Insight Vol 49 No 2 February 2007 93 1. Introduction Over the years British Energy, in collaboration with British Nuclear Group, has developed an extensive suite of computer codes for modelling the ultrasonic inspection of ferritic steel components. The models predict the echo amplitude, relative to a specifed threshold level, as a probe scans over a component containing a hypothetical smooth or rough planar defect. The models are used to help in the design of new inspections, and to provide evidence on detection capability when writing a technical justifcation for inspection qualifcation purposes. Several papers give further information about the models, their experimental validation and some typical applications (1)-(4) . This paper starts with a brief overview of the current status of the models. Two recent developments of the models are then described: the effects of fine-scale roughness on the scattering from rough defects, and the use of ‘wedge’ diffraction coefficients to model the inspection of surface-breaking cracks using the corner effect. Finally, an outline is given of a recent typical application of the models, to the technical justification of an automated ultrasonic in-service inspection of a pressuriser weld at Sizewell B Power Station. 2. Overview of current status of models A list of the current in-house models for the ultrasonic inspection of ferritic steel components is given in Table 1. All these models are implemented as computer codes running on a PC. All the models predict the echo amplitudes from planar crack-like defects, and all except TRANGLE assume that the defect is smooth. The following parameters are specified by the user when running the models: q Probe parameters, such as frequency, crystal size and pulse shape. q Defect parameters, such as size, location, tilt and skew, together with surface morphology information if the defect is rough. q Threshold parameters, such as size and depth of calibration refector (side-drilled or fat-bottomed hole), threshold level relative to calibration refector response and whether or not a DAC correction is applied. q Scan limits and increments. q Where relevant, additional parameters such as probe separation (for two-probe techniques) and component thickness. Two different theories are used to model the scattering of the ultrasonic beam by the defect: q The Geometrical Theory of Diffraction (GTD), which is a ray theory of diffraction which explicitly recognises the diffracted edge waves (‘tip diffraction’) by which smooth defects can be detected when misoriented to the incident ultrasonic beam. GTD generally works well but, in the form currently implemented in our models, it fails at the so-called caustics of the diffracted feld, where neighbouring rays cross and infnite amplitudes are predicted. Recently, work has begun at London South Bank University to refne the GTD theory near caustics and overcome this problem. MODELLING Recent in-house developments in theoretical modelling of ultrasonic inspection R K Chapman, J E Pearce, S F Burch, L Fradkin and M W Toft Paper presented at NDT 2006, the 45 th Annual British Conference on NDT, Stratford-upon-Avon, UK, September 2006. Over the years British Energy, in collaboration with British Nuclear Group, has developed an extensive suite of computer codes for modelling the ultrasonic inspection of smooth and rough planar defects in ferritic steel components. The models are used to help in the design of new inspections, and to provide evidence on detection capability when writing a technical justifcation for inspection qualifcation purposes. This paper provides an update on the current status of these models, including their experimental validation. It also briefy describes some recent developments in modelling the scattering from rough defects, allowing for the effects of fne-scale roughness, and in modelling the inspection of surface-breaking cracks using the corner effect. In both cases, experimental validation plays an important role. A recent application of the models, in the technical justifcation of an automated ultrasonic in-service inspection of a pressuriser weld at Sizewell B Power Station, is also described. Robert K Chapman and John E Pearce are with British Energy Generation Ltd, Gloucester, GL4 3RS, UK. Tel: 01452 652179; Fax 01452 652699; E-mail: bob.chapman@british-energy.com Stephen F Burch is with ESR Technology Ltd, Abingdon, OX14 4SA, UK. Larissa Fradkin is with London South Bank University, London, SE1 0AA, UK. Michael W Toft is a Consultant (Brookson Ltd), c/o British Energy address above. Code Confguration Scattering theory Probe crystal Focused probe? Defect Status PEDGE Direct pulse-echo GTD Single No Smooth Released PKIRCH Direct pulse-echo Kirchhoff Single No Smooth Released TDPKIRCH Direct pulse-echo (**) Kirchhoff Single No Smooth Prototype TRANGLE Direct pulse-echo Kirchhoff Single No Rough Prototype TWINEDGE Direct pulse-echo GTD Twin No (*) Smooth Prototype TWINKIRCH Direct pulse-echo Kirchhoff Twin No (*) Smooth Prototype FOCUSEDGE Direct pulse-echo GTD Single Yes Smooth Prototype FOCUSKIRCH Direct pulse-echo Kirchhoff Single Yes Smooth Prototype COREDGE Corner effect GTD Single No Smooth Prototype CORKIRCH Corner effect Kirchhoff Single No Smooth Released TEDGE Tandem GTD Single No Smooth Prototype TKIRCH Tandem Kirchhoff Single No Smooth Prototype TOFT Time-of-fight diffraction GTD Single No Smooth Prototype Table 1. In-house models for ultrasonic inspection of ferritic steel components (*) Other than focusing caused by toeing in of crystals (**) Time-dependent version of PKIRCH NDT.net - www.ndt.net - Document Information: www.ndt.net/search/docs.php3?id=4545