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- Active thermography (3)
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- Ultrasonic testing (3)
- Magnetic stray field (2)
- Neutron diffraction (2)
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- Round bar testing (2)
Organisationseinheit der BAM
- 8 Zerstörungsfreie Prüfung (3)
- 5 Werkstofftechnik (2)
- 5.2 Metallische Hochtemperaturwerkstoffe (2)
- 8.4 Akustische und elektromagnetische Verfahren (2)
- 5.1 Mikrostruktur Design und Degradation (1)
- 7 Bauwerkssicherheit (1)
- 7.1 Baustoffe (1)
- 8.0 Abteilungsleitung und andere (1)
- 8.5 Röntgenbildgebung (1)
- 9 Komponentensicherheit (1)
Stress corrosion cracking is a transcrystalline or intercrystalline crack formation in materials which
occurs under the influence of static tensile stress or residual stress and a specific aggressive
medium such as chloride containing substances.
This special sort of crack formation is involved in complex crack configurations, which may
only insufficiently be captured by conventional ultrasonic probes with fixed angles of incidence.
Only a number of different beam angles produce sufficient reflection to reconstruct the complete
defect shape from measured ultrasonic data. The SAFT algorithm, which was recently successfully
used in many industrial NDT-applications, is a promising tool for the reconstruction process. In
addition, the combination of phased array technique and SAFT was developed in several projects
by BAM, Berlin. Investigations using phased array equipment were performed to look into crack
configurations in test blocks with different surface curvatures. UT-SAFT has been used for the
analysis of spark-eroded notches simulating stress corrosion cracks at the thermo sleeve weld of a
nozzle. For comparison, UT-SAFT has also been applied for the analysis of real reflectors at the
same position in a nozzle used in a power plant, which was repaired later on. SAFT-scans received
from reconstructed ultrasonic measurement data confirm the practical usefulness of the SAFTalgorithm
developed by BAM.
Adhesively bonded aluminum components have been widely used in the aerospace industry for weight-efficient and damage-tolerant structures. Automated squirter jet immersion ultrasonic testing is a common inspection technique to assure the bond integrity of large, contoured assemblies. However, squirter jet inspection presents several limitations in scanning speed, related to water splash noise over protruding stiffeners and splash interference crosstalk in multi-channel inspection systems. Air-coupled ultrasonic testing has been evaluated as an alternative, possibly offering the benefits of increased throughput by enabling higher speeds, and eliminating the contamination concerns and maintenance issues of water couplant systems. Adhesive joints of multi-layer aluminum plates with artificial disbonds were inspected with novel air-coupled ultrasonic probes based on cellular polypropylene. Disbonds of various sizes were engineered in several multi-layer configurations and at various depths. Results were compared with squirter jet immersion and conventional piezoelectric transducer designs in terms of scan contrast, resolution and inspection time.
Airborne ultrasonic inspection is performed in through transmission, where the test piece (e.g. adhesive joint or polymer-based composite plate) is placed between the transmitter and the receiver. However, many structures with difficult shapes allow only one-sided inspection. The strong reflection of the signal from the surface overshadows the signals from the inside, so that broadband pulses are required. Thermoacoustic transmission, where the thermal energy of an electrically heated electrode is transformed into the acoustic energy of an ultrasonic wave, opens the possibility to excite broadband pulses and thus to inspect objects with one-sided access.
We present various thermoacoustic transducers consisting of an electrically conductive film on a solid substrate. The first type of transducer is a transmitter with an indium-tin-oxide electrode on a glass substrate combined with a laser Doppler vibrometer as a receiver. The second type of transducer combines thermoacoustic transmission and piezoelectric reception, having a titanium electrode as a transmitter deposited onto charged cellular polypropylene serving as a piezoelectric receiver.
Using a focusing thermoacoustic transmitter and a separate cellular polypropylene receiver, a through-transmission inspection of a 4 mm thick CFRP test piece with inserts as small as 1 mm was performed. The same emitter and a laser vibrometer as a receiver were used for a one-sided inspection of a Plexiglas block with a cross hole at 15 mm depth. A twin probe consisting of a thermoacoustic transmitter on a cellular polypropylene receiver was applied to a profile measurement on a step wedge with flat bottom holes. The smallest detected diameter of a flat bottom hole was 1 mm. Sound pressure level above 140dB was achieved with each of these transmitters. Thermoacoustic transmitters enable a step towards one-sided air-coupled ultrasonic inspection.
Ultrasonic testing of adhesively bonded joints using air-coupled cellular polypropylene transducers
(2014)
In air-coupled ultrasonic testing, the impedance mismatch between the transducer and the air is commonly being solved by adding matching layers to composite transducers. To avoid the difficult technological procedure regarding matching layers, some new piezoelectric materials have been proposed. Most promising are ferroelectrets, which are charged cellular polymers, having ferroelectric and consequently piezoelectric properties. In particular, the extreme softness of cellular polypropylene (cPP) leads to a high piezoelectric constant and to a good impedance match with the air, making matching layers redundant. Its elasticity modulus below 1 MPa causes an additional effect not observed with common piezoelectric materials: that is the electrostrictive effect, here defined as the thickness change due to the attractive force between the transducer electrodes. This effect exceeds the piezoelectric effect at excitation voltages over 1 kV. The extreme softness of cPP leads also to high flexibility, enabling easy focusing by bending the transducer.
We have developed air-coupled ultrasonic transducers based on cPP. This includes the electrical matching networks for the transmitter and for the receiver. The transmitter is excited with voltages up to 2.5 kV, so that the electrostrictive effect dominates, leading to sound pressure around 145dB at the transducer surface. These transducers have been applied for testing carbon-fiber-reinforced polymer plates, adhesive joints and other composite structures. Here we report about ultrasonic transmission of two types of adhesive joints. The first one is multi-layer aluminium components with some artificial disbonds, which are common in aerospace industry, and the second one is an aluminium-steel joint with polyurethane adhesive, which is used in automotive industry.
Active thermography with lock-in excitation is a non-destructive testing method that is also feasible for testing of carbon fiber reinforced polymers (CFRP). For validating the method extensive investigations were done during a research project in order to advance a standardization process.
The most important parameters of optical and ultrasonic excitation thermography were investigated. For example, the appropriate selection of spectral sensitivity of the used infrared camera systems is important for recording undisturbed thermal signals. Regarding excitation,
influences of excitation power and ultrasonic frequency were studied. Furthermore, material parameters such as CFRP layup are known for strongly affecting the results of
measurement.
The paper presents numerical methods to detect and classify defects and inhomogeneities by means of active thermography. The objective is to determine the wall thickness of structure elements with an inaccessible back wall, for example, of pipes or Containers. As test specimens we used approximately 2 cm thick PVC samples with spatial variations in the back wall geometry. Flash lamps provided the heating. To know the thickness of the wall, we used two inversion methods and compared the results achieved. One is an iterative echo defect shape method and already tested on Steel test specimens with good reconstruction results. The second one is the Levenberg-Marquardt method, applied here to thermographic data for non-destructive testing. Since data capturing using active thermography and the presented numerical methods can easily be automated, the combination of these two procedures is a promising approach providing a broad area of application.
Within a know-how transfer project funded by the government conventional ultrasonic
technique was replaced by phased array technique for automated round-bar testing. Instead of
applying a great number of conventional probes to achieve acceptable volume coverage we used
curved linear arrays. The benefits of phased array technique such as programmable skew angles,
beamforming and beam positions, led not only to a significant decrease in inspection time, but also the
number of probes could be substantially reduced . Finally, the testing parameters for a large range of
bar-diameters could be adapted by software control instead of time-consuming mechanical
replacement. The probe-design was carried out by a proprietary modelling program. Both the
theoretical calculations as well as the latter experimental verifications revealed significant advantages
of curved arrays versus the planar types. A radial oriented probe offers perfect adaption to the
cylindrical shape of the specimen allowing wide variations of the sound field. Thus beam direction,
beam size and beam position could be optimized with respect to a minimum of inspection cycles, as
inspections have to be executed in-line during the production. A number of laboratory tests were
carried out on special test components. In order to achieve an optimal performance of the reference
rod we implemented three different types of reference reflectors: (i) flat-bottom-holes with diameters
of 0.8 mm and 1.2 mm, (ii) side-drilled-holes with a diameter of 0.7 mm for the detection of
volumetric flaws, and (iii) notches with a depth of 0.2 mm and 0.5 mm for the detection of surfaceoriented
defects. All laboratory tests were carried out with the COMPAS-XXL inspection system, a
proprietary development of BAM.
Eddy Current Technique is a powerful method for detection of surface notches and of buried flaws during inspection of metallic parts. Recent EC array probes have demonstrated a fast and efficient control of large surfaces. Nevertheless, when the size of flaws decreases or the defect is rather deep, traditional winding coil probes turn out to be useless. Magnetoresistive sensors present the advantages of flat frequency response and micron size. These sensors are hence very attractive for the detection of buried defects that require low frequencies because of skin depth effect. An optimization of the probe with magnetoresistive sensors as receivers has been made by simulations using CIVA software and finite elements methods with OPERA. EC probes for buried flaw detection have been designed. Experimental results have been compared with simulations.
It is widely accepted that the magnetic state of a ferromagnetic material may be irreversibly altered by mechanical loading due to magnetoelastic effects. A novel standardized nondestructive testing (NDT) technique uses weak magnetic stray fields, which are assumed to arise from inhomogeneous deformation, for structural health monitoring (i.e., for detection and assessment of damage). However, the mechanical and microstructural complexity of damage has hitherto only been insufficiently considered. The aim of this study is to discuss the phenomenon of inhomogeneous “self-magnetization” of a polycrystalline ferromagnetic material under inhomogeneous deformation experimentally and with stronger material-mechanical focus. To this end, notched specimens were elastically and plastically deformed. Surface magnetic states were measured by a three-axis giant magnetoresistant (GMR) sensor and were compared with strain field (digital image correlation) and optical topography measurements. It is demonstrated that the stray fields do not solely form due to magnetoelastic effects. Instead, inhomogeneous plastic deformation causes topography, which is one of the main origins for the magnetic stray field formation. Additionally, if not considered, topography may falsify the magnetic signals due to variable lift-off values. The correlation of magnetic vector components with mechanical tensors, particularly for multiaxial stress/strain states and inhomogeneous elastic-plastic deformations remains an issue.