Showing papers by "Zheng Fan published in 2016"

Guided Wave Tomography Based on Full Waveform Inversion

Abstract: In this paper, a guided wave tomography method based on full waveform inversion (FWI) is developed for accurate and high-resolution reconstruction of the remaining wall thickness in isotropic plates. The forward model is computed in the frequency domain by solving a full-wave equation in a two-dimensional (2-D) acoustic model, accounting for higher order effects such as diffractions and multiple scattering. Both numerical simulations and experiments were carried out to obtain the signals of a dispersive guided mode propagating through defects. The inversion was based on local optimization of a waveform misfit function between modeled and measured data, and was applied iteratively to discrete frequency components from low to high frequencies. The resulting wave velocity maps were then converted to thickness maps by the dispersion characteristics of selected guided modes. The results suggest that the FWI method is capable to reconstruct the thickness map of a irregularly shaped defect accurately on a 10-mm-thick plate with the thickness error within 0.5 mm.

Numerical and experimental investigation of nonlinear ultrasonic Lamb waves at low frequency

Abstract: Nonlinear ultrasonic Lamb waves are popular to characterize the nonlinearity of materials. However, the widely used nonlinear Lamb mode suffers from two associated complications: inherent dispersive and multimode natures. To overcome these, the symmetric Lamb mode (S0) at low frequency region is explored. At the low frequency region, the S0 mode is little dispersive and easy to generate. However, the secondary mode still exists, and increases linearly for significant distance. Numerical simulations and experiments are used to validate the nonlinear features and therefore demonstrate an easy alternative for nonlinear Lamb wave applications.

Limited-view ultrasonic guided wave tomography using an adaptive regularization method

Abstract: Ultrasonic guided waves are useful to assess the integrity of a structure from a remote location. Recently, tomography techniques have been developed to quantitatively estimate the thickness map of plate-like structures based on the dispersion characteristics of guided waves. In many applications only limited locations are available to place transducers. The missing viewing angles lead to artifacts which can degrade the image quality. To address this problem, this paper applies the regularization method to synthesize the missing components. The regularization technique is performed by an adaptive threshold approach to the limited view reconstruction. The effectiveness of this method combined with the full waveform inversion method is demonstrated by using numerical simulations as well as experiments on an irregularly shaped defect and two flat-bottom defects. The results indicate that the additional components obtained from the regularization method can significantly reduce the artifacts, leading to bette...

Numerical studies of nonlinear ultrasonic guided waves in uniform waveguides with arbitrary cross sections

Abstract: Nonlinear guided waves have been investigated widely in simple geometries, such as plates, pipe and shells, where analytical solutions have been developed. This paper extends the application of nonlinear guided waves to waveguides with arbitrary cross sections. The criteria for the existence of nonlinear guided waves were summarized based on the finite deformation theory and nonlinear material properties. Numerical models were developed for the analysis of nonlinear guided waves in complex geometries, including nonlinear Semi-Analytical Finite Element (SAFE) method to identify internal resonant modes in complex waveguides, and Finite Element (FE) models to simulate the nonlinear wave propagation at resonant frequencies. Two examples, an aluminum plate and a steel rectangular bar, were studied using the proposed numerical model, demonstrating the existence of nonlinear guided waves in such structures and the energy transfer from primary to secondary modes.

Dispersion loci of guided waves in viscoelastic composites of general anisotropy

Abstract: Guided waves play an important role in many applications of NDE to structures of flat and cylindrical geometry. In order to develop and optimise the inspection, it is essential to have a good understanding of the wave modes that can propagate in the target structure. These can be complicated, especially in structures composed of multiple layers, anisotropic properties or materials that exhibit damping that absorbs the energy of the waves. Dispersion curves in anisotropic viscoelastic media are presented here. They have been computed by using a pseudospectral collo-cation method, details of its implementation are briefly outlined and references to the relevant literature given.

Residual Stress Profiling of Sub-surface Treated Nickel-Based Superalloy using Electromagnetic NDE Method

Abstract: To evaluate the whole residual stress profile of sub-surface processes, existing methods such as X-ray diffraction (XRD) or central hole drilling method (CHD) require material removal, which is inherently destructive. A non-destructive residual stress measurement method using eddy current is utilized to capture the stress depth profile. The technique is demonstrated on nickel-based superalloy Inconel 100 (IN100) treated using deep cold rolling and shot peening to show that the method developed for stress profiling can be independent of the subsurface treatment. This paper presents the experimental results obtained with an experimental set-up based on the commercially available Nortec eddy current instrument and pencil surface probes. Test samples prepared from IN100 by deep cold rolling and shot peening for two different treatment conditions will be presented. The calculated residual stress profile using eddy current (EC) is compared with the equivalent residual stress profile using CHD for penetration depths from (100-550)μm to validate the measurement technique.

Feature guided waves (FGW) in fiber reinforced composite plates with 90° transverse bends

Abstract: Fiber reinforced composite materials have been increasingly used in high performance structures such as aircraft and large wind turbine blades. 90◦ composite bends are common in reinforcing structural elements, which are prone to defects such as delamination, crack, fatigue, etc. Current techniques are based on local inspection which makes the whole bend area scanning time consuming and tedious. This paper explores the feasibility of using feature guided waves (FGW) for rapid screening of 90◦ composite laminated bends. In this study, the behavior of the bend-guided wave in the anisotropic composite material is investigated through modal studies by applying the Semi-Analytical Finite Element (SAFE) method, also 3D Finite Element (FE) simulations are performed to visualize the results and to obtain cross validation. To understand the influence of the anisotropy, three-dimensional dispersion surfaces of the guided modes in flat laminated plates are obtained, showing the dependence of the phase velocity with ...