Ultrasonic testing (UT)
State of the art
How powerful is ultrasonic testing?
Material testing with ultrasound is used to detect defects near the surface as well as defects in the volume of materials.
Cracks, Pores, Blister, Blowholes, Segregations, Inclusions, Doubled slag lines, Inhomogeneities and lack of bond, Wall thickness variations
- From approx. 0.5 mm below the surface (vertical test)
- From approx. 0.1 mm below the surface (angle check)
- From approx. 0.05 mm defect expansion (high frequency testing)
- From approx. 0.2 mm defect expansion (conventional testing)
Non-destructive testing with ultrasound (UT)
Ultrasonic testing is the preferred method if a component is to be inspected 100% for internal material defects, surface cracks or wall thickness measurement. Ultrasound can be used to inspect components within the production process (e.g. car bodies in automotive production) as well as in operation (e.g. installed pipes).
Components made of materials that conduct sound, such as metal or polymer, can be inspected. This is because non-destructive testing makes use of this property for defect detection and location. Here, sound that is inaudible to humans, so-called ultrasound, is used in the frequency range from approx. 0.5 to 50 MHz. Sound refers to the vibration of molecules and, in contrast to light waves etc., is bound to material. Ultrasound is therefore transmitted in the form of mechanical sound waves. Defects in the material cause a change in these waves (reflection, refraction, absorption), which are recorded by a so-called ultrasonic inspection device. This makes internal defects visible.
we support you
VOGT Ultrasonics is your service provider for ultrasonic testing with extensive experience
We reliably detect internal and external defects, e.g. in weld seams, spot welds, cast and forged parts, semi-finished products, pipes, rods, shafts, gears, differentials, turbine disks, sheet metal, CFRP rims, pressure cylinders or bearing rings. Another application is wall thickness measurements on pressure vessels, pipelines, e.g. chemical and petrochemical plants.
In our VOGT testing center, we provide individual and serial testing of your components in 3-shift operation with our large ultrasonic plant. We have specialized in particular in mechanized ultrasonic testing. Here we can inspect complex, rotationally symmetrical or flat components reliably and quickly. Due to the possibility of storing customer-specific testing parameters, a cost-efficient and fast time-delayed serial testing in batches is possible without any problems.
We are accredited as an independent testing laboratory according to DIN EN ISO/IEC 17025, certified according to EN 9100 for aviation and ISO 9001 for industry. We also have MTU and Rolls-Royce approval. Our testing personnel are trained in accordance with internationally approved certification systems. We perform your inspection task in accordance with the standard as well as customer specifications.
How is ultrasound generated?
Two general methods are used for ultrasonic testing:
- The Pulse-echo method: In this method, the ultrasonic impulse is transmitted by a probe and also received again. This method is usually used for non-destructive material testing.
- The through-transducer method:
This method uses two probes connected to the same ultrasonic device. One probe transmits the impulse, the other receives the impulse on the exact opposite side of the component. This method is very special. It is used, for example, to inspect CFRP sheets for delamination or steel components for doubling. Another application is the spot weld inspection during the welding process (an example is SPOTline).
Ultrasound is generated with the aid of the piezoelectric effect. There are crystals that become electrically positively or negatively charged on the surfaces when subjected to tension or compression. If the crystal is subjected to tensile and compressive stress in series, an alternating voltage is generated – the piezoelectric effect.
Such a piezoelectric crystal is installed in an ultrasonic probe. This usually has the shape of a disk. Its top and bottom sides are coated with a metal layer (e.g. gold). Electrical connections run from this metal layer to the connection socket. Piezoelectric effects are also provided by ceramic transducers, PVDF polymer film transducers as well as piezocomposite materials.
By connecting an ultrasonic inspection device to the connection socket of the probe, a high-frequency voltage can be generated at the piezoelectric crystal. In time with the high voltage, the disk expands and contracts again. The disk thus converts electronic energy into mechanical energy (ultrasonic vibrations), and vice versa. The generated ultrasonic wave is transmitted into the material via the protective layer of the probe.
How is ultrasound received?
After the probe has sent the ultrasonic signal, it immediately switches to receiver mode to pick up reflected sound waves (echoes). Thus, if the ultrasonic wave is reflected by the material (e.g., on the back wall or a defect in the component), the piezoelectric disk is compressed in time with the sound wave. This creates a voltage on the metal layers, which can be measured and recorded at the connection socket with the test system. A so-called A-scan is generated. On the screen of the ultrasonic device, the A-scan is adjusted so that the x-axis represents the time (or the sound path) and the y-axis the intensity of the sound signal. This makes it easy to detect and measure defects. Based on these recordings, wall thicknesses and defects in the material can be made visible.
Automated and manual UT in the VOGT test center
Detectable defects using ultrasonic testing
- Cracks are caused by external forces or stresses in the material. Ultrasound can detect cracks on the surface as well as within the material.
- Pores occur when gases are still present in the melt during solidification.
Lunger are cavities in the material that have occurred during production. Mostly lungers are larger than pores and can be detected excellently with ultrasound.
- Doublet slag lines are defects which can be caused by splitting of the material in rolled steel. These defects are very well detected by their horizontal position with a simple vertical sonication into the material.
- Segregations are segregations of a melt during metal production. In the A-scan, these defects are visible by individual echoes and/or echo mounds and/or also by a strong attenuation of the backwall signal.
- Inclusions in the material can be found if the acoustic impedance of the included material clearly distinguishes itself from the surrounding material.
Slag lines are non-metallic inclusions that can be formed during the casting process.
- Wall thickness variations can occur during production or subsequently due to friction, e.g. corrosion or erosion. Wall thickness can be measured ultrasonically by measuring the distance to the back wall.
Pro & Contra
What are the advantages of ultrasonic testing?
Ultrasonic technology can be used to non-destructively detect surface and internal defects. Ultrasonic testing works with almost all materials. The only prerequisite is that this conducts the sound.
In this way, very good defect resolutions can be achieved. In the extreme, defects from approx. 0.1 mm up to 10 m below the surface can be found. With conventional testing, defects from approx. 0.2 mm can be made visible. When using high-frequency ultrasound, even defects as small as 0.05 mm are possible.
Rotationally symmetrical components in particular can be inspected cost-effectively using ultrasound. Mechanized to fully automated ultrasonic inspection systems are used for this purpose, which suggest inspection results based on preset parameters and then sort components in production into OK and NOK before the next production step.
A major advantage of using modern ultrasonic testing systems is the iso-compliant, 100% and complete documentation of the test results.
what it can do
What are the limits of ultrasonic testing?
A coarse-grained structure makes testing more difficult: In ultrasonic testing, sound waves are transmitted into the material by the probe and their reflections are picked up again. On the basis of the reflections and the resulting A-scan, C-scan and D-scan, the inspector can make conclusions about the quality and possible defects and wall thicknesses of the component. If a material has a very coarse structure, the sound waves are deflected. A so-called noise is generated. As a result, fewer sound waves return to the probe, making evaluation more difficult.
The defect size is smaller than half the wavelength
Defects that are smaller than half the wavelength cannot be detected or can only be detected with difficulty: A defect can only be reliably detected if it has a minimum size in relation to the wavelength to reflect enough sound. Defects that have an extension smaller than 25% of the used wavelength are therefore not detectable even under optimal conditions. In practice, a minimum value of half a wavelength or a whole wavelength is used as a standard.
Vertical scanning for precise defect measurements
The exact defect dimension is only detected when the defect is scanned perpendicularly: If an ultrasonic wave hits a defect perpendicularly, e.g. a crack, it is partially reflected back to the probe, providing the inspector with information about its type, size and location.
However, if the wave hits a defect at an angle, the ultrasonic wave is deflected and consequently does not return to the probe. If a backwall can also be monitored at the same time, shadowing of the backwall signal would occur at this point due to the defect. The backwall echo breaks in, and the inspector thus detects a defect in front of it that does not reflect. However, he cannot tell at what depth this is located. In order to obtain more information, the inspector can then use so-called angle probes or a phased array probe to scan at different angles in order to obtain the best possible reflection of the oblique defect.
Reliable and fast testing with VOGT Ultrasonics
choosing the ideal probe
The core of every ultrasonic inspection system is the ultrasonic transducer, also called ultrasonic probe. There are different types of probes that are used depending on the defect situation or requirements for accuracy and imaging. The frequency, the oscillator material, the oscillator size, the damping of the oscillator and the geometric design are all factors that play a role.
Depending on which type is used, they offer the following features:
- low acoustic impedance, good attenuation, short ultrasonic pulse, good for sound scattering materials, but also for wall thickness measurements
- high coupling factor, thus a lot of sound is transmitted into the component, high sensitivity, small defects are detected at great depths
- high post-resolution capability, small defects are detected closer to the surface
Fig: Ultrasonic immersion probe with angle mirror
Ultrasonic Dual Element Probes
These probes are also called dual element probes, because they have both a transmitter and a receiver integrated. These probes allow high defect detectability close to the surface of the transducer in the component. Due to the separate arrangement of the transmitter and receiver, there is no reflection at the transducer surface. The so-called dead zone of the single transducer / standard transducer is eliminated. Thus, a dual element probe is sensitive to near-surface defect detection. Its disadvantage, however, lies in the detection of defects in the far field, i.e. deep in the material.
Fig.: Dual element probe for manual testing, here of the residual wall thicknesses
Angle probes are available as dual element probes or as single element probes. Angle probes transmit the ultrasound at a defined angle into the component. This allows the detection of defects that lie at an angle to the surface. Common angle probes have 35°, 45°, 60° and 70° angles.
Angle probes are used in industry primarily for the inspection of weld seams.
Single element probes
Unlike dual element probes, these probes have only one oscillator. This oscillator acts as both a transmitter and a receiver. In contrast to the dual element probe, these probes have a dead zone. This is caused by the wide entrance echo on the surface of the probe side. This interferes with the detection of near-surface defects because they are masked by the wide entry echo.
Single element probes are the most commonly used probes. They are used in a variety of ways in contact technology by hand or machine, bubbler technology and immersion technology.
Phased Array Probes
Phased array probes usually contain 16 to 128 small oscillator elements or piezoelectric crystals. In a phased array probe, these can be controlled independently of each other. By interconnecting several of these oscillator elements, “virtual probes” are created which behave like a single oscillator with the corresponding properties of sound field size, sound direction and focusing. Thus, single element probes, dual element probes or even angle probes are emulated. This allows electronically fast steerable wavefronts to be generated, which can move through the component like a searchlight. Thus, one phased array probe can replace several standard ultrasonic probes. With phased array probes, the sound can be swiveled and focused at the same time. This also opens up new fields of application and unimagined possibilities. The inspections become faster or finer due to a high resolution of the ultrasonic transmissions.
a bit of theory
Functionality of the ultrasonic testing systems
Modern ultrasonic inspection systems can be operated manually, mechanized, partially or fully automated. They are suitable for Industry 4.0 and are used across all industries for cost-conscious production-accompanying component inspection in the aerospace, automotive or metalworking industries.
The main task is the complete and fast inspection of components in order to ensure a production flow, a 100% inspection of a component and, if necessary, an automated evaluation of the quality of the component.
They are also used to analyze materials in research and development or in the quality laboratory. For example, ultrasound is used to measure the degree of purity, evaluate microstructures and test for micro and macro cracks.
Inspection example bearing rings
Ultrasonic inspection plays a major role in the production of bearing rings. Here it is used three times: before production, after production and recurrently. Among other things, prematerials, tapered bearings, cylindrical bearing rings, spherical roller bearings, ball bearing rings, barrels, rollers, balls and rolling elements are inspected non-destructively with ultrasound.
Before component production, the prematerial, such as flat or bar material, is first subjected to a macroscopic ultrasonic cleanliness measurement according to SEP 1927. Typical defect classes in this case are KSR 0.3 – 0.5 mm.
After production of the bearing rings e.g. for wind turbines or railroads, an outgoing goods inspection is performed by means of ultrasound. Here, the functional and core area of the rolling bearings is inspected.
Fig.: Visually displayed test result of a purity grade measurement with ultrasound of prematerial in accordance with SEP 1927.
The inspection is usually performed using an immersion technique. For this purpose, the component is placed in an immersion tank and rotated 360° along a mechanically manipulable probe with the aid of a turntable. The probe follows the geometry of the component so that the angle of incidence into the material or the angle of impact on the functional and core area remains the same over the height of the bearing ring.
In addition to this outgoing goods inspection, ultrasonic testing is used as part of component optimization. In the test laboratory, ultrasonic inspections of components are carried out frequently to accompany long-term stress testing and correlated with the stress results.
Manufacturers of highly stressed components are subject to high quality requirements. Downtimes caused by early component defects can lead to considerable costs. For this reason, rolling bearings such as those used in wind turbines are subjected to recurring ultrasonic inspections during their life cycle. These inspections allow defects that are not visible from the outside to be detected at an early stage and reduce cost-intensive damage. Depending on the component, they must be dismounted for the inspection or can be inspected directly in operation on a mobile basis..
Tasks of VOGT Ultrasonics as an independent test laboratory
As an independent accredited testing laboratory, VOGT Ultrasonics GmbH is a reliable partner whose top priority is quality assurance with reliable personnel and equipment.Thus, we support the customer to focus on quality and to keep it. Of course, thereby we do not lose sight of the costs. Thus, the use of sufficient but not overqualified personnel as well as sufficient equipment and machine performance must be kept in mind. This results in a service for the customer that combines high quality with sensible cost aspects.
In non-destructive testing, the possible detection of defects depends on the correct selection of the test method and on the training and experience of the test personnel. Detecting defects means finding them and then evaluating them correctly. If the wrong “inspection tool” is used, defects will remain undetected. If the wrong evaluation tool is used, defect sizes are incorrectly estimated.
Thus, the success of locating defects and evaluating them correctly depends on the company’s knowledge of the entire non-destructive testing process. A larger number of experts in the company is essential. With 50 employees, VOGT Ultrasonics offers a wide range of expertise to help you manufacture safe components and operate components safely.
In addition to component-specific data, quality documentation includes information on the test method used and its application parameters. It also includes information about the inspection company and the qualification of the inspector. Of course, the essential information about the inspection result must not be missing. The quality documentation, usually in the form of an inspection report, is as important as the inspection itself. It has to be prepared properly, truthfully and comprehensively.
The quality documentation is usually stored for the lifetime of a component so that it can be used for analysis in the event of a component failure.
In production, the fastest way to intervene with regard to any quality defects is through in-line monitoring with non-destructive testing methods during the manufacturing process. If this is not possible, components are rejected and inspected or finally inspected after production. Often the inspection processes are also split up so that pre-material or semi-finished goods are inspected, which reduces additional costs in the case of defective components.
Process monitoring also goes beyond the simple detection of defective components. The results of non-destructive testing are used to detect fluctuations in production, trends in quality or quality characteristics, and to provide feedback at an early stage before rejects occur. In this way, production can be immediately modified and the quality improved.
Display of the inspection results
Test results can be documented in a variety of ways. In the past, the use of analog test devices made it impossible to store data. Thus, the test report consisted of the presentation of the results in tables, with prose or the simple information “no indications requiring registration” or “N.I.O. / I.O.”.
With digitization, more information about the inspection was saved. In ultrasonic testing, for example, the A-scans (HF ultrasonic images), but increasingly also imaging information, as known from medical technology, are documented. Two-dimensional information as development up to 3D images of components with colored representations of the defects are now state of the art.
VOGT Ultrasonics is your partner for NDT
In our test center we handle your serial quality assurance in 3-shift operation.
if there are still questions
Ultrasonic testing is a nondestructive testing method that is suitable for almost all materials. It offers a very high defect resolution and is ideal for automated inspection in production. Preferably, straight, flat or rotationally symmetrical components are inspected where steady motion over one direction is possible. Complicated geometries are rather more difficult to inspect.
Ultrasonic testing (UT) is a method of non-destructive material testing for quality assurance. It can be used to reliably detect external and internal defects. During the inspection, an ultrasonic probe transmits sound waves into the material and then receives them again. The pulse-echo method uses one probe and the through-transmission method uses two probes (transmitter/receiver). Based on the amplitude and transit time measurements of the transmitted ultrasonic waves, the inspector can make conclusions about possible defects and make them visible.
Ultrasound is used, among other things, to locate and evaluate external and internal defects in the material and to measure wall thicknesses. The type of components is wide ranging: rods, tubes, bars, plates, sheets, bearing rings, shafts, gears, rollers, differentials, piston rods, engine blocks, turbine disks, rotors, lamellae carriers, weld seams, spot welds, adhesive joints, pressure vessels, bridge girders, pressure cylinders, rims, pipelines, tanks and railroad wheel sets are inspected.
strictly acc. to specifications
Standards for ultrasonic testing
The following standards refer to ultrasonic testing in general. In addition to these standards, there are a large number of company-specific or association-specific specifications or even guidelines that describe ultrasonic testing and its application for specific components, defect types and test methods.
- DIN EN ISO 16810, Non-destructive testing – Ultrasonic testing
- DIN EN 1330-4, Non-destructive testing – Terminology – Part 4: Terms used in ultrasonic testing
- DIN EN 1712, Non-destructive testing of welded joints – Ultrasonic testing of welded joints – Acceptability limits (replaced by DIN EN ISO 11666)
- DIN EN 1713, Non-destructive testing of welds – Ultrasonic testing – Characterization of indications in welds (replaced by DIN EN ISO 23279)
- DIN EN 1714, Non-destructive testing of welds – Ultrasonic testing of welds (replaced by DIN EN ISO 17640)
- DIN EN 10160, Ultrasonic testing of flat steel products with a thickness greater than or equal to 6 mm (reflection method)
- DIN EN 10228-3, Non-destructive testing of steel forgings – Part 3: Ultrasonic testing of ferritic or martensitic steel forgings
- DIN EN 10228-4, Non-destructive testing of steel forgings – Part 4: Ultrasonic testing of austenitic and austenitic-ferritic stainless steel forgings
- DIN EN 10308, Non-destructive testing – Ultrasonic testing of bars out of steel
- DIN EN 12223, Non-destructive testing – Ultrasonic testing – Description of calibration block No. 1
- DIN EN ISO 22232-x, Non-destructive testing – Characterization and verification of ultrasonic testing equipment
- DIN EN 12680, Foundry engineering – Ultrasonic testing
- DIN EN 14127, Non-destructive testing – Ultrasonic thickness measurement
- DIN EN 15617, Non-destructive testing of welds – Time-of-flight diffraction technique (TOFD) – Acceptability limits (replaced by DIN EN ISO 15626)
- DIN EN ISO 7963, Non-destructive testing – Ultrasonic testing – Description of calibration block No. 2
- DIN EN ISO 10863, Welded joints – Application of time-of-flight diffraction technique (TOFD) for testing welded joints
- DIN EN ISO 18563-x, Non-destructive testing – Characterization and verification of ultrasonic testing equipment using phased array
We stand for quality
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Certified quality management for the aviation industry
for the inspection of turbine disks (MTU)
for the ultrasonic testing of turbine disks in our testing center in Burgwedel (Hanover, Germany)