Ankit Srivastava - Academia.edu (original) (raw)

Papers by Ankit Srivastava

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Stress dependence of guided waves in rails

This paper presents numerical results on the dynamic behavior of continuously welded rails (CWR) ... more This paper presents numerical results on the dynamic behavior of continuously welded rails (CWR) subjected to a static axial stress. The results quantify the sensitivity of guided waves to stress variations and could be potentially used to estimate the stress level in CWR or alternatively the rail Neutral Temperature (stress free rail temperature). This work represents the initial concept phase of a research and development study funded by the Federal Railroad Administration. The ultimate objective of this study is to develop and test a prototype system that uses non-contact dynamic sensing to measure in-situ rail stress in motion, to determine rail Neutral Temperatures (NT) and the related Incipient Buckling Risks in CWR.

Research paper thumbnail of Nonlinear ultrasonic guided waves for stress monitoring in prestressing tendons for post-tensioned concrete structures

Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete ... more Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete structures. Prestressing tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress in order to detect dangerous stress losses. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper presents a technique based on nonlinear ultrasonic guided waves in the 100 kHz - 2 MHz range for monitoring prestress levels in 7-wire PS tendons. The technique relies on the fact that an axial stress on the tendon generates a proportional radial stress between adjacent wires (interwire stress). In turn, the interwire stress modulates nonlinear effects in ultrasonic wave propagation through both the presence of finite strains and the interwire contact. The nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation at (ω). Experimental results will be presented to identify (a) ranges of fundamental excitations at (ω) producing maximum nonlinear response, and (b) optimum lay-out of the transmitting and the receiving transducers within the test tendons. Compared to alternative methods based on linear ultrasonic features, the proposed nonlinear ultrasonic technique appears more sensitive to prestress levels and more robust against changing excitation power at the transmitting transducer or changing transducer/tendon bond conditions.

Research paper thumbnail of On the existence of longitudinal or flexural waves in rods at nonlinear higher harmonics

Journal of Sound and Vibration, 2010

This article theoretically studies the conditions for existence of longitudinal or flexural waves... more This article theoretically studies the conditions for existence of longitudinal or flexural waves in nonlinear, isotropic rods and presents numerical simulations corroborating the theoretical results. It has been known that the existence of guided waves at nonlinearity induced double harmonics is subject to constraints which arise from the potential of power flux transfer from the primary generating mode to the generated higher order modes. The knowledge about the behavior of waves in rods at harmonics higher than double is still limited. This gap was addressed here by the method of perturbation coupled with wavemode orthogonality and forced response. This reduces the nonlinear problem to a forced linear problem which was subsequently investigated to formulate an angular order-based constraint as the condition of existence/nonexistence of nonlinearity-driven higher harmonics of longitudinal and flexural waves in rods.

Research paper thumbnail of Structural health monitoring by extraction of coherent guided waves from diffuse fields

Journal of The Acoustical Society of America, 2008

Recent theoretical and experimental studies in a wide range of applications have demonstrated tha... more Recent theoretical and experimental studies in a wide range of applications have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. This letter demonstrates the passive-only reconstruction of coherent Lamb waves ͑dc-500 kHz͒ in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. It is further shown that the passively reconstructed waves are sensitive to the presence of damage in the plate as it would be expected in a typical "active" guided wave test. This proof-of-principle study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

Research paper thumbnail of Passive-only wave-based structural health monitoring from ambient noise

It has recently been demonstrated theoretically and experimentally that Green's functions (impuls... more It has recently been demonstrated theoretically and experimentally that Green's functions (impulse responses) can be estimated from coherent processing of random vibrations using only passive sensors studies in various applications (ultrasonics, acoustic, seismic...). This article investigates the passive-only estimation of coherent guided waves waves (DC-500 kHz) in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. Furthermore these passively reconstructed waveforms can also be used for damage detection in the plate similarly to conventional active testing. Based on this study, passive structural health monitoring techniques for aircraft panels can be developed using random vibrations.

Research paper thumbnail of Higher harmonic generation in nonlinear waveguides of arbitrary cross-section

Journal of The Acoustical Society of America, 2010

This article concerns the generation and properties of double harmonics in nonlinear isotropic wa... more This article concerns the generation and properties of double harmonics in nonlinear isotropic waveguides of complex cross-section. Analytical solutions of nonlinear Rayleigh-Lamb waves and rod waves have been known for some time. These solutions explain the phenomenon of cumulative double harmonic generation of guided waves. These solutions, however, are only applicable to simple geometries. This paper combines the general approach of the analytical solutions with semi-analytical finite element models to generalize the method to more complex geometries, specifically waveguides with arbitrary cross-sections. Supporting comparisons with analytical solutions are presented for simple cases. This is followed by the study of the case of a rail track. One reason for studying nonlinear guided waves in rails is the potential measurement of thermal stresses in welded rail.

Research paper thumbnail of On the existence of antisymmetric or symmetric Lamb waves at nonlinear higher harmonics

Journal of Sound and Vibration, 2009

This article theoretically studies the symmetry characteristics of Rayleigh–Lamb guided waves in ... more This article theoretically studies the symmetry characteristics of Rayleigh–Lamb guided waves in nonlinear, isotropic plates. It has been known that the nonlinearity driven double harmonic in Lamb waves does not support antisymmetric motion. However, the proof of this has not ...

Research paper thumbnail of Wave propagation models for quantitative defect detection by ultrasonic methods

Ultrasonic guided wave testing necessitates of quantitative, rather than qualitative, information... more Ultrasonic guided wave testing necessitates of quantitative, rather than qualitative, information on flaw size, shape and position. This quantitative diagnosis ability can be used to provide meaningful data to a prognosis algorithm for remaining life prediction, or simply to generate data sets for a statistical defect classification algorithm. Quantitative diagnostics needs models able to represent the interaction of guided waves with various defect scenarios. One such model is the Global-Local (GL) method, which uses a full finite element discretization of the region around a flaw to properly represent wave diffraction, and a suitable set of wave functions to simulate regions away from the flaw. Displacement and stress continuity conditions are imposed at the boundary between the global and the local regions. In this paper the GL method is expanded to take advantage of the Semi-Analytical Finite Element (SAFE) method in the global portion of the waveguide. The SAFE method is efficient because it only requires the discretization of the cross-section of the waveguide to obtain the wave dispersion solutions and it can handle complex structures such as multilayered sandwich panels. The GL method is applied to predicting quantitatively the interaction of guided waves with defects in aluminum and composites structural components.

Research paper thumbnail of Quantitative Structural Health Monitoring by Ultrasonic Guided Waves

Journal of Engineering Mechanics-asce, 2010

Francesco Lanza di Scalea is a Professor in the Department of Structural Engineering of the Unive... more Francesco Lanza di Scalea is a Professor in the Department of Structural Engineering of the University of California, San Diego (UCSD). He obtained his Laurea (summa cum laude) and doctorate degrees in mechanical engineering from the University of Palermo, Italy. He ...

Research paper thumbnail of Overall dynamic constitutive relations for layered elastic composites

A method for homogenization of an elastic composite with periodic microstructure is presented, fo... more A method for homogenization of an elastic composite with periodic microstructure is presented, focusing on the Floquet-type elastic waves. The resulting homogenized frequency-dependent elasticity and mass-density then automatically satisfy the overall conservation laws and by necessity produce the exact dispersion relations. The method is used to calculate the dynamic effective parameters for a layered composite by using the exact solution.

Research paper thumbnail of Overall Dynamic Properties of 3-D periodic elastic composites

A method for the homogenization of 3-D periodic elastic composites is presented. It allows for th... more A method for the homogenization of 3-D periodic elastic composites is presented. It allows for the evaluation of the averaged overall frequency dependent dynamic material constitutive tensors relating the averaged dynamic ?eld variable tensors of velocity, strain, stress, and linear momentum. The formulation is based on micromechanical modeling of a representative unit cell of a composite proposed by Nemat-Nasser & Hori (1993), Nemat-Nasser et. al. (1982) and Mura (1987) and is the 3-D generalization of the 1-D elastodynamic homogenization scheme presented by Nemat-Nasser & Srivastava (2011). We show that for 3-D periodic composites the overall compliance (stiffness) tensor is hermitian, irrespective of whether the corresponding unit cell is geometrically or materially symmetric.Overall mass density is shown to be a tensor and, like the overall compliance tensor, always hermitian. The average strain and linear momentum tensors are, however, coupled and the coupling tensors are shown to be each others' hermitian transpose. Finally we present a numerical example of a 3-D periodic composite composed of elastic cubes periodically distributed in an elastic matrix. The presented results corroborate the predictions of the theoretical treatment.

Research paper thumbnail of Universal theorems for total energy of the dynamics of linearly elastic heterogeneous solids

Mechanics of Materials, 2011

In this paper we consider a sample of a linearly elastic heterogeneous composite in elastodynamic... more In this paper we consider a sample of a linearly elastic heterogeneous composite in elastodynamic equilibrium and present universal theorems which provide lower bounds for the total elastic strain energy plus the kinetic energy, and the total complementary elastic energy plus the kinetic energy.

Research paper thumbnail of Overall dynamic constitutive relations of layered elastic composites

Journal of The Mechanics and Physics of Solids, 2011

A method for homogenization of a heterogeneous (finite or periodic) elastic composite is presente... more A method for homogenization of a heterogeneous (finite or periodic) elastic composite is presented. It allows direct, consistent, and accurate evaluation of the averaged overall frequency-dependent dynamic material constitutive relations. It is shown that when the spatial variation of the field variables is restricted by a Bloch-form (Floquet-form) periodicity, then these relations together with the overall conservation and kinematical equations accurately yield the displacement or stress modeshapes and, necessarily, the dispersion relations. It also gives as a matter of course point-wise solution of the elasto-dynamic field equations, to any desired degree of accuracy. The resulting overall dynamic constitutive relations however, are general and need not be restricted by the Bloch-form periodicity. The formulation is based on micro-mechanical modeling of a representative unit cell of the composite proposed by Nemat-Nasser and coworkers; see, e.g., [1] and [2].

Research paper thumbnail of Homogenization of periodic elastic composites and locally resonant sonic materials

Physical Review B, 2011

A method for homogenization of an elastic composite with periodic microstructure is presented, fo... more A method for homogenization of an elastic composite with periodic microstructure is presented, focusing on the Floquet-type elastic waves. The resulting homogenized frequency-dependent elasticity and mass density then automatically satisfy the overall conservation laws and by necessity produce the exact dispersion relations. It is also shown that the dispersion relations and the associated field quantities can be accurately calculated using a mixed variational approach, based on the microstructure of the associated unit cell. The method is used to calculate the dynamic effective parameters for a layered composite by using both the exact solution and the results of the mixed variational formulation. The exact and approximate results are shown to be in close agreement, which makes it possible to use the approximate method for the proposed type of homogenization in cases where an exact solution does not exist. The homogenized frequency-dependent effective parameters give rise to the concept of dynamic Ashby charts that can be used to illustrate the effect of the microstructural architecture on the dynamic properties of a composite. In particular, the charts vividly display how this effective stiffness and density vary with frequency and may attain negative values within certain frequency ranges which can be changed as desired using the microarchitecture while keeping the volume fraction of the unit cell’s constituents constant.

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr–2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on ...

Research paper thumbnail of Flow behaviour of autoclaved, 20% cold worked, Zr2.5Nb alloy pressure tube material in the temperature range of room temperature to 800 °C

Journal of Nuclear Materials, 2011

Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2... more Pressure tube material of Indian Heavy Water Reactors is 20% cold-worked and stress relieved Zr-2.5Nb alloy. Inherent variability in the process parameters during the fabrication stages of pressure tube and also along the length of component have their effect on micro-structural and texture properties of the material, which in turn affect its strength parameters (yield strength and ultimate tensile strength) and flow characteristics. Data of tensile tests carried out in the temperature range from room temperature to 800 °C using the samples taken out from a single pressure tube have been used to develop correlations for characterizing the strength parameters' variation as a function of axial location along length of the tube and the test temperature. Applicability of Ramberg-Osgood, Holloman and Voce's correlations for defining the post yield behaviour of the material has been investigated. Effect of strain rate change on the deformation behaviour has also been studied.

Research paper thumbnail of Stress dependence of guided waves in rails

This paper presents numerical results on the dynamic behavior of continuously welded rails (CWR) ... more This paper presents numerical results on the dynamic behavior of continuously welded rails (CWR) subjected to a static axial stress. The results quantify the sensitivity of guided waves to stress variations and could be potentially used to estimate the stress level in CWR or alternatively the rail Neutral Temperature (stress free rail temperature). This work represents the initial concept phase of a research and development study funded by the Federal Railroad Administration. The ultimate objective of this study is to develop and test a prototype system that uses non-contact dynamic sensing to measure in-situ rail stress in motion, to determine rail Neutral Temperatures (NT) and the related Incipient Buckling Risks in CWR.

Research paper thumbnail of Nonlinear ultrasonic guided waves for stress monitoring in prestressing tendons for post-tensioned concrete structures

Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete ... more Many bridges, including 90% of the California inventory, are post-tensioned box-girders concrete structures. Prestressing tendons are the main load-carrying components of these and other post-tensioned structures. Despite their criticality, much research is needed to develop and deploy techniques able to provide real-time information on the level of prestress in order to detect dangerous stress losses. In collaboration with Caltrans, UCSD is investigating the combination of ultrasonic guided waves and embedded sensors to provide both prestress level monitoring and defect detection capabilities in concrete-embedded PS tendons. This paper presents a technique based on nonlinear ultrasonic guided waves in the 100 kHz - 2 MHz range for monitoring prestress levels in 7-wire PS tendons. The technique relies on the fact that an axial stress on the tendon generates a proportional radial stress between adjacent wires (interwire stress). In turn, the interwire stress modulates nonlinear effects in ultrasonic wave propagation through both the presence of finite strains and the interwire contact. The nonlinear ultrasonic behavior of the tendon under changing levels of prestress is monitored by tracking higher-order harmonics at (nω) arising under a fundamental guided-wave excitation at (ω). Experimental results will be presented to identify (a) ranges of fundamental excitations at (ω) producing maximum nonlinear response, and (b) optimum lay-out of the transmitting and the receiving transducers within the test tendons. Compared to alternative methods based on linear ultrasonic features, the proposed nonlinear ultrasonic technique appears more sensitive to prestress levels and more robust against changing excitation power at the transmitting transducer or changing transducer/tendon bond conditions.

Research paper thumbnail of On the existence of longitudinal or flexural waves in rods at nonlinear higher harmonics

Journal of Sound and Vibration, 2010

This article theoretically studies the conditions for existence of longitudinal or flexural waves... more This article theoretically studies the conditions for existence of longitudinal or flexural waves in nonlinear, isotropic rods and presents numerical simulations corroborating the theoretical results. It has been known that the existence of guided waves at nonlinearity induced double harmonics is subject to constraints which arise from the potential of power flux transfer from the primary generating mode to the generated higher order modes. The knowledge about the behavior of waves in rods at harmonics higher than double is still limited. This gap was addressed here by the method of perturbation coupled with wavemode orthogonality and forced response. This reduces the nonlinear problem to a forced linear problem which was subsequently investigated to formulate an angular order-based constraint as the condition of existence/nonexistence of nonlinearity-driven higher harmonics of longitudinal and flexural waves in rods.

Research paper thumbnail of Structural health monitoring by extraction of coherent guided waves from diffuse fields

Journal of The Acoustical Society of America, 2008

Recent theoretical and experimental studies in a wide range of applications have demonstrated tha... more Recent theoretical and experimental studies in a wide range of applications have demonstrated that Green's functions (impulse responses) can be extracted from cross-correlation of diffuse fields using only passive sensors. This letter demonstrates the passive-only reconstruction of coherent Lamb waves ͑dc-500 kHz͒ in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. It is further shown that the passively reconstructed waves are sensitive to the presence of damage in the plate as it would be expected in a typical "active" guided wave test. This proof-of-principle study suggests the potential for a structural health monitoring method for aircraft panels based on passive ultrasound imaging reconstructed from diffuse fields.

Research paper thumbnail of Passive-only wave-based structural health monitoring from ambient noise

It has recently been demonstrated theoretically and experimentally that Green's functions (impuls... more It has recently been demonstrated theoretically and experimentally that Green's functions (impulse responses) can be estimated from coherent processing of random vibrations using only passive sensors studies in various applications (ultrasonics, acoustic, seismic...). This article investigates the passive-only estimation of coherent guided waves waves (DC-500 kHz) in an aluminum plate of thickness comparable to aircraft fuselage and wing panels. Furthermore these passively reconstructed waveforms can also be used for damage detection in the plate similarly to conventional active testing. Based on this study, passive structural health monitoring techniques for aircraft panels can be developed using random vibrations.

Research paper thumbnail of Higher harmonic generation in nonlinear waveguides of arbitrary cross-section

Journal of The Acoustical Society of America, 2010

This article concerns the generation and properties of double harmonics in nonlinear isotropic wa... more This article concerns the generation and properties of double harmonics in nonlinear isotropic waveguides of complex cross-section. Analytical solutions of nonlinear Rayleigh-Lamb waves and rod waves have been known for some time. These solutions explain the phenomenon of cumulative double harmonic generation of guided waves. These solutions, however, are only applicable to simple geometries. This paper combines the general approach of the analytical solutions with semi-analytical finite element models to generalize the method to more complex geometries, specifically waveguides with arbitrary cross-sections. Supporting comparisons with analytical solutions are presented for simple cases. This is followed by the study of the case of a rail track. One reason for studying nonlinear guided waves in rails is the potential measurement of thermal stresses in welded rail.

Research paper thumbnail of On the existence of antisymmetric or symmetric Lamb waves at nonlinear higher harmonics

Journal of Sound and Vibration, 2009

This article theoretically studies the symmetry characteristics of Rayleigh–Lamb guided waves in ... more This article theoretically studies the symmetry characteristics of Rayleigh–Lamb guided waves in nonlinear, isotropic plates. It has been known that the nonlinearity driven double harmonic in Lamb waves does not support antisymmetric motion. However, the proof of this has not ...

Research paper thumbnail of Wave propagation models for quantitative defect detection by ultrasonic methods

Ultrasonic guided wave testing necessitates of quantitative, rather than qualitative, information... more Ultrasonic guided wave testing necessitates of quantitative, rather than qualitative, information on flaw size, shape and position. This quantitative diagnosis ability can be used to provide meaningful data to a prognosis algorithm for remaining life prediction, or simply to generate data sets for a statistical defect classification algorithm. Quantitative diagnostics needs models able to represent the interaction of guided waves with various defect scenarios. One such model is the Global-Local (GL) method, which uses a full finite element discretization of the region around a flaw to properly represent wave diffraction, and a suitable set of wave functions to simulate regions away from the flaw. Displacement and stress continuity conditions are imposed at the boundary between the global and the local regions. In this paper the GL method is expanded to take advantage of the Semi-Analytical Finite Element (SAFE) method in the global portion of the waveguide. The SAFE method is efficient because it only requires the discretization of the cross-section of the waveguide to obtain the wave dispersion solutions and it can handle complex structures such as multilayered sandwich panels. The GL method is applied to predicting quantitatively the interaction of guided waves with defects in aluminum and composites structural components.

Research paper thumbnail of Quantitative Structural Health Monitoring by Ultrasonic Guided Waves

Journal of Engineering Mechanics-asce, 2010

Francesco Lanza di Scalea is a Professor in the Department of Structural Engineering of the Unive... more Francesco Lanza di Scalea is a Professor in the Department of Structural Engineering of the University of California, San Diego (UCSD). He obtained his Laurea (summa cum laude) and doctorate degrees in mechanical engineering from the University of Palermo, Italy. He ...

Research paper thumbnail of Overall dynamic constitutive relations for layered elastic composites

A method for homogenization of an elastic composite with periodic microstructure is presented, fo... more A method for homogenization of an elastic composite with periodic microstructure is presented, focusing on the Floquet-type elastic waves. The resulting homogenized frequency-dependent elasticity and mass-density then automatically satisfy the overall conservation laws and by necessity produce the exact dispersion relations. The method is used to calculate the dynamic effective parameters for a layered composite by using the exact solution.

Research paper thumbnail of Overall Dynamic Properties of 3-D periodic elastic composites

A method for the homogenization of 3-D periodic elastic composites is presented. It allows for th... more A method for the homogenization of 3-D periodic elastic composites is presented. It allows for the evaluation of the averaged overall frequency dependent dynamic material constitutive tensors relating the averaged dynamic ?eld variable tensors of velocity, strain, stress, and linear momentum. The formulation is based on micromechanical modeling of a representative unit cell of a composite proposed by Nemat-Nasser & Hori (1993), Nemat-Nasser et. al. (1982) and Mura (1987) and is the 3-D generalization of the 1-D elastodynamic homogenization scheme presented by Nemat-Nasser & Srivastava (2011). We show that for 3-D periodic composites the overall compliance (stiffness) tensor is hermitian, irrespective of whether the corresponding unit cell is geometrically or materially symmetric.Overall mass density is shown to be a tensor and, like the overall compliance tensor, always hermitian. The average strain and linear momentum tensors are, however, coupled and the coupling tensors are shown to be each others' hermitian transpose. Finally we present a numerical example of a 3-D periodic composite composed of elastic cubes periodically distributed in an elastic matrix. The presented results corroborate the predictions of the theoretical treatment.

Research paper thumbnail of Universal theorems for total energy of the dynamics of linearly elastic heterogeneous solids

Mechanics of Materials, 2011

In this paper we consider a sample of a linearly elastic heterogeneous composite in elastodynamic... more In this paper we consider a sample of a linearly elastic heterogeneous composite in elastodynamic equilibrium and present universal theorems which provide lower bounds for the total elastic strain energy plus the kinetic energy, and the total complementary elastic energy plus the kinetic energy.

Research paper thumbnail of Overall dynamic constitutive relations of layered elastic composites

Journal of The Mechanics and Physics of Solids, 2011

A method for homogenization of a heterogeneous (finite or periodic) elastic composite is presente... more A method for homogenization of a heterogeneous (finite or periodic) elastic composite is presented. It allows direct, consistent, and accurate evaluation of the averaged overall frequency-dependent dynamic material constitutive relations. It is shown that when the spatial variation of the field variables is restricted by a Bloch-form (Floquet-form) periodicity, then these relations together with the overall conservation and kinematical equations accurately yield the displacement or stress modeshapes and, necessarily, the dispersion relations. It also gives as a matter of course point-wise solution of the elasto-dynamic field equations, to any desired degree of accuracy. The resulting overall dynamic constitutive relations however, are general and need not be restricted by the Bloch-form periodicity. The formulation is based on micro-mechanical modeling of a representative unit cell of the composite proposed by Nemat-Nasser and coworkers; see, e.g., [1] and [2].

Research paper thumbnail of Homogenization of periodic elastic composites and locally resonant sonic materials

Physical Review B, 2011

A method for homogenization of an elastic composite with periodic microstructure is presented, fo... more A method for homogenization of an elastic composite with periodic microstructure is presented, focusing on the Floquet-type elastic waves. The resulting homogenized frequency-dependent elasticity and mass density then automatically satisfy the overall conservation laws and by necessity produce the exact dispersion relations. It is also shown that the dispersion relations and the associated field quantities can be accurately calculated using a mixed variational approach, based on the microstructure of the associated unit cell. The method is used to calculate the dynamic effective parameters for a layered composite by using both the exact solution and the results of the mixed variational formulation. The exact and approximate results are shown to be in close agreement, which makes it possible to use the approximate method for the proposed type of homogenization in cases where an exact solution does not exist. The homogenized frequency-dependent effective parameters give rise to the concept of dynamic Ashby charts that can be used to illustrate the effect of the microstructural architecture on the dynamic properties of a composite. In particular, the charts vividly display how this effective stiffness and density vary with frequency and may attain negative values within certain frequency ranges which can be changed as desired using the microarchitecture while keeping the volume fraction of the unit cell’s constituents constant.