C. Ramadas

Interaction of the primary anti-symmetric Lamb mode (Ao) with symmetric delaminations: numerical and experimental studies

The interaction of an ultrasonic guided Lamb wave mode with delamination type defects in a quasi-isotropic laminated composite plate has been studied, using both simulations and experiments. In a laminated composite plate with a symmetric delamination, when the primary anti-symmetric mode, Ao, is incident at the entrance and exit of a delamination, it generates a new mode, So, that is confined only to sub-laminates and undergoes multiple reflections in the delaminated region. It was observed that only the incident and mode-converted Ao modes propagate in the main laminate. The two modes reverberate between the two ends of the delaminations while undergoing multiple mode conversions, leading to a trail of signals that is captured by the finite element model. The numerical observations were validated using experiments conducted using air coupled ultrasonic transducers.

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

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.

Sizing of Delamination Using Time-of-Flight of the Fundamental Symmetric Lamb Modes

In this study, a procedure is devised, using the fundamental symmetric Lamb mode (So) to estimate the size of an impact-induced delamination at the bottom interface of a composite laminate, which has been subjected to a low-velocity impact. When S o mode incidents at the edge of delamination, it propagates independently in 0° layer and the rest of layers. Based on the propagation of So mode in the delaminated region, an expression is derived, which can estimate the size of interface delamination using measured arrival times or Time-of-Flight (ToF) of So modes. Numerical simulations were carried out on glass/epoxy (GFRP) cross-ply laminates ([0/902] s) to confirm the derived expression and eventually it was validated through experiments carried out on [0/90/0] GFRP laminates. Moreover, the minimum detectable size of delamination through the proposed method was estimated analytically and verified through numerical simulations.

Artificial neural network based multi-parameter inversion for the characterization of transversely isotropic composite lamina using velocity measurements of Lamb waves:

Artificial neural network (ANN) based multi-parameter inversion method is proposed to characterize transversely isotropic composite lamina using Lamb wave group velocity measurements. The ANN is first trained using numerical simulations and known micromechanics based formulae before being deployed on experimental samples. The group velocities obtained from the experiments were fed to the trained network. The network so trained, predicted the elastic properties, fiber volume fraction, and density.

INTERACTION OF GUIDED LAMB WAVES WITH DELAMINATIONS AND DISCONTINUITIES IN COMPOSITE PLATE‐LIKE STRUCTURES

Delamination is one of the critical failure modes that laminated composites structures encounter during their fabrication and/or in‐service. When guided Lamb wave (Ao) is employed for sizing of delamination, it is necessary to understand the wave interaction with the defect. Studies were carried out on the interaction of Ao mode with symmetric and asymmetric delaminations and transmission of turning modes in the sub‐laminates. An attempt was also made to understand the propagation of Ao mode in composite structural T‐joint. During the interaction of Ao mode with structural discontinuity in a T‐joint, generation of a new mode, So, and propagation of turning modes were observed. Length and width of interface delamination in composite T‐joint have been worked out using D‐scan and B‐scan respectively. All studied were carried out through numerical simulations and experimental verifications through Non‐ Contact Ultrasound (NCU) technique.