GRADIENT SURFACE PLY MODEL OF SH WAVE PROPAGATION IN SAW SENSORS

Investigation of the propagation of the wave in SAW sensors is a basis for the research and design of the sensors. With the advance of the sensor, both the effect of environment on the surface ply and the geometry of waveguide are complicated. To consider the complication, a model with gradient surface ply and multilayer waveguide of SH wave propagation in sensor is proposed. The equation of wave velocity is derived by a transfer matrix method. Through the equation, the function of wave velocity increment via the change of parameters in the surface ply is obtained. The effect of the inhomogeneity on the function is also studied. Finally, some influencing factors of the behavior of the sensor are discussed.

Stochastic resonance in coupled weakly-damped bistable oscillators subjected to additive and multiplicative noises

With coupled weakly-damped periodically driven bistable oscillators subjected to additive and multiplicative noises under concern, the objective of this paper is to check to what extent the resonant point predicted by the Gaussian distribution assumption can approximate the simulated one. The investigation based on the dynamical mean-field approx- imation and the direct simulation demonstrates that the pre- dicted resonant point and the simulated one are basically co- incident for the case of pure additive noise, but for the case including multiplicative noise the situation becomes some- what complex. Specifically speaking, when stochastic res- onance （SR） is observed by changing the additive noise in- tensity, the predicted resonant point is lower than the sim- ulated one; nevertheless, when SR is observed by chang- ing the multiplicative noise intensity, the predicted resonant point is higher than the simulated one. Our observations im- ply that the Gaussian distribution assumption can not exactly describe the actual situation, but it is useful to some extent in predicting the low-frequency stochastic resonance of the coupled weakly-damped bistable oscillator.

MECHANISM ANALYSIS OF THICKNESS EFFECT ON MIXED MODE Ⅰ/Ⅱ FRACTURE OF LC4-CS ALUMINUM ALLOY

Mixed mode Ⅰ/Ⅱ fracture erperiments of LC4-CS aluminum alloy were conductedby using tension--shear specimens with thicknesses of 2, 4, 8 and 14mm. Fracturemechanisms of thickness effect on mixed mode Ⅰ/Ⅱ fracture were first examined fromfracture surface morphology to correlate with the macroscopic fracture behavior andstress state. It is found that specimen thickness has a strong influence on mixed modefracture. As thickness varies from thin to thick the macroscopic fracture surfacesappear the characteristics of plane stress state (2mm, 4mm--thick specimen), three--dimensional stress state (8mm--thick specimens), and plane strain state (14mm--thickspecimens), respectively. The specimens of all kinds of thicknesses are typical of ten-sile type failure under mode Ⅰ loading condition and shear type failure under mode Ⅱloading condition. Two distinct features coexist on the fracture surfaces under mixedmode loading conditions, and the corresponding proportion varies with loading mix-ity. Void--growth processes are the failure mechanism in both predominately tensile-and shears--type fractures. The size and depth of dimples on the fracture surface varygreatly with thickness. Therefore, it is extraordinary necessary to take into accountthe thickness effect when a mixed mode fracture criterion is being established.

SCATTERING OF ANTI-PLANE SHEAR WAVES BY A SINGLE CRACK IN AN UNBOUNDED TRANSVERSELY ISOTROPIC ELECTRO-MAGNETO-ELASTIC MEDIUM

A theoretical treatment of the scattering of anti-plane shear(SH) waves is provided by a single crack in an unbounded transversely isotropic electro-magneto-elastic medium. Based on the differential equations of equilibrium, electric displacement and magnetic induction intensity differential equations, the governing equations for SH waves were obtained. By means of a linear transform, the governing equations were reduced to one Helmholtz and two Laplace equations. The Cauchy singular integral equations were gained by making use of Fourier transform and adopting electro-magneto impermeable boundary conditions. The closed form expression for the resulting stress intensity factor at the crack was achieved by solving the appropriate singular integral equations using Chebyshev polynomial. Typical examples are provided to show the loading frequency upon the local stress fields around the crack tips. The study reveals the importance of the electro-magneto-mechanical coupling terms upon the resulting dynamic stress intensity factor.

STUDY ON INFLUENCE OF BENDING-TORSION COUPLING IN AN IMPACTING-RUB ROTOR SYSTEM

A mathematical model of an impacting-rub rotor system with bending-torsion coupling was established. It was compared with the model without bending-torsion coupling through the modem nonlinear dynamic theory . It is observed that periodical, chaotic, period adding phenomena in them and the two models have a similar bifurcation process in their bifurcation figures . But the influence of bending-torsion on the dynmaic characteristics of the system is not neglected. The results have considerable meanings to analyze and improve the characteristics of an impacting- rub rotor system .

Low strain rate compressive behavior of high porosity closed-cell aluminum foams

The impact of a rigid body(protected structure) together with cushion material(cellular metal foam) on hard ground from a fixed height was investigated.An analytical one-degree-of-freedom colliding model(ODF-CM) was established to analyze the protection ability and energy absorption by the foam under low velocity impact conditions.For validation,drop hammer experiments were carried out for high porosity closed-cell aluminum foam specimens subjected to low velocity impact loading.The dynamic deformation behavior of the specimen was observed and the velocity attenuation of the drop hammer was measured.The results demonstrated that the aluminum foam had excellent energy absorption capabilities,with its dynamic compressive behavior similar to that obtained under quasi-static loading conditions.Finite element method(FEM) was subsequently employed to obtain stress distributions in the foam specimen.As the propagating period of stress in the specimen was far less than the duration of attenuation,the evolution of the stress was similar to that under quasi-static loading conditions and no obvious stress wave effect was observed,which agreed with the experimental observation.Finally,the predicted velocity attenuation by the ODF-CM was compared with both the experimental measurements and FEM simulation,and good agreements were achieved when the stress distribution was considered to be uniform and the "quasi-static" compressive properties are employed.