Showing papers by "Zheng Fan published in 2018"

Deep-subwavelength control of acoustic waves in an ultra-compact metasurface lens.

Abstract: Space-coiling acoustic metasurfaces have been largely exploited and shown their outstanding wave manipulation capacity. However, they are complex in realization and cannot directly manipulate acoustic near-fields by controlling the effective path length. Here, we propose a comprehensive paradigm for acoustic metasurfaces to extend the wave manipulations to both far- and near-fields and markedly reduce the implementation complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate. A semi-analytical approach for such a design is established using a microscopic coupled-wave model, which reveals that the acoustic diffractive pattern at every slit exit is the sum of the initial transmission and the secondary scatterings of the coupled fields from other slits. For proof-of-concept, we examine two metasurface lenses for sound focusing within and beyond the diffraction limit. This work provides a feasible strategy for creating ultra-compact acoustic components with versatile potentials. Here, the authors propose an acoustic metasurface design to extend the wave manipulations to both far- and near-fields while reducing the complexity with a simple structure, which consists of an array of deep-subwavelength-spaced slits perforated in a thin plate.

Effect of deep cold rolling on mechanical properties and microstructure of nickel-based superalloys

Abstract: Mechanical surface treatments are performed on aerospace components for fatigue life enhancement by introducing beneficial compressive stress profiles and material strengthening. This paper studies the influence of Deep Cold Rolling (DCR) on the residual stress distribution, work hardening and the microstructure modification of two nickel-based superalloys, IN100 and RR1000. Two different diameters (6.3 mm and 12.6 mm) of the rolling ball inserts were investigated in this analysis. The hardness and residual stresses at the subsurface after DCR were analyzed along the rolling and transverse directions. Electron BackScatter Diffraction (EBSD) technique was used to characterize the microstructure of the samples both qualitatively and quantitatively. The degree of work hardening of fine grain RR1000 after DCR was characterized using full width at half-maximum (FWHM) of the X-ray diffraction peaks and Grain Orientation Spread (GOS) profiles acquired by EBSD characterization. The results clearly indicate that deep cold rolling introduces compressive residual stresses as deep as 1 mm, with significant work hardening at the subsurface in the coarse grain IN100. DCR resulted in the grain refinement, increase in low angle grain boundaries and clustering of dislocation density around the carbides in IN100. Depending on the ball diameter, DCR of RR1000 induced compressive residual stresses up to 700 µm and work hardening till the depths of 400 µm. Additionally, severe deformation of grains occurred near the rolling surface. The larger diameter of the rolling ball resulted in high degree of work hardening and better residual stress distribution deeper into the materials.

Quantitative imaging of Young's modulus in plates using guided wave tomography

Abstract: An ultrasonic tomographic imaging technique is proposed for measurement of the distribution of Young's modulus in an isotropic plate. This technique is based on velocity mapping which is performed by applying full waveform inversion on the ultrasonic signals captured by transducers around the stiffness variations. The resulting wave velocity maps are then converted to Young's modulus maps by the known dispersion relation of selected guided modes. Finite element simulations were carried out to investigate the reconstruction performance of A0 mode propagating through various smoothly varying stiffness defects in a plate. It is shown that for selected cases an average through-thickness Young's modulus can be accurately reconstructed and its potential to describe flexural stiffness of the plate is discussed. The model was validated by experiments, where Young's modulus was varied in a steel plate via heating. The map of the Young's modulus was reconstructed from temperature measurements and ultrasound data and results from the two methods showed excellent agreement.

Investigation of nonlinear ultrasonic guided waves in open waveguides based on perfectly matched layers.

Abstract: Nonlinear ultrasonic guided waves have been investigated widely in closed waveguides such as plates, pipes, etc. However, the description of nonlinear ultrasonic guided waves remains challenging for open waveguides, as energy may leak into the surrounding medium. In this work, the properties of nonlinear ultrasonic guided waves in open waveguides are investigated. Mathematical framework is first established based on real reciprocity relation and modal expansion with perfectly matched layers. Numerical models are then implemented, including nonlinear semi-analytical finite element (SAFE) method to predict the properties of nonlinear ultrasonic guided waves, and time domain finite element models to simulate the nonlinear guided wave propagation and cross validate the predictions from the nonlinear SAFE method. Two examples, an aluminum plate attached to an elastomer and an aluminum plate with water loaded on one side, are studied to demonstrate the proposed methods and reveal some interesting phenomena that only exist in open waveguides. It is interesting to find out that the amplitude of the attenuated second harmonic wave in immersed waveguides can keep constant with propagation distance, only if the primary wave is non-leaky, which may bring potential non-destructive test applications for underwater inspections. Such a feature is validated by well-designed experiments in one-sidedly immersed plates.