Research & Exploitation Newsletter October 2013

Research Centre in Non-destructive Evaluation University of Strathclyde, Glasgow, G1 1XW & Imperial College, London SW7 2AZ www.rcnde.ac.uk peter.thayer@ndevr.org.uk 0141 942 5398

Inside this issue

Illuminating the future of ultrasonic inspection 1-2

Inspection trials using BRAIN post-processing software 3-4

Enhancing automated inspection performance & analysis 4-5

Contact information 5

Illuminating ultrasonic NDE Researchers at the University of Nottingham have developed a new approach to ultrasound-based NDE which has many advantages over currently available systems. Traditional ultrasonic inspection relies on piezoelectric transducers and although they have been used successfully in the industry for non-destructive testing, piezoelectric transducers have several limitations. These include requiring an ultrasonic couplant, needing close access to the component, cabling (which adds to the access difficulties), and their size, weight and material properties (e.g. maximum operating temperature). They are also expensive and can be too costly for large, extensive distributed permanent installations. The technology being developed in Nottingham is the CHOT (CHeap Optical Transducer), which overcomes these limitations. They are ultrasonic transducers that are excited optically by means of lasers and can be used to enhance, control and detect ultrasound. They are very small, remotely activated devices of a few millimetres in length and nanometres in height. These attributes and their low cost mean they can be used in large numbers, used as disposables or left permanently mounted on the component. CHOTs can offer an effective alternative to piezoelectric transducers where size restrictions are severe, access is difficult or the environment is hostile, and potentially can enable on-site and in-service ultrasonic testing currently unavailable to the industry. The CHOT is based on the principles of laser ultrasonics. In conventional laser ultrasonics, the pulsed laser light is absorbed directly by the irradiated component which heats up and expands rapidly, at times that are compared to the rise time of the laser pulse (nanoseconds duration). This fast thermo-elastic expansion is the source of the generated ultrasonic wave. The wave then travels through the component and is detected optically, usually by some type of laser interferometer. There are certain advantages in this configuration: the surface of the component remains unaltered and scanning is possible. However there are certain disadvantages: the generation and the detection system rely on complicated optical setups and a pre-determined stand-off distance, and interferometers, in particular, are very difficult to operate in industrial settings, where environmental vibrations are very difficult to isolate. CHOTs bridge the world of transducers and laser ultrasonics. They are simple 2-D patterns attached onto the

surface of the component and they act as optically excited transducers. The advantage is that the pattern controls the generation and detection of ultrasound and they require only collimated light for illumination, so eliminating the need of complicated optical setups and alignment. There are two types of CHOTs: one that generates ultrasound by using light (g-CHOT) and another that detects the ultrasonic signal by translating it into optical information (d-CHOT). The pattern of the g-CHOT creates an absorption contrast on the surface of the component: it creates areas that absorb and areas that reflect (or absorb less of) the incident laser light. In other words, the g-CHOT controls the spatial distribution of light on the component and structures the thermo-elastic generation source. In this way the g-CHOT may actively enhance the directivity, the wave-mode and the frequency content of the desired ultrasonic signal that is generated. The pattern of the d-CHOT is designed to reflect light and is composed of areas with height difference (steps) of a few tens of nanometres. The d-CHOT is a diffraction grating with geometric characteristics matching the ultrasonic wave to be detected and the height is tailored to the optical wavelength illuminating it. The d-CHOT sets up a diffraction pattern for the reflected light and the propagating ultrasonic wave modulates the height of the grating characteristics. This modulation is directly related to the propagating ultrasonic wave and allows the ultrasonic signal to be detected.

CHOTs for focused longitudinal waves. Black/white areas correspond to (left) areas with absorption difference for the g-CHOT and (right) height difference for the d-CHOT.

RCNDE Newsletter Page 2

As already mentioned, both types of CHOT are operated remotely by directing a laser onto the surface of a CHOT. A pulsed laser is required for generating ultrasound, and a continuous-wave (CW) laser is used to detect an ultrasound signal. Aligning these lasers onto their respective CHOT device is a simple task (‘point and shoot’) and, provided the CHOTs are illuminated by part of the laser beam, the ultrasound signal will be generated or detected as appropriate; only simple, low-cost optics are required for this process.

CHOTs themselves can be produced using a range of methods. For example, CHOTs can be contact printed, laser etched or produced by photolithography directly onto a surface, or they can be attached to a surface as printed stickers and left in place. Therefore, CHOTs are cheap to produce and can be regarded as disposable. CHOTs have the prospect of opening up a new range of potential markets for ultrasonic inspection. One of the great advantages of CHOTs over piezoelectric transducers is that they have the potential to be used for in-service, in-situ continuous monitoring. The small size and cost of CHOTs systems will mean that multiple CHOTs can be placed on components allowing multipoint defect detection or scanning. The small size of CHOTs means that they have minimum impact when installed onto a component. They can also cope with testing components of unusual shape and can also produce a range of ultrasonic waves – currently longitudinal and surface acoustic waves (SAW). A CHOT system could be placed in a position to inspect critical regions of a component and constantly monitor the component throughout its lifetime. The longevity and robustness of CHOTs comes to the fore for operation in hostile environments (e.g. temperature, radiation, etc.). The attributes of CHOTs also mean that as well as in-situ in-service inspection, CHOTs could also be used to check for manufacturing defects on the production line to improve manufacturing quality, and applications in other industries (e.g. food and drink, pharmaceutical) can be envisaged where CHOTs are incorporated into the packaging.

Illuminating ultrasonic NDE (continued)

CHOTs for generation & detection of SAWs. (left) g-CHOT for generation of 80MHz SAWs on glass. The design is the same both for g-CHOT and d-CHOT. However, the black/white areas correspond to areas with absorption deference for the g-CHOT & height difference for the d-CHOT.

Schematic cross section of the sample showing CHOTs for generation and detection of SAWs (above).

(a) Schematic of the CHOTs pulser. (b) Photo of the light delivery & collection fibre system – (1) generation laser fibre, (2) detection laser delivery fibre, (3) detection light collecting fibre bundle,(4) visual image fibre bundle. (c) Photo of the portable CHOTs pulser & (d) inside the CHOTs pulser.

A recent development for the deployment of CHOTs has been a portable prototype endoscopic fibre-coupled laser pulser and detector initially aimed at in situ ultrasonic inspection inside aero engines. The University of Nottingham research group are now looking to bring the portable CHOTs pulser to industrial facilities and demonstrate its capabilities, and would welcome enquiries. For further information contact: Teti Stratoudaki t.stratoudaki@nottingham.ac.uk

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RCNDE Newsletter Page 3 Inspection trials using BRAIN post-processing techniques

Novel approaches in NDE can be useful to increase the capability of an inspection. However, any new technique will require experimental testing to provide evidence of its capability and to identify its region of applicability. At an early stage, a technique is often tested against a known defect population to show that it works for a range of defect species. The technique is optimised until the defects are detected and determined to be of the correct size and character. The results are published and the technique is labelled as being proficient in the detection and analysis of those particular defect types. Often, this can lead to the perception that a technique is capable of detecting a particular species of defect in all circumstances. However, the capability of a technique is not just limited to the nature of the defect, since it can also depend upon other parameters such as geometry, frequency, size and placement of the ultrasonic probe, the material and its microstructure, etc. Before an industrial end-user of NDE can rely on a technique, it must have a defined inspection procedure to remove variation between inspections that could reduce this capability. Novel techniques using ultrasonic phased arrays have seen much academic research in recent years. In particular, the Total Focusing Method (TFM) and Scattering Coefficient Matrix (SCM) post-processing techniques have been developed through RCNDE research at Bristol University for improved defect imaging and small defect character-isation. These techniques could offer many advantages over conventional pulse-echo and phased-array sector scanning techniques. As a next step, AMEC organised a ‘blind trial’ of the TFM and SCM techniques on test specimens to understand better the limitations and capabilities of these techniques in a realistic industrial inspection. This RCNDE-funded project aimed to test the ultrasonic techniques incorporated within Bristol University’s phased-array Full Matrix Capture (FMC) software package called Bristol Array Inspection (BRAIN) which involves several post-processing algorithms including TFM and SCM. The project required the following tasks: • Development of a working inspection procedure to

provide a repeatable inspection against which the performance can be assessed

• ‘Open’ trial to allow the development of the inspection procedure

• Witnessed ‘blind’ trial to test the performance of the operator following the procedure

• Destructive examination of defects in one of the test specimens to improve confidence in the capability of the techniques.

Bristol University was responsible for developing the procedure and for carrying out the open and blind trials. AMEC assessed the procedure and the open and blind trials, and provided feedback and guidance where required, and undertook the destructive examination.

Three 600mm x 200mm x 60mm ferritic steel specimens were used by this project, each with a double-V ferritic butt weld (with the caps ground flat) running across the width of the specimen. A series of typical realistic manufacturing and in-service volumetric and planar defects were located along each weld. These defects included a lack of fusion at the weld interface, porosity, slag, and rough and smooth cracks. The aim of the first inspection was to develop the draft procedure to comply with the inspection specification and to define the parameter settings for use within BRAIN. A blind inspection of the test specimen was performed using the draft inspection procedure, and the results were submitted to AMEC in the form of TFM images (B- and D-scan) together with tabulated results listing the suspect defect volume dimensions and judgements on defect character. Feedback was provided on the submitted results compared with the intended defect population, and revisions were made by Bristol to the draft inspection procedure. The test specimen was re-inspected in an open trial using the revised inspection procedure, and the results once again submitted to AMEC for assessment.

Destructive examination of realistic defects (a) lack of fusion, (b) rough crack, (c) rough crack, and (d) slag.

The second inspection on another of the test specimens was to test the revised procedure against an additional set of defects and allow further development of the procedure. The results of the blind inspection using the revised inspection procedure were submitted to AMEC for assessment, and feedback was given on the comparison of the submitted results with the intended defect population. Following a further revision to the defect characterisation procedure, a final inspection procedure was produced. A further blind trial was run on the first test specimen but from the opposite inspection surface to that inspected previously. [The identity of the specimen was not revealed to the inspector.] This blind inspection used the final inspection procedure, and results were once again submitted for assessment comparing the results with the intended defect types and locations in the test specimen. The final inspection on the third specimen was solely a test of

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RCNDE Newsletter Page 4 Inspection trials using BRAIN post-processing techniques (continued)

Automated data acquisition is increasingly common in industrial NDE, and it is driving a significant increase in the availability of inspection data. This is particularly true for ultrasonics, where the stored data now typically comprise a full sequence of time-amplitude traces (A-scans), rather than a single amplitude-position (C-scan) image. The issue may be further exacerbated by the acquisition of multiple channels of data, using different techniques, viewing angles or probe types, including many-element array probes. The analysis of the collected data is largely done manually by a skilled inspection engineer. The engineer will inevitably spend significant effort examining data where there are no defects to report. Not only can this be costly, but human factors, such as fatigue causing inattention, can increase the probability of a defect indication being missed. Nick Brierley is an EngD Research Engineer hosted by RWE npower and supervised by Prof. Peter Cawley of Imperial College. His EngD research is investigating the scope for improving inspection reliability and reducing the interpret-ation time by partially automating the data analysis process. The project is focusing on ultrasonic methods and the key elements are: • exploiting information in both newly acquired and

historic data • applying an understanding of the underlying physics • capitalising on the capabilities of computer simulation • identifying and segregating regions of the inspected

part likely to be defect free from regions of more questionable integrity

• focusing inspection engineers’ skills on these latter key regions to reduce the impact of adverse human factors affecting reliability

• performing a detailed statistical analysis to monitor the probability of detection and the false-call rate

• recommending improvements to inspection procedures to ensure more reliable data acquisition.

The objective of the project is to develop a system for the analysis of automatically acquired data from a wide range of inspection types with the primary aim of improving the reliability of the overall inspection. Although initially aimed at RWE npower’s inspection requirements, the approach is

the first known testing of the BRAIN software and TFM-based inspection procedures for use in a realistic industrial inspection application. The trial offers valuable insights into the strengths and weaknesses of the FMC/TFM algorithms and has identified areas for further research and development. Most of all, the trial has identified routes for facilitating the rapid transfer of the FMC/TFM imaging technology into industry as part of a diverse and redundant inspection procedure. Enhancing automated inspection performance & analysis

defect sizing and characterisation. The defect locations were provided to Bristol and an open inspection performed on the four identified defect regions the specimen using the final inspection procedure. These results were reported to AMEC for assessment. The open and blind trials and associated procedure development provided important information on the strengths and weaknesses of FMC imaging techniques for the inspection of ferritic butt welds and their potential use as part of an inspection procedure for such welds. The importance of good inspection design was reinforced to ensure full-weld coverage and optimised defect discrimination and characterisation for the full range of potential defect types. The inspections were independently witnessed throughout, and there were no performance weaknesses that could be attributed to human reliability factors. The sizing of defects provided good results in general. One defect in the initial trial was grossly oversized. However, with a revised procedure, when the same specimen was inspected as part of the third blind trial (i.e. from the opposite surface), a good result was achieved on this defect. On average, defects were oversized more often than they were undersized, and by a larger amount. This is likely to be partly due to the small size of most defects in the test samples. Some false calls relating to spurious echoes were noted in the analysis of FMC data, and most of these indications were sentenced as insignificant point indications following the application of the characterisation process. Some of these echoes were also noticed and sentenced as such during conventional pulse-echo inspections. Inevitably, with improved FMC imaging techniques, the cost of higher sensitivity is the increase in false-call rate from small insignificant reflectors. This is reminiscent of the TOFD technique. Therefore, care is needed for selecting a reliable calibration technique regarding inspection sensitivity and for selecting a robust characterisation process. Also, further optimisation of the FMC, TFM and SCM inspection procedure for the full range, types and orientations of likely defects would increase the amplitude difference between very small (innocuous) and larger defects, and so simplify the identification and discrimination of defects. The testing of the FMC/TFM post-processing algorithms available within the BRAIN software in this trial has lead to the development of effective FMC/TFM inspection procedures for ferritic welds which can be further optimised for improved defect detection, sizing and characterisation/discrimination. The trial demonstrates the capabilities of the FMC/TFM algorithms and procedures in a simulated industrial application, and it is

RCNDE Newsletter Page 5 Enhancing automated inspection performance & analysis (continued)

For further information about membership of RCNDE, please contact: Peter Thayer at NDE Research Association (NDEvR) at peter.thayer@ndevr.org.uk Alternatively, contact Keith Newton at k.newton@imperial.ac.uk www.rcnde.ac.uk

identifying indications, and does not utilise signals acquired from different directions relating to the same part of the component. The first step of the new approach considers the alignment of datasets to a common coordinate system i.e. data registration, which is critical for any combination or comparison of datasets. This comprises feature extraction, feature pairing, evaluation of the quality of the registration fit and optimisation. The subsequent data fusion stage is designed to exploit fully the available data to detect possible indications, for sequential examination by a skilled engineer, and ultimately to improve the Probability of Detection (POD) without incurring a Probability of False Alarm (PFA) penalty. Data fusion involves breaking the dataset down into resolution element (resels), developing a local data model (e.g. using the component’s hoop symmetry to examine each resel relative to the distribution of resels in the same hoop) and applying a consensus test.

intended for generic applications commonly encountered throughout industrial NDE. An example has been applying the approach to the inspection of Rolls-Royce titanium aeroengine disks during manufacture, for which Nick has collaborated with Trevor Tippetts, also of Imperial College.

The disks undergo ultrasonic immersion inspection partway through manufacturing. The acquired dataset comprises hundreds of thousands of A-scans. The current analysis scheme relies on a globally consistent threshold for

A radial cross-section of the disk, displayed in terms of resels (each ~5mm across), counting the number of times each is interrogated over the full set of scans acquired.

The performance of the analysis process may be evaluated using the Relative Operating Characteristic (ROC) plot that relates POD and PFA. The results shown were obtained for a dataset acquired from a test disk made from a contaminated billet, and containing multiple known indications. The first two plots relate to the most prominent known indication, and the third to one that is much harder to detect. The analysis scheme developed in the project using registration and data fusion steps out-performs traditional approaches – a higher POD and/or lower PFA is readily achievable. The proposed process requires almost no

the PFA. Fusing both inputs provides a further improvement and demonstrates the benefit of the data fusion.

a PFA that is as low as can be estimated from the available data.

current detection system.

(Right) Two input scans viewing the indication are shown. Both individually out-perform the fixed-threshold approach by using locally derived thresholds based on the local normalisation of amplitudes, and significantly reduce

(Right) Using all six available scans, two are individually worse, even using the locally deriv-ed threshold, than the global threshold. Fusing all the channels gives an ROC curve that effect-ively represents a perfect detector: a step-function at

(Right) Five scans of a different indication (far harder to detect), the locally derived thresh-old shows 3 channels are individually worse than the global threshold. Fusing all the available channels still yields an ROC curve that substantially outperforms any of the individual scans or the

An aerospace engine titanium disk (approx. 1m dia.)

changes to the data collection procedure, and the partial automation of the analysis is more reliable, faster and lower cost. The next step will be to transfer the system from the university environment to an NDE laboratory in Rolls-Royce to establish the nature of any new indications being detected. This will provide greater understanding of defect populations leading to improved component use.

Fusion

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Locally derived thresholds

Locally derived thresholds

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Scan surfaces & probe angles

Fusion

Effectively, PFA=0 for these data

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