Training course
Fundamental principles of FMC and TFM technologies in
Ultrasonic Inspection
© ZETEC, Inc. 2017. All rights reserved. All the information herein is subject to change without prior notification.
Table of Contents
Chapter 1 – Introduction............................................................................................................................... 4
2
3
1.1
Introduction .................................................................................................................................. 4
1.2
UT inspection ................................................................................................................................ 4
1.3
Terminology .................................................................................................................................. 4
1.3.1
Data collection ...................................................................................................................... 4
1.3.2
Data processing ..................................................................................................................... 5
1.4
An overview of standard PA UT .................................................................................................... 6
1.5
Basics of FMC UT data collection .................................................................................................. 9
FMC Characteristics ............................................................................................................................ 11
2.1
Signal characteristics ................................................................................................................... 11
2.2
Scale factor of FMC ..................................................................................................................... 11
2.3
FMC Data size .............................................................................................................................. 12
2.4
FMC versus HMC data collection ................................................................................................ 13
2.5
HMC data .................................................................................................................................... 14
2.6
How to use FMC data .................................................................................................................. 14
2.7
Typical FMC signal explained ...................................................................................................... 16
TFM Characteristics............................................................................................................................. 18
3.1
Signal characteristics ................................................................................................................... 18
3.2
TFM Frame parameters and FMC ............................................................................................... 18
3.3
TFM and delay laws..................................................................................................................... 19
3.4
Focusing capability ...................................................................................................................... 19
3.5
Coverage capability ..................................................................................................................... 20
3.6
Impact of frame parameters on amplitude ................................................................................ 21
3.6.1
4
A little theory….................................................................................................................... 21
3.7
Amplitude subject to resolution ................................................................................................. 24
3.8
Amplitude and Interface/Dead zones ......................................................................................... 27
3.9
Use cases ..................................................................................................................................... 27
3.9.1
Weld examinations ............................................................................................................. 27
3.9.2
Corrosion examinations ...................................................................................................... 28
3.9.3
Other examples ................................................................................................................... 29
General information ........................................................................................................................... 35
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4.1
Codes........................................................................................................................................... 35
4.2
Calibration ................................................................................................................................... 35
4.3
The future ................................................................................................................................... 35
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Chapter 1 – Introduction
1.1 Introduction
In this training course, you will learn about Full Matrix Capture (FMC) and the Total Focusing Method
(TFM) in phased array ultrasonic (PA UT) non-destructive testing. FMC and TFM are recent technological
advancements in phased array testing. This technology is already available, but still requires
considerable refinement to make it industrially deployable. Improvements in inspection speed,
calibration, validation, as well as the proper information technology (IT) support are still required before
it becomes fully viable.
Full Matrix Capture (FMC) is a data-acquisition process where each array element is sequentially used as
a single emitter and all array elements are used as receivers creating a matrix of A-Scan data. FMC has
the advantage of acquiring high amounts of data that may be reused later in
FMC is UT data collection
many ways.
and TFM is data processing
Once the data of this matrix is collected, the signal is processed using the Total
Focusing Method (TFM) to produce an image (or frame) where each pixel is
one dedicated and focused focal law in the region of interest. TFM is particularly useful for
reconstructing the data for defect characterization.
This training session will explain the theory behind FMC and TFM, discuss the strengths and weaknesses,
and explore its capabilities including corrosion and welding examinations.
1.2 UT inspection
The basic steps of a UT inspection are:
•
•
•
Gather useful and complete data – the general rule is: do it once, do it well. Make sure all
regions/sectors of the weld or of the pipe are covered by the inspection
Once the data is acquired, transfer the data to imaging software to visualize the defects in a
colour coded image – the image is an important part of identifying the defects
Produce a defect report
1.3 Terminology
Before explaining FMC and TFM, it is a good idea to become familiar with certain terms that are used
throughout the course. It is important to have a general understanding of the terms before beginning to
explain FMC and TFM.
1.3.1 Data collection
Focal Law
A set of delays that describe how the signal from individual probe elements must be offset (or delayed)
so that a constructive interference is created along a given path and focal point. More information
about delays is shown in section 1.4.
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Dynamic Depth focusing (DDF)
A variation of a focal law. A single focal point is still used for pulsing but during reception, the delay laws
are adjusted dynamically to focalize at multiple depths at once.
Summed A-Scan
Traditional phased array processing where signals from every element are
A traditional PA summed Asummed together using a focal law to generate an A-Scan.
Scan can be compared to an
Elementary A-Scan
X-ray of a broken bone in
A-Scan signal from a single pulsed element received by a single receiving
your arm (uni-directional,
one pulse/one capture result,
element.
and no post-treatment of
Full Matrix Capture (FMC)
data) while FMC can be
A data-collection technology. Every element is pulsed in sequence, and the
compared to an MRI (more
elementary A-Scan data is collected for each combination of pulsing and
data, multiple 360°
receiving elements.
pulses/captures, and post-
1.3.2 Data processing
examination treatment of
data is possible)
Sweep
A collection of summed A-Scans displayed in an organized way. For example,
you can have a Sectorial, Linear or Compound Sweep in phased array.
Frame
The typical output of the TFM method. Defined as a rectangular area where each point in the grid (or
frame) is displayed as a pixel.
Total Focusing Method (TFM)
PA-processing method using collected FMC data to generate a frame of pixels, where each pixel is
computed using a dedicated focalized focal law.
The following terms may or may not be familiar to you. They are relatively new to the PA UT market, and
are primarily taken from today’s industry marketing materials:
Sectorial Total Focusing (STF)
Also called the Almost-Total Focusing Method (A-TFM)
A processing method using FMC collected data to generate a sweep of an A-scan, where each sample of
every A-scan is computed using a dedicated focalized focal law.
Linear Total Focusing (LTF)
A processing method using FMC collected data to generate a linear sweep of A-scans, where each
sample of every A-scan is computed using a dedicated focused focal law.
Compound Total Focusing (CTF)
A processing method using FMC collected data to generate a compound sweep of A-scans, where each
sample of every A-scan is computed using a dedicated focused focal law.
Adaptive TFM (A-TFM again…but it means something else)
Variation of the TFM where the surface of the component is not known before examination. FMC data is
used in a first phase to deduct the surface shape, generate a set of delay laws and in a second phase, reprocess the same set of FMC data to obtain the final TFM frame.
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Adaptive TFM (A-TFM but it still means something else again)
Same name as above, but a different concept.
It is a variation of the TFM where an amplitude normalization algorithm is applied on every pixel to
improve signal quality and reflector/indication shape.
1.4 An overview of standard PA UT
Before explaining FMC and TFM, it is relevant to make a review of Standard PA inspection. In Standard
PA UT, multiple, independent, UT transmitting-receiving channels are used for acoustic beamforming
and data reception. Only the summed and digitized A-Scan signal is transferred to the computer for
display and recording to create an image.
In Figure 1, (a) Pulsing shows an acquisition unit emitting individual pulses using delays to elements
(shown as the blue squares) through a probe/wedge/angle configuration. These elements send pulses
creating acoustic waves through the probe element into the material being inspected creating incident
wave fronts which overlap, creating strong areas, weak areas, and as well, areas where no wave fronts
are detected at all (dead areas). These are called constructive and destructive wave fronts. These wave
fronts combine into a single wave front called an incident wave front that detects the defects. This
action happens in a fraction of a second, allowing many different angles and delays to be changed very
rapidly.
In (b) Return, the returning wave fronts, combined into a reflected wave front, is detected by the same
individual elements (also using delay laws) by individual echo signals and transmitted to the PA unit,
where the A-Scans are generated and sent to the acquisition unit. Note that the number of elements
determine the total number of pulses/return signals.
(a) Pulsing
(b) Return
Figure 1 PA acquisition unit and elements detecting an indication
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Focal law calculators in the acquisition/PA unit establish specific delay times for the firing pulses,
generating the desired beam and accounting for the probe and wedge characteristics, as well as the
type of material. A delay time must be integrated into the pulsing and receiving to compensate for the
variations in distance caused by the wedge shape and material, and achieve effective angle within the
material, called Steering.
This is a simplified demonstration of PA UT. In summary, standard PA UT can be summarized as follows:
1. Delay laws (or Focal Laws) are required to compensate for the probe and wedge configurations
to produce linear, sectorial or compound A-Scans.
2. The acoustical properties of the material also have an influence on the delay law.
3. The instrument pulses every relevant probe element. Each element is pulsed with a delay
defined by the focal law.
4. Pulsing: Inside the wedge and component, energy from each probe element is summed
together through constructive and destructive interference (wave fronts).
5. Instrument digitalizes a signal for each relevant probe element.
6. Receiving: Instrument performs a summation of signal from each element, using delay laws
again.
7. The result is a summed A-Scan.
8. The process is repeated for every focal law to generate a complete Sweep.
The following is a series of figures showing Standard PA UT, demonstrating a sector scan with
preprogrammed focal laws (beams) and showing a “live” image during an inspection.
Figure 2
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Standard PA UT – 40 to 70 SW
Figure 3
Note: Standard PA UT 40 to 70 SW means that a resolution of 1 focal law per degree is used;
there are a total of 31 focal laws (70-40+1)/1. In the image, the angle of the current focal law
(the black line) is at 43 degrees.
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1.5 Basics of FMC UT data collection
Full Matrix Capture (FMC) consists in capturing and recording A-Scan signals from every transmitterreceiver pair in the array:
Figure 4
From raw A-scan signals now stored on a computer drive, it is possible to generate (or later, re-generate,
construct and reconstruct) UT imaging for any given focal law/beam (aperture, angle, focus depth), and
for improved algorithms (e.g. TFM) through post processing. This is the main advantage of FMC.
For FMC:
1. Delay laws are generated, but not used during data collection
2. FMC - for each element in the probe
• Instrument pulses a single element
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• Instrument digitalizes elementary A-Scan for each element
3. FMC data can be processed in the instrument or transferred to host computer for processing later or
on the spot
4. FMC data is read and processed by:
• Using regular focal laws to reconstruct the standard PA beam sweep
• Using STF focal laws to reconstruct STF beam sweep
• Using TFM focal laws to reconstruct TFM frame
In the previous
• Using other innovative algorithms
comparison, we saw
that an MRI is
composed of a great
deal of signal data. FMC
also has a great deal of
stored signal data, and
like an MRI, the image
can be reconstructed at
will to demonstrate a
new image of any of the
area that was recorded.
This is one of its main
advantages
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2 FMC Characteristics
2.1 Signal characteristics
Elementary A-Scan signals gathered by FMC require specific characteristics, so that they can be used
later for treatment (such as reconstruction, or regeneration).
1. First, RF rectification must be used. FMC requires phase
information to compute constructive and destructive
The same set of FMC
interference
data can be processed
2. The minimum frequency is 100 MHz digitalization (= 10 ns
multiple times to
between samples). Sample resolution is crucial for proper delay
produce different
law forming in post processing
results, using different
3. High amplitude fidelity. 12 bits or better is recommended, along
reconstruction
with very low noise, is required since individually, each signal is
weak
parameters
4. No compression. The amplitude must maintain the best
resolution
5. No smoothing. Smoothing is done only on rectified signal, which is not possible on elementary
A-Scan
FMC capability is also frequently limited, due to current era electronics, imposing further limitations:
(However, we expect that such limitations will change over time, as electronic capabilities improve)
1. There is limited or no gain control. Gain is usually identical for each elementary A-Scan
generated from a single FMC data collection.
2. There is currently no way to calibrate probe elements electronically (discussed more in detail
later).
3. There is no filtering possible. Filtering, usually done in FIR on traditional Phased array, not
possible due to the scale of parallel data acquisition acquired at the same time. Analog filtering
component is currently possible, but it takes up space on acquisition boards and is often
discarded by manufacturer.
4. No Averaging. Averaging would require many multiple firings to occur per probe, and would
require a huge amount of memory to process (See FMC scale factor).
2.2 Scale factor of FMC
To properly position FMC, the following discussion is required to understand the scope of the acquired
data:
Let’s assume a probe with the number n (16, 32, 64) elements Therefore, the probe must be pulsed n times, or once per element
This implies that the acoustic travel time of an ultrasound pulse occur n times
For each pulse, n, an elementary A-Scan is digitalized (one per element)
This implies that the total number of elementary A-scans gathered after the full firing cycle = n2
Let’s look at some real examples:
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For a 16-element probe Probe will be pulsed 16 times
256 elementary A-Scans are gathered
For a 32-element probe Probe will be pulsed 32 times
1024 elementary A-Scans are gathered
For a 64-element probe Probe will be pulsed 64 times
4096 elementary A-Scans are gathered
Therefore, the number of scans gathered is a factor in both storing the data, and the ability to
regenerate images later.
2.3 FMC Data size
In its typical configuration using today’s technology, FMC is performed with a 64-element probe. This
enhances aperture size, beam forming capability and focusing power. Next, a typical elementary A-Scan
length can be around 80 µs. Each elementary A-scan must have a sufficient time base so that the
maximum applied delay law + depth of coverage is fully contained. This usually translate to an A-scan
with 8192 samples, 2 bytes per sample (12 to 16 bits amplitude)
Therefore, the amount of storage required for a single FMC probe location is: 64 MB
4096 * 8192 * 2 (elementary A-scan * sample count * byte count)
This file size is for each probe location along a scan line.
The large amount of storage required for FMC (up to 64 MB of data for a 64-element probe) is subject to
further scale factoring. To achieve sufficient acquisition speed, the instrument data throughput must be
very high.
For example, to achieve 30 Hz, 1.8 Gigabyte/second of data throughput is required (64 MB * 30 Hz)
E.g., To scan a typical 12” pipe with typical resolution, huge files are produced
(12” * 3.1416 / 0.039” scan resolution) * 64 MB = 60 GB
Although the file size is enormous, with the proper IT capabilities, this can be managed.
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2.4 FMC versus HMC data collection
Using FMC, every element is used to pulse and to receive. As previously demonstrated, this results in
enormous file sizes, which have the capacity to be problematic if proper data storage is not sufficient.
How can the file sizes and data accumulation be reduced or at least become more manageable?
Let’s look at the basics of FMC firing and receiving. The description of element combinations can be
represented in a firing matrix as shown:
•
•
Rows usually represent firing elements
Columns usually represent receiving elements
In the firing matrix shown below, there is pair of combinations who have the equivalent ultrasound path
and characteristics, resulting in equivalent A-scan signal:
•
•
A21 is equivalent in path and signal to A12
A41 is equivalent in path and signal to A14
Figure 5
Therefore, there is approximately half of the data which is equivalent in nature.
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2.5 HMC data
Using properties of equivalent elementary A-Scan, alternative, more compact methods of performing
FMC have been proposed.
One such method is called Half Matrix Capture (HMC).
Using HMC, for each equivalent pair, only one combination of elements is kept:
Figure 6
This reduces by nearly one-half the data that is required.
2.6 How to use FMC data
Due to the large amount of data produced per second, there are typically 2 ways of handling FMC data
today
1. Use and discard
• Some instruments are using the FMC data for a single probe location within the instrument,
using dedicated high-speed hardware.
• The FMC is converted on the fly into a TFM frame
• TFM frame is sent to the software for viewing and for storage
• Then FMC data is discarded to preserve usable space and speed
• It offers the best inspection speed, but there is no data validation or reuse capabilities
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2. Transfer, keep and reuse
• Some instruments also support a storage capability for FMC (usually with IT capabilities)
• This requires a high-speed data link between the instrument and the computer. However, the
highest Ethernet link is still too slow, which constrains the acquisition speed
• Amount of data stored on hard disk of host computer is high
• But since the FMC is stored, it can be reused in analysis at will, with different sets of
reconstruction parameters.
• This method offers absolute flexibility of use but at the cost of speed
In the next section, FMC data is shown as a visual representation of the data as seen on the acquisition
device’s display screen.
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2.7 Typical FMC signal explained
Table 1 Breakdown of the FMC signal data as represented on images
This is an example of a 16-element probe and the
FMC data that is displayed on the acquisition
device’s display screen. This information is not
useful in this form without the underlying
explanations of what the coloured marks mean.
In the highlighted red rectangle, the figure shows
element 1 firing; receiving is performed on
elements 1 to 16
In the red area, now element 2 is firing; receiving
on elements 1 to 16
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Red area shows element 16 firing; receiving on
elements 1 to 16 (elements 3 to 15 are the same)
The red area demonstrates the bang from the
probe onto the wedge
This is the signal inside the wedge, demonstrating
the interface between the wedge and the
component (or material)
This is the signal inside the component or
material; this red area represents the actual data
used by TFM for image construction.
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3 TFM Characteristics
3.1 Signal characteristics
The Total Focusing Method (TFM) requires elementary A-Scans for proper reconstruction.
Assuming elementary A-scans are present, reconstruction can be performed on-the-spot during data
collection, or post-processed during analysis.
TFM can be performed on multiple probe configurations:
• Pulse Echo
• Pitch & Catch
• Tandem and Self Tandem
However, TOFD (Time of Flight Diffraction) does not seems to be supported by this technology at the
moment.
3.2 TFM Frame parameters and FMC
We have seen that the advantage of FMC is that a single set of FMC data (elementary A-Scans) can be
reused multiple times in TFM to construct different type of results. In reality, how are these scans used?
Each application of TFM can have different parameters at every data reconstruction:
• A new frame location
• A new frame size
• A different path reconstruction mode
It is possible to reconstruct the direct path, but also indirect and mode converted paths, and in any
combination.
Different TFM frames can be volumetrically merged together to enhance detection and sizing capability,
as shown in Figure 7:
Figure 7
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3.3 TFM and delay laws
Every frame has an overlay grid containing individual pixels. A delay law is created for every pixel in the
TFM frame. Each pixel has its own unique region of interest and signal characteristics.
The summation of constituting FMC elementary A-Scans is done for every pixel, using a local focal law.
The result is that every pixel has a dedicated focal law perfectly focused to its location. This results in
better definition and localization of defects in all areas of the frame due to the energy sent in all
directions within the component.
Here are several images showing the pixel focal laws in more detail:
Figure 8 Dedicated pixels for various focal laws
3.4 Focusing capability
“Total” focusing…or something very close to it anyways….
Total focusing is theoretically not possible.
Let’s look at total focusing:
• Every pixel in a TFM frame is created using a dedicated focal law
• This result in 1 focal point per pixel
• In theory, each and every pixel is then perfectly focalized
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However, in practice, the laws of physics still apply – the near field length in each material defines the
maximum depth at which a sound beam can be focused – a beam cannot be focused beyond the near
field
The following is always true and must be considered:
• Probe aperture size determines near-field depth
• No probe can focalize past its near field depth
• Past the near field depth, a pixel will still have content, but it will be essentially similar to regular
non-focused phased array
Note: The near field depth must take into account the wedge thickness!
Let’s look at some sample values:
LM-5MHz, Wedge, 64 elements, 38.4 mm aperture.
Near-field depth at 55 degrees = 147 mm
LM-5MHz, Wedge, 32 elements, 19.2 mm aperture.
Near-field depth at 55 degrees = 28 mm
LM-2.25MHz, Wedge, 32 elements, 19.2 mm aperture. Near-field depth at 55 degrees = 6.3 mm
The laws of physics will ALWAYS apply….
Where (for circular elements)….
N = near field depth
D = element aperture or diameter
F = frequency
Lambda = wavelength = V/F
V = sound velocity in medium
Near field depth is
very like the way
your eyeglasses
work – you can
see very well at
exactly one point
or closer. You
cannot see well
beyond this focal
point with your
glasses
3.5 Coverage capability
“Total” coverage (…or something very close to it anyways….!)
Recall that every pixel in a TFM frame is a created using a dedicated focal law.
This is always true.
•
•
This results in a complete rectangular area with ultrasound data
In theory, every pixel is then perfectly positioned
But remember once again, those laws of physics will still apply!
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Ultrasonic beams are limited to the amount of steering (see section 1.4) that can be applied. The
maximum beam steering is a function of the beam spread of a single element aperture. Past this
maximum steering angle, the focal laws can still be computed, but the beam does not really cover the
expected area. This leads to situation where pixels contain data related to a TOTALLY different location.
See the following example:
Example: LM 5MHz, Wedge SW55, Direct path attack
Where: V is velocity, D is diameter, and F is frequency
Figure 9
In the example on the left, the data is properly located in the frame due to the probe and wedge’s
configuration. In example on the right, the defect is improperly located in the HAZ (Heat Affected Zone).
3.6 Impact of frame parameters on amplitude
Several phenomena can impact the precision of the amplitude reading on a given TFM pixel within a
frame:
•
•
Effect of frame resolution versus probe frequency
Effect on interface signal
Before going further, a little background is useful to explain amplitude in signals.
3.6.1 A little theory….
The Nyquist Theorem, also known as the Sampling Theorem, is a principle that engineers follow in the
digitization of analog signals. For analog-to-digital conversion (ADC) to result in a faithful reproduction of
the signal, slices, called samples, of the analog waveform must be frequently taken.
•
Signal frequency can be expressed as period or wavelength:
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5 MHz signal = 200 ns period or 1.2 mm Wavelength (LW in steel)
•
•
•
•
A waveform cannot be preserved with less than two samples per period
Even when the waveform is preserved, the signal peak amplitude can be missed, resulting in an
amplitude error in digitized signal
Higher digitizing frequency reduces the amplitude error
In Ultrasound methodology, an effective digitizing frequency of 5 times the probe frequency is
required to limit this amplitude error
5 MHz signal = 25 MHz minimum digitizing frequency (1 sample every 40 ns)
Figure 10
At same digitizing frequency, exact error is dependent on signal timing:
•
•
If the sampling in synchronized with the peak signal (as in (a)) there is no amplitude error
If the sampling in exactly halfway before and after the peak (as in (b)) the maximum amplitude
error occurs
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(a)
(b)
Figure 11
Maximum error occurs at random occurrences, depending on signal generator location (or the flaw
location).
Maximum error can be calculated:
•
•
•
•
•
•
Phase spread = number of degrees between two consecutive samples (see Figure 12)
Phase error = number of degrees between a sample and peak location
Maximum Phase error = Phase spread / 2
Occur when samples are at same distance before and after the peak
Amplitude error ratio in % = cos (Maximum Phase Error)
Amplitude error in dB = 20 X log (cos (Maximum Phase Error))
Figure 12 Sampling taken exactly halfway before and after the peak
Sample Error level:
•
•
•
•
•
5 MHz signal, digitized at 25 MHz = 5 points per wavelength
Phase spread = 360 / 5 = 72 degrees
Maximum Phase error = 72 / 2 = 36 degrees
Amplitude error ratio in % = cos (36) = 80.9%
Amplitude error (AE) in dB = 20 X log (cos (36)) = -1.85 dB
Other examples of samples:
•
•
20 samples per period: AE = -0.1 dB
5 samples per period: AE = -1.85 dB
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•
•
•
4 samples per period: AE = -3.0 dB
3 samples per period: AE = -6.0 dB
2 samples per period: AE = Infinite loss
3.7 Amplitude subject to resolution
Remember what has been said on several occasions:
Every pixel in a TFM frame is a created using a dedicated focal law
Therefore, the corollary is: Each pixel has ONLY one focal law to cover it
The focal law is tuned for the center of the pixel. Depending on the instrument and software capability,
certain issues may be present. One such issue is the size of the pixel versus a wavelength. The ratio of
size of a pixel versus the wavelength of the ultrasonic beam is important. A large pixel will statistically
miss the peak amplitude of a signal, which may be very small, as shown in Figure 13.
•
•
•
The Nyquist theorem must be respected for peak signal to be detected
Systems with low and fixed frame resolutions (i.e.: 256 * 256) are more susceptible to this issue
Systems with higher and flexible frame resolutions (x * y) can overcome this issue and generate
higher quality images
Given a pixel size, the acoustic wave may peak right at its center, or elsewhere within the pixel area as
shown in Figure 13. When the signal is at its peak elsewhere in the pixel, the amplitude used in TFM
construction is lower than expected. When the size discrepancy between the pixel and the wavelength is
large, a large error in amplitude is statistically expected.
•
•
20% (2 dB) drop is measured when the pixel size is smaller than 1/5 the wavelength of the
ultrasound beam
100% of the amplitude can be lost when the pixel size is 1/2 the wavelength of the ultrasound
beam
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Figure 13
Let’s look at a couple of examples:
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Nyquist example - Tilted Notch
•
•
•
•
•
Single FMC data collection
TFM processed twice
Only parameters modified is pixel size in TFM frame (6 versus 2 pixels)
All other parameters are constant
Same indication is visualized
Figure 14
Tilted notch
•
•
•
•
Using the same notch as shown in Figure 14
Using the same parameters
The probe’s position was moved 0.5 mm
Amplitude pattern changed drastically when under sampling:
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Figure 15
3.8 Amplitude and Interface/Dead zones
Contrary to STF, where all the beams transit through a small area and caused saturation interference,
TFM has a relatively smooth interface signal.
In TFM, even when the frame is located below the probe, the contribution to the top row of pixels is
done in such way that the energy is distributed and usually prevents saturation. In effect, the top rows
of a frame dedicated to corrosion mapping will have an interface signal less intrusive than with an
equivalent phased array approach.
This effectively reduce the dead zone in the top section of the frame compared to regular phased array.
3.9 Use cases
Now let’s look at a couple of examples – one for welding and one for corrosion inspection.
3.9.1 Weld examinations
For welding examinations, FMC/TFM inspections have the following strengths and weaknesses:
Strengths
•
•
•
Every pixel of the image is a focal point (as long as Near field rule is respected)
The definition of the focal law is easier in the calculator (No focalization parameters, no beam
steering parameters)
Multiple frame reconstruction types can be overlaid to cover multiple mode conversions and
may improve detection
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Weaknesses
•
•
•
•
•
•
•
•
The number of acoustic paths to perform the acquisition is dependent on the number of
elements, not number of laws. A Typical 40-70 SW (1-degree resolution) requires 31 travel times
with regular phased array, and 64 with a typical TFM 64-element probe. This typically results in a
slower inspection speed
For a small TFM Frame (256 x 256), size and resolution can limit depth of weld that can be
examined due to Nyquist rule for a maximum amplitude drop of 2dB (Nyquist X5)
Using Shear Wave in Steel, Wavelength is 0.646mm. Pixel size must be 0.13 mm and maximum
Frame dimension is 33 mm X 33 mm (for a 256 X 256 frame). Insufficient to cover welds over
16.5 mm thick with direct and rebound coverage
Using Longitudinal Wave in Steel, with proportional effect on wavelengths, pixel size and frame
size, a maximum frame dimension of 60 mm X 60 mm is attained (for a 256 X 256 frame). Able
to cover welds up to 30 mm thick can be achieved with full skip coverage
Inaccurate pixel location at extreme angle
Live TFM does not store FMC data, which prevents data reuse and add additional risk to
qualification effort
Post-processing TFM has FMC data, but is typically too slow for a production environment
inspection
TFM does not provide A-Scan data, which also impacts signal identification and qualification
effort
3.9.2 Corrosion examinations
For corrosion examinations, FMC/TFM inspections have the following strengths and weaknesses:
Strengths
•
•
•
•
•
•
Any one pixel of the image is a focal point (Subject to near field rule)
The definition of focal law is easier in the calculator (No focalization parameters, no beam
steering parameters)
The number of acoustic paths to perform the acquisition is dependent on the number of
elements, not the number of laws. The net effect is the opposite of the weld. A Typical 0 LW, 64element probe requires a 58 travel time (for 64-element probe minus active aperture) with
regular phased array, or 116 travel time when using improved resolution. A typical TFM 64element probe will require only 64 travel time no matter the final resolution. This may result in a
faster inspection speed depending on TFM processing speed
Coverage is performed at 0 degrees, but also covers a wide range of angles, which increase
detection and rendering quality of the backwall
The averaging effect helps produce a crisper image
Even with small TFM Frame (256 x 256), size and resolution is within typical inspection range
and is not an issue
o Using Longitudinal Wave in Steel, with proportional effect on wavelength, pixel size and
frame size, maximum frame dimension of 60mm X 60mm is attained (for a 256 X 256
frame). Able to cover a component up to 60 mm thick with direct path.
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Weaknesses
•
•
•
•
With typical frame resolution, lateral resolution is better than with regular phased array, but
depth resolution is typically worse
Live TFM does not store FMC data, which prevents data reuse and adds additional risk to the
qualification effort
Post-processing TFM has FMC data, but is typically too slow for a production environment
inspection
TFM does not provide A-Scan data, which also impacts signal identification and qualification
effort
3.9.3 Other examples
3.9.3.1 Plate weld inspection
Carbon steel, T = 19 mm, with realistic welding defects & SDH
Linear array LM 5 MHz (64 elements) on 55SW wedge
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LOF, Incomplete Penetration, Toe-Crack, Porosity
Std PA UT, Merged data from Sector 40 to 70SW, focusing HP 50 mm
LOF, Incomplete Penetration, Toe-Crack, Porosity
Reconstructed FMC data, Merge from Sector STF 40 to 70SW
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LOF, Incomplete Penetration, Toe-Crack, Porosity
Reconstructed FMC data, Merge TFM Frames SW
à
LOF, Incomplete Penetration, Toe-Crack, Porosity
Reconstructed FMC data, Merge from TFM Frames SW
Rebounds included
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3.9.3.2 Thick Vessel Weld
Carbon steel, T = 120 mm, Narrow Gap Weld
Realistic Welding Defects:
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Flaws F & NS5
Standard PA, Merge Sector 40 to 70SW, focusing HP 50 mm
Flaws F & NS5
Reconstructed FMC data, Merge STF Sector 40 to 70SW
(STF = Sectorial Total Focusing)
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Flaws F & NS5
Reconstructed FMC data, Merge TFM Frame SW
(TFM = Total Focusing Method)
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4 General information
4.1 Codes
The combination of FMC and TFM is not currently supported by codes. Code-related work does not yet
allow this technique.
However, a Working FMC/TFM Group has started discussions within ASME section V. The schedule is to
have a mandatory appendix for publication by December 2019. However, draft
proposals are not yet redacted, and 2019 is not far off. There are still many
Code-related
issues to be resolved. For example, rapidly, a standardization of the names
inspection work
used for the technology should be undertaken.
does not yet
support the
FMC/TFM
technique
Zetec Inc. and other manufacturers are participating in the dialogue and the
Working Group.
4.2 Calibration
Currently, no system supports TFM calibration. A draft proposal for amplitude calibration is currently
under redaction (and due for mid-2017). However, there are still no proposals yet for Wedge or Velocity
calibration. This creates many issues for amplitude-based sizing. However, it is anticipated that this will
change as the technology evolves.
4.3 The future
Many of the limitations of current-era FMC/TFM can and will be reduced as the technology matures:
•
•
•
•
Calibration will be introduced
Frame Size and Pixel resolution can be improved
Slow scanning speed can be improved
Live TFM may also store FMC data in the future
However, don’t forget….
The underlying laws of physics will still apply and must always be a part of the discussions!
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