Recently, active soft materials have attracted widespread attentionthe dielectric energy and the elastic energy described by the Mooney-Rivlin model. Secondly, the relation between the peak value of stationary response and the step voltage/pressure is analytically derived through the first integral of system. The intersection point of the relation curve and the static equilibrium curve gives the critical voltage/pressure. The instability of the spherical dielectric elastomer balloon under different situations is discussed in detail. For any given voltage, there exists a threshold of material constant, below which the structure will remain stable forever. A similar threshold of material constant exists for any pressure with the value of nonzero. For the case with absent pressure, however, there exists a critical voltage, beyond which the structure will be dynamically instable. The influences of the imposed voltage/ pressure, material constant and pre-stretch on the critical state of the dynamic instability are investigated systematically. Pre-stretch will improve the dynamic stability of the spherical dielectric elastomer balloon. This work paves the way for the theoretical research on dynamic instability of soft material, and provides some guidance for the structural and functional design of soft sensors and actuators. in the flexible device manufacturing industry. As a typical active soft material, the dielectric elastomer, with two compliant electrodes covering its upper and lower surfaces, constitutes the dielectric elastomer structure. Although the dielectric elastomer structure possesses excellent electromechanical properties, various failures occur frequently when operating in the dynamic environment, which greatly hinders its extensive application. The dynamic stability of an ideal spherical dielectric elastomer balloon is investigated here, which is related to the phenomenon of electro-mechanical instability. Firstly, the governing differential equation with respect to the stretch ratio of the ideal spherical dielectric elastomer balloon is derived according to the principle of virtual work. The free energy function is expressed as the summation of

The cohesive zone model is widely used in fracture mechanics. When the fracture process zone (FPZ) in front of the crack tip is too large to be neglected, the nonlinear behavior must be considered. That is to say, in this circumstance the linear fracture mechanics is no longer valid. In order to take into account the nonlinear behavior in FPZ, many fracture models have been proposed, among which, the cohesive zone model (CZM) might be one of the simplest and has been widely used. However, there still remain some problems in the existing numerical methods; for instance, length of the fracture process zone cannot be obtained accurately; dense meshes are required, etc. In order to get over these difficulties, a new analytical singular element is proposed in the present study and further extended into the cohesive zone model for crack propagation problems. In this singular element, the cohesive traction is approximately expressed in the form of polynomial expanding though Lagrange interpolation. The special solution corresponding to each expanding term is specified analytically. Each special solution strictly satisfies the requirements of both differential equations of interior domain and the corresponding traction expanding terms. The real cohesive traction acting on the cohesive crack surface is thus expressed in a natural and strict way. Then the special solution can be transformed into nodal forces of the present singular element. Assembling the stiffness matrix and nodal force into the global FEM system, the cohesive crack problem can be analyzed. An efficient iteration procedure is also proposed to solve the nonlinear problem. Finally, the cohesive crack propagation under arbitrary external loading can be simulated, and the length of FPZ, crack tip opening displacement (CTOD) and other parameters in the cohesive crack problem can be obtained simultaneously. The validity of the present method is illustrated by numerical examples.

Nondestructive testing of subsurface defects plays an important role in ensuring the performance and safety of a component or a structure. However, the subsurface defects are often deeply buried and invisible, and therefore, are difficult to be identified. Laser ultrasonic technology (LUT), which is further developed from the traditional piezoelectric ultrasonic technology, has a series of advantages, such as non-contact, broadband, high sensitivity and high spatial resolution. And LUT has been used in detecting surface defects for a long time. The interaction of laser excited surface acoustic wave (LESAW) with surface defects is well understood, but its interaction with subsurface defects is still unclear, even though these two groups of defects have some similar features. In this paper, the interaction of LESAW with subsurface defects has been studied in detail, and the quantitative detection of such defects by LUT has been discussed. The finite element method and the thermoelastic model of laser ultrasound have been employed to simulate the interaction, and to discuss the influences of burial depth and vertical size of the rectangular defect. Firstly, simulation has been made for the initial interaction of LESAW with the frontier of subsurface defect at the vertical edge. The impact of burial depth on the waveform has been analyzed, and the scattered feature caused by the defect has been extracted and interpreted. It is shown that the arrival time of the reflected surface wave is insensitive to the burial depth when the depth is less than the center wavelength. Then for a fixed burial depth, the influences of vertical size and dimension on scattered echo have been simulated and analyzed. It is found that there exists a specific linear relationship between the arrival time of reflected surface wave and the vertical size of defect. At last, based on the results of the study, an equation for the evaluation of vertical size of defect has been presented. The findings on the interaction between LESAW and subsurface defects may provide possible means for nondestructive testing.