A mathematical model of shear wave propagation in the incompressible transversely isotropic thermoelastic half-spaces

This article deals with the problem of reflection and transmission of shear waves at a plane interface between two dissimilar incompressible transversely isotropic thermoelastic half-spaces. Two coupled quasi-shear waves are found to propagate due to the incompressibility of such materials. Applying appropriate boundary conditions at the plane interface, amplitude ratios of the reflected and transmitted quasi-shear waves are obtained. It has been observed that these ratios are functions of the angle of incidence, elastic and thermal parameters of the materials. These ratios are computed numerically for a particular model to see the effects of specific heat and thermal expansion on quasi-shear waves in incompressible transversely isotropic thermoelastic materials. The results are also presented graphically

The reflection coefficient from interface layers in NDT of adhesive joints.

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The application of laser-generated ultrasound to the study of aluminium-epoxy bonded systems

The poor performance of acoustic wave techniques in predicting adhesively-bonded joint failure under destructive loading is a long-standing problem, known to derive from unreliable adhesive defect detection. This thesis examines the feasibility of applying a relatively new technique, generating ultrasound with pulsed Nd:YAG lasers, to the study of aluminium alloy adherends joined by epoxy layer bonds. Laser generation is a non-contacting method which produces highly repeatable ultrasonic sources in metals, without damping motion at the sample surface. Pulses created in this fashion have bandwidths around 20 MHz and radiate both along the sample surface and into the material bulk. Displacements at the sample surface recorded by a broad bandwidth non-contact detector, such as a 532 nm wavelength laser Michelson interferometer, are therefore able to resolve details in time-varying traces which are not visible when narrowband transducers are used. In particular, individual reverberations between the interfaces of epoxy layers less than 100 μm thick are detected in transmission through adhesively bonded joints, on time domain traces. An epoxy layer sandwiched between two thick aluminium adherends presents a three layer case which is seldom discussed in the literature. I have therefore adapted theory developed for surface waves in thin layers overlying deep substrates, and for waves transmitted through multilayer structures, into an explicit formulation for an elastic layer embedded in adherend half-spaces that can be used for both through-transmission and interface-parallel waves. The case of travelling waves in a viscoelastic layer has not yet been examined as the current formulation requires unfeasibly long computation times. A numerical solution assuming elastic behaviour, gives strong indications that embedded epoxy layers support travelling waves directed along the interfaces, despite the fact that a single interface between epoxy and aluminium will not support non-dispersive Stoneley interface waves. Experimental work presented in Chapters 4 to 7 is preceded by a review of laser generation and non-contact detection methods, which introduces techniques that I have employed. As well as using laser interferometers, 1 have also built my own electromagnetic acoustic transducers (EMATs), to provide a cheaper alternative detection scheme. Chapter 4 concentrates upon on-epicentre detection of direct through transmission pulse arrivals, using analysis both of the entire reverberation wavetrain following the main arrival and of consecutive pulses within it, in order to extract information on the bonds' cohesive and adhesive properties. Chapter Five examines variations in surface-travelling waveforms on unbonded, free-surface aluminium plates with thicknesses varying from 63 mm down 28 μm, in a search for non- dispersive waves that would be suitable for probing adhesive bonds. Rayleigh arrivals on samples over 10 mm thick and the symmetric zero-order Lamb mode on plates under 200 μm thick both propagate from the NdtYAG laser source as sharp pulses, but intermediate plate thicknesses only allow waves with highly dispersive characteristics, which tend to mask any dispersion due to bonds. The plate wave experiments allow a full intercomparison between interferometer and EMATs, both out-of-plane motion sensitive and in-plane motion sensitive. Chapter 6 uses Rayleigh-like surface waves travelling along 25 mm thick adherends to initiate interface-parallel travelling waves in an adhesive layer bonding a second adherend to the surface, which are subsequently detected on emerging at the free surface beyond the bond. These surface-interface-surface travelling (SIST) waves penetrate under increasingly longer bonds as the wave frequency decreases, a fact confirmed by the behaviour of pulses given a narrowband frequency modulation when generating laser beams interfere to produce a spatially modulated source. The interference source optical arrangement, described in Chapter 5, can be altered to give Rayleigh arrival modulation frequencies from 20 MHz to below 1 MHz. Finally, Chapter 7 examines alternative pathways for surface waves incident upon the edge of a bonded joint region, and demonstrates that SIST waves are an efficient mechanism for transferring ultrasound between the two adhesive-adherend interfaces, given the observed emergence of clearly discernible SIST waves on the second adherend of a lapped bond joint. I conclude that through transmission pulse analyses arc capable of extracting quantitative information about bond properties and should be developed as the basis for laser generated ultrasonic bond testing. SIST waves, however, require further research before they can be employed in a practical manner

Nondestructive Evaluation and Health Monitoring of Adhesively Bonded Composite Structures

As the growth of fiber reinforced composite materials continues in many industries, structural designers will have to look to new methods of joining components. In order to take full advantage of composite materials, such as increased stiffness, decreased weight, tailored material properties and increased fatigue life, mechanical fasteners will need to be replaced by adhesive bonding or welding, when possible. Mechanical fasteners require the drilling of holes, which damages the laminate and becomes the source of further fatigue damage. Also, an increase in laminate thickness or inclusion of other features is required for the material to withstand the bearing stress needed to preload fasteners. Adhesives transfer the load over a large area, do not require additional machining operations, provide increased stiffness through the joint, provide corrosion protection when joining dissimilar materials, and provide vibrational damping. Additionally, the repair of composite structures, which will become a major concern in the near future, will require the use of adhesive bonding for thermoset composites. In order for adhesives to be used to join primary aerospace structures they must meet certification requirements, which includes proof that the joint can withstand the required ultimate load without structural failure. For most components, nondestructive inspection is used to find critical flaws, which is combined with fracture mechanics to ensure that the structure can meet the requirements. This process works for some of the adhesive flaws, but other critical defects are not easily detected. Weak interface bonding is particularly challenging. This type of defect results in an interphase zone that may be only a dozen microns in thickness. Traditional bulk wave ultrasonic techniques cannot easily distinguish this zone from the interface between adherend and adhesive. This work considers two approaches to help solve this problem. Guided elastic wave propagation along laminate structures is highly dependent on the boundary conditions at the surface and between plies, especially at high frequencies. This work investigates how interfacial defects can alter the propagation of guided waves through bonded fiber reinforced composite materials. As well as how this information can be used to determine the interface properties and correlate the results with fracture parameters. The second approach investigates how structural health monitoring can be used to detect the growth of disbonds from service loads. A mode selection technique is proposed for selecting frequency ranges for electromechanical impedance spectroscopy

Ultrasonic nondestructive evaluation of metal additive manufacturing.

Metal Additive Manufacturing (AM) is increasingly being used to make functional components. One of the barriers for AM components to become mainstream is the difficulty to certify them. AM components can have widely different properties based on process parameters. Improving an AM processes requires an understanding of process-structure-property correlations, which can be gathered in-situ and post-process through nondestructive and destructive methods. In this study, two metal AM processes were studied, the first is Ultrasonic Additive Manufacturing (UAM) and the second is Laser Powder Bed Fusion (L-PBF). The typical problems with UAM components are inter-layer and inter-track defects. To improve the UAM process, an in-situ quality evaluation technique was desired. Several NDE techniques were tested in a lab environment before ultrasonic NDE was chosen as a practical, robust, and cost-effective NDE tool. An in-situ monitoring setup was designed and built on an UAM system. NDE results showed interesting features that were simulated through analytic and finite element wave-propagation models. AM layers with defects were characterized as an intact layer and a finite interfacial stiffness spring. The spring stiffness coefficient is a quality parameter that was used to characterize AM layers through a model-based inversion method. In-situ and post-process NDE provided an understanding of defect generation and propagation in UAM. A novel solid-state repair mechanism based on Friction Stir Processing (FSP) was proposed and demonstrated. The quality of L-PBF components depends on several factors including laser power, scan speed, hatch spacing, layer thickness, particle shape/size distribution and other build conditions. Developing process parameters for a new material is an expensive and complex optimization problem. Post-process ultrasonic NDE tests revealed that the model-based in-situ quality monitoring developed for UAM is also applicable to L-PBF Additive Manufacturing. A similar NDE set-up was designed and installed on an open-architecture L-PBF system. A layer-by-layer bond quality evaluation demonstrates the ability to detect good-quality bonds hidden behind poor-quality regions for Inconel 625 alloy. A cost-effective, process parameter development methodology has been proposed and demonstrated

Tomography applied to Lamb wave contact scanning nondestructive evaluation

The aging world-wide aviation fleet requires methods for accurately predicting the presence of structural flaws that compromise airworthiness in aircraft structures. Nondestructive Evaluation (NDE) provides the means to assess these structures quickly, quantitatively, and noninvasively. Ultrasonic guided waves, Lamb waves, are useful for evaluating the plate and shell structures common in aerospace applications. The amplitude and time-of-flight of Lamb waves depend on the material properties and thickness of a medium, and so they can be used to detect any areas of differing thickness or material properties which indicate flaws. By scanning sending and receiving transducers over an aircraft, large sections can be evaluated after a single pass. However, while this technique enables the detection of areas of structural deterioration, it does not allow for the quantification of the extent of that deterioration. Tomographic reconstruction with Lamb waves allows for the accurate reconstruction of the variation of quantities of interest, such as thickness, throughout the investigated region, and it presents the data as a quantitative map. The location, shape, and extent of any flaw region can then be easily extracted from this Tomographic image. Two Lamb wave tomography techniques using Parallel Projection tomography (PPT) and Cross Borehole tomography (CBT), are shown to accurately reconstruct flaws of interest to the aircraft industry. A comparison of the quality of reconstruction and practicality is then made between these two methods, and their limitations are discussed and shown experimentally. Higher order plate theory is used to derive analytical solutions for the scattering of the lowest order symmetric Lamb wave from a circular inclusion, and these solutions are used to explain the scattering effects seen in the Tomographic reconstructions. Finally, the means by which this scattering theory can be used to develop Lamb wave Tomographic algorithms that are more generally applicable in-the-field, is presented

On the behaviour of porcine adipose and skeletal muscle tissues under shock compression

The response of porcine adipose and skeletal muscle tissues to shock compression has been investigated using the plate-impact technique in conjunction with manganin foil pressure gauge diagnostics. This approach has allowed for measurement of the levels of uniaxial stress imparted to both skeletal muscle and rendered adipose tissue by the shock. In addition, the lateral stress component generated within adipose tissue during shock loading has also been investigated. The techniques employed in this study have allowed for equation-of-state relationships to be established for the investigated materials, highlighting non-hydrodynamic behaviour in each type of tissue over the range of investigated impact conditions. While the adipose tissue selected in this work has been shown to strengthen with impact stress in a manner similar to that seen to occur in polymeric materials, the skeletal muscle tissues exhibited a ow strength, or resistance to compression, that was independent of impact stress. Both the response of the adipose material and tested skeletal muscle tissues lie in contrast with the shock response of ballistic gelatin, which has previously been shown to exhibit hydrodynamic behaviour under equivalent loading conditions. Plate-impact experiments have also been used to investigate the shock response of a homogenized variant of one of the investigated muscle tissues. In the homogenized samples, the natural structure of skeletal muscle tissue, i.e. a fibrous and anisotropic composite, was heavily disrupted and the resulting material was milled into a fine paste. Rather than matching the response of the unaltered tissues, the datapoints generated from this type of experiment were seen to collapse back on to the hydrodynamic response predicted for skeletal muscle by its linear equation-of-state (Us = 1.72 + 1.88up). This suggests that the resistance to compression apparent in the data obtained for the virgin tissues was a direct result of the interaction of the shock with the quasi-organized structure of skeletal muscle. A soft-capture system has been developed in order to facilitate post-shock analysis of skeletal muscle tissue and to ascertain the effects of shock loading upon the structure of the material. The system was designed to deliver a one-dimensional, at-topped shock pulse to the sample prior to release. The overall design of the system was aided by use of the non-linear and explicit hydrocode ANSYSR AUTODYN. Following shock compression, sections of tissue were imaged using a transmission electron microscope (TEM). Both an auxetic-like response and large-scale disruption to the I-band/Z-disk regions within the tissue's structure were observed. Notably, these mechanisms have been noted to occur as a result of hydrostatic compression of skeletal muscle within the literature

The exploitation of acoustic-to-seismic coupling for the determination of soil properties

Laboratory measurements of three predicted wave types (two compressional or P-waves and one shear S-wave) have been made in artificial soils. The Type-I P and S-wave are predicted to be most sensitive to the macroscopic elastic properties of the frame, whilst the Type II P-wave is predicted to be most sensitive to the hydrodynamic material properties. A loudspeaker source has been used for the preferential excitation of the Type II P-wave whilst preferential excitement of the Type-I P-wave has been accomplished using a mechanical shaker. Probe microphone measurements of the Type-II wave allowed the flow resistivity and tortuosity of the material to be determined using a rigid frame model, whilst deduction of elastic moduli has been made from signals received at buried geophones. It has been shown that microphone signals include Type-I P-wave energy in a high flow resistivity soil. Acoustically deduced soil properties are consistent with mechanically derived values. A systematic investigation of outdoor measurements of acoustic-to-seismic coupling ratio has been made. From the measurements, it has been found that the geophone-ground coupling has a great effect upon the measured coupling ratio. In-situ calibration methods have been developed to overcome this problem. whilst the novel use of a Laser Doppler Vibrometer has been proposed to provide a completely non-invasive method of measuring motion in soils. The measured coupling ratio has been compared with theoretical predictions, using a modified Bio-Sto11 formulation. The model can be used to predict values of flow resistivity, porosity, bulk and shear moduli and layer depths. Reasonable agreement has been obtained between the model and data. Procedures that exploit acoustic-to-seismic coupling data and models to determine soil properties have been developed and used to measure the soil properties of friable agricultural soils where more standard investigation techniques have proved unsuitable

Linear and nonlinear elastic wave propagation in a fluid-filled borehole

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1993.Includes bibliographical references (leaves 180-187).by Sergio Kostek.Ph.D