Peridynamics is a non-local theory proposed for the effective handling of discontinuities such as propagation, branching, and coalescing of cracks. Collective information of the displacements of peridynamic particles is used to calculate the deformation gradient, and the equation of the motion is naturally integrodifferential. Using the integrodifferential equations, peridynamics can describe the discontinuities without additional criteria, while the classical continuum mechanics, finite element method (FEM), requires the conditions to predict the discontinuities. However, the computational efficiency of peridynamics is expensive compared to FEM because peridynamics calculates stress fields using the collective information of the displacements of the neighboring particles within the certain distance of the target particle, while FEM limits the interaction to adjacent nodes in the element. Therefore, coupling peridynamics and FEM provides both advantages of peridynamics in solving the discontinuities without additional criteria as well as the high computational efficiency of FEM.
We develop bimaterial composites with enhanced impact resistance by mimicking a unique hierarchical geometry inherent in nacre. Three-dimensional models of the nacre-like composites are developed using pattern-generating algorithms, and the corresponding experiment specimens are fabricated by means of an FDM-based 3D printer. Under drop weight impact tests, it is found that the impact resistance of the nacre-like composite is significantly improved compared with a monolithic stiff specimen. The performance enhancement is also verified through numerical simulation with the use of a commercial finite element code. Mimicking the natural hierarchical architecture can render a guideline toward the development of high-performance material systems.
We investigate the nonlinear wave propagation through micro-cracks that are compressed by external forces by means of nonlinear ultrasonic modulation technique. The nonlinear modulated waves are generated by the truncation of the waves passing through cracks due to the opening and closing of the cracks, and the nonlinear ultrasonic modulation technique has been known to be effective in detecting finer cracks in comparison with other linear ultrasonic methods since the technique utilizes the breathing of the cracks rather than wave reflections or refractions. However, if the cracks are strongly compressed, the crack opening is hindered due to the excessive initial stress and the nonlinearity does not show up.
In this study, the improvement of the nonlinear modulation wave technique for the detection of micro-cracks under compression is devised. By analyzing photomicrographs of the cracks with crack width measuring algorithm, a realistic crack model is generated, and a chirp signal is applied to find the resonant frequencies which are used as the excitation frequencies. Experimental tests are conducted to verify the numerical results. The aluminum plate is compressed in the direction normal to the cracks’ lateral surfaces and is excited using piezoelectric patches attached on the surface aluminum plate. The experimental and numerical results show good agreement for various excitation frequencies and different compressions.
We investigate the nonlinear wave caused by interaction of surfaces of a fatigue crack, and study the effect of the crack’s contact compression on the magnitudes of nonlinear waves. Nonlinear wave modulation is generated when two ultrasonic waves having different frequencies passing through a crack, and the so-called nonlinear ultrasonic wave modulation technique is developed using this nonlinear waves. However, the magnitude of the nonlinear wave decreases as the crack contact compression increases because the large compression prevents the cracks from opening in motion. Even if the nonlinear wave modulation occurs in the damaged structures under compression, the magnitude might be different with the magnitude without compression. Consequently, finding the range of contact compression and the excitation directions with which the nonlinear wave modulation might occur is essential to use the technique for structural components under constraint compression. In order to examine the relations between the constraint compression and nonlinear wave modulation, we conduct numerical simulations under various compression by changing the excitation directions. The numerical model consists of a thin aluminum plate with a fatigue crack under constant compression, and the crack surfaces are modeled mimicking the shapes of real cracks. The roughness of the crack is determined using the crack widths obtained from optical measurement of fatigue cracks. Effective range of contact compression to generate nonlinear wave and the requirements to make the magnitudes of nonlinear waves non-trivial are described.
We present a technique for microcrack modeling in the finite element framework, and numerically investigate the occurrence of nonlinear wave modulation. Typically, fatigue cracks are initiated and developed when structures are exposed to repeated loading; the crack widths of the fatigue cracks are extremely small in the early development stage. As the fatigue cracks grow by combining and coalescing, the overall size increases. Enlarged cracks undermine the safety of the structure. Therefore, fatigue crack detection is very important to ensure the integrity of structures. Although the nonlinear ultrasonic wave modulation technique has been widely used due to its high detecting sensitivity, the basic principle is not fully understood. To reveal the mechanism of nonlinear wave modulation, the movements of the crack surfaces are calculated through numerical simulation. The shape of the crack surface can determine the intensity of the wave modulation. In this study, we investigate the variation of the crack widths due to fatigue failure using microscopic imaging of real fatigue cracks, and use these images to create realistic models of the fatigue cracks.
This study presents nonlinear ultrasonic wave modulations that can be effectively used for crack detection in thin structural components. Fatigue cracks occur when structure is exposed to repeated load although the load causes the smaller stress than the yield stress. The existence of cracks deteriorates the integrity of structures and reduces the safety. To detect these cracks, several kinds of nonlinear ultrasonic wave modulation techniques have been proposed for many years. However, the fundamental reason of the nonlinearity has not been well explained theoretically yet. Mostly, the phenomenon has been investigated experimentally. In order to find the reasons of the observed modulation, numerical studies are performed considering a variety of sizes of crack widths and depths using a commercial FEA program.
Acoustic radiation force is a physical phenomenon caused by propagation of ultrasound in an attenuating medium. When
ultrasound propagates in the medium, the momentum of propagating ultrasound is transferred to the medium due to
absorption mechanism. As a result, acoustic radiation force is generated in the principal direction of waves. By focusing
the ultrasound at a specific location for a certain period, we can exert the acoustic radiation force at the location and
generate the source of the shear waves. Characteristics of the shear wave critically depend on the material properties.
Therefore, the shear wave propagation in the medium containing an inclusion shows differences compared to the wave in
the pure medium. We simulate acoustic radiation force and generate shear waves by using the finite element method. The
purpose of this study is to simulate the effect of the radiation force and to estimate the properties of the inclusion through
analyzing the change of the shear wave induced by the radiation force in the almost incompressible materials.
Ultrasonic guided waves have been widely utilized for the structural health monitoring (SHM) of structural components such as plates and pipes. In particular, the noncontact excitation of the pipe surfaces using laser pulses has shown several advantages in experiments by eliminating the bonding process of the dielectric patches on the curved surfaces and the complicated interpretation of the temperature effect on the bonding layers. However, the numerical simulation of the methodology requires thermo-mechanical coupling and large-scale computation. Therefore, the numerical efficiency of the spatial partitioning by deploying thermo-mechanical elements and mechanical elements is investigated. Then, the laser excitation on the surface is modeled in the form of heat flux, and the generated wave forms are observed. The formation and propagation of the guided waves are also represented numerically.
Thermoelastic guided waves, induced by a laser pulse on a material surface, demonstrate advantages for nondestructive
evaluation of structures due to the non-contact feature and effectiveness to generate broadband signals. Both symmetric and
asymmetric Lamb waves can be generated by the local thermal expansion from the laser energy absorption. We consider the
thermomechanical equation and solve the problems using the finite element method. The capability of the finite element
method for the modeling of guided wave propagation induced by laser pulses is demonstrated, and the computational
efficiency is improved significantly by partitioning the plate into thermomechanical and mechanical subregions. Numerical
results are compared with experimental observations of Lamb waves generated by unfocused and line-focused laser sources.
Using the wavelet transform, the group velocity of A0 mode is obtained from the detected signal and compared to the
solution of Rayleigh-Lamb equations. Lamb waves in a plate with defects of different lengths are examined.
Ultrasonic guided waves have been utilized for accurate diagnosis of structural integrities for thin structural components. In
this paper, wave propagation excited by surface-mounted instruments such as PZT and MFC transducer rings in an infinite
isotropic hollow cylinder is investigated. A mathematical model of the wave propagation system is studied both analytically
and numerically. A detailed derivation of the characteristic equation of the system is conducted, and the development of
waves is simulated using the Finite Element (FE) method. Compared with the analytical results, the accuracy of the
numerical modeling is verified and Lamb wave propagation in pipes with defects is studied as well.
Recent collapse of the I-35W bridge in Minneapolis on August 1, 2007 raised public concern enormously regarding
the structural safety of highway bridges. However, the maintenance, which is mostly performed by visual inspection,
is very expensive and labor intensive process. The structural health monitoring system has been developed
to monitor the integrity of those structures effectively. Although the two-dimensional numerical simulation has
been widely used for the last decade, the three-dimensional simulation is indispensable for the development of a
Lamb wave phased array system. We develop a virtual Lamb wave phased array considering three-dimensional
coupled numerical models of metal plates and PZT patch arrays using finite element methods. The optimized
geometry and operation techniques are investigated for the focusing of Lamb waves by Lead-Zirconate-titanate