Crack detection on aerospace composites by means of photorefractive interferometry.

Seresini, Tommaso; Katholieke Universiteit Leuven; Belgium

Seresini, T.; University of Leuven; Belgium
Sunetchiieva, S.; University of Leuven; Belgium
Pfeiffer, H.; University of Leuven; Belgium
Wevers, M.; University of Leuven; Belgium
Xiong, J.; Nanjing University of Science and Technology; China
Glorieux, C.; University of Leuven; Belgium

ID: ECNDT-0588-2018
Download: PDF
Session: Nonlinear Ultrasonics 1
Room: G2
Date: 2018-06-12
Time: 11:10 - 11:30

Aircraft industry requires fast, robust and reliable tools for assessing structural integrity. Linear ultrasonic techniques exploit the reflections resulting from the acoustical impedance mismatch caused by an open crack or delamination. However, when the fracture is closed then no reflection occurs. Most techniques rely on contact transducers both for excitation and detection, which is an important limitation when it comes to probing areas that are difficult to reach by the operator. Thus, the need for a technique that has higher sensitivity and ease of use than the available ones.
Surface waves travelling across a crack experience a nonlinear modulation of their frequency content if such crack is subject to periodical opening and closing (clapping). In this work, the phenomenon of clapping is used to assess the presence of defects, such as cracks or delamination, using a fully optical method. A low frequency, high amplitude surface wave is used to act on the defect, opening and closing the two limbs, while a high frequency, low amplitude surface wave is used as a probe, whose transmitted amplitude is modulated at the rhythm of opening and closing. The modulation results in frequency mixing. Although full field optical interferometry allows to visualise the dynamics of frequency mixing, most interferometric schemes have a nonlinear response between the light intensity and the displacements of interest, making it cumbersome to distinguish nonlinear acoustic effects from effects of nonlinear optical response.
In this work an approach is presented that exploits frequency selectivity of a photorefractive interferometer scheme to detect vibrations that are excited due to acoustic nonlinear effects that occur in the neighbourhood of defects, without interference of effects of optical nonlinearity. The method is non-contact and allows to inspect both in a pointwise scanning fashion, and in full-field mode, imaging vibrations of a complete large area at once, thus detecting defects in an early stage.