This study investigates the effect of potassium ion (K+) concentration on double helix aggregation in κ-carrageenan-based hydrogels, which significantly influences their shakedown behavior. The shakedown behavior of κ-carrageenan/polyacrylamide (PAAm) hydrogels was characterized by the evolution of maximum stress and energy dissipation during cyclic load. The experimental results indicate that higher K+ concentrations significantly improve the maximum stress in the steady state, but barely influence the energy dissipation in the steady state. The improved maximum stress can be explained by the higher density of double helix aggregation. The steady energy dissipation elucidates that the K+ concentration does not affect the breaking–recovering balance of the sacrificial network in cyclic loading. These results provide mechanistic insights into how ion-triggered double helix aggregation influences the shakedown behavior of κ-carrageenan-based hydrogels.

Guided waves are suitable for non-destructive testing and structural health monitoring of tube-like structures. However, the dispersion phenomenon impedes the application of guided waves. Although the finite element (FE) method can simulate the guided wave propagation and help to study the dispersion phenomenon, boundary reflections can contaminate the wave field of interest and impede the FE simulation. In this paper, damping boundaries are developed as a set of FE frames with gradually increasing damping coefficients to alleviate boundary reflections. The wave signals simulated through the FE model with the damping boundaries only contain the waves from the transmitter to the receiver, without the interferences of the boundary reflections. The energy distribution on the frequency-velocity spectrum of the simulated signals agrees well with the analytical dispersion curves, indicating that the boundary reflections are effectively alleviated. The analytical solution of the guided wave equation and the FE modelling method presented in this paper can facilitate both research and applications of guided waves for tube-like structures.

Cracks are one of the most common damages to thin plates. Therefore, rapid and accurate detection of cracks is of great significance in the nondestructive testing of thin plates. Most existing nondestructive testing technologies focus on crack localization rather than the quantification of the crack length and orientation. Therefore, this paper proposes a damage index, considering the wave attenuation during propagation, to quantify the crack length and orientation. Lamb waves are transmitted and received by a circular sensor array. Based on the abnormal reflection of the A0 mode of Lamb waves, the reflections at cracks are extracted by the Hilbert transform. The influence of the crack length and orientation are studied parametrically, and the relations between the proposed damage index and crack length and angle are given based on regression analysis.

By monitoring the structural vibration and analyzing the modal parameter, hidden damage can be identified at an early stage, preventing abrupt failures for important structural components. However, the modal parameter has exhibited a low sensitivity to the interfacial damage, such as the delamination in the composite beam. In this paper, a signal-processing method is developed to increase the sensitivity of the modal parameter to the delamination. Both the first- and high-order modal parameters are transformed to an approximate waveform capacity dimension (AWCD). Based on the wave peaks of the modal parameters in the AWCD, the start and the end of the delamination can be accurately pinpointed. Since the AWCD method can highlight the delamination-related waveform features of the modal parameters, the delamination can be identified without referring to the baselines. Both analytical and experimental investigations demonstrate that the AWCD method can modify the waveforms of the modal parameters and lead to an accurate identification of the delamination in composite cantilever beams.