TOFD Scanning Conventions

TOFD Scanning Conventions
Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

中国工业检验检测网 http://www.industryinspection.com
Scan Conventions Non-parallel or D Scan: The scanning direction is at 90 degrees to the beam direction Parallel or B Scan: The scanning direction is parallel to the beam direction Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Non-parallel - D scan Weld Tx Rx Direction Beam Scanning This scan is used to locate flaws and obtain defect lengths in the direction of scanning but does not guarantee the highest precision for through wall height measurements Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网中国工业检验检测网

What you see! The D scan collects data from parent metal one side of the joint, the weld & then parent metal the other side of the weld Viewing this image does not give any concept of lateral location of the indications across the joint! Tx Rx Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Parallel B scan Parallel B scans are used to assess more precisely the through wall extent of a flaw. This can also aid width assessment and tilt angle of indications. Weld Scanning Direction Tx Rx Beam Direction Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Time Depth Resolution & The Effects At Near & Far Surfaces

Caution Surface breaking flaws at the scan surface are frequently missed due to subtle transitions between the lateral wave and lower tip diffraction response as they both share the same phase!!! Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Refracted Beam Spread of Bi-modal Probes

Beam Spread Calculation: 1
Calculation of incident angle in wedge = Sine q1 Sine q2 Velocity 1 Velocity 2 Sine 60 Wedge Vel Test Matl.Vel 2.7mm/ms 5.9mm/ms 0.866 (0.866x2.7mm/ms) 0.396 Inverse sine = 23.30 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Beam spread in wedge Sine q = Constant x wavelength Crystal diameter Wavelength = Shoe velocity Probe frequency 2.7mm/ms 5MHz 0.54mm 0.8 x 0.54mm 6mm 0.072 Inverse sine = 4.10 beam spread each side of the central beam axis Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

q1 q Beam spread in wedge causes peripheral edges to cross the interface at different incident angles. Main axis incident angle = 23.30 Leading edge of beam incident = = 27.4 Trailing edge of beam incident = = 19.20 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Calculation of refracted leading beam edge = Sine q1 Sine q2 Velocity 1 Velocity 2 Sine 27.40 Wedge Vel Test Matl.Vel 2.7mm/ms 5.9mm/ms 0.46 (0.46x5.9mm/ms) 1.00 Inverse sine 1.00 = 900 Sine q2 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Calculation of refracted trailing beam edge = Sine q1 Sine q2 Velocity 1 Velocity 2 Sine 19.20 Wedge Vel Test Matl.Vel 2.7mm/ms 5.9mm/ms 0.32 (0.32x5.9mm/ms) 0.69 Inverse sine = 45.90 Sine q2 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Beam Spread in Wedge Wedge Vel: 2.7mm/ms using a Crystal diameter of 6mm. @ 2MHz: +/ @ 3MHz: +/- 6.80 @ 4MHz: +/- 5.10 @ 5MHz: +/- 4.10 Based on –12dB beam edge: K=0.8 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

600 Wedge Incident & Refracted Beams
Incident (Centre beam 23.30) 2MHz: – 33.60 3MHz: – 30.10 4MHz: – 28.40 5MHz: – 27.10 Refracted (Centre beam 600) 2MHz: – 900 3MHz: – 900 4MHz: – 900 5MHz: – 900 Based on –12dB beam edge: K=0.8 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

450 Wedge Incident & Refracted Beams
Incident (Centre beam 18.80) 2MHz: 8.50 – 29.10 3MHz: – 25.60 4MHz: – 23.90 5MHz: – 22.90 Refracted (Centre beam 450) 2MHz: – 900 3MHz: – 70.70 4MHz: – 62.30 5MHz: – 58.20 Based on –12dB beam edge: K=0.8 Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

What is Probe Centre Separation ( PCS ) ?
PCS is the distance between probe index points when aligned in the jig. PCS Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

How Is PCS Determined ? The following factors are considered: Material thickness Number of scans Consideration of beam angle Area of scrutiny required Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

How Is PCS Determined?: General Applications
Probe separation adjusted to focus centre of beams at 2/3 thickness PCS d 2/3d Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Calculation Of PCS For Initial Scan On Linear Joints
For 2/3 d ( d = thickness of sample) then PCS = (4/3)d tanq s 2s = PCS q 2/3d d Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Focus At A Point Of Interest
For focusing at a predetermined depth ( D = depth of interest) then PCS = 2D tan J s 2s = PCS q D d Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Coverage Beam coverage has to be assessed considering the following: Frequency Transducer diameter Material thickness Lateral extent of inspection Time depth resolution needed Sensitivity Attenuation Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

TOFD Advantages & Limitations

TOFD Against Pulse-echo
The two most important differences of TOFD from conventional pulse-echo are, The almost independence of angle of the defect for detection of the diffraction signals The depth sizing is not dependent on the amplitude of the signals and the corresponding errors Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

TOFD Advantages Thus the main advantages of TOFD are, TOFD has a through wall sizing accuracy of + or - 1 mm and a crack growth monitoring capability of + or- 0.3 mm Efficient detection of defects of all orientations Permanent digital record of the inspection data with cross-section type views through the metal. Sensitive to small and large flaws Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

TOFD Limitations The main disadvantages of TOFD are, No simple amplitude threshold for selecting reportable defects All the TOFD inspection data has to be visually analysed in order to select the reportable defects Not so suitable for defects near to the inspection surface since they may be hidden by the lateral wave and the sizing accuracy deteriorates rapidly near to the inspection surface. Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Limitations Continued:
Small flaws at the backwall can be hidden by the backwall image 5. TOFD can be a dangerous tool applied as the only detection mechanism. Use of automated pulse-echo or phased array transducers fixed onto the same probe holder will allow the blind zones of TOFD to be investigated simultaneously with all data viewable using data acquisition and analysis software. Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Lateral Wave Dead Zone Due to the pulse duration the lateral wave causes a blind zone at the top of the inspection area. This can be minimised by: Reducing the PCS, Selecting higher frequency transducers Adjusting the pulse duration Considering if transducers with more damping is feasible. Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

Flaw Visualized By Removal Of The Lateral Wave

Back Wall Blind Zone Flaws that exist off centre to the D scan can be obscured by the backwall image This is due to the ellipse of equal time Use of wide PCS values minimises this problem and observance of indications in the mode converted signals often reveals small flaws at extremities of the beam profile Signals generated from points at any position along the elliptical arc occur at exactly the same timing Undetected defect Copyright © 2005 Lavender International NDT Ltd: 2018/12/9 中国工业检验检测网

TOFD Scanning Conventions

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