001-Introduction to Phased Array

February 2011

Basic Principles of Ultrasonic Phased Array

Prepared by: Applications Department February 2011 1

Outline • Conventionnal Ultrasonic Testing • Phased Array Ultrasonic Testing • Imaging and views • Calibrations • Basics of TOFD

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February 2011

Goal and Motivation  Train the attendees to better understand the veo and UT Studio software  Provide trainees with a good understanding of Phased Array UT  Provide trainees with hands-on experience  Provide trainees with a good understanding of the competition and market place  Overview of PA applications

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Conventional Ultrasonic Testing Basic Concepts Conventional ultrasound has been commercially available for about 50 years. The technology has almost remained unchanged for that period of time. • Single crystal (or two) • Single beam for inspection • Single pulser/receiver (spike or square) • Single angle using a mountable wedge for refraction • A-scan signal representation, B-scan on some units • Aperture and frequency define the acoustic field 4

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Conventional Ultrasonic Testing Basic Concepts

Amplitude

Sound waves are mechanical vibrations propagating into the piece under test. Waves are generated by exciting a piezoelectric transducer. When a change (boundery) in the medium occurs, waves are reflected back to the transducer and converted into an electric signal displayed as an A-scan.

Time or sound path

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Conventional Ultrasonic Testing Sound Field Characteristics The resulting ultrasonic beam is composed of three main components.

Far Field

DOF

Near Field Depth of Field

• Near Field: Unstable sound field. • Far Field: Gradual decay of sound field energy. • Focal spot, also known as DOF region. Region with the highest energy.

6 Source Image: http://en.wikipedia.org/wiki/Ultrasonic_sensor

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February 2011

Conventional Ultrasonic Testing Sound Field Characteristics The characteristics of the sound field are mainly driven by the probe aperture and frequency. Increasing Aperture • Increase near field lenght • Narrow beam width • Shorten DOF

Increasing Frequency • Increase near field lenght • Narrow beam width • Increase resolution • Shorten DOF

Conventional Ultrasonic Testing Focusing By focussing a sound beam, it is possible to achieve a higher sensitivity (energy concentration) and resolution (smaller beam width). Focussing can be achieved by using curved radiator or more commonly by using curved lens. Focusing Rules • A plane radiator can only be focused to a distance shorter than its near-field length. • When focusing, the near-field is compressed into the space between the radiator and the new focus.

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February 2011

Phased Array Ultrasonic Testing Basic Concepts For Phased Array, the physics remains exactly the same than with conventional UT. The main differences come from the fact that the crystal is splitted into multiple ones and each of these crystals are driven by a pulserreceiver circuitry.

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Phased Array Ultrasonic Testing Basic Concepts and Advantages With PA we have the ability to control the beam. It gives the ability to steer and focus the beam. This is achieve by controlling the electronic delay applied to each crystal. This is the equivalent of using a focusing lens in conventional UT. Main Advantages • • • • • •

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No safety issue Full waveform recording Automated reporting Covers all angles of conventional UT and more Combines various techniques (UT, TOFD, PA) Higher Probability of Detection (POD)c

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Phased Array Ultrasonic Testing Beam Steering Beam steering is the ability of controlling the angle at which beams are fired. Delays

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Phased Array Ultrasonic Testing Beam Steering Maximum Beam Steering Angles • As a rule of thumb, beam steering is limited to ±20° each side of the natural refraction angle. • The steering angles limits can be defined as the maximum and minimum refracted angles in the test piece that can achieve a 6 dB drop between two adjacent side drilled holes. 6 dB

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Phased Array Ultrasonic Testing Beam Focusing Beam focusing is the ability of concentrate the beam to a size smaller than the aperture. Delays

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Phased Array Ultrasonic Testing Beam Focusing Maximum Focal Distance • It is very important to remember that focusing is only effective within the near-field length. • Focusing beyond the near field is equivalent to work with the natural focus point. •The aperture is of key importance when focusing. 10 elmts

16 elmts

32 elmts

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February 2011

Phased Array Ultrasonic Testing Beam Focusing – Photoelastic visualization

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Phased Array Ultrasonic Testing Beamforming One Particular Beam = One Focal Law

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February 2011

Phased Array Ultrasonic Testing Sectorial Scan The Sectorial scan or S-scan: • A different focal law per angle. • Give the ability to cover a whole weld volume without any probe movement. • Useful for inspection of complex geometries. • Can be used as a screening tool with no focusing or as a sizing tool using its various focusing patterns.

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Phased Array Ultrasonic Testing Type of Scan – Sectorial Scan

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February 2011

Phased Array Ultrasonic Testing Sectorial Scan – Second skip inspection Example of weld examination in second skip.

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Phased Array Ultrasonic Testing Sectorial Scan – Focalisation Patterns Three focalisation patterns are available on the veo unit. 1. Constant Path For generic focusing or fusion face. 2. Constant Depth For corrosion/erosion/ lamination detection. 3. Constant Offset For crack detection in pins, detection of indication in weld center.

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Phased Array Ultrasonic Testing Sectorial Scan – Constant Path Focusing

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Phased Array Ultrasonic Testing Sectorial Scan – Constant Depth Focusing

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February 2011

Phased Array Ultrasonic Testing Sectorial Scan – Constant Offset Focusing

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Phased Array Ultrasonic Testing Linear Scan The Linear scan or L-scan, also called E-scan: • The same focal law is sweep along the array. • Ability to perform a fast rastering without any probe movement. • Useful for inspection of weld bevel/fusion face and corrosion mapping. • Precise resolution along the array axis

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Phased Array Ultrasonic Testing Type of Scan – Linear Scan

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Phased Array Ultrasonic Testing Linear Scan – Example of Corrosion

Delays

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February 2011

Phased Array Ultrasonic Testing Linear Scan – Delamination in Composite

Phased Array Ultrasonic Testing Sectorial Scan vs Linear Scan S-scan • The whole array aperture is used to generate each beam • One different focal law per angle. • Varying angle • 3 focalisation patterns

L-scan • A subset of the aperture is used, called active aperture. • The same focal law is multiplexed across a group of active elements • Constant angle • Only one focalisation pattern 28

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Imaging and Views Color Encoding An A-scan waveform represents the reflections from one sound beam position in the test piece. Imaging capability is provided for the rectified A-scan signal by color encoding the amplitude.

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Imaging and Views Projected views – Extraction Box In addition to the S-scan and L-scan views, the veo has the capability of displaying projected views. These views are generated by the “extraction box”.

Extraction box

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Imaging and Views Projected views – Top View The top view is a 2D projection seen from a plan view. Scan Axis

Depth Axis

Index Axis

Index Axis

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Imaging and Views Projected views – End View The End-view is a 2D projection seen from the back of the probe for all angles within the extraction box. Scan Axis DepthAxis

Depth Axis

Index Axis

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February 2011

Imaging and Views B-Scan View

Depth Axis

The B-scan view is a 2D projection seen from the back of the probe at one angle.

Sound Path Axis

Scan Axis

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Calibrating the Scan

Calibrate The Calibrate tab allows access to calibration wizard. In stop mode, you can clear existing calibrations, while in play mode you can create/modify them. The items in this menu are sorted in the order the calibrations should be performed. If you are using a multi-scan setup, each scan must be calibrated independently.

Stop Mode

Play Mode

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February 2011

Calibrating the Scan

Calibrate The table below is a summary of the wizards available along with the scan type they apply to.

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Calibrating the Scan – Velocity Wizard

Calibrate The first wizard to start with is the velocity wizard. The velocity wizard shall be used when the velocity is unknown. Otherwise, the velocity can be entered manually in the Part tab. Scan Selection For multi-scan setup, the first step is to chose the scan that needs to be calibrated. 36

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Calibrating the Scan – Velocity Wizard

Calibrate Reflectors Selection Select the type of reflectors used to calibrate the velocity. Tip: Ideally, choose a block with reflectors that do not require probe movement or a minimal probe movement.

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Calibrating the Scan – Velocity Wizard

Calibrate Reflectors Position According to the selected reflectors, set the distance at which they should be found. Tip: Chose reflectors that have a separation distance long enough to obtain accurate results. 38

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Calibrating the Scan – Velocity Wizard

Calibrate Scan Settings Make sure that the Range Path is long enough to detect both reflectors. Typically, the middle angle is chosen to calibrate the velocity.

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Calibrating the Scan – Velocity Wizard

Calibrate Reflector 1 Make sure that the peak is within the gate and then maximize the reflector. The gate is automatically positioned by the software, but some adjustments are sometimes required. Tips: The gate can be set from the menu or by pressing and then use click wheel to move it freely. The worst case is to use 2 SDH and add 40 couplant in between 2 reflectors.

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February 2011

Calibrating the Scan – Velocity Wizard

Calibrate Reflector 2 Make sure that the peak is within the gate and then maximize the reflector. Tip: Properly maximizing the indication is crucial to get an accurate result.

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Calibrating the Scan – Velocity Wizard

Calibrate Validate Result The last step of the velocity calibration wizard is to validate the calculated velocity. If the calculated velocity doesn’t correspond to the expected value, go back to Reflector 1 step. Tips: When the velocity is known, it can be entered in the Part tab. 42

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February 2011

Calibrating the Scan – Wedge Delay Wiz.

Calibrate The Wedge Delay wizard aims to compensate for the sound path variation in the wedge. The calibration ensures that indications are displayed at the right depth. Wedge delay calibration is performed using only one reflector.

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Calibrating the Scan – Sensitivity Wizard

Calibrate The Sensitivity Wizard aims to compensate for the sound attenuation due to the wedge and the angle variation in the S-scan. The calibration ensure a uniform amplitude response for each focal law for a given reflector. Sensitivity calibration is performed using only one reflector. 44

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February 2011

Calibrating the Scan – TCG Wizard

Calibrate The TCG wizard aims to equalize the amplitude level of a given reflector size along the sound path. TCG equalizes the A-scan % FSH of a reflector as well as its representation in S-scan or L-scan.

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Basics of TOFD TOFD stands for Time Of Flight Diffraction. It was originally developed as a sizing technique for the nuclear industry in the 70’s. The technique is now well recognized by the industry and many codes and standards are available. The combination of Phased-Array and TOFD is becoming a very popular and efficient inspection technique.

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February 2011

Basics of TOFD Advantages • Cover a wide area • Fast encoding speed • Accurate sizing capability in height • Permanent data recording • Detection and sizing almost orientation independent

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Basics of TOFD Limitations • Blind areas • Near Surface: The width of the lateral wave can be a limitation on thin components. • Back wall: The large signal reflected from the back wall can hide indications.

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February 2011

Basics of TOFD How does it work? TOFD is based on signal diffraction. Incident wave

Diffracted waves

Reflected wave

FLAW

Diffracted waves

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Basics of TOFD How does it work? TX

RX

Lateral wave

Back-wall reflection BW

LW

Upper tip

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Lower tip

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Basics of TOFD The Lateral Wave • The Lateral Wave travels at the compression velocity speed. • Always arrives first. • On curves surfaces, will travel straight across the metal. Not a true surface wave, but a bulk wave generated at the edge of the wide beam generated by the send transducer. • Becomes weaker with increased PCS. 51

Basics of TOFD Color Encoding • Imaging capability is provided for the non-rectified Ascan signal by color encoding the amplitude. +

White

Time

-

Black

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February 2011

Basics of TOFD TOFD View The TOFD view is a “B-scan” parallel or particular to the beam axis.

Scan Axis

Beam Axis

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Basics of TOFD Types of TOFD Scan Two types of TOFD scan are possible. Non-parallel – Movement of probes at right angles to direction of the beam.

Parallel – Movement of probes in same direction as the beam.

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Basics of TOFD Typical TOFD Scans – Near Surface Crack

1 2

1

2

The crack blocks the Lateral Wave And the lower tip appears on the A-scan

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Basics of TOFD Typical TOFD Scans–Incomplete Root Pen. 1

2 3 4

1

2

1 3

4 2

Note the two signals from the top & bottom 56

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February 2011

Basics of TOFD Typical TOFD Scans – Lack of Root Pen. 1

1 2 3

2 3

Note the inverted phase between LW and defect 57

Basics of TOFD

Typical TOFD Scans – Lack of Fusion on the Side Wall 1 1

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2 3

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4

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Note the two signals from the top & bottom 58

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February 2011

Basics of TOFD Typical TOFD Scans – Porosity 1 1

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2 3

Porosity may image in many forms whether individual or cluster 59

Basics of TOFD Typical TOFD Scans – Transverse Crack 1

1 2

1

3

2 3

2

4

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I n t he LW we can observe t he wide beam effect on t he crack 60

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February 2011

Basics of TOFD Typical TOFD Scans – Concave Root

1 1

2 2 3

3

Distortion of back-wall echo 61

Basics of TOFD Typical TOFD Scans – LOF - Interpass

1 2 3

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