An octagonal photonic crystal fiber (O-PCF) structure with eight air-holes on the first ring is proposed based on a unit
isosceles triangle. The mode effective index and chromatic dispersion of the O-PCF are numerically investigated by
employing the finite element method. It is found that under the same value of air filling fraction (AFF), the octagonal
photonic crystal fiber has smaller mode effective index and higher amplitude of dispersion. On the other hand, the
dispersion of O-PCF can be easily controlled through adjusting appropriately the air-hole pitches and air-hole diameters.
This paper utilized the Finite Element Method to investigate the transient scattering of Rayleigh wave by a surface crack in a plate. The incident wave models the guided waves generated by a pulsed line source laser irradiation on the top surface of the plate. The pulsed laser is assumed to be transient heat source, and the surface acoustic wave is calculated based on the thermoelastic theory. We have computed the different results of the Al plates with the varied depth surface-breaking crack, then attained the temporal characteristics of reflected waves and transmitted waves which are generated by the initial surface acoustic waves interacted with the surface breaking cracks with different depth. The artificial neural networks (ANN) are applied to establish the mapping relationship between the characteristic of the reflected waveform and the crack depth. The results of crack damage detection for Al plates show that the method developed in this paper can be applied to online structural damage detection and health monitoring for various industrial structures.
Non-contact and nondestructive monitoring of subsurface defects are urgently required especially in the line production of such products. In this paper a two-dimensional plane stress finite element model with absorbing boundary condition has been developed to investigate the ultrasonic wave generated by nanosecond pulsed laser propagation in two-layer material. The pulsed laser is assumed to be transient heat source, and the Surface acoustic wave is computed based on the thermoelastic theory, which propagated on the top surface of the plate. The defects located on the interface can be modeled as the subsurface slits of the Al/Cu layers. The numerical results include the three cases which the subsurface slit located in different positions of the interface of the Al/Cu layers. After performing Wigner-Ville analysis on the displacement data, frequency domain feature analysis is done. This study of the SAW was demonstrated to be promising in evaluating the bond quality as well as identifying the location of subsurface lateral defects.