Browsing by Author "Bernhardt, Yannick"

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Open Access
Flow front monitoring in high-pressure resin transfer molding using phased array ultrasonic testing to optimize mold filling simulations
(2023) Littner, Linus; Protz, Richard; Kunze, Eckart; Bernhardt, Yannick; Kreutzbruck, Marc; Gude, Maik
During the production of fiber-reinforced plastics using resin transfer molding (RTM), various characteristic defects and flaws can occur, such as fiber displacement and fiber waviness. Particularly in high-pressure RTM (HP-RTM), fiber misalignments are generated during infiltration by local peaks in the flow rate, leading to a significant reduction in the mechanical properties. To minimize or avoid this effect, the manufacturing process must be well controlled. Simulative approaches allow for a basic design of the mold filling process; however, due to the high number of influencing variables, the real behavior cannot be exactly reproduced. The focus of this work is on flow front monitoring in an HP-RTM mold using phased array ultrasonic testing. By using an established non-destructive testing instrument, the effort required for integration into the manufacturing process can be significantly reduced. For this purpose, investigations were carried out during the production of test specimens composed of glass fiber-reinforced polyurethane resin. Specifically, a phased array ultrasonic probe was used to record individual line scans over the form filling time. Taking into account the specifications of the probe used in these experiments, an area of 48.45 mm was inspected with a spatial resolution of 0.85 mm derived from the pitch. Due to the aperture that had to be applied to improve the signal-to-noise ratio, an averaging of the measured values similar to a moving average over a window of 6.8 mm had to be considered. By varying the orientation of the phased array probe and therefore the orientation of the line scans, it is possible to determine the local flow velocities of the matrix system during mold filling. Furthermore, process simulation studies with locally varying fiber volume contents were carried out. Despite the locally limited measuring range of the monitoring method presented, conclusions about the global flow behavior in a large mold can be drawn by comparing the experimentally determined results with the process simulation studies. The agreement between the measurement and simulation was thus improved by around 70%.
Open Access
Integrated defect sensor for the inspection of fiber-reinforced plastics using air-coupled ultrasound
(2020) Bernhardt, Yannick; Kreutzbruck, Marc
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.
Open Access
Resonant airborne acoustic emission for nondestructive testing and defect imaging in composites
(2021) Solodov, Igor; Bernhardt, Yannick; Kreutzbruck, Marc
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.
Open Access
Ultrasonic anisotropy in composites : effects and applications
(2022) Bernhardt, Yannick; Littner, Linus; Kreutzbruck, Marc
Stiffness anisotropy is a natural consequence of a fibrous structure of composite materials. The effect of anisotropy can be two-fold: it is highly desirable in some cases to assure a proper material response, while it might be even harmful for the applications based on “isotropic” composite materials. To provide a controllable flexibility in material architecture by corresponding fibre alignment, the methodologies for the precise non-destructive evaluation of elastic anisotropy and the fibre orientation are required. The tasks of monitoring the anisotropy and assessing the fibre fields in composites are analyzed by using the two types of ultrasonic waves suitable for regular plate-shaped composite profiles. In the plate wave approach, the effect of “dispersion of anisotropy” has been shown to make the wave velocity anisotropy to be a function of frequency. As a result, the in-plane velocity pattern measured at a certain frequency is affected by the difference in the wave structure, which activates different elasticity against the background of intrinsic material anisotropy. Phase velocity anisotropy and its frequency dependence provide a frequency variation of the beam steering angle for plate waves (dispersion of beam steering). In strongly anisotropic composite materials, the beam steering effect is shown to provide a strong focusing of ultrasonic energy (phonon focusing). For bulk shear waves, the orthotropic composite anisotropy causes the effect of acoustic birefringence. The birefringent acoustic field provides information on stiffness anisotropy which can be caused by internal stresses, texture, molecular or/and fibre orientation. On this basis, a simple experimental technique is developed and applied for mapping of fibre orientation in composite materials. Various modes of acoustic birefringence are analyzed and applied to assessing the fibre fields in injection moulding composites and to identify the fibre lay-ups in multiply materials. The birefringence pattern is also shown to be sensitive and applicable to characterizing impact- and mechanical stress-induced damage in composites.