Sound it out - Ultrasonic fault detection

Materials World magazine
,
25 Sep 2012

Ultrasonic technology is increasingly being used to detect flaws in materials and structures in industries including oil and gas, defence and  transport. Ian Bowbrick of the Royal Academy of Engineering examines its applications.             

Early measurement and detection of latent and developing flaws in materials and structures is crucial in a wide range of applications in the energy engineering industries. Ultrasonic technology plays a key role in achieving this, and its use in industry is being supported by a new UK initiative developed by the University of Warwick and the Royal Academy of Engineering – the Centre for Industrial Ultrasonics, now known as the Warwick CIU.             

Comprised of member companies and organisations of all sizes, including multinationals, the CIU will predominantly include members from smaller UK-based companies that use ultrasonic technology. Its applications cover a wide range of industries, including energy markets, nuclear and other steam-raising plants, oil and gas, renewable energy, medical, defence and security, and rail and transport. While the structure of the CIU is yet to be fully established, it is hoped this will be  finalised through discussion with focus groups from key UK industrial companies and organisations involved in ultrasonic technology.             

A very common application for ultrasonics is in measuring remaining wall thickness of plant for the power generation industry, one of a large number of  industrial sectors that employs ultrasonic gauging to measure this parameter. A new development is the first generation of a special adapter that allows noncontact electromagnetic acoustic transducers (EMATs) to be used with conventional ultrasonic flaw detectors. These are typically used with contacting piezoelectric transducers and are unsuitable for use with an EMAT (except under some highly  favourable conditions).             

 

 

The advantage of non-contact transducers is that they allow measurements to be taken on ferritic boiler tubes without the need for expensive surface preparation. Furthermore, there is no requirement  for liquid couplant, and probe misalignment causing refraction is not an issue with non-contact ultrasonic probes. This produces significant savings in direct inspection costs and reduced inspection times while delivering increased inspection reliability. The latest  version of the adapter attaches to an EMAT, which is magnetically attached to a section of boiler tube that has been removed from service.             

This next generation of adapters allows operation on a much wider range of samples, such as ferritic  boiler tubes that do not have a favourable, well-adhered magnetostrictive magnetite surface layer. This is achieved using some of the latest field-programmable gate array (FPGA) technology inside the adapter, coupled to specialised amplifiers and signal conditioning electronics. Think of the FPGA adapter as behaving like a fast, relatively low-power computer, capable of performing intelligent signal  processing and combining features such as automatic gain control with signal averaging.        

The signal-to-noise performance of the system is boosted considerably by such developments, which enable operations on samples that simply could not have been considered previously, even when using state-of-the-art amplifiers with inefficient transducers. The flaw detector pulses at several hundred Hz, while its screen only updates at around 20Hz. Rather than lose this valuable information, the FPGA unit  automatically adjusts the amplifier gain then performs signal averaging, before  producing improved time-domain data as an analogue signal to the flaw detector. This is all done in such a way that as far as the operator is concerned, the system is simply behaving as though it were being used with a conventional piezoelectric probe in real time.             

Thickness gauging of metallic plant components will continue to play an extremely important role in power generation for both new-build nuclear plant and the next generation of fossil-fuelled plant in performing essential routine safety inspections. Furthermore, it will help engineers understand and track plant deterioration over time.             

On the same level             
Cans or containers moving fast down a production line are usually tested by passing  a wide beam of gamma rays through the container and measuring the beam intensity – the higher the liquid level, the lower the beam intensity. The CIU has developed a method that replaces the gamma-ray inspection method by using non-contact EMATs that generate a wide beam of ultrasound on one side of the can – a method that is  not only safer, but comes at a lower cost.             

A signal can be detected by a simple law of reflection that travels directly across the can, and another that bounces off the liquid-air interface inside it. Using simple trigonometry to measure how long it takes for the sound to cross the known can thickness, it is possible to measure the fill height using a self-calibrating approach. This gives accuracies  of fill level better than 1mm, even for cans moving at  high speeds.             

The same type of approach can be applied to measuring fill levels in large- or small-scale containers. In this case, the ability of the EMATs to  generate and detect sound at a metallic can wall without touching it enables the system to be built into conveyor belts on fast production lines.             

Go with the grain             
Measurements can be performed on the crystallographic grain alignment (or texture) in a metal sheet via non-contact ultrasonic transducers, which generate guided ultrasonic Lamb waves. These measurements provide valuable information on the mechanical and plastic properties of a material, and can potentially be used to monitor manufacturing processes and reduce costs. The crystallographic texture of a metal is determined  by the manufacturing processes involved, and has important implications for the way in which it behaves both elastically and when it is plastically deformed. By measuring the group velocity of  ultrasonic Lamb waves, the elastic anisotropy in the sheet can be quantified and related to predictions of formability. Using electron backscatter diffraction (EBSD) methods, the grain alignment in metal sheets can be measured.             

Good correlation has been obtained that averages  between the predicted elastic properties of the  material and the ultrasonic measurements – a direct measurement of the elastic properties of the sheet. The variation from maximum to minimum velocity is  approximately 1% or 50 ms-1, an accuracy that can only be obtained because of the non-contact nature  of the measurement.             

Although the ultrasonic method will never provide a complete and quantified measurement of grain  alignment and orientation distribution function, it does provide a measure of elastic anisotropy and can be used to predict formability – or even to produce simulated X-ray pole figures. While there is insufficient information in the ultrasonic data to produce a truly representative pole figure, it is possible to convey some of the key features expected from such an X-ray based measurement.             

Off the rails          
Most inspection of rail in-service is ultrasonic and is  usually restricted to low inspection speeds of around  20–30mph, which limits the viability of regularly  testing many tracks. In addition, the most serious defects that can develop in the rail head, such as rolling contact fatigue, can be difficult to detect using the inspection equipment currently available. A solution to this comes in the pitch-catch, low  frequency-wideband Rayleigh wave EMAT system, developed by the CIU for detection and depth  gauging of transverse cracks in rail head, such as  gauge corner cracking. The ultrasonic waves used  in this technique have similar characteristics to a classical Rayleigh surface wave. The waves propagate  along the surface of the rail, penetrating down to  a depth approximately equal to their wavelength,  between 2–15mm. The wideband surface generated contains a range of frequencies in a single pulse. The depth of a crack can be estimated by measuring the surface wave at a particular frequency that passes underneath the crack, relative to the frequency content of a wave that has propagated along a defect-free region.      

 

A key concern when deploying ultrasonic transducers on rail is the required proximity of the   transducer to the rail. Although they do not have to touch the rail, the EMATs should sit no more than a few millimetres above it. Furthermore, the surface of the rail can have steps, dips, spalls and other   discontinuities. This issue has been mitigated to some degree by modifying the EMAT design to separate the small, light coil from the large, bulky permanent magnet. The EMAT coil is attached to a light metal spring and protected by a titanium sheet such that the coil is held lightly, but close to the rail surface. The permanent magnet and housing can then be held at a safer distance, a few centimetres away from the rail surface.      

Applying the science      
All of these are areas of special interest for Professor Steve Dixon, previously Warwick’s Ultrasonics Group Leader in Physics, who is now leading the CIU project, which is a joint initiative between the Department of Physics and the School of Engineering. He says, ‘The latest developments in ultrasonics relevant to materials science include methods that are capable of not only detecting, but characterising small defects in components. Some methods can measure and quantify changes in material properties that could be characteristic of material degradation.’ He believes that this ability to reliably detect and monitor the progress of material property changes before defects develop, ‘would provide a sea change in the way that we can maintain safety, manage assets and ensure quality.’       

He adds, ‘Tied in with this is the need to be able to perform such measurements on complex samples,  often in hostile environments, that may have limited  access and require the measurement to be performed with the assistance of some kind of automated or robotic deployment. We are collaborating closely with  industry, trying to develop methods and hardware  that can tackle some of these challenges.’