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Rapid Analysis Of Wood Using Transient Infrared Spectroscopy And Photoacoustic Spectroscopy With Pls Regression, Stanley J. Bajic, Roger W. Jones, John F. Mcclelland, Bonnie R. Hames, Robert R. Meglen Aug 1997

Rapid Analysis Of Wood Using Transient Infrared Spectroscopy And Photoacoustic Spectroscopy With Pls Regression, Stanley J. Bajic, Roger W. Jones, John F. Mcclelland, Bonnie R. Hames, Robert R. Meglen

Ames Laboratory Conference Papers, Posters, and Presentations

In the forest products industry, improved methods are needed for rapid analysis of wood and paper products. Currently, the best methods for determining chemical and physical properties of wood-based materials require considerable sample preparation and analysis time. Consequently, quantitative information is often not obtained on a time scale suitable for process monitoring, control, and quality assurance. The primary barriers to practical utilization of conventional infrared methods are the opaqueness and poor reflection properties of the wood-based materials. This paper demonstrates how photoacoustic and transient infrared spectroscopies have been combined with chemometric techniques to overcome the limitations of conventional infrared spectroscopies ...


Interaction Of Acoustic Beam With Elastic Structures , Han Zhang Jan 1997

Interaction Of Acoustic Beam With Elastic Structures , Han Zhang

Retrospective Theses and Dissertations

This thesis describes experiments and calculations performed on the interaction of acoustic beams in water and air with planar and cylindrical elastic structures. Ultrasonic reflection measurements have been used to elucidate the phenomena of guided wave generation and reradiation by selecting beam incidence at, or near, phase-matching conditions. Under these circumstances reasonant mode conversion of accoustic wave to guided wave mode energy can occur. This interaction has been studied in rubber-coated steel, aluminum, plexiglas, and graphite-epoxy composite. The acoustic coupling media used in these experiments has been either water or air;Some theoretical modeling has also been undertaken to explain ...


Sonochemical Reactions: Mass Transfer And Kinetic Studies Of A Solid-Liquid System , Leigh Christine Hagenson Jan 1997

Sonochemical Reactions: Mass Transfer And Kinetic Studies Of A Solid-Liquid System , Leigh Christine Hagenson

Retrospective Theses and Dissertations

Ultrasound has been shown to have desirable effects on both homogeneous and heterogeneous reactions, such as increasing the conversion, enhancing the selectivity, and improving the yield. Enhancements due to ultrasound may be attributed to chemical effects or mechanical effects, or to both simultaneously. The chemical effects of ultrasound are attributed to the implosion of microbubbles, generating free-radicals with a great propensity for reaction. Mechanical effects are caused by shock waves formed during symmetric cavitation, or by microjets formed when the bubble implodes asymmetrically. Research emphasis in this area attempts to discern the mechanisms behind ultrasound's mechanical effects by selecting ...


Ultrasonic Field Reconstruction From Optical Interferometric Measurements, C. Mattei, L. Adler Jan 1997

Ultrasonic Field Reconstruction From Optical Interferometric Measurements, C. Mattei, L. Adler

Review of Progress in Quantitative Nondestructive Evaluation

Acousto-optical measurement inside transparent media is a well known non invasive method for acoustic fields probing. The optical index variations induced by acoustic wave propagation in a liquid or transparent solid can be detected using several techniques: optical deflection, diffraction methods, etc ... Last year, X. Jia, G. Quentin and Laszlo Adler [1] described the ability of a interferometric detection technique to provide local and quantitative measurements of pressure fields in water as well as dilatational fields inside transparent solids. This method combines the acousto-optical effect with heterodyne interferometry. It measures the phase shift of a laser beam passing through an ...


Thermoelastic/Ablatic Laser Generated Ultrasound In Graphite/Polymer Composites Detected With A Cfp-Based System In Reflection Configuration, James N. Caron, Yuqiao Yang, James B. Mehl, Karl V. Steiner Jan 1997

Thermoelastic/Ablatic Laser Generated Ultrasound In Graphite/Polymer Composites Detected With A Cfp-Based System In Reflection Configuration, James N. Caron, Yuqiao Yang, James B. Mehl, Karl V. Steiner

Review of Progress in Quantitative Nondestructive Evaluation

A laser-based system was used to study thermoelastic (TE) and ablative (AB) ultrasonic generation mechanisms in graphite/polymer composites. When laser pulses of sufficiently low energy are incident on the sample, elastic waves are generated by the thermal expansion which accompanies absorption of the laser energy. At higher amplitudes, where the laser energy is high enough to melt and vaporize material in the sample, elastic waves are also generated by an ablative process. Thermoelastic and ablative ultrasound generation are described in the literature [1]. A review article by Srinivasan [2] includes excellent photographs of ablation of pure polymers.


A Model Of Temporal Intensity Modulation For Laser Generated Ultrasound, Terry Sanderson, Dr. Charles Ume, Dr. Jacek Jarzynski Jan 1997

A Model Of Temporal Intensity Modulation For Laser Generated Ultrasound, Terry Sanderson, Dr. Charles Ume, Dr. Jacek Jarzynski

Review of Progress in Quantitative Nondestructive Evaluation

Q-switched lasers are often used as a non-contact ultrasound source in non-destructive evaluation (NDE) of materials [1]. Q-switched lasers typically have ns pulse durations and generate broadband ultrasound waves, though longer laser pulses, of 100 microseconds or greater, have also been used [2] for NDE. These longer pulses tend to produce somewhat lower center frequencies than do Q-switched pulses, though they are still a broadband source. But it would be desirable in some NDE applications to narrow the signal bandwidth to improve the signal to noise ration (SNR), and also to have direct control over the center frequency of the ...


Progress Towards A Fiber-Based Laser-Ultrasonics System For Rapid Nde Of Large-Area Composites, A. D. W. Mckie, R. C. Addison Jr. Jan 1997

Progress Towards A Fiber-Based Laser-Ultrasonics System For Rapid Nde Of Large-Area Composites, A. D. W. Mckie, R. C. Addison Jr.

Review of Progress in Quantitative Nondestructive Evaluation

State-of-the-art integrally stiffened composite materials, manufactured for use in the next generation of commercial and military aircraft, are being increasingly used for structural components such as wings and fuselages. However, due to the complexity of the manufacturing process, small variations in the shape of integrally stiffened composite structures often occur. Thus, a prioriknowledge of the part shape often does not provide sufficient tolerance to allow an automated conventional ultrasonic inspection. Many of the advantages of laser-based ultrasonics, including its noncontacting nature and applicability to rapid scanning of contoured and integrally stiffened structures, have been described previously [1–5]. To further ...


Analysis And Correction Of The Source Parameter Effects For Optimizing The Laser Ultrasonic Mechanical Characterization, A. Hammoutene, F. Enguehard, L. Bertrand Jan 1997

Analysis And Correction Of The Source Parameter Effects For Optimizing The Laser Ultrasonic Mechanical Characterization, A. Hammoutene, F. Enguehard, L. Bertrand

Review of Progress in Quantitative Nondestructive Evaluation

The laser-ultrasonic technique is a promising tool in the field of the mechanical characterization of materials [1,2]. The use of a laser is associated to three temporal convolutions related to the source parameters (the spot size, the pulse duration and the wavelength of the excitation) [3,4]. For a given wavelength, the evaluation of the mechanical properties of low-damage threshold materials in the thermoelastic regime, requires a compromise between the space distribution and the pulse duration of the excitation. An enlargement of the spot or an increase of the pulse duration introduces delays on the arriving times of the ...


Robust Laser-Based Ultrasound Sensor Using Integrated Photo-Induced Emf Detection And Time-Delay Interferometer, G. J. Dunning, D. M. Pepper, M. P. Chiao, P. V. Mitchell, J. W. Wagner, F. M. Davidson Jan 1997

Robust Laser-Based Ultrasound Sensor Using Integrated Photo-Induced Emf Detection And Time-Delay Interferometer, G. J. Dunning, D. M. Pepper, M. P. Chiao, P. V. Mitchell, J. W. Wagner, F. M. Davidson

Review of Progress in Quantitative Nondestructive Evaluation

The important benefits of quantitative nondestructive evaluation can be realized by a manufacturer if the system is fully compatible with the manufacturing environment and cost effective. In many cases an in-line real-time process control would be desirable. There have been a variety of ultrasonic inspection techniques which have successfully met the aforementioned conditions. These techniques are able to capitalize on a large body of established ultrasonic methods and signal analysis. However, there are environments and processes which cannot use these conventional procedures because they require the part under inspection to be either in physical contact with the transducer, via an ...


Mathematical Modeling Of Laser Ablation In Liquids With Application To Laser Ultrasonics, R. J. Conant, S. E. Garwick Jan 1997

Mathematical Modeling Of Laser Ablation In Liquids With Application To Laser Ultrasonics, R. J. Conant, S. E. Garwick

Review of Progress in Quantitative Nondestructive Evaluation

The use of pulsed lasers to produce ultrasonic waves in materials has proven to be attractive in many applications. However, one of the limitations of laser ultrasonics is the weak signal strength produced by thermoelastic sources. One way to improve signal strength is to use laser intensities that are high enough to ablate the material surface. While ablation leads to surface damage in solids, it is generally not a problem in liquids. Consequently ablation is a viable means of enhancing signal strength for laser ultrasonics applications such as high temperature materials processing involving molten metals. Experiments carried out at the ...


Quantitative Acoustic Emission And The Fracture Of Concrete, Eric Landis, Surendra P. Shah Jan 1997

Quantitative Acoustic Emission And The Fracture Of Concrete, Eric Landis, Surendra P. Shah

Review of Progress in Quantitative Nondestructive Evaluation

The fracture behavior of concrete is often attributed to a fracture process zone. This fracture process zone manifests itself in the the form of nonlinear stress-strain behavior, post peak strain softening, size effect, and numerous toughening mechanisms. Features of the fracture process zone include arrays of microcracks, aggregate interlocking, crack bridging, and grain boundary sliding friction. From a material modeling standpoint, properties of the fracture process zone must be known in order to accurately predict the response of the material to stress. Since the fracture process zone characteristics are critical to material properties, a better understanding of those characteristics will ...


Wavelet Inverse Neutron Scattering Study Of Layered Metallic Nic-Ti Composites, G. Saab, B. Defacio, H. Kaiser, D. L. Worcester Jan 1997

Wavelet Inverse Neutron Scattering Study Of Layered Metallic Nic-Ti Composites, G. Saab, B. Defacio, H. Kaiser, D. L. Worcester

Review of Progress in Quantitative Nondestructive Evaluation

Composites are prevalent in high technology devices such as aircraft, computers, automobiles and communications systems. They improve brittleness and provide a lower density which enhances mechanical strength. Electron and light manipulating composites will be used more and more in the future. It is necessary to have a capability of inspecting composites, both to assure production quality and as a baseline for later NDE. In this paper, we present a study using wavelet, inverse neutron optics and the grazing angle neutron spectrometer, GANS, at the Missouri University Research Reactor, MURR.


Investigation Of The Anisotropic Nature Of Laser Generated Ultrasound In Hcp Crystals And Unidirectional Carbon Epoxy Composites, David H. Hurley, James B. Spicer, James W. Wagner, Todd W. Murray Jan 1997

Investigation Of The Anisotropic Nature Of Laser Generated Ultrasound In Hcp Crystals And Unidirectional Carbon Epoxy Composites, David H. Hurley, James B. Spicer, James W. Wagner, Todd W. Murray

Review of Progress in Quantitative Nondestructive Evaluation

Laser generated ultrasound has been used to determine material properties and to characterize material defects [1–3]. To a large extent, the success of laser ultrasonics has been the researcher’s ability to correctly predict the temporal evolution of the displacement waveform resulting from pulsed laser irradiation. Theories that assume isotropic elastic properties work well for crystalline materials that have grain sizes that are small compared to the wavelength of the interrogating ultrasonic wave [4–5]. For single crystal samples or carbon epoxies, the elastic anisotropic nature must be taken into account. Royer and Dieulesaint [6] have shown, using a ...


High Resolution Laser Ultrasound Detection Of Metal Defects, Peter W. Lorraine, Ralph A. Hewes, Denis Drolet Jan 1997

High Resolution Laser Ultrasound Detection Of Metal Defects, Peter W. Lorraine, Ralph A. Hewes, Denis Drolet

Review of Progress in Quantitative Nondestructive Evaluation

The standard for sensitive detection and resolution of defects in metal components is scanned focused immersion inspection to produce C-scans. Laser ultrasound, although successfully applied to composite inspection, has previously not produced comparable results in this arena.


The Elastic Modulus Of Nano-Sized Zinc Determined By Laser Ultrasonic Method, X. R. Zhang, J. F. Xu, Y. W. Du Jan 1997

The Elastic Modulus Of Nano-Sized Zinc Determined By Laser Ultrasonic Method, X. R. Zhang, J. F. Xu, Y. W. Du

Review of Progress in Quantitative Nondestructive Evaluation

The nano — sized materials are the advanced materials developed in the eighties[1]and being called nanocrystalline materials, ultra — fine grained materials or nanophase materials. Because there are a lot of interfaces within the nano — scaled materials, the volume fraction occupied by the interface is comparable with that of particles. The particle size effect and disordering effect of interface exist in the materials. They are referred to have“gaslike” structure. So the nano — sized materials have a number of advantages excelling to the traditional materials properties. Many new phenomena have been discovered from the investigations of their optical and electric ...


Experimental And Theoretical Investigation Of Interfacial Losses In Concrete With Laser Ultrasonics, Joseph O. Owino, Laurence J. Jacobs Jan 1997

Experimental And Theoretical Investigation Of Interfacial Losses In Concrete With Laser Ultrasonics, Joseph O. Owino, Laurence J. Jacobs

Review of Progress in Quantitative Nondestructive Evaluation

The strength of concrete is controlled by three factors: strength of the matrix, strength of the aggregate, and strength of the bond between matrix and aggregate. It is well known that material interfaces play an important role in the overall mechanical behavior of bonded materials e.g concrete. Reflected and transmitted ultrasonic waves have been used to develop non-destructive ultrasonic techniques to examine the size and location of flaws and to provide measurements of the internal structure of concrete among other things. In the studies of elastic wave scattering by interfaces, it is generally assumed that the inclusion is perfectly ...


The Laser Ultrasonic Inspection System (Luis) At The Sacramento Air Logistics Center, C. J. Fiedler, T. Ducharme, J. Kwan Jan 1997

The Laser Ultrasonic Inspection System (Luis) At The Sacramento Air Logistics Center, C. J. Fiedler, T. Ducharme, J. Kwan

Review of Progress in Quantitative Nondestructive Evaluation

The proportion of composite parts used in new Air Force aircraft is increasing significantly with each airplane that is designed [1]. In addition, composite parts are also being used for critical applications, where the loss of the part could cause the loss of the plane. These parts are susceptible to delaminations, disbonds, and impact damage. As part of its mission to overhaul aircraft, the Sacramento Air Logistics Center needs to be able to efficiently validate the integrity of composite aircraft parts.


Laser Generated Bulk Waves In Plates, Terry Sanderson, Charles Ume, Jacek Jarzynski Jan 1997

Laser Generated Bulk Waves In Plates, Terry Sanderson, Charles Ume, Jacek Jarzynski

Review of Progress in Quantitative Nondestructive Evaluation

The use of lasers as ultrasound sources is of interest in nondestructive materials testing, primarily because lasers provide a noncontact ultrasonic source. The generation and reception of ultrasonic waves has by now been widely demonstrated, as discussed in References 1, 2, and 3. Comprehensive numerical models are needed in order to quantify the waveforms generated by laser sources in the thermoelastic regime, and as an aid in designing nondestructive materials evaluation systems. Comprehensive models have been developed, for example by McDonald [4, 5], Schliechert et al. [6], and Spicer [7]. These numerical models have been experimentally confirmed by a number ...


Optical Detection Of Ultrasound By Two-Wave Mixing In Photorefractive Semiconductor Crystals Under Applied Field, A. Blouin, P. Delaye, D. Drolet, J. P. Monchalin Jan 1997

Optical Detection Of Ultrasound By Two-Wave Mixing In Photorefractive Semiconductor Crystals Under Applied Field, A. Blouin, P. Delaye, D. Drolet, J. P. Monchalin

Review of Progress in Quantitative Nondestructive Evaluation

The optical detection of transient surface motion has many practical applications which include, in particular, the vibration monitoring of engineering structures (aircraft, power plants,...) and the detection of ultrasound produced by piezoelectric transducer or by pulse laser excitation. This last application where ultrasound is generated and detected by lasers, presents many advantages over conventional piezoelectric based methods. First, laser-ultrasonics is a remote sensing technique. Consequently it can be used, for example, for inspecting hot materials and products moving on a production line. Second, surfaces of complex shapes can also very easily be probed. For many applications, these advantages compensate the ...


A Linear Systems Approach To Laser Generation Of Ultrasound In Composites, Todd W. Murray, Kevin C. Baldwin, James W. Wagner Jan 1997

A Linear Systems Approach To Laser Generation Of Ultrasound In Composites, Todd W. Murray, Kevin C. Baldwin, James W. Wagner

Review of Progress in Quantitative Nondestructive Evaluation

Laser ultrasonic generation and detection systems have been shown to be effective in the inspection and evaluation of both metals and composite materials [1–3]. Advantages of these noncontact systems include rapid scanning capability, the inspection of parts with complex geometries, and the ability for use in hostile environments. Unfortunately, laser ultrasonic systems are somewhat less sensitive than conventional contact piezoelectric systems. In order to increase the sensitivity, careful consideration must be paid to the choice of both generation and detection laser systems. Although the sensitivity of current laser ultrasonic systems has been shown to be sufficient for several applications ...


A Fiber-Based Laser Ultrasonic System For Remote Inspection Of Limited Access Components, Liu-Sheng Wang, J. Scott Stechenrider, Jan D. Achenbach Jan 1997

A Fiber-Based Laser Ultrasonic System For Remote Inspection Of Limited Access Components, Liu-Sheng Wang, J. Scott Stechenrider, Jan D. Achenbach

Review of Progress in Quantitative Nondestructive Evaluation

Surface and plate waves are commonly used to nondestructively inspect the near-surface region of a solid component for cracks and other defects due to, for example, structural fatigue. One particularly attractive method of generating and detecting such ultrasonic signals is laser based ultrasonics (LBU) [1]. In particular, because it is non-contact (i.e., does not require couplant), LBU can be implemented for inspection of limited access components using optical fibers, requiring only a small cross-sectional area for access. An example can be found in the inspection of internal surfaces of an aircraft wing as shown in Figure 1 where a ...


Application Of Measurement Models To Specification Of Ultrasonic Inspections, Timothy A. Gray Jan 1997

Application Of Measurement Models To Specification Of Ultrasonic Inspections, Timothy A. Gray

Review of Progress in Quantitative Nondestructive Evaluation

for economic reasons, there is an increasing tendency to perform automated ultrasonic scans of near net-shaped forgings, which can have rather complex shapes, as opposed to inspection of simpler, sonic shapes, which typically have only planar inspection surfaces. A difficulty in the former approach is that the surface curvature of forgings causes the ultrasonic beam to focus or defocus within the component and, therefore, the ultrasonic sensitivity to internal defects changes as compared to inspection through a flat surface. It is certainly possible to account for this sensitivity variation by using curved calibration or reference blocks. However, a more convenient ...


Interaction Of Acoustic Beams With Lossy Fluid-Loaded Layered, Cylindrical Shells, Han Zhang, Dale E. Chimenti Jan 1997

Interaction Of Acoustic Beams With Lossy Fluid-Loaded Layered, Cylindrical Shells, Han Zhang, Dale E. Chimenti

Review of Progress in Quantitative Nondestructive Evaluation

Nonspecular reflection of bounded beams has attracted interest since the work of Schoch [1] and Bertoni and Tamir [2] gave an accurate description of the nonspecular reflection of well collimated Gaussian beams from half spaces at or near the Rayleigh critical angle. Investigations on plates and half spaces concentrated on planar interfaces [3,4], while our previous work extended to the interaction of divergent Gaussian beams with planar surfaces (half space and plate) and of collimated beams with curved surfaces (solid cylinders and shells) [5,6].


Dispersion Compensation In Acoustic Emission Pipeline Leak Location, Lance E. Rewerts, Ron R. Roberts, M. Amanda Clark Jan 1997

Dispersion Compensation In Acoustic Emission Pipeline Leak Location, Lance E. Rewerts, Ron R. Roberts, M. Amanda Clark

Review of Progress in Quantitative Nondestructive Evaluation

The general practice of acoustic leak location relies on two different physical phenomena for determining source location: 1) reduction in signal amplitude with increasing distance from the source (attenuation-based methods), and 2) increase in signal transit time with increasing distance from the source (time-of-flight-based methods). The work discussed here describes efforts at ISU directed at gaining first-principle understanding of the underlying physical phenomena of multi-mode dispersion in fluid filled pipes and to developing time-of-flight source location data processing for such dispersive systems. Results are presented for work detailing the characteristics of pipe propagation, as well as the effect of those ...


Effect Of Rough Surfaces On Guided Waves In Plates, Dale E. Chimenti, O. I. Lobkis Jan 1997

Effect Of Rough Surfaces On Guided Waves In Plates, Dale E. Chimenti, O. I. Lobkis

Review of Progress in Quantitative Nondestructive Evaluation

The interaction of ultrasound with rough surfaces is being actively investigated, from both a theoretical and experimental standpoint. The problem is important to several areas, including ocean acoustics [1,2] and dielectric waveguides, such as optical fibers. In either of these problems the fields can be assumed to be represented by a single scalar potential. The problem we address here has previously received very little attention, and concerns the propagation of guided elastic waves in a planar solid waveguide having randomly rough surfaces with compressional and shear potentials that are coupled at each interface. The rough surface is intended to ...


Automated Source Identification Using Modal Acoustic Emission, Steve Ziola, Ian Searle Jan 1997

Automated Source Identification Using Modal Acoustic Emission, Steve Ziola, Ian Searle

Review of Progress in Quantitative Nondestructive Evaluation

As defense budgets are reduced, military aircraft are being flown well beyond their expected lifetimes. An example of this is the Boeing B-52 bomber. Introduced in 1954, the B-52 is still one of the Air Force’s primary bomber aircraft, as evidenced in Desert Storm. Currently, it is expected to fly until the year 2024, well past is original design goal. Because of this extended duty, fatigue crack growth in the airframe has become a concern, and methods to detect these defects are being implemented.


Quantitative Fracture Mode Analysis Of Composites By Acoustic Emission, M. Enoki, H. Fujita, T. Kishi Jan 1997

Quantitative Fracture Mode Analysis Of Composites By Acoustic Emission, M. Enoki, H. Fujita, T. Kishi

Review of Progress in Quantitative Nondestructive Evaluation

The acoustic emission (AE) source characterization has been developed to understand the dynamic process of microfracture in composites. AE signals can be represented as the convolution integral of the source function due to microfracture of materials, the dynamic Green’s function of the media and the transfer function of the measuring system. We developed the advanced analysis system to evaluate AE signals quantitatively. Source location of each AE is determined from the signals recorded using multi-transducers. Each dynamic Green’s function of the specimen is calculated by a finite difference method. The transfer function of the measuring system is calibrated ...


Wave Propagation In A Newtonian Fluid With Viscosity Gradients Profiles, Vimal V. Shah, Krishnan Balasubramaniam, R. Daniel Costley, Jagdish P. Singh Jan 1997

Wave Propagation In A Newtonian Fluid With Viscosity Gradients Profiles, Vimal V. Shah, Krishnan Balasubramaniam, R. Daniel Costley, Jagdish P. Singh

Review of Progress in Quantitative Nondestructive Evaluation

Plane compressional wave propagation in a fluid is significantly affected by the shear viscosity of the fluid [1]. Several different theories have been developed [2,3,4] to understand effects of viscosity on wave propagation characteristics. Presence of concurring phenomena such as thermal conductive losses and molecular relaxations [5] has frurther complicated the study of wave propagation. In liquids however, the effect of thermal conductivity is not comparable to the viscous losses [1]. In such cases, it has been possible to associate viscosity with the wave propagation characteristics.


A Wave Splitting/Invariant Embedding Approach To An Ultrasonic Inverse Problem, Paul Stucky, William Lord Jan 1997

A Wave Splitting/Invariant Embedding Approach To An Ultrasonic Inverse Problem, Paul Stucky, William Lord

Review of Progress in Quantitative Nondestructive Evaluation

Material property measurement is an important area of basic and applied research and can be defined as an inverse problem in which knowledge of the system input and output can be used to determine material properties.


Lamb Waves From Microfractures In Composite Plates, Dawei Guo, Ajit Mal, Kanji Ono, Michael Gorman Jan 1997

Lamb Waves From Microfractures In Composite Plates, Dawei Guo, Ajit Mal, Kanji Ono, Michael Gorman

Review of Progress in Quantitative Nondestructive Evaluation

Plate elements are common engineering structures and it is important to monitor damage evolution in these plates to ensure the integrity of the structure. Due to stress concentration in the vicinity of existing defects such as voids, inhomogeneities and surface cracks, microfracture events may occur locally in a plate highly stressed before final failure takes place. As a result, the stored strain energy radiates out in the form of elastic waves which carry information regarding the nature of the source. Due to the highly dispersive character of Lamb waves, their forms change as the disturbance propagates away from the source ...