The S0 Lamb mode can propagate over distances of the order of 1 m in composite laminates and so has the potential to be used in long‐range nondestructive inspection. This paper discusses the interaction of the S0 Lamb mode with delaminations. The dispersion curves and the corresponding stress and displacement mode shapes of the lower order Lamb modes are obtained analytically and the interaction of the S0 mode with delaminations at different interfaces in a composite laminate is then studied both by finite element analysis and by experiment. It is shown that the amplitude of the reflection of the S0 mode from a delamination is strongly dependent on the position of the delamination through the thickness of the laminate and that the delamination locations corresponding to the maximum and minimum reflectivity correspond to the locations of maximum and minimum shear stress across the interface in the S0 mode.

The interaction of Lamb waves with delaminations in composite laminates

A deconvolutional neural network (DNN) is integrated into the ultrasound (UT) pulse-echo Lamb wave nondestructive evaluation (NDE) imaging technique to achieve subwavelength super-resolution defect images for the application of anisotropic composite airplane-laminated structures. First, numerical simulation has been performed to simulate the multilayer velocity–frequency dispersion relation of the symmetric and antisymmetric Lamb wave modes. Then, a super-resolution DNN is developed with all Lamb wave modes as the convolutional kernels to obtain the subwavelength defects image of the laminated structures. After that, the effectiveness of the pulse-echo Lamb wave NDE imaging technique is verified by the experimental test of a standard aluminum metal plate with three calibration holes of 2/3/5 mm diameters at the UT center frequency of 2 MHz and a line defect. Finally, the experiment of the pulse-echo Lamb wave NDE imaging technique is carried out with an L-joint coupon sample made of anisotropic graphite–epoxy airplane materials. The comparison between the experimental result and the conventional pitch-catch C-scan shows that the Lamb wave NDE technique can reveal more details of the defects, indicating its promising application in anisotropic layers’ structures defects inspection.

Super-Resolution Ultrasound Lamb Wave NDE Imaging of Anisotropic Airplane Laminates via Deconvolutional Neural Network

Composite materials are incorporated in various applications and their industry is widely growing. They offer cost savings and are more environmentally friendly than conventional metal structures. Some of the concerns this industry faces are the energy and time spent on long curing cycles to achieve permanent bonding between the matrix and fibers. In our previous work, a reusable sensing polytetrafluoroethylene (PTFE) system that can monitor the degree of cure of the composite while curing was developed and tested through Lamb waves analysis. This thin film is now used to monitor the same cure parameters for a shorter curing cycle than that suggested by the CFRP manufacturer. The results show that the three cure parameters: Minimum viscosity, full gelation, and vitrification are offset by the same time deducted from the cycle, highlighting the feasibility of using such technology. To verify the viability of this approach, tensile testing and dynamic mechanical analysis are performed on these composites. Tensile testing results show that the average tensile modulus for the shortened cycle is of similar values if not slightly higher than that of the normal cycle. Dynamic mechanical analysis (DMA) results verify both previous conclusions: Time shift of cure parameters and enhanced mechanical properties of the shortened cycle.

Assessment of cure cycle shortening for advanced composites via ultrasonics and dynamic mechanical analysis

A quick method to construct 3-D velocity contour map for assessing anomalies in concrete

Lamb waves have emerged as a valuable tool to examine long plate-like structures in a faster way compared to conventional bulk wave techniques, which make them attractive in non-destructive testing. However, they present a multimodal and dispersive nature, which hinders signal identification. Oblique incidence is one of the most known methods to generate and receive Lamb waves and it is applied in different experimental arrangements with different types of sensors. In this work, several setups were conducted and compared to determine the optimal ones to launch and detect ultrasonic Lamb waves, especially in non-homogeneous specimens. The chosen arrangements were contact with angle beam transducers, immersion in a water tank, localised water coupling using conical containers and air coupling. Plates of two different materials were used, stainless steel and Portland cement mortar. Theoretical and experimental dispersion curves were compared to verify the existence of Lamb modes and good correspondence was achieved.

https://www.mdpi.com/1424-8220/19/19/4068/pdf

Santiago Vázquez

Papers

Comparative Study of Coupling Techniques in Lamb Wave Testing of Metallic and Cementitious Plates.