Optimal Excitation Frequency of Lamb Waves for Delamination Detection in CFRP Laminates

Abstract

To improve the reliability of delamination detection techniques based on Lamb waves for carbon fibre reinforced plastics (CFRP) composite laminates, the influence of excitation frequency of Lamb waves on the intensity of reflected waves from a delamination in cross-ply CFRP delaminated beams was investigated. The A0 Lamb wave mode was chosen although the conclusion in this work is also applicable to S0 mode. A Chebyshev pseudospectral Mindlin plate element proposed by authors was employed to simulate the A 0 mode propagation and the interaction of the A0 mode with the delamination. Numerical results show that the sensitivity of A0 mode to the delamination at different frequencies is remarkably different. Each delamination with different lengths or different locations along the through-thickness direction of the laminates generally has at least one or more optimal excitation frequencies which can generate the strongest reflected wave signals from the delamination. The reason for this phenomenon was also investigated by analyzing the natural vibration of the local upper- and lower-delaminated regions. The optimal excitation frequencies were identified to be close to the resonant frequencies of the local delaminated regions with pure flexural vibration modes similar to the deformation pattern of A0 mode.

On the Directivity of Lamb Waves Generated by Wedge PZT Actuator in Thin CFRP Panel

“…The sources, which have been used to adequately build the transducer's structure and replicate its dynamics in FE model, were the technical notes about the ultrasonic principles important to wedge transducer application and design, including the scheme and main requirements to the active element, backing, matching layers and the sound path presented by Olympus Co. ® [48], the outer dimensions of the used actuator (see Figure 3), the frequency response functions for the electric current through PZT active plate, and out-of-plane displacement amplitude on the actuator's footprint (see Figure 4a,b), which have been determined experimentally. Only averaged normal displacement amplitudes of the actuator's contact surface were measured experimentally by using the VibroGo single-point vibrometer (Polytec Co. ® , Irvin, Figures 1b and 2b demonstrates that the wavelength of A0 Lamb waves mode is 2-5 time less than the wavelength of SS0 horizontally polarized shear wave, i.e., spatial defects resolution of A0 waves can be significantly better that is fully consistent with the results of the papers [11,20,22,30].…”

On the Directivity of Acoustic Waves Generated by the Angle Beam Wedge Actuator in Thin-Walled Structures

“…Plate excitation at the different eigenfrequencies of the actuator leads to the significant modification of the waves' directivity pattern. The well-defined directivity of the generated waves and their length of 1-2 cm make it possible to use them for identification of relatively large imperfections in the structure of the material, such as delamination, local change of the mechanical properties, inclusions, and another defects [6,7,9,10,17].…”

“…Among these degrees of freedom, which should provide the selected wave motion excitation method, the directivity of wave propagation is most difficult to implement as desired. In order to excite the acoustic waves in a structure, a variety of different techniques is used, including piezoelectric transducers in the form of circular [4,6], annular tablets [13], or rectangular piezoelectric (PZT) patches [12,13], by PZT wedge transducers [11,[14][15][16], piezoelectric wafer active (PWAS) and macro-fiber composite (MFC) actuators [17][18][19], and by laser beam excitation [20]. For the sensoring of the waves, which are reflected or scattered on the defects, small piezoelectric transducers or non-contact laser Doppler vibrometry (LDV) are most often used.…”

On the Directivity of Lamb Waves Generated by Wedge PZT Actuator in Thin CFRP Panel

“…Meanwhile, the collective skills of many researchers confirm that in some cases the Lamb wave based SHM techniques can provide more reliable information about the damage presence and the severity than another methods, and provide the possibility of determining a damage location due to their local response nature. These imperfections that can be detected by the acoustic SHM are the delaminations, the inclusions, the uneven resin cure areas and the porosity [11], the dry spots, which can arise during liquid composite molding [12,13] and even the errors of lay-up stacking orientation [14].…”

“…The paper [8], which experimentally studied the Lamb waves attenuation in the quasi-isotropic plastic, demonstrated a sufficient decrease of the propagating wave amplitude with angle between the main axis of anisotropy and wave path. Hu with coworkers [11] investigated the A0 and S0 waves propagation and their sensitivity to the delamination in cross-ply laminates. It should be noted that all of the aforementioned works [2,8,11,14], as well as the other articles [16 -18] that investigated the influence of the anisotropy of the elastic properties of the material on the propagation of acoustic waves, tested the proposed models and calculation methods on quasi-isotropic and cross-ply laminates.…”