State Key Laboratory of Mechanical Structure Strength and Vibration

Water-based ionic polymer–metal composites (IPMCs) exhibit an anode deformation followed by a back-relaxation deformation under a direct current voltage. The latter is strongly related to the water content. This Letter proposes an evaluation method of the critical water content for relaxation deformation. Based on Nafion-IPMC doped with various alkali cations, continuous deformations are measured under a periodic pulse voltage in air. A transformation of the relaxation deformation is found to exist between two successive pulses. The critical water content is confined between the two pulses and evaluated gravimetrically. Removal of no more than 4 wt. % water or 1.4–2 water molecules per cation from the saturated material can completely eliminate the large relaxation deformation in Nafion-IPMC doped with alkali cations.

Ionic Polymer-Metal Composite (IPMC) has been well-documented of being a promising functional material in extensive applications. In its most popular and traditional manufacturing technique, roughening is a key process to ensure a satisfying performance. In this paper, based on a lately established multi-physical model, the effect of roughening process on the inner mass transportation and the electro-active output of IPMC were investigated. In the model, the electro-chemical field was monitored by Poisson equation and a properly simplified Nernst–Planck equation set, while the mechanical field was evaluated on the basis of volume strain effect. Furthermore, with Ramo-Shockley theorem, the out-circuit current and accumulated charge on the electrode were bridged with the inner cation distribution. Besides, nominal current and charge density as well as the curvature of the deformation were evaluated to characterize the performance of IPMC. The simulation was implemented by Finite Element Method with Comsol Multi-physics, based on two groups of geometrical models, those with various rough interface and those with different thickness. The results of how the roughening impact influences on the performance of IPMC were discussed progressively in three aspects, steady-state distribution of local potential and mass concentration, current response and charge accumulation, as well as the curvature of deformation. Detailed explanations for the performance improvement resulted from surface roughening were provided from the micro-distribution point of view, which can be further explored for the process optimization of IPMC.

The effects of thickness and delamination on the fracture toughness of X60 pipeline steel were investigated experimelltally by using of compact tension specimens with thicknesses of 3, 6, 9, 12 and 15 mm. The couple effect of delamination cracks and out-of-plane stress constraint was discovered. Delamination became serious as thickness increasing, and the three-dimensional stress constraint near the crack-tip is nearly low-constraint plane stress state during fracture. Appearance fracture toughness is independent on thickness. It is impossible to attain plane strain fracture toughness by increasing specimen thickness when delamination cracks turn up. Because the couple effect of thickness and delamination is related closely with the relative direction of crack growth and thickness, the safe assessment of pipeline by using test data from through-thickness crack is not very reliable. The plasticity modified stress intensity factor Ke and J integral were found to be equivalent in description of the cracking growth of such ductile steel.

The method of using impulse excitation of internal force to investigate rod structure was proposed in this paper. This method rectifies the defect in wide band random excitation method, i.e. the difficulty in exciting each modal for energy disperse, and slow at sine excitation. The modal experiment on the large space flexible truss structure was carried out using impulse excitation of half sine wave and from which the natural frequencies and modals of the system were obtained. It is found that the experimental result agrees well with the theoretical result, thus validating the correctness of theoretical calculating result. The experimental result shows that using the impulse excitation of internal force can effectively excite the natural frequencies of the structure. The vibration parameters obtained from the experiment is helpful for further research on the vibration active control of the large space flexible truss structure.