Jan Hettler
Katholieke Universiteit Leuven
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Featured researches published by Jan Hettler.
Materials | 2016
Jan Hettler; Morteza Tabatabaeipour; Steven Delrue; Koen Van Den Abeele
The amount and variety of composite structures that need to be inspected for the presence of impact damage has grown significantly in the last few decades. In this paper, an application of a probabilistic ultrasonic guided wave imaging technique for impact damage detection in carbon fiber-reinforced polymers (CFRP) is presented. On the one hand, a linear, baseline-dependent, technique utilizing the well-known correlation-based RAPID method and an array of piezoelectric transducers is applied to detect impact-induced damage in plate-like composite structures. Furthermore, a baseline-independent nonlinear extension of the standard RAPID method is proposed, and its performance is demonstrated both numerically and experimentally. Compared to the conventional RAPID, the baseline-free version suffers from a somewhat lower imaging quality. However, this drawback is compensated by the fact that no damage-free (intact) baseline is necessary for successful imaging of damage.
Ultrasonics | 2016
Steven Delrue; Morteza Tabatabaeipour; Jan Hettler; Koen Van Den Abeele
Friction stir welding (FSW) is a promising technology for the joining of aluminum alloys and other metallic admixtures that are hard to weld by conventional fusion welding. Although FSW generally provides better fatigue properties than traditional fusion welding methods, fatigue properties are still significantly lower than for the base material. Apart from voids, kissing bonds for instance, in the form of closed cracks propagating along the interface of the stirred and heat affected zone, are inherent features of the weld and can be considered as one of the main causes of a reduced fatigue life of FSW in comparison to the base material. The main problem with kissing bond defects in FSW, is that they currently are very difficult to detect using existing NDT methods. Besides, in most cases, the defects are not directly accessible from the exposed surface. Therefore, new techniques capable of detecting small kissing bond flaws need to be introduced. In the present paper, a novel and practical approach is introduced based on a nonlinear, single-sided, ultrasonic technique. The proposed inspection technique uses two single element transducers, with the first transducer transmitting an ultrasonic signal that focuses the ultrasonic waves at the bottom side of the sample where cracks are most likely to occur. The large amount of energy at the focus activates the kissing bond, resulting in the generation of nonlinear features in the wave propagation. These nonlinear features are then captured by the second transducer operating in pitch-catch mode, and are analyzed, using pulse inversion, to reveal the presence of a defect. The performance of the proposed nonlinear, pitch-catch technique, is first illustrated using a numerical study of an aluminum sample containing simple, vertically oriented, incipient cracks. Later, the proposed technique is also applied experimentally on a real-life friction stir welded butt joint containing a kissing bond flaw.
Food Research International | 2015
Annelien Rigolle; Imogen Foubert; Jan Hettler; Erik Verboven; Ruth Demuynck; Koen Van Den Abeele
The quasi-isothermal crystallization process of cocoa butter was monitored by an ultrasonic shear reflection technique utilizing a custom-built experimental set-up in a temperature controlled environment. To facilitate the interpretation of the measurement results, the propagation of shear waves was first theoretically studied in different configurations of gas, liquid or solid layers with varying thickness for the case of normal incidence, yielding theoretical equations of the shear wave reflection coefficient (swRC) for different layering conditions. The typical experimentally observed pattern of the swRC during quasi-isothermal cocoa butter crystallization was subsequently linked to the theoretical equations. The remarkable oscillatory damped response in the swRC as function of the crystallization time could be explained by constructive and destructive interference of a first reflection at the boundary between a plexiglass delay line and the crystallized cocoa butter and a second reflection occurring at the interface between crystallized and liquid substance. This hypothesis was supported by the excitation frequency dependence of the oscillations. The quality of the fit of the theoretical model to the experimental results was very good and also the reproducibility between different independent measurements was acceptable. Finally, measurements at different temperatures (18°C and 20°C) suggested that the technique was able to detect differences in crystallization behavior, as measurements at 18°C displayed faster oscillations compared to measurements at 20°C. Moreover, this was also confirmed by the theoretical model, as a higher value of the crystallization rate parameter K, exhibited more rapid oscillations.
Journal of the Acoustical Society of America | 2015
Koen Van Den Abeele; Jan Hettler; Morteza Tabatabaeipour; Steven Delrue
When seeking out evidence for nonlinear behavior, various signal processing techniques can be applied for the comparison of two signals, one being a slight distortion of the other. For instance, the pulse inversion technique compares the responses to two out-of-phase excitation signals. Alternatively, one can compare the response at a finite (nonlinear) excitation amplitude to a scaled response at a very low (linear) excitation, as performed in the scaling subtraction technique. In this report, several examples are given in which these nonlinearity based signal processing techniques are used in practice to visualize damage features in solids. In view of kissing bond defect detection in friction stir welds, the pulse-inversion method was employed in a contact pitch-catch mode using a chirp signal. B-scan spectral heat maps obtained after pulse inversion allow to easily identify and size damage zones along the weld path. Second, the scale subtraction technique will be illustrated in combination with an ultr...
Ndt & E International | 2016
Morteza Tabatabaeipour; Jan Hettler; Steven Delrue; K. Van Den Abeele
Physics Procedia | 2015
Steven Delrue; Morteza Tabatabaeipour; Jan Hettler; K. Van Den Abeele
Innovative Food Science and Emerging Technologies | 2016
Annelien Rigolle; Imogen Foubert; Jan Hettler; Erik Verboven; Arvid Martens; Ruth Demuynck; Koen Van Den Abeele
Journal of Nondestructive Evaluation | 2017
Jan Hettler; Morteza Tabatabaeipour; Steven Delrue; Koen Van Den Abeele
Acta Acustica United With Acustica | 2017
Morteza Tabatabaeipour; Jan Hettler; Steven Delrue; Koen Van Den Abeele
Physics Procedia | 2015
Morteza Tabatabaeipour; Jan Hettler; Steven Delrue; K. Van Den Abeele