|NDT.net - September 1999, Vol. 4 No. 9|
|TABLE OF CONTENTS|
The TOFD technique is a fully computerized system able to scan, store, and evaluate indications in terms of height, length, and position with a grade of accuracy never achieved by other ultrasonic techniques.
The TOFD technique is based on diffraction of ultrasonic waves on tips of discontinuities, instead of geometrical reflection on the interface of the discontinuities.
This phenomena makes TOFD ideal for identifying cracks, lack of fusion located along the vertical axis of the weld (in particular for narrow gap preparation) or with any other orientations, because detection is not affected by the negative consequence of ultrasonic beam deviation from the receiver due to unfavourable orientation of the discontinuity.
These features have extended the use of TOFD to replace Radiography and complex Ultrasonic inspection by tandem technique wherever planar defects (cracks, lack of fusion) are the main object of examination.
The main disadvantages to both the aforementioned techniques are expensive manipulation for Radiography on big parts and the amount of time required to perform the tandem technique due to the accuracy necessary for reliable testing.
TOFD overcomes both techniques in terms of speed of examination and higher accuracy.
The British Standard Institute has issued the first draft: BS No 7706 (4)
ASME Code has included in Section V, art. 4, Appendix E, the computerized ultrasonic system and TOFD technique for ultrasonic examination of welds (5).
An "ad hoc" Workgroup has been nominated from within the ASME Technical Committee to prepare a document establishing the conditions for using computerized systems, such as TOFD in replacing Radiography examinations of welds, in the over 4" thickness range (see Code Case).
CEN (comite Europeen de Normalisation) has issued the document CEN/138/WG on practical application of TOFD for ultrasonic examination of welds (6).
In addition to higher sensitivity, the TOFD technique allows the full examination data to be recorded on hard disk, displaying all discontinuities in C/scan images. This enables "off line" evaluation of indications by computer using dedicated software. Very accurate sizing of defects can be achieved and printed for documentation.
Another advantage and current application of TOFD is its use in monitoring welds during the service life of components. Stored data acquired from initial examinations, made during the final stage of construction, can be compared with new data obtained from in-service inspection. This allows the stability of existing indications to be determined with high accuracy and reliability.
The more accuracy guaranteed by TOFD in sizing thruwall extention of flaws allows more reliable fracture mechanic calculation for the residual life evaluation.
Companies like Exxon, Shell, Fluor Daniel, Texaco, Chevron etc. are using TOFD to replace radiography in examination of welds after PVMT or hydraulic testing. The main inspection agencies have approved procedures of the TOFD Technique and its validation.
Fig 1: TOFD waves pattern and corresponding A-Scan
Fig 2: Compressional waves (left) crossing the tip of a discontinuity generate a wide beam of spherical waves (right)
Fig. 3: TOFD map of the calibration on ASME test block of 185 mm (7.28 inch) with three holes of 7.5 nun (1/3 inch.) diameter. The TOFD map shows the three holes with symmetrical mode conversion indication. The straight lines at left and right of the TOFD map are respectively the later wave (scanning surface) and the back-wall waves (opposite surface). The space between these two line represents the thickness of the weld.
In Figure 1 TOFD waves pattern and the corresponding A-SCAN image are represented.
The Figure 2 shows how spherical waves are generated on the tips of the cracks when incident longitudinal waves cross the tip.
The Figure 3 shows a typical TOFD image obtained during calibration of equipment on two cylindrical holes.
The Figure 4 represents the mathematical and geometrical model on which TOFD software is based.
Fig. 4 - On Pitagora principle is based the software to
measure the trough thickness dimension in TOFD Technique.
Fig 5: Block diagram of TOFD ultrasonic computerized system.
Fig 5a: The TOFD system in its practical aspect.
Probes need special characteristics to emit short pulses to improve accuracy m Time Of Flight measurement. In case of small thicknesses high frequency can be used, 4-8 MHz.
Portable computer , Pentium , with minimum 3 slot free for installed the boards, color screen active matrix TFT, Window's 95.
| SCAN PLAN TAB.1|
Fig. 6 - Scan Plan indicating the main parameters value to use in relation of the thickness examined.
| Block |
| Thickness |
|PCS- Probe Center|
|1||85||90||2.5|| 1/4 T |
|3,28 mm||52°||0-90|| BC = 57 mm|
PCS = 184 mm
|2||164||170||2.5||1/4 T |
|3,28 mm|| 52° |
| 0-90 |
| BC = 57 mm |
PCS = 184 mm
BC = 125 mm
PCS = 175 mm
|Fig 7a: - Scanning direction - parallel to the weld axis.|
Fig: 7b - Scanning direction - perpendicular to the weld axis to improve sizing accuracy of indication.
When transversal cracks have to be investigated a scan in 90° orientation with respect to the primary scanning shall be applied
|Fig 8: B-Scan image in real time create and visualize on PC during the scanning of 1 meter length weld and 135 mm (5.3 inch.) of thickness.|
All data are recorded into the computer and can be transferred in standard floppy.
|Fig 9: Normal Tube Support Plate Responses in Differential and Absolute Modes|
Fig. 9 - TOFD image, visualize in post processing software (TOFD) for evaluation data and sizing indications.
Fig 10: Lack of fusion not detected by radiography test and
clearly displayed on the TOFD map. Thickness of weld 120 mm
Fig 11: Small defects not clearly identified with standard pulse
echo technique are clearly represented on TOFD map. Thickness of
weld 183 mm (7.2 inc.) Step of weld represented 1 m (39 inc.)
Fig 12: The TOFD map shows elongated lack of fusion at
different deeps clustered with isolated slag inclusions. Thickness of
weld 13 1 min (5. 1 inc.) Step of weld represented 1 m (39 inc.)
Fig 13: The TOFD map shows clustered slag inclusions located
near the root of weld. Thickness of weld 131 mm (5.1 inc.) Step
of weld represented 1 m (39 inc.)
Fig 14: TOFD map of a cladded test block of 170 mm (6.7
inch) thickness with overlay weld. The TOFD map shows on
the left a strong lateral waves, through the thickness the
three holes and before the backwall an "interference" image
like a transparency which represents the cladded layer.
The welds, circumferential, nozzle-shell, nozzle-heads and longitudinal have been examined with three techniques in order to compare the accuracy of TOFD with the traditional NDE:
The TOFD technique has detected all the indications which have been revealed by radiographic examination and traditional ultrasonic testing (pulse echo). The results referred to the single techniques are the following.
TOFD / Radiography
All indications visible on RT are visible on TOFD image. Some flaws with planar characteristics as lack of fusion, not detectable by RT have been clearly displayed on the TOFD image giving evidence of type of defect (see Figure 10).
TOFD / UT pulse echo
All indications visible with pulse-echo have been displayed on TOFD image. Some minor flaws, that are not detected or interactive with the noise using pulse echo in standard ASME calibration, have been clearly shown on the TOFD image (see Figure 11). Some other images of TOFD examination are shown in figures 12 and 13.
TOFD and new images / cladding
During the examination of cladded reactors we have been surprised about the good visibility on the TOFD images of the cladded layer (see Figure 14).
This is a really new possibility to examine and measure the thickness and discover cracks or disbonding flaws (1).
|Fig 15: Scanner on test block for calibration before scanning.||Fig 16: Picture on PC monitor showing the three ASME block holes.|
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