NDT 2007 Abstracts: 5A
Thursday 20 September

Plenary Paper

NDT: Novel Diagnostics for Teeth
J M Girkin1, S Poland1, D Hughes1, S Cochran2, J Elgoyhen2, C Longbottom3, T W Button4, H Hughes4, C Meggs4, I Young5 and D Grinev5
1Institute of Photonics, SUPA, University of Strathclyde, Glasgow
2SUPA, School of Engineering and Science, University of Paisley, Glasgow
3Centre for Clinical Innovations, University of Dundee, Dundee
4AFM Ltd, University of Birmingham, Birmingham
5SIMBIOS, University of Abertay, Dundee

Perhaps the greatest challenge for non-destructive testing is in the medical field. Here there must be minimal risk of 'damage' to the patient! One area of particular interest is in the early detection, and subsequent diagnosis (treatment planning), of early dental challenges both from tooth decay (caries) and more recently acid drink based erosion.
The examination of hard structured material in the moist, and frequently inaccessible, oral cavity is a real challenge. This presentation will outline several current approaches to this problem without the use of potentially harmful ionising radiation. The first is based upon the use of near infrared light in a novel confocal microscope utilising optical fibres to deliver and subsequently collect the returned light to record depth profiles through teeth. We will then present the first results using a novel, focused high frequency (~40 MHz) ultrasound probe, with the results compared to a new, non-clinical, 'gold standard', micro X-ray CT. This work will then be placed in context with what the dentist ultimately desires and how early diagnosis could potentially remove the need for invasive dental treatments.

Session 5A – Electromagnetics

Shear wave birefringence measurements on thin strip metal samples using EMATs
S Dixon, M Fletcher and G Rowlands
Dept. of Physics, University of Warwick, CV4 7AL, England

If a radially polarised SH wave EMAT is used to make through thickness shear wave measurements on a rolled steel or aluminium sheet, the weak elastic anisotropy splits the shear wave into two discrete polarisations that lie along and orthogonal to the rolling direction of the sheet. This phenomena of having two distinct polarisations with slightly different propagation velocities is referred to as acoustic birefringence. The slightly different propagation velocity of each polarisation leads to interference between the two shear wave signals in the time domain waveform. We have developed wideband EMAT systems with frequency content between approximately 0.5 – 12 MHz, where the SH wave pulses are temporally sharp, giving good measurement resolution. On thin sheets one possible approach to extract the reverberation frequencies is to apply a Fast Fourier Transform (FFT) to the A-scan to measure a discrete reverberation frequency. From these reverberation frequencies, one can calculate quantities that relate to the crystallographic texture or stress state of the material. We showed in earlier work that there is a significant and non-trivial interference effect between the peaks in the FFT. This is due to an interference term, that links the two distinct reverberation frequencies in the FFT. It would appear that some other factors that are present, such as mode converted compression wave modes, that limit the accuracy to which the individual reverberation frequencies can be identified. Measurements have shown that where the method is used to determine thickness from a known calibration velocity the accuracy of the calculated thickness is still excellent, but care must be taken when trying to calculate crystallographic texture parameters from such measurements.

Lift-off performance of ferrite enhanced generation EMATs
Y Fan1, X Jian2 and S Dixon3
1Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
2Phoenix Inspection System LTD., 46 Melfort Court, Warrington, WA1 4RZ, UK

Electromagnetic Acoustic Transducers (EMATs) are non-contact ultrasonic transducers capable of generating wide-band ultrasonic waves on electrically conductive and magnetostrictive samples. The lack of physical contact makes EMATs particularly suitable for online inspection applications, or situations where samples may be moving or hot. The generation efficiency of a given EMAT on a given sample is dependent on the 'lift-off', which is the distance between the EMAT and the sample surface, efficiency dramatically reducing with increased lift-off. This requirement to be in close
proximity imposes a practical limit of operation and changes in lift-off due to phenomena such as sample vibration can have practical implications in certain NDE applications.

This paper describes some results from experiments comparing the performance of a ferrite enhanced EMAT design to one of our 'standard' EMATs, where we have substituted the permanent magnet from the standard EMAT with a suitable ferrite material. When the EMAT coil is placed in proximity to the ferrite, but not wrapped around the ferrite, the increase in the generated eddy current amplitude is significant whilst the inductance or bandwidth of the EMAT is not significantly effected. Using a ferrite material eliminated the eddy current loss in the permanent magnet, and also amplifies the self-field generation mechanism, which generates a repulsive normal force on the sample surface. Direct experimental results show that the generation efficiency of the standard EMAT is higher than the ferrite enhanced EMAT when lift-off is small. However, the ferrite enhanced EMAT generation efficiency can be higher at large lift-offs and is also less sensitive to lift-off variations. Although the example we describe here only applies to EMAT generation, there are situations where ferrite could be used to enhance detector efficiencies.

The inspection of moving steel billets using Laser-EMATs
I Baillie1,2, P Griffith2, S Dixon2, X Jian2
1Corus R,D&T, Teesside Technology Centre, Middlesbrough, UK
2Department of Physics, The University of Warwick, Coventry, UK

A laboratory system has been built for ultrasonic inspection of steel billets weighing up to 250 kg as they move past a fixed point where a pulsed laser is used to generated ultrasound and non-contact EMATs are used to detect ultrasound in the sample. This approach allows billets containing a variety of different surface defects to be detected using a Laser-EMAT array, regardless of the orientation of the defect. The billets can be moved past the inspection point at either in different increments with measurements taken every 1 mm or so, or it can be moved at a constant velocity. Methods for automating the system and acquiring the data and plotting B-Scans will be discussed.

Comparisons between modelled work and those measured in the laboratory will be made.

Finally, it will be shown that after taking careful laboratory measurements, how these results can be compared with those obtained when the same billet is being moved on a pilot scale industrial rolling mill. The intention of this project is to establish on on-line inspection system for moving, hot steel products such as slabs and billets.

Depth profiling of defects in stainless steel using electric potential sensors and a non contact a.c. potential drop method
Robert Prance, Wifgi Gebrial and Charles Antrobus 
Centre for Physical Electronics and Quantum Technology, University of Sussex, Brighton, East Sussex, BN1 9QT, UK. T: 01273 872577; F: 01273 678087; E: r.j.prance@sussex.ac.uk

A new technique is described, a non-contact version of the well known a.c. potential drop method. Electric potential sensor technology is capable of measuring, non-contact, the spatial electric potential associated with a sample via the displacement current, through capacitive coupling. This can be achieved due to the extremely high input impedance of the sensors, resulting in many cases in no loss of signal when compared with a contact voltage measurement. For laboratory samples an a.c. current is passed through the sample and the spatial potential above the surface monitored as a function of position. For a full non-contact implementation this current would be induced inductively as in eddy current testing systems. Previous work on stainless steel using this new technique was able to detect 1 mm of material loss from the inside of a 24 mm-thick sample. The work described here extends this by applying multiple frequencies simultaneously to a sample and analysing the data for each frequency component separately. In this way we are able to obtain depth information about faults within a material, due to the skin depth effect. This enables the location and characterisation of faults in three dimensions to be achieved while only performing a single scan.

Detection and quantification of rail contact fatigue cracks in rails using ACFM technology
M Ph Papaelias1, M Lugg2, M Smith2, C Roberts1, C L Davis1
1Rail Research UK Centre, Gisbert Kapp Building, University of Birmingham, Birmingham, B15 2TT
2TSC Inspection Systems, 6 Mill Square, Featherstone Road, Milton Keynes, MK12 5RB

Alternating Current Field Measurement (ACFM) is an electromagnetic inspection method capable of both detecting and sizing (length and depth) surface breaking cracks in metals. The basis of the technique is that an alternating current can be induced to flow in a thin skin near the surface of any conductor. By introducing a remote uniform current into an area of the component under test, when there are no defects present the electrical current will be undisturbed. If a crack is present the uniform current is disturbed and the current flows around the ends and down the faces of the crack.

This paper presents results from the initial slow speed (0.1 m/s) tests on a set of rails which contained artificially induced surface-breaking defects, including half-face slots machined normal to the railhead surface, clusters of angled slots, and pocket-shaped defects using an ACFM system particularly designed for the inspection of railheads. Furthermore, successful ACFM tests were performed on rail samples with actual defects. Some initial results on inspection of RCF defects using a novel pulsed eddy current array are also presented for the purpose of comparison. Further developments for testing on a rotating rail rig at significantly higher speeds are currently ongoing.

Inspection of storage tank floor welds with ACFM
Martin Lugg, TSC Inspection Systems UK
Christian Laenan, Apave, France
German Salazar, SIEEND, Mexico

The Alternating Current field Measurement (ACFM) method has become well established in the oil and gas industry for routine in-service weld inspection. It offers many advantages over conventional inspection methods by avoiding the need for extensive pre-cleaning. It has recently been applied to the inspection of lap and fillet welds inside steel storage tanks and has, in some cases, outperformed the standard methods for internal weld inspection such as magnetic particle inspection or vacuum box testing.

This paper focuses on recent experience in both Europe and Central America in the use of the technique for the inspection of welds in the floors of cylindrical steel storage tanks. The inspections used conventional ACFM probes as well as new array probes recently developed to speed up inspection of these welds. Results from ACFM are compared with those from the conventional methods, including examples where through-wall cracks giving rise to measurable leaks were not detected with the conventional methods, but were found using ACFM. Conclusions are drawn as to the benefits this technique offers in terms of cost and time savings and inspection reliability.

Measurement of microstructure of steels by multi-frequency electromagnetic sensors
X J Hao, M Strangwood, C L Davis1
A J Peyton, W Yin2
P F Morris, S Dewey, C Lester3
1Department of Metallurgy and Materials, University of Birmingham
2School of Electrical and Electronic Engineering, University of Manchester
3Swinden Technology Centre, Corus, Rotherham

Most microstructural information is determined using small samples in a destructive manner (optical metallography, SEM etc). There is a desire to measure microstructure non-destructively and in a non-contact fashion to allow on-line measurements during processing. This study aims to develop a multi-frequency (10 – 106 Hz) electromagnetic (EM) sensor to measure microstructure of steels in such a way. Below the Curie temperature (≈ 770°C for carbon steel) austenite is paramagnetic and ferrite is ferromagnetic. EM sensors work on the basis of detecting the difference in relative permeability, µR, and conductivity, , between microstructural phases or changes with temperature (µR = 1 for austenite and µR ≈ 200+ for ferrite at room temperature, the differences in conductivity are much smaller). Two potential applications for EM sensors including phase transformation and fraction determination during cooling (e.g. strip or rod processing) and measurement of decarburization in rod are considered here.

Magnetic flux leakage inspection of laser welds in the automotive industry – modelling and preliminary experiments
C Carpentier, A Raude, J R Rudlin
TWI Ltd, Cambridge

The automotive industry uses laser welding to manufacture various sheet components (tailor welded blanks) and specific joins on semi-completed bodies (body in white).

The OLIWAM project was proposed to provide inspection method for these welds to be carried out in-line. One of the NDT inspection techniques selected for evaluation was magnetic flux leakage (MFL) inspection.

This paper describes modelling work to assess the feasibility of the MFL technique, together with initial investigations on flawed samples. Implication of the work on the design of suitable MFL probes and systems are considered.