TOFD Full Notes

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TIME OF FLIGHT DIFFRACTION (TOFD) CONTENTS 1. Chapter 1 – TOFD Introduction & History 2. Chapter 2 – TOFD Vs Other methods 3. Chapter 3 – Physics – Tip Diffraction 4. Chapter 4 – Fundamentals of TOFD 5. Chapter 5 – Equipment Setup & Parameters Selection 6. Chapter 6 - Errors in TOFD 7. Chapter 7 – Digitization Principles 8. Chapter 8 – Data Analysis & Sizing 9. Chapter 9 – Codes & Standards - TOFD 10.Chapter 10 – Applications of TOFD

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TIME OF FLIGHT DIFFRACTION

CHAPTER 1 INTRODUCTION TO TOFD

INTRODUCTION Time of Diffraction is an Ultrasonic Testing technique which relies on the detection of diffraction signals which are generated from the edges and corners of a flaw.

HISTORY • TOFD was invented in the UK in the 1970s initially as a research tool in the 1970’s by Maurice Silk. • The use of TOFD enabled crack sizes to be measured more accurately, so that expensive components could be kept in operation as long as possible with minimal risk of failure. • TOFD gained wider acceptance in the 1980’s and 1990’s. • Development of quality control codes related to ToFD in the late 1990’s and 2000’s.

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ENGINEERING CRITICAL ASSESSMENT •

Engineering structures were assessed and observed to fail catastrophically by rapid brittle fracture, if they contain planar propagating defects (like cracks) above a certain critical size for the load applied.

CRITICAL FLAW SIZE •

Accurate measurement of the flaw size is of great importance in ensuring the structural integrity of many engineering structures.

•

Time of Flight Diffraction (ToFD) has a good accuracy for measuring the through - wall size of crack like defects.

•

The accuracy in general is ± 1mm and it can achieve ± 0.3mm when the defects are monitored.

TOFD BEHAVIOUR

•

TOFD also offers a good probability of detection (PoD) of defects, including badly oriented defects.

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TOFD coverage can be around 90% of the through wall thickness.

•

Normally up to 10% coverage loss is observed due to the two dead zones (OD surface and ID surface), but the actual percentage depends on the TOFD setup parameter selection.

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ToFD SUPPLEMENT • Two dead zones are located near the lateral wave and the back wall reflection. • To get full coverage ToFD should be combined with pulse echo (PE) technique. • Conveniently, ToFD and PE are complimentary, the strong features of ToFD are the weak points of PE and vice versa. THE PROBLEMS WITH THE PULSE ECHO TECHNIQUE • Pulse echo (PE) techniques are based on the reflected echoes coming from planar reflectors which are suitably angled to give a specular reflection back to the transducer. • Clearly it must be quite rare for defects to be exactly normal to the beam as would be required for a perfectly smooth large specular reflector. Flaws which are not favorably oriented are found to be less significant or sometimes may be not found .

TYPICAL TOFD EQUIPMENT

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The advantages of TOFD • TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique. • Defect height can be exactly determined, thus most suitable for monitoring growth or changes in known defects. • The inspection results are immediately available, as is a permanent record. • Because of the high-test speed the costs are less than those for radiography for wall thickness above 25 mm. • High probability of defect detection. • Most efficient for inspection of thick-walled vessels where X & Gamma ray would require too much time. • TOFD method can be used to observe and report microscopic degradation caused by fatigue, stress and chemical attack. • The entire volume can be inspected using a single pass along the length of the weld.

The disadvantages of TOFD • Sensitivity level: If the instrument sensitivity (gain) is set on very low level, the TOFD image would display no diffracted echo. If the instrument sensitivity is set just above electronic noise level, the TOFD image will display a lot of diffracted echoes which are caused by very small in homogeneities of the weld seam and does not mean that the weld is really bad. • Crack size determination: In practice, diffracted echoes at crack tips are not so clear as they are displayed. Crack tip echoes are part of a noise area caused by other relevant diffracted echoes of inhomogeneity. That can make sizing with the TOFD technique difficult. • Detection of small cracks at backside: This is one of the main disadvantages of TOFD. In that case traditional UT techniques with angle beam probes are used. • Crack edges must be sharp, and they are not always. • There is a dead zone for defect detection under the surface. It means, defects close to the surface could not be detected. This may be compensated by MPT (Magnetic Particle Test).

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TOFD Summary • It is fast, efficient. 'sees' everything and records all raw data for presentation in a proportionate and representative fashion. • TOFD is an ideal detection tool which provides an accurate and invaluable 'fingerprint' of condition as a quality control function at the time of construction.

• Good for defect detection especially mid-wall type defects (Planar). • It is the best defect sizing technique available with proper application. • To be used in conjunction with pulse-echo for complete volumetric weld examination and to meet code requirements with high PoD.

DISCUSSION

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TIME OF FLIGHT DIFFRACTION

CHAPTER 2 TOFD Vs OTHER NDT METHODS

UT Vs ToFD

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RT Vs ToFD

Probability of Detection for NDT Methods

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DISCUSSIONS

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TIME OF FLIGHT DIFFRACTION

CHAPTER 3 PHYSICS - TIP DIFFRACTION

1. 0 Diffraction Diffraction of waves is a phenomenon in which waves spread out at the edges when they pass through an aperture or round a small obstacle.

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1.1 Mechanism Diffraction is a resultant of wave displacement at edges and superposition of waves along the plane of Propagation. This is described by the Huygens-Fresnel principle. 1.2 Effects of Diffraction No change in frequency, wavelength and velocity of the waves. But a change in the direction and amplitude of the waves upon diffraction.

1.3 Ultrasonic diffraction •

When an ultrasonic wave interacts with a crack-like flaw it results in diffracted waves from the crack tips, in addition to specular reflected waves from the surface of the crack.

•

This diffracted wave from the tip of the crack is used to accurately size the depth of the crack from the ID or the OD.

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The diffracted Waves are much weaker than specularly reflected waves used for conventional ultrasonic inspection, but they radiate from the tips in all directions along the same plane as the incident ultrasonic waves.

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THE DIFFRACTION PHENOMENON

Important Points – Diffraction • • • • • • •

Modification in direction or deflection of sound beam Ends of defect become point sources Not related to orientation of defect Much weaker signal than reflected signals Sharp defects provide best emitters Tips signals are located accurately Time of flight of tip signals used for sizing

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• Between 45 degree and 80 degree the signal response within 6dB variation for both top and bottom tips of the crack . • The response is maximum at 65 degree for both top and bottom tips. • Lesser than 45 degree gives poor amplitude response.

Amplitude in dB

Variation of Diffraction signals with angle

Angle in degrees

DISCUSSIONS

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TIME OF FLIGHT DIFFRACTION

CHAPTER 4 FUNDAMENTALS OF TOFD

Basic Setup • 2 probes (transmitter, receiver) in pitch catch arrangement. • Wide weld volume coverage • Longitudinal waves • Probes symmetrical to the weld center • Amplifier – at receiver side

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TOFD Signals Signals received • Lateral wave (LW), subsurface • Back-wall echo(BW) • Mode converted ( shear wave echo) • Diffracted signals from defects

Receiver

Transmitter

Lateral wave Upper tip

Mode converted shear wave

Lower tip

Back-wall reflection

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The Lateral wave: • A sub / near-surface longitudinal wave generated from the wide beam of the transducer. • The lateral wave is not a surface wave. • Takes the path of least time/distance between two points - Fermat’s principle. • The frequency of the lateral wave tends to be lower than the waves at the centre of the beam and hence has a wider beam spread. • Since, lateral waves are weak waves, the amplitude would decay exponentially with distance from the inspection surface. • For large probe separation distance, lateral waves and may not even be seen. • For a curved surface it will travel straight across the metal between the two probes.

Back wall Signal • A large and strong longitudinal wave reflected from the back wall. •

The back wall is observed after the lateral wave because of the greater distance travelled.

Diffracted wave - Defect signals • If any crack is present in the weld, diffraction occurs at the top and bottom tips of the defect and are seen between the lateral wave and the back wall. • These signals are generally weaker than the back wall signal but stronger than the lateral wave. • For small defects (small height) the signals from the top and bottom may not be clearly resolvable and are more subjective. • It is sometimes easiest to concentrate on the two or three most predominant cycles.

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Mode converted shear signals • Appears after the back wall signal is a much larger signal. • Mode converted shear waves at a defect are generally observed between the longitudinal back wall and the mode converted shear back wall. • Mode converted signals takes a longer time to arrive at the receiver.

Importance of having mode converted shear waves

• Because of the basic pitch-catch probe arrangement the signals from the near surface region are very compressed in time and these signals may be hidden beneath the lateral. • Thus the importance of a minimum number of cycles in the signals in order to improve the resolution of the signals from the top and bottom of small defects. • lt is often very useful to collect signals in this region since genuine defect signals are repeated at longer times and near surface defect signals may be clearer since they are spread out in time more for the shear waves.

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Phase - Relationship between waves • Whenever a signal is reflected at an interface due to higher acoustic impedance a phase change of 180 degrees occurs. • For example, if the lateral wave starts with a positive cycle before it hits the tip of the defect, then the diffracted wave will start with a negative cycle after reflection. • In a few cases the top and bottom diffraction signals may not have a phase change of 180 degrees. • The recognition of phase change depends on the amplitude of the signals, and it is generally difficult if the signal is saturated.

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Transmitter

Receiver Lateral wave

Back-wall reflection BW

LW

Upper tip

Lower tip

ToFD Data Visualization (A-Scan to B-Scan) Image (B-Scan) is a collection of large amount of A-Scan data. The image is in gray scale which consists of Phase information of signals (unrectified A-Scan)

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A-Scan to B-Scan A-Scan

B-Scan

B-Scan image - ToFD

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An Example for Phase reversal • Detection and sizing of a lack of fusion by TOFD - phase reversal of upper and lower defect edges is displayed in gray levels.

TOFD Dead Zones

TOFD dead zones due to lateral waves and backwall. Dead zone size depends on frequency, pulse length, probe center separation, material thickness, and velocity. Errors can occur with TOFD if the defect is not symmetrically placed between the two probes.

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ToFD Probe angle • Longitudinal wave in gerneral is used. • Depends on the desired focus depth. • Depends on the needed weld volume. Transducer size • Depends on the desired focus depth. • Depends on the needed weld volume. • Influences the beam spread. Probe frequency • Depends on the probe characteristics. • Depends on needed focus depth and needed coverage area.

MATHEMATICAL CALCULATIONS - TYPICAL TOFD SETUP PCS S

S

t

PCS - Probe-Centre Separation t – Thickness of the material S – Distance between weld centerline and probe index

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Time of Arrival calculation 1. The first arrival time from the lateral wave signal to the receiver: tL = 2S/V 2. The second arrival time from the top tip diffracted signal to the receiver: t1 =(L1+L2)/V (i.e distance / Velocity) L2 = (d2 + S2)1/2 t1 = 2 [(d2 + S2)1/2 ] / V {assuming L1 = L2) 3. The third arrival time from the bottom tip diffracted signal to the receiver: t2 =(L3+L4)/V (i.e distance / Velocity) L3 = {(d+h)2 + S2}1/2 t2 = 2 [{(d+h)2 + S2}1/2 ] / V {assuming L3 = L4) 4. The fourth signal is Bac kwall tbw = 2 [(T2 + S2)1/2 ] / V

Note: V- speed for the longitudinal wave in steel L1, L2 – Half of the path of the diffracted signal so it takes time t1/2 S – Half distance of the probes separation. t1 – The arrival time of the top tip diffracted signal

Time of Arrival calculation with Probe Delay 1. tL = 2S/V + 2 t0 2. t1 = 2 [(d2 + S2)1/2 ] / V + 2 t0 3. t2 = 2 [{(d+h)2 + S2}1/2 ] / V + 2 t0 4. tbw = 2 [(T2 + S2)1/2 ] / V + 2 t0 Note: V- speed for the longitudinal wave in steel L1, L2 – Half of the path of the diffracted signal so it takes time t 1/2 S – Half distance of the probes separation. t1 – The arrival time of the top tip diffracted signal t0 - Probe delay

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DISCUSSION

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TIME OF FLIGHT DIFFRACTION

CHAPTER 5 EQUIPEMENT SETUP AND PARAMETERS SELECTION

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Probe Transducers / probe that are used for TOFD are different from the conventional manual ultrasonic testing. The various properties of TOFD probe, the effect of change in frequency , diameters and PCS will be discussed in the following slides. Parameter to be considered before selection of a probes: • •

To achieve wider beam coverage low frequency and small diameters probe can be used . But as the frequency of the probe decreases the test sensitivity will also be decreased . High frequency probes are used in ToFD as it gives better sensitivity and resolution.

Use of the high frequency probes for testing is a compromise on the wider beam coverage.

Beam spread Higher the bream spread more volume on the test material can be covered. High dampened and broad band probes are generally used in ToFD to get wider beam coverage ( larger volume coverage). Effect of increase in frequency and diameters on the beam spread is discussed in following slides.

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Calculate the beam spread using different frequency and diameter and see the changes in beam spread. Sample calculation to find the beam spread: Sample1: Use the following data for solving the problems Formulae to find beam spread: Sinø = [K x( v / f)]/ D K = 0.7, D = 6mm, f = 5MHz Perspex velocity is 2760 m/s Carbon steel velocity is 5960 m/s

Use Snell's law to find the incident angle of Perspex, for 5 MHz probe with refracted angle is 600 in steel is used to carry out ToFD :

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1.Calculate the beam spread in steel for longitudinal wave and shear wave for the following probes : i.450 ii.600 iii.70

Given: Velocity in Perspex = 2760 m/s Velocity in steel = 5960 m/s(longitudinal wave) Velocity in steel = 3240 m/s(shear wave) Frequency of the probe = 5 MHz Diameter of the probe = 6mm

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Difference in beam spread with respect to frequency Increase in frequency reduces the beam spread 5MHz, 6mm, 60°

10MHz, 6mm, 60°

Probe frequency Higher the frequency, resolution will be better, however with increase in frequency attenuation will also increase.

The following information act as a guide for different thickness : • • •

Less than 10mm – 10 - 15 MHz 10mm to 30mm – 5 to 10 MHz 30mm to 70mm – 2 to 5 MHz

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Probe angle • • • • •

•

Choice of probe angle depends on the material thickness and the component geometry. Choice of probe angle also depends on the technique and scan plan i.e whether the volume is covered in one pass or multiple passes. 70 degree probe will have wider coverage and 45 degree probe will have the smallest beam coverage. 70 degree probe will have the least time spread and will have poor resolution. 45 degree probe will have the maximum time spread and will have good resolution. 60 degree probe is considered as a good choice considering the volume coverage and time spread.

Probe angle To achieve a satisfactory data many scans may be required with different probe angles and different PCS.

Effect of change in PCS & beam angle for same frequency and diameter size of probe in 20mm plate.

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Recommended Probe selection parameters for thickness up to 70mm

Probe-Centre Separation (PCS): Distance between index point of transmitter and receiver in TOFD setup. For curved objects it is the shortest distance between the index points. This is generally based on focusing point in the examining material (or a typical weld) PCS Equal to 2S PCS Depends on the focus depth PCS Depends on the probe angle For the initial scan, 2/3T rule is used for PCS using the following formula : PCS (2S) = 2x t x Tan ø x 2/3 Or PCS (2S) = 1.33 x t x Tan ø If the focus is other than 2/3T, PCS should be calculated based on depth (d) : PCS (2S) = 2x d x Tan ø

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Probe-Centre Separation Change in PCS will effect the focus and the coverage of the volume. Following figures shows the effect on depth of focus for increase and decrease in PCS.

PCS 76 mm for depth of focus at 76 % of thickness.

PCS increased to 98mm

PCS decreased to 36 mm

When PCS is increased weld coverage increases, if all other parameters are remains the same. Low PCS gives a very good near surface resolution.

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Settings Gain setting • Setting gain (dB) in TOFD is difficult as its works on the principle of diffraction, the diffracted signals received are quite weak as compared to the reflected signals. • Amplitude of the defects in TOFD cannot be used to decide the size of the defect and for its evaluation. Time window settings • For full-thickness testing using only one set-up, the time window recorded should start at least 1μseconds prior to the time of arrival of the lateral wave, and should where possible extend up to the first mode converted back wall signal. • For more than one set-up used, the time windows shall overlap at least 10 % of the depth-range. • The start and extent of the time windows have to be verified on the test object.

Time-to-depth conversion (Screen Calibration) • For a given PCS, setting of time-to-depth conversion is best carried out using the lateral wave signal and the back wall signal with a known material velocity and thickness (usually V1 block). • For curved components geometrical corrections may be necessary.

Sensitivity settings • For all examination levels the sensitivity shall be set on the test object. • The amplitude of the lateral wave shall be between 40% and 80 % full screen height (FSH).

Note Any change of the TOFD set-up, e,g. probe centre separation (PCS), thickness requires a new setting.

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Calibration The following parameters shall be set based on the thickness of the item being tested : Type of Scan : Non Parallel / Parallel Type of Ultrasonic Wave :Voltage : Pulse Width : PRF : Filters : Averaging : Sizing curves : Measurement cursors etc.

Thickness of the item : PCS : Probe Angle : Probe Frequency : Wedge : Material Velocity :

Set the lateral wave, back wall and the mode converted signals on the screen using the reference block and verify the timing of lateral wave and the back wall signals against the manually calculated timings. Range start and range (parameters in the equipment) shall be used to set the screen.

Calibration Pre inspection and post inspection calibration shall be carried out using A2 / V1 blocks or other blocks similar in thickness as the object being tested. Scan area of 0 to 50mm shall be selected. 1mm or 2mm thickness tolerance may be allowed depending on the thickness of the object being used, if the readings exceeds the tolerance then check the system and the accessories and repeat the process. This is the system performance check to ensure the equipment (its software) as well as accessories like probe, wedges, preamplifier (if used), encoder, cables etc. are functioning properly.

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Scanning Longitudinal scan - Non-Parallel or D-scan or line scan • Scan direction is Perpendicular to the probe beam direction. • Most frequently used for weld inspection. • Detection Initial sizing. • High speed inspection.

Weld

Limitations • Defect depth only accurate when the probes are symmetrically positioned with regard to the defect. • Defect lateral position is unknown.

Parallel scan - Lateral, transverse or B-scan Weld • Movement of probes is parallel to the probe beam direction. • Precise sizing and positioning • Time will be minimum when probes are symmetrically positioned over the defect. Limitation Weld inspection: weld cap often reduces or makes impossible the extend of the scan.

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Mechanical Scanner • Very simple to use. • Magnetic wheels. • Manual (or motorized). • One axis position encoding. • Basically 2 probes, must be able to hold more (PE). • Easy and precise adjustment of probe separation is needed.

Scan Resolution (Sampling Interval) •

In manual as well as mechanical scanning the quality of the scan depends on scan resolution (sampling interval). The latest equipments offer a choice of as low as 0.1mm scan resolution, this means one A-scan is collected at an interval of 0.1mm, this will result in size of the data file too big and the scanning speed will be reduced which may lead to missing data . • Scan resolution of 1mm is considered to be good in terms of ToFD scans to get information about the discontinuities. • Scan resolution has to be decided together with other factors like averaging, PRF, length of the weld etc. Data amount per scan depends on:  Length of the scan, Resolution of scan.  Length of the gate [μs] (function of wall thickness).  Sample rate (digitization rate).

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Parameters for a ToFD Scan

Based on the material and the thickness of the weld the following parameters should be selected : 1.

Probes : Suitable probe frequency, size and the refracted angle should be selected for coverage, resolution and penetration in the material.

2.

Calculate the initial PCS focusing at 2/3T and subsequent PCS at the required depth or coverage.

3.

Select the type of scan i.e parallel or non parallel scan. Initial scans for welds are non parallel scans.

Parameters for a ToFD Scan – Contd., 4. The following parameters are selected from the equipment software : a.

b.

c.

d.

Material Velocity : Initially the velocity is set, the actual velocity against the thickness is verified when performing wedge delay and velocity calibration. Digitization frequency : The digitization frequency has to be at least 5 times of the probe frequency. Most of the ToFD equipments has provisions of setting higher digitization frequencies thus improving the sampling rate. Pulse width : Pulse width helps in forming the shape of the signal. It is taken for half of the period of the probe frequency. For 5 Mhz it is 100ns and for 10 Mhz it is 50ns. Voltage : Set as per manufacturers recommendation or can start with a lower value (50 V)

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Parameters for a ToFD Scan – Contd., e. PRF : Set at optimum. PRF can be high for low thickness and set low for high thickness. f. Averaging : High averaging will give a good digitize signal, by eliminating random electrical noise. However it has to be selected keeping in mind the size of the data file. g. Cursors / Sizing curves : These are important for measurement and sizing. h. Set the lateral wave, back wall and mode converted signal in the screen by using range start and range (parameters in the equipment). Setting the screen is important as our area of interest is from lateral wave to back wall. Most of the times mode converted signals also shows indications of the discontinuities which are detected between the lateral wave and back wall by the compression wave and sometimes it shows the indications which are not detected by compression wave, this helps to change the existing settings and detect the discontinuity which was not covered by the compression waves.

Parameters for a ToFD Scan – Contd., i.

Set the lateral wave amplitude as 50 to 60 % FSH by increasing or decreasing the gain. This is for scanning without setting the gain by the use of blocks as mentioned in BS EN 583 – 6

j.

Select the encoder and calibrate the encoder to record the position of the probes. Defect locations may be wrong if encoder is not calibrated properly. Verify the encoder resolution after calibration with the resolution manufacturers given by the manufacturer.

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Geometric Consideration in TOFD Single transducer diffraction (called “back method” in Japan)

Diffraction” or the “tip echo

Twin transducer TOFD with both transducers on the same side of the defect/weld. Complex inspections, e.g. nozzles

Parameters to be considered in TOFD Transducer size • • • •

Decreasing transducer diameter - decreases output Decreasing transducer diameter - increases beam divergence Decreasing transducer diameter - decreases near field length Decreasing transducer diameter - decreases contact area, emission point closer to front of probe wedge

Probe frequency • • • • • • •

Decreasing frequency - increases wavelength Decreasing frequency - decreases resolution Decreasing frequency - increases time duration and intensity of lateral wave Decreasing frequency - increases beam divergence Decreasing frequency - decreases near field length Decreasing frequency - increases penetration Decreasing frequency - decreases acoustic scatter

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DISCUSSIONS

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TIME OF FLIGHT DIFFRACTION

CHAPTER 6 ERRORS IN TOFD

OUTLINE Errors in the timing Near Surface errors Dead zone errors Resoultuion of top and bottom tips Off-Axis depth error PCS errors Multiple arcs Other errors

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Errors in Timing Problem • All depth calculations are based on the assumption that the defects are symmetrically(centrally) located to the two probes. • Depth error is high in near surface area as compared to the mid wall. Solution

• Un-symmetrical defect errors can be reduced by offset scan To compensate the depth error to some extent • Lowering PCS - but weld coverage compromised, may require more scans. • Higher frequency probe – minimizes ringing effect in other words increases resolution. • Higher digitizing rate - availability.

Near Surface Problems Problems •

Presence of defects near to the surface are masked by lateral waves.

•

Small dimensional defects may be missed as the signal may go around the defect.

Solutions • • • •

Lowering the PCS improves timing measurements. Separate scan for near surface using high frequency probes. Reducing the lateral wave rings ( number of cycles), using a highly damped broad band probes. Using software application to remove lateral wave – subjective.

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Dead Zones – Lateral wave and Back wall dead zones Problem • •

Defect signal is hidden beneath the lateral wave signal. Defect signal is masked by back wall signal.

Solution • •

Using a smaller PCS will decrease the dead zone. Using a probe with short pulse length.

Dead Zone - Calculation

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Distance for dead zone = 1.2174 μs x velocity = (1.2174 x 10-6 ) x (5960 x 103 ) = 7.2mm

Calculate the dead zone for the lateral wave with 2 cycles, 10 MHz probe

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Resolution of top and bottom tip of defects Problem Some times it is difficult to resolve top and bottom tips for the defects – particularly for volumetric defects . Solution • Decreasing PCS. • Decreasing pulse length.

Defect position uncertainty

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Off axis – depth effect Problem Error in depth measurement due to defect not centrally located(Off-Axis) between transmitter and receiver probes. Related • The error will be higher when the defect is at the boundary of the beam on the same ellipse. • The depth error can vary from almost zero to 60% and greater. If the flaws are only present in the weld volume then the depth error is less than 1% to 3%. Solution • Parallel scan where the defect will be at the center of the probes for more accurate information on depth. • Using a large PCS

In practice: • Maximum error on absolute depth position lies below 10%. • Error on height estimation small defect is negligible.

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Index point migration effects • The probes are not point sources. Due to beam spread, the probe separation (S) will vary accordingly to related signals. • In the wedge and in the object, beam will have main beam in the centre, trailing and leading edge at the edges of the beam. • This causes a shift in the index point for near surface area (leading edge) and the back wall area (trailing edge) this will lead to change in PCS and result in negligible error. • The ultrasonic beam transmitted and received from the edges of wedge, behaves like two transmitter and two receivers in the near field area and shows four separate arcs (multiple arcs)in a parallel scan.

Effects due to couplant film thickness • Like conventional UT, in TOFD couplant is used to efficiently transmitter and receive ultrasound. • This coupling film is so thin that its influence on the timing of the ultrasonic signals is negligible. • However thick films of couplant lead to lateral wave being not straight (wavy) as ultrasound may take more time to pass through the areas where the couplant film is thick.

Effects due Probe wedges • The wedge causes a delay (probe delay). Probe delay is not constant and is different for lateral wave, backwall echo, flaw tips etc., because of beam spread, and angle. This adds to all arrival times.

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Inspection surface characteristics • Usually it is assumed that the inspection surface is flat plane. Minor departures flatness will obviously degrade the accuracy somewhat as the probes will be displaced up or down from the assumed positions. The depth error will be of the same order as , or less than, the displacements of the probes.

Effects of velocity • For an uniform, homogeneous, isotropic material, the velocity accuracy is easily met by timing the interval between back wall reflections for a beam normal to the surface. • In more complex geometries or with material with less ideal properties, the inaccuracy of velocity estimates may become significant source of error. • The error is reduced if the PCS is reduced. Independent calibration of the velocity by measurement of the delay of the back wall echo, with a known wall thickness, greatly reduces this error.

Overall effects

Overall effects will be all the effects (errors) discussed in this lesson, if all the effects are added the overall depth effect may be derived. However some of the major effects ( like timing effect) may contribute more than the other effects which may be minor or negligible. Other effects • using different transducers or changing transducers • changes in probe angle due to wear and tear of the wedge • changes in probe position • angle of diffractions • changes in angular velocity • attenuation in the material

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No Signals – Common Faults • • • • • • •

Probes not properly fixed or Fixed in opposite direction. Damaged Cables. No couplant or not enough couplant on the surface of examination. Wedge is not in proper contact with the surface of examination. Rough surface. Preamplifier is switched off (if used). Cable from the transmitter /receiver probe was not connected in right socket in the equipment /preamplifier.

DISCUSSION

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CHAPTER 7 DIGITIZATION PRINCIPLES

Need for digitization • • • •

•

As analog signals cannot be stored, digitization is required before storing the signals. for a permanent record of data for re-analysis. Compare the result with previous inspection which helps in maintenance operations of life assessment of plant or equipment. Images are digitized and stored in static form (freeze option in many conventional instrument) or dynamic form (in real time as indications are formed on the screen). Sending results to far consultants to analysis and receive the advise / consulting without having time and money to spend in travelling.

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Key Parameters of digitizer • Frequency (Digitizer) • Processor (no. of bits or amount of information that can be handled) • Averaging • PRF • Acquisition Rate • Soft Gain

Digitization • Conversion of analogue A-scan (amplitude) to digital numbers (digits) by taking samples of a signal at a regular interval. • Analog signals are continuous electrical signals; digital signals are non-continuous. • Digital information exists as one of two digits, either 0 or 1. These are known as bits and the sequences of 0s and 1s that constitute information are called bytes. • Analog signal can be converted to digital signal by ADC. The reading of an analog signal at regular time intervals (frequency), is the sampling value of the signal at the point. • Each such reading is called a sample (a particular combination of 0’s and 1’s) and is considered to contain exact information for that stage;

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Digitization – Continued.,

• The sampling frequency has to be greater than the bandwidth of the signal being sampled. • Nyquist Sampling Theorem - for a correct representation of a digitized signal, the sampling frequency has to be at least twice as high as the bandwidth. • It is recommended that digitizer frequency (Ideal minimum frequency) is at least 5 times the probe central frequency to reduce the amplitude error to within 10%.

Analog to Digital Conversion (ADC) • An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts a continuous physical quantity (usually voltage) to a digital number that represents the quantity's amplitude. The conversion involves sampling of the of the data, so it necessarily introduces a small amount of error.

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Ultrasonic features of TOFD General

Pulser – Receiver

Digitizer

Gain/ booster

Voltage

Digitizing frequency

Type of Wave

Pulse width

Averaging

Material Velocity

Rectification

Repetition rate (PRF)

Ultrasonic Start

Band-pass filters

Acquisition rate

Ultrasonic range

The feature might be found under different tabs, depending on instrument and software version.

Pulse Width • An ultrasonic probe consists of a piezoelectric material which when set into vibration with a voltage pulse produces a burst of ultrasound. • The use of different voltages ranging dependents on the probe frequency and the type of crystal element. • The pulse width helps to optimise the shape of the received signal. The first edge of the rectangular pulse sets the crystal element into oscillation. • The second edge of the rectangular pulse also sets the crystal element into oscillation again but the phase of the burst of ultrasound is 180 degrees out of phase with the first set of oscillations.

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Pulse Width – Continued. • The pulse width is generally set to 1 period of the wave frequency after which the two signals will be out of phase and a smaller amplitude signal will be obtained. • This will reduce the ringing of the probes and have better resolution of signals from the top and bottom tips of small defects.

Pulse Repetition Rate •

The repetition rate (PRF , or pulse-repetition frequency) is the firing frequency of the ultrasonic signal.

•

PRF depends on averaging, acquisition time, gate length, processing time, and the update rate of the parameters.

•

In general, the PRF should be set as high as reasonable, ensuring that any ghost echoes are out of the acquisition range.

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Effect of PRF •

PRF is rate of voltage pulses transmitted from pulser to transducer (remember this is not probe frequency!!!!!!!!).

•

Selecting low PRF results in loss of data or missing scan data which are caused due to high scan speed, wide beam angles chosen, high resolution, low communication speed.

•

Increasing PRF too high results in ghost or phantom signals.

Under sampling Sub-sampled image: • Nyquist is not met • Amplitude error, phase shift, distortion

Over sampling • A higher sampling rate will result in more data points, thus larger files.

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Aliasing Effect • An alias is a false lower frequency component that appears in sampled data acquired at too low a sampling rate. • The Nyquist theorem states that a signal must be sampled at a rate greater than twice the highest frequency component of the signal to accurately reconstruct the waveform; otherwise, the high-frequency content will alias at a frequency inside the spectrum of interest (passband).

The dotted line indicates the aliased signal recorded by the ADC and is sampled as a 1 MHz signal instead of a 5 MHz signal

For analog signal to digital by digitizing the amplitude • Signal amplitude is quantized into a sequence of samples before signal processing. • The precision of samples depends on no. of bit levels. • As number of bit increases dynamic range and file size increases . • The dynamic range of the 3-bit system is approximately 18 dB, while that of the 8bit is 48dB. That means that if the signal of interest is below that value it can never be retrieved simply because it was never sampled properly in the first place.

For Pulse Echo (FWRF), The 8 bit digitizer steps are 0 to 255 and for RF wave form -127 to +128

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dB relation to Processor bit • For an 8 bit digitizer, it is 28 digital numbers i.e 256 (0 to 255, ToFD in RF mode -127 to +128) • For an 10 bit digitizer, it 210 digital numbers i.e 1024 (0 to 1023, ToFD in RF mode -511 to + 512) • For ToFD data in the RF mode, how many dB’s it takes to display the data for an 8 bit digitizer : dB = 20 x log A1 / A2 • ToFD data is displayed as -127 to +128, we consider 1 to 128 dB = 20 x log 128 / 1 = 42dB

Signal Averaging

•

•

• •

Averaging is the number of samples (A-Scans) summed for each acquisition step on each A-Scan displayed. Averaging increases the signal to noise ratio by reducing random noise. High averaging reduces acquisition speed. Averaging does not affect file size or amount of data collected.

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Date file size per scan depends on: Length of the scan  Resolution of scan  Wall thickness of the material  Sample rate (digitization rate) 

Sampling rate Calculation

• If a digitization frequency is 25MHz – Which means 25 million samples per 1 sec – Or we can write as 25 samples per 1 micro sec – Or we can otherwise write as 1 sample per 0.04 micro sec

• Now, please calculate the sampling rate for 45MHz, 70MHz and 150MHz

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Number of samples and data size Consider an example: A pair of TOFD probe with frequency of 1MHz and digitization frequency of 50MHz. If one A-Scan is collected every one mm of weld. What would be the data size for 10m of weld • • • •

1 time period = 1/1MHz = 1 micro sec Sampling rate = 1/50MHz = 0.02 micro sec So for 1 A-Scan per mm, no. of samples = 1/0.02 = 50 For 10m (10000mm), No. of samples = 50 X 10000 = 500000 bytes = 0.5MB

Calculating number of samples • • •

No. of samples depend on the time required for one wave length (i.e. we call as no. of samples per one time period) If you divide time period by sampling rate you will get no. of samples Calculate the no. of samples for – Probe frequency of 5MHz at digitization rate of 50MHz, 75MHz and 125MHz – Probe frequency of 10MHz at digitization rate of 40MHz, 65MHz and 110MHz

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Effect of parameters on scanning speed • • •

If collection rate is one A-scan per mm, For 150mm/sec (scanning speed) = 150 pulses per second (minimum PRF) If you use averaging of 16, then the no. of pulses increases by 16X150mm/sec = 2400 pulses per sec (minimum PRF)

•

So what should you do to avoid “missing data” : • Reduce averaging • Reduce scan speed

Signal Processing - Filters • •

A filter is a device or process that removes from a signal some unwanted component or feature. The drawback of filtering is the loss of information associated with it.

Low Pass Filter : Low pass filter will allow the signals which are lower than the set frequency.

Using the full band signal will increase noise level

High Pass Filter : High pass filter will allow the signals which are higher than the set frequency. Band Pass Filter : Only frequencies in a frequency band are passed

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Filters thumb rule : • High pass filter is set at 0.5 times of the probe center frequency • Low pass filter is set at 2.0 times or more of the probe center frequency Choosing according to probe center frequency • •

Actual choice depends on beam spread and attenuation and beam path. Best solution is to try different filters to optimize the image

DISCUSSION

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CHAPTER 8 INTEPRETATION, ANALYSIS & SIZING

Transmitter

Receiver Positive

Negative

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Top Tip Bottom Tip

Interpretation and analysis of TOFD images Interpretation and analysis of TOFD images is generally performed as follows: • Assessing the quality of the TOFD-image; • Identification of relevant indications and discrimination of non-relevant indications; •

Classification of relevant indications in terms of:  embedded (linear, point-like);  surface breaking;

•

Determination of location, length and height (sizing)

•

Evaluation against acceptance criteria.

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Assessing the quality of the TOFD image • A TOFD-testing has to be carried out such that satisfactory images are generated which can be evaluated with confidence. Satisfactory images are defined by appropriate:  coupling  sensitivity setting,  time-base setting.

Identification of relevant indications • TOFD can image discontinuities in the weld as well as geometric features of the test object. To identify indications of geometric features, detailed knowledge of the test object is necessary. • Indications are identified by patterns or disturbances within the TOFD image. To decide whether an indication is relevant (caused by a discontinuity), patterns or disturbances have to be evaluated considering shape and signal amplitude relative to general noise level.

Classification of relevant indications • Consideration to be given on amplitude, phase, location, presence of mode converted signals and pattern of relevant indications as it may contain information on the type of discontinuity. • Relevant indications are classified as  surface-breaking indications o disturbance of the lateral wave o disturbance of the back wall reflection; 

embedded discontinuities - indications between lateral wave and back wall reflection.

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Surface breaking discontinuities •

Scanning surface discontinuity: This type shows up as an elongated pattern generated by the signal emitted from the lower edge of the discontinuity and a weakening or loss of the lateral wave (not always observed). The indication from the lower edge can be hidden by the lateral wave, but generally a pattern can be observed in the mode converted part of the image. For a small discontinuities, only a small delay of the lateral wave may be observed.

•

Opposite surface discontinuity: This type shows up as an elongated pattern generated by the signal emitted from the upper edge of the discontinuity and a weakening, loss, or delay of the back wall reflection (not always observed).

•

Through wall discontinuity: This type shows up as a loss or weakening of both the lateral wave and the back wall reflection accompanied by diffracted signals from both ends of the discontinuity.

Embedded discontinuity indications • Point-like discontinuity: This type shows up as a single hyperbolic shaped curve which may lie at any depth. • Elongated discontinuity with no measurable height: This type appears as an elongated pattern corresponding to an apparent upper edge signal. • Elongated discontinuity with a measurable height: This type appears as two elongated patterns located at different positions in depth, corresponding to the lower and upper edges of the discontinuity. The indication of the lower edge is usually in phase with the lateral wave. The indication of the upper edge is usually in phase with the back wall reflection. Indications of embedded discontinuities usually do not disturb the lateral wave or the back-wall reflection. Other indications that cannot be classified may require further testing and analysis.

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TOFD – an example for good image Time window shall start at least 1μSec prior to Lateral Wave.

Gain Settings

Low

High

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Missed Scan

Loss of Signal

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Effect of thick couplant layer • Causes uneven lateral wave making measurements difficult.

• To some extent lateral wave can be straightened using software options.

Screen Calibration •

Identify the Phase of the lateral and back wall for screen calibration ( suggested to keep the cursor on good portion prominently displaying phase information in the D-scan and then go to from A-scan to place the blue and red cursors before performing the screen calibration option).

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Lateral wave synchronization

Before

After

Lateral wave removal – done to view masked defects by lateral wave. This operation is not always completely successful.

Before

After

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Sizing Techniques Location & Length measurements • Flaw length from a non-parallel scan( DScan), is measured from end to end of the signal after compensating for beam spread. • If the flaw is curved, then it is difficult to accurately measure the length and done more often with errors. • Length of the flaw is defined by the difference of the x-coordinates of the extremities of the indication.

Diffracted flaw signal

Resultant flaw signal

Curved flaw

Height measurements • • • •

Uses the accurate time of arrival of the signal unlike the length measurement technique. The height is defined as the maximum difference of the z- coordinates. For indications displaying varying z-coordinates along their length, the height should be determined at the x-position where the difference of the z-coordinates is greatest. Another method is counting the number of rings when the resolution of the tips is not seen.

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Synthetic Aperture Focusing Technique (SAFT) •

A technique to measure length of a flaw on the reconstructed TOFD D-scan signal using the 6 dB drop method.

•

SAFT produces a collimated beam from the transducer with a beam width of approximately half the crystal diameter.

•

This greatly reduces the beam spread of a normal transducer allowing accurate measurements particularly for flaws smaller than the normal beam spread.

•

SAFT process also greatly improves signal to noise ratio.

TOFD image of surface notch's

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TOFD image of opposite surface discontinuity

TOFD image of opposite surface discontinuity with a larger depth

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TOFD image of a through wall discontinuity

TOFD image of point like discontinuities

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TOFD image of a change in thickness

TOFD image of misaligned pipe joint

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TOFD image of a root corrosion

DISCUSSIONS

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CHAPTER 9 CODES & STANDARDS - TOFD

•

Many national and International standards or European Standards are available :EN, ASME, ASTM, AWS, ISO, etc.

• European Standard EN 583-6, based on BS 7706 – check for latest editions. TOFD as a method for defect detection and sizing • CEN/TS 14751 (CEN Technical specification) - – check for latest editions. Welding – Use of TOFD for examination of welds is replaced by BS EN ISO 10863:2011 • ASME V, Art. 4 & 5 Mandatory Appendices I & II • ASME XIII requires RT, Code Case 2235 lists conditions under which RT may be replaced by UT

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•

Code Case 2235-9 Inquiry: Under what conditions and limitations may an ultrasonic examination be used in lieu of radiography. Requirements For ½” and thicker materials. Coverage of HAZ Scan plan Calibration/Validation block Computer-based data acquisition required with data recording Flaw sizing is required Acceptance criteria

• When ToFD is used to replace RT, then should also be used with an additional surface technique. Magnetic Particle Testing Penetrant Testing Manual UT

DISCUSSIONS

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CHAPTER 10 APPLICATIONS OF TOFD

Applications • •

• •

Inspection of complex geometries like nozzle-shell, pipe-flange, reducer-pipe etc., In more recent years this expertise has been adapted for non nuclear applications including vessels for the chemical/process industries, complex forgings and castings (eg turbine discs) and nodal configurations on tubular structures. Detection cracks in service pipes, pressure vessels etc., Monitoring of cracks during service Continuous Condition Monitoring

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TOFD scanning mechanisms There are several types of scanning mechanisms: • Manual scanning • Semi-automatic • Mechanical scanning

Manual Scanning •

Manual scanning is achieved by the use of jigs which has the probe holders and it allows the probes to be moved (adjusted) horizontally to set the PCS. Encoders are fixed to the jigs to record the position of the probes.

•

In manual scanning we can mark the PCS and the edges of the probe so that the inspector knows if he is deviating from the marked positions. Use of guides (magnetic strips, rulers etc.) will also help to get the scan straight. Always set the probes such that the weld axis is in the centre of the two probes.

•

Manual scanning needs practice and experience to get a good scan as it is not easy to maintain the constant movement and direction. However the inspector can stop and restart the scan from the areas where the data is not captured properly or if there is missing data.

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Mechanical Scanning • • •

•

Mechanical scanning is achieved by the use of scanners, similar to manual jigs it has the probe holders and it allows the probes to be moved (adjusted) horizontally to set the PCS. Encoders can be fixed to the scanner to record the position of the probes. The positions are set and the inspector has to ensure the scanner is not deviating from the marked location. Mechanical scanning can be semi automatic where the inspectors uses mechanical means to move the scanner like a handle or through the gear mechanism. It can be automatic where the scanner is operated using motor. Some of the advance systems allows the motor to be operated from the equipment itself, where the speed and its movements are controlled.

DISCUSSIONS

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