Non-destructive Testing of 3D-Hybrid Components Using Air-coupled Ultrasound

The increasing use of hybrid lightweight structures in the automotive industry brings new challenges in the area of multi-material boundaries. Insufficient bonding quality between two kinds of materials, especially in highly loaded structures, affects the component’s performance. Hence, it is very important to monitor the bonding quality in multi-material structures. Flat specimen, consisting of sheet steel and organic sheet layers were examined using air- coupled ultrasound transducers in reflection setup. By slanted incidence of the ultrasound on the profile’s surface, guided waves were excited in the specimen and delaminated areas could be visualized due to changed phase values in the received signal. Furthermore, a 3D-Hybrid cap profile consisting of sheet steel, organic sheet layers and injection molded reinforcing ribs was examined using a special transducer setup. Results show that distinctions in the received signal could be made between bonded and delaminated organic sheets as well as intact and broken ribs behind the organic sheet layer. Several frequencies from 200 kHz up to 500 kHz were then used and compared with this new setup. Simulations of the guided waves propagation support the findings

Local Defect Resonance-based Shearography to Increase the Selectivity of Defects

Abstract. The optically excited lock-in shearography (OLS) is an optical method for remote non-destructive testing by monitoring the displacement field of the object to be inspected. OLS is designed for quick and one-sided non-destructive testing of large components. The surface displacement is measured by means of an expanded laser beam. For non-destructive testing, usually not the displacement of the whole inspected object is of interest, but only the changes in the displacement field that are caused by defects. To determine the depth of defects we combined it, in the past, with the lockin technique where modulated excitation generates a thermal wave in the sample which is monitored by a shearography sensor. By using the optical excitation, in some cases, the defects signals are superimposed by the large displacement fields. Additionally, conventional application is often too long which makes the new approach with measuring times of about a few seconds a promising alternative. Thus, the elimination of these drawbacks will be helpful to broaden the range of applications of shearography. Non-destructive testing is an important part of the whole quality strategy of manufacturing and operation of CFRP components. However, routine application is often too long with regard to industrial inspection requirements. One possible solution to this problem is local defect resonance (LDR), a recently proposed method, which is combined in this paper with shearography. The LDR-based shearography (LDRS) measurements are performed within a few seconds. In practical use, the component is excited at frequencies in the range of 5- 50 kHz and the component surface observed as live video. We report on achievable detection sensitivities when using the LDR and present testing results of different types of CFRP-components

Non-Contact Inline Monitoring of Thermoplastic CFRP Tape Quality Using Air-Coupled Ultrasound

Besides the aerospace industry, fibre reinforced plastics have also spread towards many further applications such as automotive, civil engineering as well as sports and leisure articles. Their superior strength and stiffness to mass ratio made them the number one material for achieving high performance. Especially continuous fibre reinforced plastics allow for the construction of structures which are custom tailored to their mechanical loads by adjusting the paths of the fibres to the loading direction. The two main constituents of CFRP are carbon fibres and matrix. Two possibilities for matrix material exist: thermosetting and thermoplastic matrix. While thermosetting matrix may yield better properties with respect to thermal loads, thermoplasticity opens a wide range of applications due to weldability, shapabilty, and compatibility to e.g. injection moulded thermoplastic materials. Thin (0.18 mm) thermoplastic continuous fibre CFRP tapes with a width of 100 mm were examined using air-coupled ultrasound. Transducers were arranged in reflection as well as transmission setup. By slanted incidence of the ultrasound on the tape surface, guided waves were excited in the material in fibre direction and perpendicular to the fibre direction. Artificial defects – fibre cuts, matrix cuts, circular holes, low velocity impacts from tool drop, and sharp bends – were produced. Experiments on a stationary tape showed good detectability of all artificial defects by guided waves. Also the effects of variation in material properties, fibre volume content and fibre matrix adhesion being the most relevant, on guided wave propagation were examined, to allow for quality assessment. Guided wave measurements were supported by destructive analysis. Also an apparatus containing one endless loop of CFRP tape was constructed and built to simulate inline testing of CFRP tapes, as it would be employed in a CFRP tape production environment or at a CFRP tape processing facility. The influences of tape conveying speed on detectability of artificial defects as well as material properties were elaborated and recommendations for implementation in production scale inline monitoring are given

The magnetoresistance of homogeneous and heterogeneous silver-rich silver selenide

The magnetoresistance (MR) effect of the low-temperature phase of silver selenide (-Ag2 + Se) is measured as a function of composition. Very small composition variations in the order of = 10–6 are achieved by coulometric titration and can be performed simultaneously during the MR measurement. A homogeneous Ag2 + Se shows an ordinary magnetoresistance (OMR) effect, which can be well described by the two-band model. For silver selenide with a heterogenous silver excess, we found quite a different MR behavior. Up to a minor silver excess of 1×10–4 10–2) shows again an OMR effect

Routes for GMR-Sensor Design in Non-Destructive Testing

GMR sensors are widely used in many industrial segments such as information technology, automotive, automation and production, and safety applications. Each area requires an adaption of the sensor arrangement in terms of size adaption and alignment with respect to the field source involved. This paper deals with an analysis of geometric sensor parameters and the arrangement of GMR sensors providing a design roadmap for non-destructive testing (NDT) applications. For this purpose we use an analytical model simulating the magnetic flux leakage (MFL) distribution of surface breaking defects and investigate the flux leakage signal as a function of various sensor parameters. Our calculations show both the influence of sensor length and height and that when detecting the magnetic flux leakage of µm sized defects a gradiometer base line of 250 µm leads to a signal strength loss of less than 10% in comparison with a magnetometer response. To validate the simulation results we finally performed measurements with a GMR magnetometer sensor on a test plate with artificial µm-range cracks. The differences between simulation and measurement are below 6%. We report on the routes for a GMR gradiometer design as a basis for the fabrication of NDT-adapted sensor arrays. The results are also helpful for the use of GMR in other application when it comes to measure positions, lengths, angles or electrical currents

Integrated defect sensor for the inspection of fiber-reinforced plastics using air-coupled ultrasound

Air-coupled ultrasound (ACU) is a non-destructive testing (NDT) method with a rising significance in industrial use. Common cases where ACU is used are the testing of fiber-reinforced plastic or testing of weld joints between metal sheets. The advantage compared to contact ultrasound is the absence of a liquid, solid or gel-like couplant. The usage of a couplant is an obstacle for developers of automatic scanning systems for ultrasonic testing because it takes a huge effort to integrate a system that delivers a continuous flow of the couplant. In addition a further step of cleaning is often necessary. ACU needs specially adapted probes to compensate for the tremendous impedance difference between a solid and air. A standard method uses two ACU probes in a normal transmission mode. With slanted probes, it is possible to generate Lamb waves in plate-like materials. Because of the contact to the surrounding air, Lamb waves transmit ultrasound to the air on both sides of the plate continually. These so-called leaky Lamb waves can be used with only one accessible side, and by using a specific resonance angle, a higher signal-to-noise ratio (SNR) is achievable. In the past, the correct angle was determined using an iterative method, where the angle of incidence was changed manually while observing the amplitude level. With the stepper-motor-driven angle scanning system, introduced here, the determination of the resonance angle is possible automatically. The system allows changes of the incidence angle during the ultrasound scan too. This makes it possible to adapt the system to wall thickness changes and changes of the radii of the parts contour

Non-destructive Testing of 3D-Hybrid Components Using Air-coupled Ultrasound

The increasing use of hybrid lightweight structures in the automotive industry brings new challenges in the area of multi-material boundaries. Insufficient bonding quality between two kinds of materials, especially in highly loaded structures, affects the component’s performance. Hence, it is very important to monitor the bonding quality in multi-material structures. Flat specimen, consisting of sheet steel and organic sheet layers were examined using air- coupled ultrasound transducers in reflection setup. By slanted incidence of the ultrasound on the profile’s surface, guided waves were excited in the specimen and delaminated areas could be visualized due to changed phase values in the received signal. Furthermore, a 3D-Hybrid cap profile consisting of sheet steel, organic sheet layers and injection molded reinforcing ribs was examined using a special transducer setup. Results show that distinctions in the received signal could be made between bonded and delaminated organic sheets as well as intact and broken ribs behind the organic sheet layer. Several frequencies from 200 kHz up to 500 kHz were then used and compared with this new setup. Simulations of the guided waves propagation support the findings.</p

Lateral heat flux reduction using a lock-in thermography compensation method

Abstract The naturally diffusive heat flow in solids often results in differences in surface temperatures. Active thermography (AT) exploits such differences to gain information on the internal structure, morphology, or geometry of technical components or biological specimens. In contrast to sound or light waves, thermal waves are lossy; consequently, it is difficult to interpret measured 2D temperature fields. Most AT evaluation methods are based on 1D approaches, and measured 3D heat fluxes are frequently not considered, which is why edges, small features, or gradients are often blurred. Herein, we present a method for reducing the local temperature gradients at feature areas and minimizing the induced lateral heat flux in optical lock-in thermography (LT) measurements through spatial- and temporal-structured heating. The vanishing lateral gradients convert the problem into a 1D problem, which can be adequately solved by the LT approach. The proposed compensation method can bypass the blind frequency of LT and make the inspection largely independent of the excitation frequency. Furthermore, the edge sharpness and separability of features are improved, ultimately improving the feature-detection efficiency

Non-Contact Inline Monitoring of Thermoplastic CFRP Tape Quality Using Air-Coupled Ultrasound

Besides the aerospace industry, fibre reinforced plastics have also spread towards many further applications such as automotive, civil engineering as well as sports and leisure articles. Their superior strength and stiffness to mass ratio made them the number one material for achieving high performance. Especially continuous fibre reinforced plastics allow for the construction of structures which are custom tailored to their mechanical loads by adjusting the paths of the fibres to the loading direction. The two main constituents of CFRP are carbon fibres and matrix. Two possibilities for matrix material exist: thermosetting and thermoplastic matrix. While thermosetting matrix may yield better properties with respect to thermal loads, thermoplasticity opens a wide range of applications due to weldability, shapabilty, and compatibility to e.g. injection moulded thermoplastic materials. Thin (0.18 mm) thermoplastic continuous fibre CFRP tapes with a width of 100 mm were examined using air-coupled ultrasound. Transducers were arranged in reflection as well as transmission setup. By slanted incidence of the ultrasound on the tape surface, guided waves were excited in the material in fibre direction and perpendicular to the fibre direction. Artificial defects – fibre cuts, matrix cuts, circular holes, low velocity impacts from tool drop, and sharp bends – were produced. Experiments on a stationary tape showed good detectability of all artificial defects by guided waves. Also the effects of variation in material properties, fibre volume content and fibre matrix adhesion being the most relevant, on guided wave propagation were examined, to allow for quality assessment. Guided wave measurements were supported by destructive analysis. Also an apparatus containing one endless loop of CFRP tape was constructed and built to simulate inline testing of CFRP tapes, as it would be employed in a CFRP tape production environment or at a CFRP tape processing facility. The influences of tape conveying speed on detectability of artificial defects as well as material properties were elaborated and recommendations for implementation in production scale inline monitoring are given.</p

Resonant Airborne Acoustic Emission for Nondestructive Testing and Defect Imaging in Composites

A new version of an acoustic emission mode which is different from its traditional counterpart is discussed in view of applications for nondestructive testing. It is based on the effect of acoustic waves generation from the defect area in ambient air by local standing wave vibration developed in this area at the defect resonant frequency. Another approach which does not require preliminary knowledge of local defect-resonance frequency is one that uses wideband acoustic activation by a noise-like input signal. The acoustic emission field from the defect area is a “fingerprint” of the radiation source, and thus is applicable to defect detection and imaging. This enables the use of commercial microphone scanning for detecting and imaging various defects in composites. An improvement in the acoustic-emission scanning mode based on a multiple-axis robot is studied to applications to complex shape components. A rapid, full-field imaging of the acoustic-emission field is implemented by means of an array of microphones (acoustic camera). Numerous case studies validate the potential of the resonant acoustic-emission modes for integration in the defect imaging system based on inexpensive, fully acoustic instrumental components