Apostila de TOFD PCN, Manuais, Projetos, Pesquisas de Engenharia Física

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TABLE OF CONTENTS

Rev 0 Nov 2011 i

TABLE OF CONTENTS

• 1. ULTRASONIC NON‐DESTRUCTIVE TESTING PARTICULAR PAGE
• 1.1 Pulse‐Echo Detection Of Flaws
• 1.2 Flaw Sizing With The Pulse‐Echo Technique
• 1.3 Comparision Of Flaw Sizing Accuracy For Different Techniques
• 1.4 The Time Of Flight Diffraction Technique
• 1.5 History Of Tofd Development
• 1.6 Tofd Advantages And Limitations
• 2.1 Diffraction 2. THE PRINCIPLES OF TOFD
• 2.2 Waves
• 2.3 Conventional Use of Diffraction
• 2.4 Signals
• 2.5 Basics of TOFD inspection
• 2.6 A‐Scan with no Defect Present
• 2.7 A‐Scan with Defect Present
• 2.8 Lateral Wave
• 2.9 Back Wall Signal
• 2.10 Defect Signals
• 2.11 Shear or Mode Converted Shear Signals
• 2.12 Basic Principles of the TOFD Technique (TOFD: Typical Setup)
• 2.13 A‐Scan Signals
• 2.14 Some Typical Defects
• 2.15 Data Visualization
• 2.16 What do TOFD scans really look like?
• 2.17 Signals
• 2.18 Choosing an Angle
• 2.19 Depth calculation
• 2.20 Signal Time
• 2.21 What is Probe Centre Separation (PCS)?
• 2.22 How is PCS determined?
• 2.23 How is PCS Determined?: General Applications
• 2.24 Calculation of PCS
• 2.25 Focus At A Point Of Interest
• 2.26 Types of TOFD Scan
• 2.27 Transverse Scan
• 2.28 Parallel Scan
• 3. Equipment used in TOFD
• 3.1 Digital Control
• 3.2 Pulsers and Receivers
• 3.3 Pulsers
• 3.4 Tone Burst
• 3.5 Square Wave Pulsers
• 3.6 Receivers Rev 0 Nov 2011 ii
• 3.7 Gates
• 3.8 Data Acquisition and Automated Systems
• 3.9 Instrument Outputs
• 3.10 Memory and Digitisation Aspects
• 3.11 Data Processing
• 3.12 Scanning Equipment
• 3.13 Limitations of Mechanised Scanning
• 3.14 Scanning Speed
• 3.15 Encoders
• 4. EQUIPMENT REQUIREMENTS
• 4.1 Ultrasonic equipment and display
• 4.2 Ultrasonic probes
• 4.3 Scanning mechanisms
• 4.4 Equipment set‐up procedures
• 4.5 Probe choice and probe separation
• 4.6 Time window setting
• 4.7 Sensitivity setting
• 4.8 Scan resolution setting
• 4.9 Setting of scanning speed
• 4.10 Checking system performance
• 4.11 System Verification Reference blocks
• 5. TOFD Depth, Ring‐Time Issues and Errors
• 5.1 Depth and Ring‐time Calculations
• 5.2 Flaw Position Errors
• 5.3 Timing Error
• 5.4 Acoustic Velocity Error
• 5.5 PCS Error
• 5.6 Lateral Position Error
• 5.7 Frequency Content Effects
• 6. ANALYSIS SOFTWARE FEATURES & TOFD OF COMPLEX GEOMETRY
• 6.1 Linearisation
• 6.2 Lateral /Back wall Straighten and Removal
• 6.3 Parabolic Cursor
• 6.4 Synthetic Aperture Focusing Technique ‐ SAFT
• 6.5 Split Spectrum Processing
• 6.6 Locus Plots
• 6.7 Curved Surfaces and Complex Geometry
• 6.8 Complex Geometry
• 7. INTERPRETATION AND EVALUATION
• 7.1 Development of TOFD codes and standards
• 7.2 ASME Adaptations to TOFD
• 7.3 Indications from surface breaking discontinuities
• 7.4 Indications from embedded discontinuities
• 7.5 BASICS OF DIMENSIONING Rev 0 Nov 2011 iii
• 7.6 Height measurement
• 7.7 Method
• 7.8 Method
• 7.9 Method
• 7.10 Examples
• 7.11 Length measurement
• 7.12 Scanning surface discontinuity
• 7.13 Opposite surface discontinuity
• 7.14 Through wall discontinuity
• 7.15 Embedded point‐like indication
• 7.16 Flaw Tip
• 7.17 Flaw Position Errors
• 7.18 Evaluation
• 7.19 Single Flaw Images
• 7.20 Multiple Flaw Images
• 7.21 Typical Problems With TOFD
• OmniScan Orientation
• Calibrations

Rev 0 Nov 2011 Page 1 of 147

1. ULTRASONIC NON-DESTRUCTIVE TESTING

If an electric potential is applied to a piezoelectric type material it oscillates and if it is

of the right thickness will produce waves of ultrasound of frequencies most useful for

inspecting metal components. This material is the basis of ultrasonic probes which

produce longitudinal waves, generally called compression waves. If the longitudinal

waves enter metal at an angle then they refract in the metal and produce both

longitudinal and shear waves, the angles of the two types of waves depending on the

velocity of shear and longitudinal waves in the metal and the velocity of the

longitudinal waves in the probe shoe material. Shear waves are transmitted by a

periodic shear force and can only exist in materials like metals which possess shear

elasticity. Liquids cannot sustain a shear force. For normal ultrasonic inspection of

metals ultrasonic frequencies of between 2 and 5 MHz are used. The corresponding

wavelength of the waves are found from the formula,

Velocity = wavelength X Frequency

Velocity is usually defined in units of m/s and typical values in steel are 5950m/s for

longitudinal waves and 3230m/s for shear waves. Since the probe frequency is in

units of MHz (and we shall see that time is defined in microseconds in TOFD) it is

more convenient to define the velocity units as mm/μs. In these units the wavelength

in the above equation is given in mm. Thus for the above frequencies the wavelength

of longitudinal waves is in the range 1 to 3mm and for shear waves from 0.6 to

1.6mm. For reflectors of size less than half a wavelength interference can take place

in the reflected waves and hence the minimum size of cracks that can reliably be

detected is equivalent to one half of wavelength. To detect small cracks in thin higher

frequencies are used but in thick material the increasing attenuation with increase in

frequency generally prevents the use of much higher frequencies.

For conventional Pulse-echo ultrasonic inspections angled shear waves are very

important since at a given frequency they have a wavelength half that of longitudinal

waves, allowing for the resolution of smaller defects. Also, as will be seen in a later

chapter, for a given size of crystal diameter and frequency shear waves produce a

smaller beam spread and a consequently higher beam intensity and accurate sizing

ability than longitudinal waves.

1.1 PULSE-ECHO DETECTION OF FLAWS

An Ultrasonic inspection of a sample is carried out by scanning the metal with a

beam of ultrasound. Any reflectors in the metal are only detected if the sound is

reflected back from the discontinuity and returns to the crystal element of the probe,

where it vibrates the crystal and is converted into electrical signals. In order to reflect

the waves back the beam must ideally be at right angles to the reflector surface. This

is so called “Specular” reflection. If the surface is tilted with respect to the direction of

the beam of ultrasound then the reflected waves may miss the probe crystal

altogether and the discontinuity will remain undetected. The proportion of the sound

beam getting back to the crystal falls off rapidly with increasing angles of tilt and

skew from this ideal position. A tilt of only 5 degrees can cause the amplitude to fall

by a factor of about 2 (6dB) and 10 degrees or more may result in loss of detection.

Rev 0 Nov 2011 Page 3 of 147

Figure 1.2 Determination of Flaw Size by 6dB Drop Sizing

When the probe is moved towards the weld the flaw starts to appear in the ultrasonic

beam and the amplitude of the flaw signal rises. Once the flaw area fills the beam

the amplitude stays constant until the beam starts to pass the other end of the flaw,

when the amplitude starts to fall. It is assumed for this explanation that a distance

amplitude correction has been applied so that there is no variation of amplitude with

range. The maximum amplitude trace across the flaw is called an echodynamic trace

and is shown in the bottom half of the figure.

At the level where the signal amplitude is half that of the maximum signal it is

assumed that only half the flaw area is in the beam of ultrasound that that the centre

of the probe is opposite the edge of the flaw. Thus if the positions of the probe are

noted where the amplitude has dropped by 6dB the size of the flaw can be measured

and hence the term 6 dB drop sizing. If the distance between the probe positions is x

mm then the width w of the flaw is given by w=x cosα where α is the angle of the

beam centre with respect to the normal to the surface of the metal on which the

probe sits. The through wall height of the flaw (the critical measurement) h is then

h= x cosα sin α

To determine the length of the flaw along the weld the probe must be positioned so

as to obtain the maximum amplitude signal and them moved parallel to the weld to

determine the 6dB drop positions. The length is the distance between the positions.

Rev 0 Nov 2011 Page 4 of 147

Again the main problem with the 6dB drop technique is the variation in amplitude due

to the possible roughness of scattering surface and the fact that the flaw surface is

unlikely to be normal to the ultrasonic beam.

Some typical smallest sizes of flaws which can be detected and typical sizing errors

are listed below from document IIS/IIW-580-86 from commission V of the

International Institute of Welding for pulse-echo ultrasonics.

Table 1.

_Defect positions Through-thickness Length Smallest Size, mm Accuracy,

• or - mm Smallest length, mm Accuracy,
• or - mm Near surface, 0-5 mm depth 3 3 4 5 Mid-wall, mm 5- 25- 75- 3 3 5 3 3 5 4 7 10 4 7 10 Back surface wall thickness, mm 10- 25- 75- 4 4 5 4 4 5 4 7 10 4 7 10_

1.3 COMPARISION OF FLAW SIZING ACCURACY FOR DIFFERENT

TECHNIQUES

Other techniques are available for sizing flaws. For surface opening flaws, if access

is available to the surface the alternating current potential drop (ACPD) is an

accurate technique. The technique essentially measures the drop in voltage between

two surface contacts when placed on either side of the flaw, the additional voltage

drop being due to current having to flow around the flaw surface. Specialized eddy

current techniques (ACFM) can also measure the size of near surface flaws. One of

the most accurate and useful techniques for measuring flaw height is the TOFD

technique. The importance of the TOFD technique is seen from the comparison of

the accuracy of different crack sizing techniques summarised below.

Table 1.

Manual pulse-echo 4 mm ACPD (surface opening only) 1 mm TOFD 1 mm TOFD when monitoring growth 0.3 mm

Rev 0 Nov 2011 Page 6 of 147

technique. This means that a considerable number of flaws, which are actually below

this size are reported as being above this size because they appear with the pulse-

echo technique to be larger. Thus while a very high probability of detection may be

obtained for flaws above the size of interest there will be a large falls call rate. This is

made worse by the fact that the distribution curve of flaw size against number of

flaws usually rises towards the smaller sizes.

Thus in principle the detection threshold for the more accurate TOFD technique can

be set much closer to the size of interest and thus greatly reduce the falls call rate.

1.5 History of TOFD Development

Critical predicted size lifetime height of growth crack curve Pulse-echo measurements time, years Critical size predicted lifetime growth curve height of crack TOFD measurements time, years

Figure 1.3 Illustration of Importance of accurate sizing measurements

In the last section the importance of accurate sizing of cracks was described.

Especially in the nuclear industry. For this reason the national NDT Centre, Harweel

(Then part of the United Kingdom Atomic Energy Authority – UKAEA) asked Dr.

Maurice Silk to try and develop an ultrasonic sizing technique more accurate than

the conventional pulse-echo method. In the early 1970’s Dr.Silk developed the

technique known as Time of Flight Diffraction (TOFD)

The most important differences from conventional pulse-echo are

a) Its more accurate sizing accuracy (typically ±1mm and in a monitoring

situation ± 0.3mm) and the almost independence of angle of the flaw for detection.

The sizing is based on the transit time of the diffracted signal and doesn’t depend on

the amplitude.

b) It will be seen that with TOFD a simple amplitude threshold cannot be applied

for rejection or reporting flaws since the amplitude of the diffraction signals doesn’t

depend on the size of flaw and all the data must be analysed before any flaws can

be rejected. Hence the TOFD technique training and experience are essential.

Rev 0 Nov 2011 Page 7 of 147

For a number of years TOFD remained largely a laboratory tool but the realisation of

its importance and the proposed public enquiry for a PWR Reactor in UK lead to a

number of major trials in the early 1980’s to evaluate the best possible UT Technique

for the reactor pressure vessel and other major components. The trials were known

as Defect detection Trials (DDT). The trials were very important in view of

international PISC exercise in the late 1970’s, which was aimed at establishing the

capability of the ASME code Ultrasonic procedures and which obtained poor results

for the reliability and accuracy of conventional Pulse-echo inspections. Many other

trials and validations have been carried out comparing different techniques and in all

these tests TOFD has always proved to be virtually the most reliable and accurate

technique.

1.6 TOFD Advantages and Limitations

If one was to listen to some of the proponents of TOFD it would seem that TOFD is

the panacea of inspection problems. This is clearly untrue. It has its advantages and

limitations, like any NDT method. Depending on the application, TOFD may stand as

a useful option on its own. In other situations it is best used with support from other

NDT methods or as a support option to other NDT methods.

A brief list of TOFD pros and cons should help the practitioner to decide how and

when to best use this NDT tool.

Advantages:

Repeatability

TOFD (especially when used with a positioning encoded

provides measurements in real units (e.g. millimeters) that are

much more useful to engineers than dB’s or equivalent scales of

response. A scan made of a weld with a TOFD setup by one

operator will be essentially identical to TOFD scan made by

another operator (assuming both use similar probes and

settings). This makes TOFD ideal for flaw monitoring,.

Accuracy

Generally levels of accuracy attainable by TOFD are

within ±0.5mm in terms of (critical) through wall extent

and ± 0.5 to 1.0mm in terms of length. Position along the

weld and with respect to the weld centreline can usually

be established to within 0.5mm and angular dispositions

can be resolved to within a few degrees when appropriate

scan procedures are used. This accuracy and reliability

makes TOFD a suitable NDT tool for fracture mechanics

assessment (otherwise destructive methods and physical

measurement would be required).

Rev 0 Nov 2011 Page 9 of 147

the length of weld volumetrically inspected in a single

pass of the transducers and not just the scanning speed

of the probes.

Sensitivity

This item may be an advantage or disadvantage. It

depends on your point of view. TOFD is generally

configured to “see everything”. When the test specimen is

relatively clean or the material highly refined there is no

issue with the sensitivity. However, where the test

materials contains many major anomalies to be reported

or in coarse material where the grain boundaries are on

the order of size of the flaws, TOFD sensitivity can be

construed as a hindrance and, in certain circumstances,

can make interpretation and sentencing a time

consuming ordeal. When the data storage advantage is

considered in light of sensitivity it might be noted that one

of the features of digital processing is the ability to

increase gain via software. That means that small (un-

saturating) signals can be increased after data collection.

Easy discrimination of defects and geometry

A common problem experienced in manual ultrasonic

testing of welds is the issue of operator skills in

differentiating between flaw signals and signals

originating from surface geometries. When TOFD is

carried out on a butt weld with the root and cap re-

enforcement left on the TOFD data display provide un-

ambiguous indications easily discriminated from the re-

enforcement metal.

Flaw orientation

Because of the omni-directional aspect of diffracted

signals TOFD is sensitive to virtually all types of defects

regardless of orientation. This is also partly attributable to

the very wide angular coverage of the divergent beam

used. Providing the flaw falls within the effective beam

envelope, the low amplitude signals diffracted from its

edges will be captured and displayed in correct relative

position.

Coupling Status

TOFD data can be collected by manual or mechanised

methods of probe motion. Any manual ultrasonic operator

doing pulse-echo testing monitors the A-scan and can

recognise when the coupling is not as effective by a loss

Rev 0 Nov 2011 Page 10 of 147

of the grass level. However, in the case of TOFD

scanning the operator does not monitor the A-scan and

when scans are lengthy or when mechanised, the

operator has no sense of the coupling condition by simply

looking at the probes moving on the surface. By

observing the data collected for the lateral wave

amplitude and the associated “grain-noise” the TOFD

display is an effective indicator of how well the probes

were coupled. Maintaining coupling is made somewhat

more difficult than standard manual scanning because

both the transmitter and receiver must be well-coupled to

the test surface.

Reduced Operator Reliance

Since TOFD data can be collected and stored to a

computer file for later analysis it is possible to reduce the

reliance of the test on the probe operator. Many

applications can now be configured by a senior operator

and then the data acquisition assigned to a “field team.

This might consist of a person that operates the computer

data acquisition unit and another that pushes the probe

along the weld. Sufficient experience and competence is

required by this team to ensure that the data collected is

good. Then final assessment and sentencing can be

carried out at a later time by the senior operator.

Limitations:

Weak Signals

Typically the diffracted signals associated with

TOFD are 20-30dB lower than those associated

with specular reflections using pulse-echo

techniques. This tends to put a “strain” on the

ultrasonic receiver units and most are operates

near their maximum amplification capabilities.

Electrical noise is a common problem with many

TOFD systems and attempts to reduce this noise

generally involve the use of pre-amplifiers near the

probe or remote pulser/pre-amp combinations.

“Dead” Zones

The most widely accepted “limitation” to TOFD is

the loss of information due to ring time. This is

especially noticeable at the entry surface but a

similar zone occurs on the far side (back-wall).

Brown points out that TOFD does suffer from a

Rev 0 Nov 11 Page 12 of 147

2. PRINCIPLES OF TOFD

Figure 2.

2.1 Diffraction

 Modification or deflection of sound beam

 Sound striking defect causes oscillation

 Ends of defect become point sources

 Not related to orientation of defect

 Weaker signal than reflected – needs higher gain

 Sharp defects provide best emitters

 Tips signals are located accurately

 Time of flight of tip signals used to size

Rev 0 Nov 11 Page 13 of 147

Figure 2.

2.2 Waves

Figure 2.

Rev 0 Nov 11 Page 15 of 147

2.4 Signals

 Signals Received

 Lateral wave

 Subsurface

 Back-wall echo

 Mode converted (shear wave) echo

 Define top and bottom of part

 Note phase change

2.5 Basics of TOFD inspection

Figure 2.

Rev 0 Nov 11 Page 16 of 147

2.6 A-Scan with no Defect Present

Figure 2.

2.7 A-Scan with Defect Present

Figure 2.