Phased Array Ultrasonic Testing (PAUT) is a nondestructive testing technique that uses phased array technology to detect complex geometries and defect sizes in a variety of materials in real time.
Phased array ultrasonic technology is probably most familiar in everyday life from medical diagnostics. Phased array sensors are used, for example, in ultrasound examinations of internal organs or in pregnancy. Here, the testing device generates high-resolution cross-sectional images of the body using appropriate software and a so-called multi-element probe. The organs can be viewed with the ultrasound sensor at different angles of incidence and depth. This electronic focusing and/or angular panning as well as different sound fields make the multi-element probe a phased array probe. The nature of the human body, with its high water content, is ideally suited for ultrasonic testing and allows for relatively easy scanning and analysis.
Phased array ultrasonics has also been increasingly used in industry for several years. In contrast to the human body, however, the application here is considerably more complex. This is due to the fact that the materials to be inspected (e.g. metals, composites, CFRP) have different acoustic properties, geometries and thicknesses, and are also less easy to sonicate through. Volume defects and welds are examined, wall thicknesses are measured, and materials are tested for corrosion.
The difference between ultrasound and phased array ultrasonic testing
Like conventional ultrasonic testing, phased array ultrasonic testing is based on the pulse-echo method, which is the most widely used ultrasonic method. With this method, the presence as well as the dimensions of possible defects can be concluded from the transit time and the sound attenuation of the echo pulses in materials.
In contrast to conventional ultrasonic inspection, phased array technology can provide increased inspection speed, reliability, simplification of complex inspection procedures, and imaging of the material in addition to the A-scan.
Detectable defects with the help of phased array ultrasonic testing
- Fractures in the material can be caused by external forces or stresses in the material or component itself. PAUT can reliably detect both surface cracks and cracks within the material.
- Pores can occur when gases are still in the melt while it is solidifying. With the help of the ultrasonic inspection systems, they can be visually detected at first glance with the imaging technology.
- Voids are cavities in the material created during manufacturing. Since these are usually larger than pores, they can be found very easily with ultrasonic technology.
- Duplications are defects in rolled steel in the form of splitting of the material. They occur when voids such as blowholes are flattened during further processing of the material (e.g. forging). This type of defect is ideal for ultrasonic testing because perpendicular scanning into the plate or onto the forging lines allows good detectability.
- Segregations are segregations of a melt during metal production. This material defect shows up in the ultrasound by individual echoes and/or echo mounds and/or also by a strong attenuation of the backwall signal.
- Inclusions in the material, e.g. copper particles in welded joints due to copper electrodes. These defects can be found if the acoustic impedance of the enclosed material is clearly distinguishable from the surrounding material.
- Slag lines are non-metallic inclusions that can be caused by irregularities during the casting process. These can also be detected well with ultrasound.
- Wall thickness variations can occur directly during the manufacturing process or subsequently due to wear such as corrosion. Such defects can be detected very well by ultrasound by measuring the distance to the back wall.
What are the advantages of phased array?
The advantage of phased array technology over conventional ultrasonic technology lies in the electronic sonic beam control and focusing of the many individual elements of the multi-element or phased array probe. This results in the following capabilities:
- The individual elements are dynamically controlled by the testing software with the set pulse repetition frequency so that each test point of the component is optimally sonicated even with complex geometries. This means that the shape and size of the sound beam can be adapted electronically and very quickly to the expected defect location.
- The electronic focusing of the sound beam at different depths enables better sizing of defects during volume inspection.
- By means of a so-called sector scan, the component can be inspected at different angles in just one pass.
- Phased array technology is also ideally suited for inspection objects with limited accessibility due to its ability to change the angle of incidence without moving the inspection head.
- Wear is reduced by replacing mechanics with electronics.
- High reproducibility due to digital storage of adjustment data, minimizing mechanical adjustments.
These technical features not only increase the probability of finding defects, but often speed up the inspection process as well.
Phased Array probes
Standard probes or SE probes are used for conventional ultrasonic testing. Standard probes work with one oscillator element, which transmits and receives ultrasonic pulses. SE probes have two elements – a transmitter and a receiver – which avoids so-called dead times. This has the advantage that defects close to the surface can also be detected.
To create a so-called array with conventional search units, they are arranged, for example, linearly or in a cluster. This allows a larger area to be inspected at the same time or at different angles, thus reducing the inspection time. One application example is the PROline pipe inspection system, which was developed for TÜV Nord Material Testing. It works with a cluster-shaped probe arrangement and was thus able to halve the inspection time compared to the previous ultrasonic inspection.
Fig: PROline pipe inspection system with cluster-shaped inspection head arrangement
A phased array probe replaces such probe arrangements. It is a simplified combination of such individual probes or elements in only one housing – only these elements are much smaller than a normal probe.
Functionality of a Phased Array probe
A phased array probe contains 16 to 256 small oscillator elements. These are piezoelectric crystals. These oscillator elements can send and receive sound pulses independently of each other at the same or different times. They can be arranged as a line array (stripes), as a 2D matrix, as a ring array or in a complex arbitrary shape.
Thus, by interconnecting several oscillator elements, “virtual probes” are created, which behave like a single oscillator with the corresponding properties of sound field size, sound direction and focusing.
Such phased array sensors can be used in contact, bubbler, or immersion techniques with different lead lengths.
By electronically activating or controlling these individual elements, steerable wavefronts are generated. This allows test areas to be scanned flexibly, quickly and without sensor movement. One can imagine a steerable searchlight being guided through the material.
Fig: Representation of the scanning with a phased array probe: left: perpendicular scanning; right: scanning at an angle by delayed actuation of the elements.
In order to penetrate the material vertically, all four elements are controlled in the same way in the exemplary illustration. This causes the wave front to run vertically through the material.
In order to enter the material at a certain angle, the elements are activated with a delay from left to right. This causes the wave front to travel obliquely through the material. In this way, the material can be inspected flexibly at various angles without an angle probe.
In the same way as when emitting the sound pulses, the receiver function also combines the incoming pulses of these many elements into a single display.
Fig: The test result display is imaging. The so-called C-scan is generated from all A-scans (Processing of the test field).
Functionality of the phased array inspection systems
Phased array ultrasonic flaw detectors and systems, as mentioned earlier, allow flexible control of sound waves in a material. They can pan through the test material without movement and dynamically focus to different depths. This allows one phased array probe to replace several conventional ultrasonic probes, greatly simplifying complex inspection procedures.
By combining multiple sonic pulses, this technology creates a visual image of the area to be inspected. This allows imaging of inspection results in real time, greatly simplifying the inspection process.
Test example spot welds
One example of the use of phased array technology is spot weld inspection in the production of automotive bodies. For this purpose, a phased array ultrasonic inspection system such as the PHAsisNEO or PHAsisBLU is used with a phased array multi-element probe. One probe of these inspection systems holds 729 virtual probes. This allows a very high physical resolution of the weld lens more accurate than 0.35 mm. The simultaneous focusing adapts the sound field to the grid resolution, in this case 0.35 mm.
Fault detection and display of test results
With the aid of imaging phased array technology, the quality of the spot weld is made “visible” as a live image (C image) and a result image (D image, depth accurate). Behind each of these pixels of the inspection images is an A-scan. In the PHAsisNEO inspection system, for example, 729 A-scans are recorded per spot in less than one second.
This makes it much easier to assess the weld spot quality, so that even inspectors without in-depth ultrasonic knowledge can inspect reliably and quickly.
Fig: C-scan representation of a spot weld with phased array technology
The A-scan as well as the C-scan with runtime-based alignment points supports the inspector in positioning the probe vertically on the surface of the weld spot to be inspected.
Fig: Imaging display of the inspection result of a spot weld by means of C-/D-scan using phased array technology. Green = welded, red = unwelded.
By freezing the C-scan, the phased array inspection system generates a C-/D-scan as an evaluation proposal. The C-/D-scan is an image of the spot weld with colored depth representation. This allows possible defects such as pores or a lens that is too small to be detected quickly and visually.
Phased array inspection systems, such as the PHAsisNEO, create an automatic and tamper-proof documentation of the inspection results including C- and/or D-scan. By storing all A-scans for a spot weld, a possible re-evaluation and correlation to the destructive test can be made afterwards. In the management software, this test data is organized centrally and test equipment is also monitored.
What are the limitations and disadvantages of phased array ultrasonic testing?
Even though phased array technology offers many advantages, it also has disadvantages. On the one hand, these are the increased costs. PAUT systems are generally more cost-intensive than conventional ultrasonic inspection systems. This is due to the higher range of functions and the more complex construction of the phased array probes and, above all, the ultrasonic electronics.
On the other hand, depending on the PAUT inspection system, the large range of functions may require significantly more extensive training in contrast to conventional ultrasonic systems.
A possible limitation of the application is the accessibility due to the larger design of the inspection passages and thus for complex shaped components. However, the technology, like conventional ultrasonic testing of technical materials, is seldom restricted in terms of materials; fiber, composite materials, metals, plastics and ceramics can all be tested. In general, sonic materials can be tested well. Materials that are less good or cannot be tested at all are materials with strong sound scattering or weakening properties, such as fiberglass-reinforced plastics, rubber, fabric or even foam.
Material properties with influence on testability
The characteristics of the material of the component to be tested have an influence on the testability by ultrasound. This includes the so-called grain size of the microstructure. In most cases, the basic material has a fine-grained structure. During further processing, e.g. welding, the microstructure is subjected to strong heat, which leads to the formation of coarse grains. A coarse-grained material is more difficult to inspect with ultrasound. This is because coarse grain leads to scattering and absorption of the ultrasonic waves. Less information is therefore returned to the receiver of the probe.
Resolution of the smallest irregularities, which are smaller than half the wavelength
For a defect to be detected by ultrasound or phased array ultrasound, it must have a minimum size in relation to the wavelength and reflect sufficient acoustic energy to the probe. This means that defects with an extension (width and length) smaller than 25% of the wavelength used cannot be detected even under optimum conditions. In practice, this has proven a minimum value of half or a whole wavelength.
Determination of the exact extent of the defect requires perpendicular scanning of the defect
To detect a defect, the probe must be perpendicular to the defect width. This is because if an ultrasonic wave hits the interface between two different media perpendicularly, the wave is partially reflected. The information recovered by the probe through the reflection provides the inspection system with information about the type, size and location of the material defect.
If the ultrasonic wave hits a defect at an angle rather than perpendicularly, the ultrasonic waves are deflected and do not return to the sensor. Due to the missing echo, the inspector recognizes that a material defect is hidden in the component, but cannot locate it with depth accuracy.
Norms for phased array ultrasonic testing
DIN EN ISO 23243:2021-03
Nondestructive testing – Ultrasonic testing with arrays – Terminology
DIN EN ISO 13588:2019-07
Nondestructive testing of welded joints – Ultrasonic testing – Application of automated phased array technology
DIN EN ISO 19285:2017-12
Nondestructive testing of welded joints – Ultrasonic testing using phased arrays (PAUT) – Acceptability limits
DIN EN ISO 22825:2018-02
Nondestructive testing of welds – Ultrasonic testing – Testing of welds in austenitic steels and nickel alloys
DIN EN ISO 20601:2019-04
Nondestructive testing of welded joints – Ultrasonic testing – Use of automated phased array technology for thin-walled steel components
DIN EN ISO 23864:2021-09 – Draft
Nondestructive testing of welded joints – Ultrasonic testing – Use of automated total focusing method (TFM) and related technologies
DIN EN ISO 18563-1:2021-09 – Draft
Nondestructive testing – Characterization and verification of ultrasonic testing equipment using phased arrays – Part 1: Test equipment
DIN EN ISO 18563-2:2017-12
Nondestructive testing – Characterization and verification of ultrasonic testing equipment using phased arrays – Part 2: Probes
DIN EN ISO 18563-3:2016-06
Nondestructive testing – Characterization and verification of ultrasonic test equipment using phased arrays – Part 3: Complete test systems
Why phased array testing?
Inspection with Phased Array technology has various advantages over conventional ultrasonic inspection:
- Die bildgebende Darstellung der Prüfergebnisse mit farbcodierter Amplitude bzw. Fehlertiefe
- Die erhöhte Prüfgeschwindigkeit durch das Ersetzen einer mechanischen Abtastung durch eine elektronische Abtastung mit einem Phased Array Multi-Elemente Prüfkopf
- Das Ersetzen mehrerer konventioneller Prüfköpfe durch einen elektronisch fokussierbaren Phased Array Prüfkopf
- Eine verbesserte Fehlerdetektion durch elektronisches Abscannen des Prüfmaterials unter verschiedenen Winkeln. Dadurch werden mehrere konventionelle Winkelprüfköpfe durch einen Phased Array Prüfkopf ersetzt
- The imaging display of the inspection results with color-coded amplitude and defect depth, respectively.
- Increased inspection speed by replacing mechanical scanning with electronic scanning using a phased array multi-element probe.
- The replacement of multiple conventional probes with one electronically focusable phased array probe
- Improved defect detection by electronically scanning the test material at different angles. This replaces several conventional angle beam probes with one phased array probe.
What is a phased array inspection?
A phased array inspection is an ultrasonic inspection with phased array technology. Here, ultrasonic pulses can be interconnected and guided through the test material like a search beam at different angles and with different focuses.
What does focal law mean?
Focal law is defined as the interconnection of several elements of the phased array probe to which defined actuation times are applied. The resulting sound beams allow electronic focusing and scanning at various angles without mechanical movement of the probe.
Which objects are inspected with phased array?
Phased array technology is used to detect volume defects and cracks in the volume or weld seams, to determine the quality of spot welds and to inspect components for erosion, corrosion or production-related wall thickness variations.