Jung-Wuk Hong

Three-dimensional Lamb wave propagation excited by a phased piezoelectric array

A dielectric?mechanical coupled finite element formulation from partial differential equations is derived, and the formulation is implemented in the framework of FEAP (Taylor 2008 FEAP?A Finite Element Analysis Program, Version 8.2 User Manual University of California at Berkeley). For the enhancement of the calculation efficiency in the three-dimensional simulation, a partitioned solution scheme is utilized, and time integration is implemented explicitly for the mechanical part. A comparison of the numerical results in two-?and three-dimensional Lamb wave propagations is performed, and the formation of the Lamb waves and the tangential stress distribution imposed on the aluminum plate by a lead?zirconate?titanate (PZT) patch are investigated. A virtual Lamb wave phased array comprised of 16?PZT patches is developed by tuning the symmetric mode of the Lamb wave to be dominant. The 16?PZT patches are excited individually, aiming at a point, and the numerical result shows good agreement as regards the estimated times of arrival and the focused response at the target point.

Crack coalescence morphology in rock-like material under compression

This paper uses peridynamic simulations to determine the extent of coalescing damage and identify the underlying causes. The basic crack types and crack coalescence patterns in specimens with a flaw pair under uniaxial compression are systematically investigated. Various crack types including horsetail cracks, anti-wing cracks, and tensile wing cracks are successfully observed and the coalescence sequences are identified. By varying angles, six crack coalescence categories with respect to the overlapping ratios provide insightful information of different crack growths and indicate various cracking modes underlying various coalescence patterns. The arrangement of the flaw pair strongly influences the crack initiation position and trajectories, allowing for different coalescence morphologies. Coalescence formed by two internal tensile wing cracks, or transfixion, shows unbroken crack segments with a further loading, along with growing shear cracks until failure. In contrast, after the coalescence is formed through two horsetail cracks, the interior of the rhombic shape gets deformed with further loading. The peridynamic code adopted in this research can provide realistic simulation results and help researchers to conduct expanded tests as well as to enhance understanding the fracture of rock-like material.

Parallel programming of a peridynamics code coupled with finite element method

Using OpenMP (the Open Multi- Processing application programming interface), dynamic peridynamics code coupled with a finite element method is parallelized. The parallel implementation improves run-time efficiency and makes the realistic simulation of crack coalescence possible. To assess the accuracy and efficiency of the parallel code, we investigate its speedup and scalability. In addition, to validate the parallel code, experimental results for crack coalescence development sequences are compared. It is noted that this parallelized code markedly reduces computation time along with the coupling scheme. Moreover, the coupling approach used in this parallel code enables a more realistic and feasible numerical prediction of coalescing fractures. With the parallel implementation, two main types of crack coalescences between two flaws, formed by two short shear cracks and by a short central tensile segment and subsequent shear cracks are in detail discussed in terms of their development sequences. Consequently, this proposed coupled peridynamics code can be used to efficiently solve actual coalescence development sequences, thereby providing a numerical solution for fracture mechanics.

Longitudinal differences in the mechanical properties of the thoracic aorta depend on circumferential regions

Understanding the mechanical behavior of the arterial wall and its spatial variations is essential for the study of vascular physiopathology and the design of biomedical devices that interact with the arterial wall. Although it is generally accepted that the aortic wall gets stiffer along its length, the spatial variations in the mechanical behavior of the thoracic aorta are not well understood. In this study, therefore, we investigate both longitudinal and circumferential variations in the mechanical properties of the porcine descending thoracic aorta. Using a previously developed experimental method and stress-strain analysis, the stress, stretch, tangent modulus (TM), and pressure-strain elastic modulus (PSEM) are estimated in the range of in vivo pressure. The results show that the longitudinal differences of both TM and PSEM are statistically significant in the posterior region but not in the anterior region. Both moduli are greater in the posterior distal region when compared with the other test regions. The findings of this study meet a need for clarifying the region investigated, especially in circumferential region, to study the regional variations in biomechanics of blood vessels.

Modeling of three-dimensional Lamb wave propagation excited by laser pulses.

As a type of broadband source of ultrasonic guided waves, laser pulses can be used to launch all modes of interests. In this paper, Lamb waves are excited by imposing heat flux mimicking the supply of the heat from laser pulses, and effects by defects on the received Lamb waves in a plate are investigated by means of the finite element method. In order to alleviate the heavy computational cost in solving the coupled finite element equations, a sub-regioning scheme is employed, and it reduces the computational cost significantly. A comparison of Lamb waves generated by unfocused and line-focused laser sources is conducted. To validate numerical simulations, the group velocity of A0 mode is calculated based on the received signal by using the wavelet transform. The result of A0 mode group velocity is compared with the solution of Rayleigh-Lamb equations, and close agreement is observed. Lamb waves in a plate with defects of different lengths are examined next. The out-of-plane displacement in the plate with a defect is compared with the displacement in the plate without defects, and the wavelet transform is used to determine the arrival times of Lamb waves traveling at the A0 mode group velocity. A strong correlation is observed between the extent of defects and the magnitude of wavelet coefficients.

The effects of stacking sequence on the penetration-resistant behaviors of T800 carbon fiber composite plates under low-velocity impact loading

Impact damages induced by a low-velocity impact load on carbon fiber reinforced polymer (CFRP) composite plates fabricated with various stacking sequences were studied experimentally. The impact responses of the CFRP composite plates were significantly affected by the laminate stacking sequences. Three types of specimens, specifically quasi-isotropic, unidirectional, and cross-ply, were tested by a constant impact carrying the same impact energy level. An impact load of 3.44 kg, corresponding to 23.62 J, was applied to the center of each plate supported at the boundaries. The unidirectional composite plate showed the worst impact resistance and broke completely into two parts; this was followed by the quasi-isotropic lay-up plate that was perforated by the impact. The cross-ply composite plate exhibited the best resistance to the low-velocity impact load; in this case, the impactor bounced back. Impact parameters such as the peak impact force and absorbed energy were evaluated and compared for the impact resistant characterization of the composites made by different stacking sequences.

Characterization of wavelet coefficients for ultrasonic signals

A wavelet transform has been widely used to investigate the characteristics of wave signals for a decade. However, only qualitative investigation of the spectrogram was made rather than a quantitative interpretation. On the other hand, an analytical closed-form representation of the wavelet transformed wave signal can be used as a basis function in estimating parameters using nonlinear least-squares optimization. We derived a quantitative closed-form equation directly from the analytical continuous wavelet transformation of a pulse with a Gaussian spectrum. A fundamental three-dimensional shape of a wavelet in the spectrogram was obtained, and the analytical form was compared quantitatively with numerical results.

A crack model for the onset of blisters using finite surface thicknesses

In this paper, hydrogen-induced delamination of a bulk material with a finite thickness is investigated. Hydrogen implanted interface splitting is considered as the growth of the crack by forming blisters. The radius of a blister depends on the amount of the implanted hydrogen, crack surface energy, and annealing temperature. For a finite thickness of the superstrate, the evolution of the blisters is calculated adopting a smooth bell-shape function and applying the Rayleigh-Ritz method [K. K. Raju and E. V. Rao, J. Eng. Mech. 119, 626 (1993)]. The required minimum implanted gas Nmin is calculated accordingly. The calculated Nmin value is compared with an experimental result in literature.

On analytical transformations for efficiency improvements in the method of finite spheres

Publisher Summary Various meshless techniques— such as the smoothed particle hydrodynamics method, the diffuse element method, and the element free Galerkin method—have been developed to solve complex boundary-value problems because meshless techniques are attractive to circumvent mesh generation difficulties. For the general field of linear static analysis the meshless methods require complex numerical integrations resulting into relatively large solution times. This chapter introduces the method of finite spheres with the aim to reach a meshless method that is in terms of computational effort competitive with traditional finite-element methods. For the method of finite spheres to be an effective scheme, improvements in the numerical integration and an efficient auto sphere generation are essential. The chapter addresses these issues briefly and reviews the basic formulation of the method of finite spheres.

Parallelization of a finite element Fortran code using OpenMP library

A finite element analysis code FEAP is parallelized using the OpenMP library, and the performance is evaluated. The essential concepts of the OpenMP for the implementation of the parallelization are described and then our parallelization procedures are presented. The skeleton of FEAP is analyzed and the subroutines that need to be parallelized are identified. Then, the general procedures to declare the parallel variables are explained and synchronization, which is necessary for parallelization, is described. Using the parallelized FEAP code, several examples are solved, and the accuracies and the numerical efficiencies are investigated.