Non Destructive Testing Part 2 Common Techniques

engineering, year end, Origen

Following from our previous tech tip which addressed some of the key elements associated with NDT, this Tech Tip discusses some of the more commonly used NDT techniques. For External (Surface) Flaw Detection, the NDT process typically starts with Visual Inspection, which can be enhanced by Dye Penetrant. Crack like defects can be readily manifest by cleaning the surface and exposing the surface in question to a dye penetrant (coloured ink or dye) and allowing it to soak in to any local surface breaking defects. The surface is then wiped clean, followed by the application of a ‘developer’ (normally a white powder like aerosol that is sprayed onto the surface). Any crack type defects present become visible as the dye leaks out from the crack into the white developer to form a coloured ‘line’, highlighting the presence of the defect. This simple, yet powerful technique is commonly used as the first indicator of the presence of cracks in a structure and is particularly useful for weld inspection, shrinkage cracks, or surface porosity.

Following from our previous tech tip which addressed some of the key elements associated with NDT, this Tech Tip discusses some of the more commonly used NDT techniques. For External (Surface) Flaw Detection, the NDT process typically starts with Visual Inspection, which can be enhanced by Dye Penetrant. Crack like defects can be readily manifest by cleaning the surface and exposing the surface in question to a dye penetrant (coloured ink or dye) and allowing it to soak in to any local surface breaking defects. The surface is then wiped clean, followed by the application of a ‘developer’ (normally a white powder like aerosol that is sprayed onto the surface). Any crack type defects present become visible as the dye leaks out from the crack into the white developer to form a coloured ‘line’, highlighting the presence of the defect. This simple, yet powerful technique is commonly used as the first indicator of the presence of cracks in a structure and is particularly useful for weld inspection, shrinkage cracks, or surface porosity.

Magnetic particle inspection (MPI) is also routinely used for detect surface breaking flaws.  Although the technique can be used on components with a wide variety of geometries, it is particularly useful for detecting cracks in circular pipes or round bars (of steel, or other ferro-magnetic material).  The technique utilises an electro-magnetic coil (or sometimes a permanent magnet) to develop a magnetic field in the surface layers of the component.  A liquid, containing fine iron particles, is then sprayed on to the component, and the iron particles agglomerate at crack boundaries because of their associated localised magnetic fields being interrupted by the crack.  Typically, the fluid contains fluorescence particles which under UV light, readily highlights any cracks.

Eddy current NDT is another form of electro-magnetic testing, which employs an electromagnetic field to induce circulating eddy currents in the component in question.  These eddy currents are then detected by a coil in a probe, which detects changes in the eddy currents as a result of near surface and surface breaking flaws (or cracks).  The resulting change in the coil voltage can then be detected and if necessary recorded.

Flaw detection for subsurface and/or embedded defects is more complex than surface NDT.  X ray radiography has already been mentioned (which is good for detecting volumetric type defects), but is much less effective (and sometimes almost impossible) for fine crack detection.  The standard NDT approach to detect internal defects is now dominated by ultrasonic techniques.  A series of short pulses of ultrasonic sound are transmitted into the component through an acoustic fluid coupling in a targeted direction, governed by the angle of the piezo-electric probe.  This sound wave travels through the metal and is reflected back from surfaces (such as the back wall), but also crucially by crack surfaces and internal flaws.  These reflected sound waves are picked up by another piezo probe and displayed as a defect signal.  The details of the reflected signal of this ‘pulse–echo’ technique depends on the flaw (or crack) boundary and its position within the material.  Its location can be established by careful movement of the transmitter and receiver probes, but this causes amplitude variations, which can lead to detection scatter (of up to several millimetres of error), and good operator skill is required for successful interpretation.

This ultrasonic methodology can be improved by using Time of Flight Detection (TOFD), where two probes are again used, but at a fixed distance apart.  The component is ’flooded’ with ultrasound and the ends of a crack effectively act as ‘ringing’ sound sources, albeit of smaller amplitude.  Because the probes are at a fixed distance apart, the defect position can now be inferred from the time (which can be detected much more accurately than position) taken for the sound wave to travel from the defect to the probe.  This allows the flaw to be detected and sized with significantly improved accuracy, with errors typically in the order of 0.5mm (an order of magnitude improvement over conventional ‘pulse echo’ methods).

The next development in ultrasonic techniques is to control the angle at which the ultrasound is transmitted into the material.  In phased array techniques the ultrasound is introduced at a range of angles in order to detect a variety of defects, but this gets into the realm of more sophisticated ultrasonic NDT methods.  Some of the more advanced, but generally less common, techniques include, acoustic emission (AE), digital leak testing, laser testing methods (holography, shearography)), magnetic flux leakage, neutron radiation, infra-red and thermal imaging (wave length detection), electro-magnetic acoustic inspection, guided waves, automated inspection techniques, potential drop flaw detection, vibration modes/resonant inspection and alternating current field measurement (ACFM).  Each of these have their own advantages and disadvantages and are generally offered by specialist companies that specialise in the specific technique and guard the IP competitively.

This is the last Tech Tip for the year – we trust you have found the tips informative and they have been useful in helping improve integrity/prevent failure.  Please note we will be closed from 15 December 2020 and will be back on 4 January 2021.

The Origen team wishes you and yours a stress free festive season and the New Year will be a significant improvement on 2020!  Please remain vigilant during the festive season and stay COVID free.