Article by Tom Haxton HeroX Writers' Bench
Although destructive testing is used in certain industries, we are primarily interested in Non-Destructive Evaluation (NDE) techniques suitable for use ‘in-the-field’ against the typical defects found in large FiberFibre Reinforced Polymer (FRP) composite water infrastructure. The defects of interest are cracks, delamination and fiber-breakage deep within the structure (25mm depth). Some of the NDE techniques currently in use within other sectors include Terahertz Imaging, Radiography, Shearography, Thermography and Ultrasound. All have advantages and disadvantages, although none of them in their current form are entirely suitable for the rapid evaluation of large, in-place infrastructure. However, with modifications, new processing algorithms or other novel techniques, they could have a part to play. Either way, analysis of these and other techniques may help the idea generation process for a challenge winning entry to the HeroX Imperfection Detection Challenge. Let’s look at some of the main NDE techniques in use today.
Terahertz NDE of FRP composites is a new and exciting area of research. It is already deployed in the aerospace industry to check high performance non-conducting materials and was used to check the foam insulation on the Space Shuttle. The energy, frequency and wavelength of Terahertz radiation makes it particularly suited to the one-sided scanning of non-polar and non-conducting materials, including FRPs (but not carbon fiber as this is a conductor). Terahertz radiation can be used as pulsed energy or energy or a continuous wave.
The main downside of Terahertz Imaging is that the systems are not very portable, and scanning takes a lot of time (up to a few hours per square meter), although mobile systems are used for scanning artwork. The point to note here with relation to NDE of FRP infrastructure, such as pipelines, is that Terahertz NDE is in its infancy and novel techniques or new processing methods may allow revolutionary change in portability and scanning speeds. It is therefore a favorable technique for exploring with regards to the Imperfection Detection Challenge.
Radiography NDE uses X-Rays, Gamma Rays or sometimes particles such as neutrons. It is a very well-established technology used in a great many industries. Until recently, the technique required two-sided access to samples with the emitter on one side and the detector on the other but new back-scatter imaging techniques have now allowed some one-sided NDE applications. The X-Ray scanners at airports are an example of this one-sided backscatter technique. Radiography can struggle to detect defects that are perpendicular to the beam, but new processing techniques are beginning to overcome this limitation. Neutron beans are particularly exciting as they can differentiate between similar materials with ease.
Shearography is a coherent light optical technique. An image of the sample to be evaluated is taken using the coherent light source (normally a laser) and then another one taken of the exact same part of the sample but this time while it is under load. Any defects show as an ‘interference' pattern when the two images are combined. The system is quick, compact, mobile and resistant to environmental effects such as vibration, dirt, rain and wind. Shearography is used extensively in the aerospace industry, particularly for helicopter rotors, and in the renewable energy sector for wind-turbine blade testing.
Active thermography measures heat flow through an object with an IR sensor or camera. It works on the principle that defects will interfere with heat flow and this can be detected and processed using sophisticated software to give a near real-time image. The system is cheap and quick, and the heat can be applied in a number of different ways. Inspection depth can be limited, and this may be a restriction for certain large and thick pieces of FRP infrastructure. However, there are many unexplored areas such as alternative methods of active heating and different IR detection and processing systems.
(Image – Manuel de la Fuente, Pixabay)
There are a number of different ultrasound NDT techniques. Some have been around for a while, but there are newer techniques such as laser ultrasonics. All work on the principle that an ultrasound wave will be reflected or change in some other way when it encounters a change in material, density or shape. A detector can convert this change into an image that can be used by a trained operator to evaluate the integrity of the test object. Some of the newer ultrasound techniques may well be suitable for NDE of pipelines and the like if they can be adapted to work in the field at high speed and without an excessive training burden.