NDT 2007 Abstracts: 5C
Thursday 20 September
Session 5C – Modelling
Ultrasonic inspection of tilted defects using the 'corner effect' – modelling and experimental validation
Robert K Chapman
British Energy Generation Ltd, Gloucester, GL4 3RS, UK. Tel: 01452 652179; Fax: 01452 652699
e-mail bob.chapman@british-energy.com
Stephen F Burch and Neil J Collett
ESR Technology Ltd, Abingdon, OX14 4SA, UK
Larissa Ju Fradkin
London South Bank University, London, SE1 0AA, UK
In ultrasonic NDT, the 'corner trap' mechanism is a common means of detecting planar cracks which break the back-wall surface of the component. The mechanism is most efficient when the crack is normal or near-normal to the back-wall, so that a specular reflection is obtained. However, a response is still obtained when the crack is more tilted, due to diffracted edge wave signals from the crack tip and from the 'wedge' formed between the crack and the back-wall.
In order to model the wedge mechanism using the Geometrical Theory of Diffraction, it is necessary to know the diffraction coefficients from the elastic wedge. For ferritic steel, these wedge diffraction coefficients are given by solving the 'canonical problem' of the scattering of a plane elastic wave by a wedge vertex of arbitrary angle in a homogeneous isotropic elastic medium. We have obtained a theoretical solution to this very challenging mathematical problem. The underlying mathematical theory has been implemented in software, which is used to generate look-up tables of diffraction coefficients for use in our ultrasonic NDT models.
The theory has recently been validated against experiment, using two sector-shaped steel test specimens having different vertex angles. Pulse-echo measurements of both amplitude and phase of the diffraction coefficients were made, for both compression and shear waves, and compared with theory. Good agreement between theory and experiment was obtained.
TOFD simulation tools in CIVA software
Steve Mahaut1, Michel Darmon1, Philippe Benoist1, Larissa J Fradkin2
1CEA/LIST, 91191 Gif-sur-Yvette, France
2Waves and Fields Research Group, ECCE, FESBE, London South Bank University, London SE1 0AA, UK
TOFD techniques are commonly used in non-destructive evaluation to detect and characterise flaws. Simulation of TOFD inspection can be helpful to analyse and improve these methods especially for complex configurations.
A new TOFD modelling tool has been developed in CIVA software. This module is based on the geometrical theory of diffraction (GTD) and can predict the ultrasonic response of disoriented planar or complex flaws for various specimen shapes (planar, parametric as nozzle or CAD). Besides diffraction echoes, the module available in CIVA allows to simulate lateral surface waves and backwall echoes and takes into account the shadowing effects from flaws. Various experimental measurements were carried out in order to validate this modelling tool.
Experimental validations of the module were performed on planar specimens containing rectangular and CAD contour planar flaws with different orientations (tilt, skew, vertical orientation). Simulation and experiments were also performed on nozzles configurations. The paper presents the simulation tools validated for nozzle inspections and the development under progress.
Integrating flaw-response modelling tools used for inspection qualification into a multi-agent system software platform
James P McKenna1, Anthony Gachagan1, Stephen D J McArthur2 and Gordon Hayward1
1Centre for Ultrasonic Engineering
2Institute for Energy and Environment
University of Strathclyde, Glasgow, G1 1XW, UK. T: +44 (0)141 548 3477; F: +44 (0)141 548 2950; E: j.mckenna@eee.strath.ac.uk
This paper details the application of a multi-agent software system used in the qualification of an ultrasonic NDT inspection using theoretical flaw-response modelling. Flaw-response models provide a fast and cheap alternative to test-piece trials but their use involves prior knowledge about the model's theory and application, requiring an experienced NDT engineer. Due to the disparate nature of these software models they are unable to integrate with other NDT software tools making the selection and application of the most appropriate model for a given inspection complex.
A multi-agent system approach creates a flexible software environment that permits the integration of software tools that display autonomous behaviour, allowing them to share data and collaborate with other tools despite their heterogeneous characteristics. A prototype inspection qualification multi-agent system has been developed that is capable of performing flaw-response model selection through the use of a rule-based software system incorporated within a model facilitator agent. Importantly, the design and implementation of this system utilises expert knowledge elucidated from senior NDT engineers. The performance of the model facilitator will be analysed through case studies, illustrating the benefits of this approach with regard to computational speed and accuracy over conventional techniques used in the ultrasonic qualification process.
A novel algorithm for robot guidance along weld
Chee Chung Chan, Markus Friedrich, Prof S Marshall, Prof G Hayward
A novel method of guiding a mobile robotic device along a weld using visual techniques is presented in this paper. The visual system consists of a USB (Universal Serial Bus) based camera for image capturing, an image processing algorithm and a guidance control algorithm. Experimentation shows that the intensity levels across the cross section of the weld in captured images shows a unique characteristic. Taking advantage of this, a non-linear recognition algorithm known as the Hit-and-Miss Transform was used for segmenting the characteristic of the weld. The guidance control relied on the results of the Hit-and-Miss Transform, where the point of overlap provided the basis of control. The results of the Hit-and-Miss Transform created a steering vector used for the directional commands for the robot. This has effectively integrated image processing with guidance and control. The outcome of this approach produced a reliable real-time tracking system that is robust and accurate.
Finite element modelling of elastic wave propagation and defect interaction in large, complex components
J Russell1,2, P Cawley2, M Drozdz2, M Lowe2, N Habgood1
1Rolls-Royce plc
2RCNDE, Imperial College
An Inspection Qualification (IQ) must be completed before the inspection of a safety critical component can take place. Modelling can be used to support the IQ process and demonstrate inspection capability in a cost and time efficient manner. One appropriate modelling technique to simulate elastic wave propagation and the interaction between the wave and a defect is finite element (FE) modelling.
In practical NDE applications, large welded components with complex geometries are often inspected. Due to its size and complexity it is typically not possible to use an FE modelling approach to model the entire component. Original work by Castaings et al has been further developed by Drozdz et al to generate the Absorbing Layers using Increasing Damping (ALID) technique to facilitate the modelling of acoustic waves in unbounded elastic media. ALID is used to model the inspection of a large welded component containing a range of known defects. The simulated results are compared to experimental data and demonstrate the validity of the modelling approach.
Simulation and modelling of ultrasonic wave propagation in austenitic steel welds
George Connolly, Mike Lowe, Andrew Temple
Imperial College, RCNDE, Department of Mechanical Engineering, London SW7 2AZ, United Kingdom
Stanislav Rokhlin
The Ohio State University, Edison Joining Technology Center, 1248 Arthur E Adams Drive, Columbus OH 43221
Inspection of austenitic welds is particularly difficult because the materials tend to form anisotropic grains resulting in an inhomogeneous structure whose properties are specifically unknown. These grains, which are of a dimension comparable to the applied ultrasonic wavelength, are responsible for both scattering and beam-steering. A reliable method of ultrasonic NDE would remove the need for radiography, reducing inspection time and cost.
Numerical methods such as finite element analysis and ray-tracing are increasingly used to gain understanding of wave propagation and scattering in complex structures, aiding the development of inspection techniques and qualification of procedures.
This paper will present several case studies comparing a semi-analytical ray-tracing function and a finite element model using a simplified anisotropic continuously inhomogeneous weld model in which elastic constants are oriented in the direction of maximum heat flow during cooling. Simulations comparing the two approaches will be shown, to include model studies of reflections from defects, demonstrating the significance of incorporating the inhomogeneity of the weld in the modelling. Future directions will also be discussed.
Validation of theoretical models for focused/phased array ultrasonic inspection
Channa Nageswaran and Charles Schneider
TWI Ltd, Granta Park, Great Abington, Cambridge, CB21 6AL. T: +44 (0)1223 899 000; E: channa.nageswaran@twi.co.uk
SimulUSTM and CIVA are two theoretical models currently being used to assist in the design of ultrasonic inspections. They are being used to simulate the sound fields created by transducers in various inspection scenarios. Both models are designed to simulate the pressure field from both single element and array transducers. For inspection personnel to gain confidence in the use of these models, it is important to validate their output. This paper presents the validation of sound fields from SimulUSTM and CIVA for single crystals and focussed phased arrays. The validation is performed by comparison with validated theoretical models and experiments. In general, the models agree well with experiment, within their expected range of validity. Where discrepancies occur, possible reasons for this are discussed.
