Delamination Size Detection using Time of Flight of Anti-symmetric (A_o) and Mode Converted A_o mode of Guided Lamb Waves

Abstract

In this article an attempt has been made to quantitatively assess the extent of delamination in composite laminates, using time-of-flight of fundamental Lamb wave modes, without recourse to baseline data from a healthy structure. An expression has been derived to determine the delamination size, from group velocities of primary Lamb modes in the sub-laminates and time-of-flight of transmitted Ao signal and mode converted Ao signal, which is generated when Ao mode propagates through a delamination. The effectiveness of the expression, when group velocities of primary Lamb modes in the main laminate were used, has been verified through numerical simulations carried out on a quasi-isotropic glass/epoxy laminate with various delamination interfaces. The effectiveness of the expression has also been verified experimentally, on two GFRP cross-ply laminates of [0/90/0] lay-up with 40 mm and 50 mm delamination sizes, using air coupled ultrasonic transducers. The predicted delamination sizes were found to be in good agreement with the actual delamination sizes. Using the proposed technique absolute identification of delamination is possible. A supplementary equation for determining the minimum length of delamination has also been derived and presented in the article.

Attenuation of Ao Lamb mode in hybrid structural composites with nanofillers

“…This confirms that there is no significant change in elastic properties (Young's moduli, shear moduli and Poisson's ratios) of the hybrid laminates in comparison to pristine ones. This observation is in order with the earlier findings [9,20], which report that addition of nanofillers does not influence the elastic properties.…”

“…From the above it is inferred that even though the nanomaterials help in increasing the mechanical strength properties of composites [20] (CNFs in glass/epoxy composite have shown an improvement in compression strength by 40% and interlaminar shear strength by 33%), they result in more attenuation of Lamb wave (A o ). Consequently, distance of propagation of the Lamb wave reduces considerably in a hybridized composite laminate in comparison with its pristine counterpart.…”

“…It was ensured that the formulations possess right tackiness to stick to the dry fabric for the subsequent composite fabrication and appropriate rheological characteristics. We observed a fine amount of enhancement in mechanical strength properties of composite structures, when formulations were made to have NS and CNFs at a loading fraction of 0.5 wt% and 1 wt% in epoxy resin, respectively [9,20]. They are then made into films of desired thickness using a casting machine and subsequently used for composite fabrication through RFI process.…”

3D finite element modelling of guided wave scattering at delaminations in composites

“…It was found that the reflection of the S0 mode from an artificial delamination is strongly dependent on the position of the delamination through the thickness of the laminate [22]. Using the first anti-symmetrical (A0) guided wave mode, the wave scattering at a delamination area was studied both numerically and experimentally, and mode conversion from the A0 mode to the S0 mode observed [23]. From numerical simulations to characterize the scattering pattern generated at a circular delamination, it was found that the amplitudes around a delamination showed a large forward scattered wave relative to the reflected pulse [24].…”

Evaluation of the Thickness and Bond Quality of Three-Layered Media using Zero-Group-Velocity Lamb Waves

“…Ultrasonic inspection is a promising method for characterizing bonding conditions. Lamb waves, which propagate in a plate structure, were used for detecting defects at an interface and succeeded in detecting a relatively large defect compared to wavelength of Lamb waves [1]. Reflection waves at interfaces between an adhesive and adherents owing to acoustic impedance mismatch have also been utilized [2].…”