ECT can also be divided into two groups depending on whether they are used to measure
SQUID Another sensor that has been used in ECT since the 1980s is the Superconducting Quantum
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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. Enquiry No. 2202 Turn to page 80a or log on to www.equipment-news.com to enquire
March 2014 asia pacific metalworking equipment news
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