tofd presentation

Tarek R. Abdel-Alim GL Doha Station 1

TOFD BETWEEN SIMPLICITY AND COMPLCITY

BY

Eng. Tarek R. Abdel-Alim

Tarek R. Abdel-Alim GL Doha Station 2

TOFD Introduction

Time of flight Diffraction (TOFD) is an accurate defect sizing UT technique not depend on sound amplitude reflected from defect.

TOFD Uses longitudinal (compression) waves. TOFD Pair of transducers are considered as part of AUT

system arrangements ( one or two channels of the multi channel system).

TOFD could be used with both of Pulse echo or phased Array Techniques.

Sizing of defect not related to its orientation.

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The Diffraction Phenomenon

Huygens' principle:

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TOFD CONCEPT & THEORY

The Time of Flight Diffraction technique employs the use of two compressional wave transducers using angles between 30°and 70°set at a calculated distance (Probe Centre Spacing “PSC”) apart in the pitch and catch mode.

When the compressional wave strikes a defect diffracted waves are generated from the extremities of the defect (oscillation of defect ends).

The diffracted waves are detected by the receiving transducer.

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TOFD CONCEPT & THEORY

The first signal received (1) is from the lateral wave.This is a creeping wave at the surface with the same velocity of the compressional or longitudinal waves.The second (2) and third (3) signals are the diffractedwaves from the tips of the flaw. The fourth (4) signalis the back-wall representing one complete sound path. The signals past the back-wall are mode converted Shear wave.

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TOFD Transducer Arrangement

Dual Transducer Technique.Transmitter and Receiver mounted typically opposite of and symmetrically about the volume of interest.Typically mounted at fixed probe centre spacing (PCS).

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Crack Tip Diffraction

Diffraction occurs at both top and bottom tips

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Volumetric Discontinuities

Reflection produces response from the top of the discontinuityCreeping wave produces response from the bottom of the discontinuity.

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Manual TOFD Hand Scanner

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TOFD Standing Wave Pattern

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Typical TOFD Display

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TOFD Waveform with Mid-wall Discontinuity

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TOFD Depth Measurement

Constant depth loci form an ellipse with the beam index points the focal points.Consequently, some minor error for lateral position offset.

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Amplitude Independent

•Depth calculations based on pulse transit time•therefore, so long as gain is sufficient for detection•size calculation will not change

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TOFD Implementation –Weld Inspection Application

Beam angle, transducer size, and PCS set to flood weld volume.Uni-axial scan along weld length “B or D Scanning mode”.Dead zone near test surface due to inability to resolve flaws from lateral wave.Maximum thickness (single scan) is about 2-2.5 inches.

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Near-Surface Dead Zone

For near-surface flaws, time difference between lateral and flaw is not sufficient to separate the two, therefore, near-surface flaws are hidden by lateral wave.increased PCS worsen the effect.

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Improved Near-Surface Detection and Sizing Resolution

• Narrower PCS.• Higher frequency probes.• heavier probe damping (broader bandwidth).

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Depth Zone Coverage

Deeper zones generally use:1. larger transducers.2. lower frequency.3. lower refracted angle.4. larger PCS.

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TOFD Imaging

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2-Dimensional Raster Scan TOFD

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TOFD Image Showing No Indications

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Creep Macro-Crack

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TOFD technique advantages

TOFD technique offers the following advantages in comparison to radiography:

I- Radiation hazard-free area – This allows work to be performed by other personnel in the vicinity of the examination area without any radiation safety precautions.

II- Fast, accurate results – This process provides information at the examination area without the need to develop film, determine radiograph quality conformance (i.e. density and sensitivity) and film interpretation.

III- Less expensive than radiography per foot of weld.

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TOFD technique advantages

IV- Volumetric information – As opposed to radiography that provides a 2-D view of a weldment, and radiography’s detection difficulty with cracks not directly oriented to the cone of radiation, this process can provide accurate length and depth information of recorded indications.

V- Permanent record – The scan image files can be saved for future reference and evaluation.

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Codes and standard

British Standard. European ASME ASTM API 1104 “19th Edition”

1. BS 7706 (1993) Guide to calibration and setting-up of the ultrasonic time-of-flight diffraction (TOFD) technique for detection, location, and sizing of flaws. British standards Institute 1993.

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Codes and standard

2. pr EN 583-6 (1995) Non-destructive testing ultrasonic examination - Bart 6: Time-of-flight diffraction technique as a method for defect detection and sizing.

3. ASME VIII Code Case 2235 (2000 Edition) Ultrasonic examination to be in accordance with ASME Section V, Article 4 "Alternatively, for techniques that do not use amplitude recording levels, ...'. This has opened the door for TOFD to be used on Section VIII pressure vessels

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Codes and standard

4. In 1998 ASTM E-1961 was published describing the various aspects involved in pipeline inspection using mechanised UT.

5. In the 19th edition API 1104 they revised its description of UT requirements to include mechanised systems.

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TOFD Limitations

1. Blind area - near surface, back-wall.

2. Weak signals.

3. Couplant loss.

4. Flaw classification limitation.

5. Interpretation of defects.

6. Sensitive to grain noise.

7. Coarse grained materials.