An investigation on the vibroacoustic behavior of systems in similitude

Similitude theory allows engineers to establish the necessary conditions to design a scaled - up or down - model of a full-scale prototype structure. In recent years, the research on similitude methods, which allow to design the models and establish similitude conditions and scaling laws, has grown so that many obstacles associated with full-scale testing, such as cost and setup, may be overcome. This thesis aims at, on the one hand, expanding the possibilities of similitude methods by means of their application to new structural configurations; on the other hand, at the investigation of new approaches. Therefore, similitude conditions and scaling laws of thin aluminium plates with clamped-free-clamped-free boundary conditions, first, and aluminium foam sandwich plates with simply supported and free-free boundary conditions, then, are derived. Particularly, two sets of conditions are derived for the sandwich plates: the first by expliciting all the geometrical and material properties, the second by combining some parameters into just one with physical meaning, that is, the bending stiffness. These conditions and laws are successively validated by means of dynamic experimental tests, in which reconstructions of the natural frequencies and the velocity response of the prototype are attempted. Also the prediction of the radiated acoustic power is performed for the sandwich plates. All the tests highlight that these laws do not work fine when the models are distorted, i.e., when the similitude conditions are not satisfied. Therefore, the potentialities of machine learning are investigated and used to establish degrees of correlation between similar systems, without invoking governing equations and/or solution schemes. In particular, artificial neural networks are used in order to predict the dynamic characteristics, first, and the scaling parameters, then, of beams, as test (since they do not exhibit distorted models), and plates. In the latter case, the predictions of the artificial neural networks are validated by the results provided by the experimental tests. The networks prove to be robust to noise, very helpful in predicting the response characteristics, and identifying the model type. Finally, the similitude methods are used as a tool for supporting, and eventually validating, noisy experimental measurements, not for predicting the prototype behavior. In this way, they can help to understand if a set of measurements is reliable or not. Therefore, the sandwich plates are analysed with digital image correlation cameras. Then, with the help of an algorithm for blind source separation, the force spectra and velocity responses are reconstructed. It is demonstrated that the similitude results are coherent with the quality of the experimental measurements, since the curves overlap when the spatial patterns are recognizable. Instead, when the displacement field is too polluted by noise, the reconstruction exhibits discrepancies. This proves that the application of similitude methods should not be underestimated, especially in the light of the expanding range of approaches which can extract important information from noisy observations

Application de la tomographie sonique au diagnostic du béton

Recognition of the many problems in concrete structures requires the reconstruction of their internal image. For this reason, some years ago, a new nondestructive method, sonic tomography, has been developed for scanning concrete structures. This method is most often based on transient stress wave propagation for verifying a concrete body, as well as measuring the wave velocity. The relative variation in wave propagation velocity in the material provides information about changes in the structure, and therefore the state of degradation. Capacity and limitation of this method for reconstructing the internal image of the concrete structures are addressed in this thesis. This study discusses some experiments in which sonic tomography were performed on various concrete models in laboratory and the structures in service in-situ. In this case, two important aims are the main focus: (1) Degraded area is usually marked by fracture and delaminations or poorly performed sections within the structure. Unfortunately, most of the degradation in concrete structures remain undetectable to the naked eye. Sonic tomography is a strong technique to show the damaged areas in body concrete. In addition, this technique is able to evaluate internal reparation (e.g. control of grout-injection zones). (2) This is limited by the measurement and reconstruction of image conditions and their parameters (frequency, pixels size, rays type, measurement step,...). For illustrating the real image it is necessary to know precisely these conditions and the quality of their influence on the tomographic image

Testing concrete foundation piles by sonic echo

In 2 volsSIGLEAvailable from British Library Document Supply Centre- DSC:D41472/82 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Estimación ingenieril de la intensidad sísmica en la vecindad de una formación morfológica

A pesar de la evidencia experimental y teórica de la importancia de la topografía en la respuesta sísmica, sus efectos son poco considerados en la ingeniería práctica. En la actualidad los pocos códigos que tienen propuestas al respecto desconocen el efecto que sobre la magnitud de la variación sísmica por efecto de la topografía tienen la frecuencia del movimiento y la relación entre la longitud de onda de la excitación y el tamaño del accidente topográfico. En este trabajo, a partir de conceptos de difracción de una cuña infinita, se propone una expresión bi-paramétrica para cuantificar la variación en la respuesta sísmica en puntos localizados en la vecindad de un accidente topográfico. La expresión es derivada a partir de un análisis paramétrico en el dominio de la frecuencia sobre geometrías en forma de montaña y cañón. Las expresiones propuestas son aplicadas sobre dos casos realistas para obtener espectros de respuesta. Para medir la calidad de la propuesta, los resultados son contrastados con los obtenidos con una modelación computacional.Despite the relevance of topographic effects in seismic response by experimental and theoretical studies, these parameters is not widely used in practice engineering. Nowadays, just a few codes consider these effects, but they do not take into account the frequency of the movement and the relation between the dimensions of the topographic accident and wavelength of the incident field. A two-parameter expression for the quantification of topographic effects in ground response analysis at sites located in the vicinity of a topographic feature is presented. The expression is derived from parametric analysis of convex and concave canonical shapes after identifying that the diffracted field in these geometries remains relatively stable in the low dimensionless frequency regime. The proposed expression is used in the estimation of topographic effects at two sites located in realistic topographic scenarios over a given period range. To show the quality of the approximate formula results are compared with numerical simulation values

Optimization of Indentation for the Material Characterization of Soft PVA-Cryogels

Over the past few years a variety of clinical procedures aiming at tissue repair and other relevant therapies have been under active investigation [12,32]. Success of procedures aimed at soft tissue repair depend on the combined response of biochemical and biomechanical properties of the organs neighbouring the tissue [53]. Using human or animal cadaveric tissue for this purpose is very challenging due to issues pertaining to biodegradability and infection or biohazard risk factors [135,205].As such, tissue mimicking materials (e.g. Polyvinyl Alcohol Cryo-gel (PVA-C)) have been investigated to satisfy the need for the said clinical applications. Advantages of using tissue-mimicking materials include (a) biocompatibility, (b) being not biodegradable and long term shape preservation and (c) having similar biomechanical properties of human tissue [76,126]. To assess biomechanical compatibility of tissue mimicking materials, various mechanical testing techniques have been proposed. Among them, indentation testing has shown great potential for this purpose and it has been used broadly for tissue biomechanical characterization [158]. This method has become more popular because it allows for cost effective, non-destructive, quick, and quantitative assessment of soft tissue biomechanics [64,193]. Soft tissue is idealized as non-linear [46], isotropic [72] and incompressible [198] material. Given its interesting properties and biocompatibility, PVA-C has attracted a great deal of attention as a biocompatible material suitable for clinical applications such as tissue repair, tissue engineering etc. As such, many studies have been conducted to understand this material’s mechanical properties and its suitability for fabricating artificial cornea replacement [54], heart valve [90], lung [164], breast [167], kidney [169], brain [195], stomach [160], bladder [18], prostate [36] and articular cartilage [20] This stems from that this material has similar characteristics to human soft tissue [44,46,129]. Similar to biological tissues, the internal structure of PVA-C leads to nonlinear behavior [66, 80]. This nonlinearity becomes predominant while it undergoes large deformation [205]. Several analytical, semi-analytical and computational models have been proposed to understand tissue mechanical behavior, including its linear and nonlinear behavior, under indentation testing[60]. These include the methods proposed by Boussinesq [27],Sneddon[176],Hayes[77], Cao [34]. This thesis aims at gaining in-depth insight into the mechanical behavior of PVA-C under indentation testing. To this end it presents development of an inverse Finite Element (FE) techniques solved using numerical optimization to characterize the mechanical properties of PVA-C specimens. used to understand the indentation response of PVA-C at different thickness and conditions. The investigation reported in this thesis includes numerical analysis where displacement influence factor was employed in conjunction with linear elastic model of finite thickness. In the analysis, effects of Poisson’s ratio, specimen aspect ratio and relative indentation depth were investigated and a novel mathematical term was introduced to Sneddon’s equation. Results indicate that the developed models have been successful to characterize PVA-C material while they can be used effectively in characterizing the mechanical behavior of biological tissue specimens obtained from medical intervention

Energy Absorption of Macrocomposite Laminates

PhDThe aim of this project was two-fold. Firstly to provide an understanding of the behaviour of SMC when subjected to drop weight impact and secondly to investigate the effect of a surface layer of a metallic material (stainless steel, aluminium, brass and copper) and a layer of Ionomer on the impact behaviour of SMC. Tensile, flexural, compression, shear, charpy and drop weight impact tests were carried out on SMC (Sheet Moulding Compound). The response of SMC and various combinations of SMC and metal sheet (stainless steel, aluminium, brass and copper) and SMC with a layer of Ionomer to impact load have been assessed using an Instrumented Falling Weight Impact test machine. Slow indentation tests and a variety of destructive and non-destructive test techniques were used to monitor the initiation and propagation of damage and relate them to the major features of typical force-time curves obtained during impact. The deformation of the metallic layer was compared under impact and slow test and a calibration curve was produced. By using the calibration curve the energy absorbed by SMC and SMC as a layer in SMC+metal laminate was compared and results were related to stiffness and ductility of the metallic layer. The energy absorbed by the SMC-metal laminates were analysed and the energy absorbed by each constituents was determined. The effect of impact damage on tensile and compressive residual strength was assessed by conducting tension and compression test on the damaged specimens. Finally, a number of simple models and fInite element technique were used to predict the impact response of SMC and SMCmetal laminates to impact. The results of the research programme indicated a strong macrocomposite effect resulting in greatly improved energy absorbing capabilities for SMC. The indications were that a metal layer was required that would be stiff, thereby putting the SMC into compression and also ductile in order to support extensive deformation in the SMC whereby microcracking could accumulate

Numerical modelling of additive manufacturing process for stainless steel tension testing samples

Nowadays additive manufacturing (AM) technologies including 3D printing grow rapidly and they are expected to replace conventional subtractive manufacturing technologies to some extents. During a selective laser melting (SLM) process as one of popular AM technologies for metals, large amount of heats is required to melt metal powders, and this leads to distortions and/or shrinkages of additively manufactured parts. It is useful to predict the 3D printed parts to control unwanted distortions and shrinkages before their 3D printing. This study develops a two-phase numerical modelling and simulation process of AM process for 17-4PH stainless steel and it considers the importance of post-processing and the need for calibration to achieve a high-quality printing at the end. By using this proposed AM modelling and simulation process, optimal process parameters, material properties, and topology can be obtained to ensure a part 3D printed successfully

On the generation of homogeneous, inhomogeneous and Goodier-Bishop elastic waves from the geometrical ray theory

In this paper, a new group of exact and asymptotic analytical solutions of the displacement equation in a homogeneous elastic media, considering the most general solution of the Helmholtz equation, which have not been shown in papers and standard texts, are presented. Moreover, the authors show from the ray theory point of view the meaning of such solutions. These solutions could be helpful in future conceptual works about generation and emerging phenomena in elastic waves such as scattering and diffraction, among others, specifically in the analysis of the boundary conditions. Here, new kinds of P-S body waves that oscillate elliptically and propagate outward from sources in a full-space are found where, as special cases, the grazing longitudinal (Py) and transversal (SVy) waves of the Goodier-Bishop type, the analytic expressions for the Rayleigh wave and surface P waves, for which the amplitude decays from sources, are obtained. Also, the standard expressions for the homogeneous plane wavefronts, surface P waves, and Rayleigh surface waves, are achieved. © 2006-2015 Asian Research Publishing Network (ARPN)

Elastocapillary Phenomena in Soft Elastic Solids

Soft elastic solids play an important role in a wide range of applications such as in tissue scaffolds to grow artificial organs, in wearable contact lenses, as adhesives, in soft robotics and even as prototypical models to understand the mechanics of growth and morphology of organs. For a soft elastic material like hydrogel with its shear modulus in the range of tens of pascals, its surface tension also contributes to the mechanics of its deformation in addition to its elasticity. As opposed to a hard solid that is very difficult to deform, for the case of these soft solids, even a weak force like gravity can bring about significant deformation. Many of these aspects of the deformation and behavior of these ultrasoft materials are still not very well understood. Thus, the objectives of this dissertation were to understand the role of elastocapillarity (i.e, joint roles of solid surface tension and elasticity) and elastobuoyancy (i.e, joint roles of gravity and elasticity) that manifest in such solids. In this dissertation, we studied the role elastocapillarity in adhesion-induced instability in thin elastic films bonded to rigid substrates and also in surface oscillation modes of soft gel spheres set to vibration; the elastobuoyancy effect; elasticity mediated interaction of particles in soft solids as well as on thin films supported over a pool of liquid. We also presented some new results on how soft spherical gels undergo restricted spreading on rigid substrates with varying surface energies. In the first section, we studied how a thin confined layer of a soft elastic film loses adhesion from a rigid substrate by forming interfacial instabilities when a tensile stress is applied to it. We performed experiments to quantify the characteristic lengthscale of the patterns formed and found that they were significantly larger than the wavelengths of purely elastic instabilities. A linear stability analysis of the elastic field equations by taking into account the role of surface tension showed that the amplification of the wavelength is due to the role of elastocapillarity where the surface tension, elasticity, and film thickness contribute jointly in a non-trivial way. In addition, we found experimentally as well as theoretically that the stress required to adhesively fracture these films is much larger than Griffith’s fracture stress for stiffer elastic films, which is also due to the effect of elastocapillarity. We also studied the surface fluctuation of sessile hydrogel spheres subjected to mechanically-induced Gaussian white noise to understand the role of elastocapillarity in their oscillation modes. An important finding of this study is that they give a direct evidence that the surface tension of these elastic hydrogels is almost like that of water, which is the integral solvent in the swollen network of the polymeric gel. In the subsequent section, we introduced the new phenomenon of Elastobuoyancy. When a rigid sphere is placed on the surface of an ultrasoft hydrogel, it plunges into the soft substrate to an equilibrium depth where the elastic strain energy of the surrounding medium balances its weight. We refer to this state of the sphere as ‘Elastobuoyant’. By performing systematic experiments where we varied the sphere size and the elasticity of the substrate, we obtained scaling laws of the depth as a function of the radii, elastic modulus and the spheres buoyant weight, which were also supported by asymptotic analyses of the same. Following the section on elastobuoyancy, we reported a new set of principles to design self-assembly of particles by using the combined roles of surface tension, elasticity, and gravity in soft substrates. We used three different systems to study this elastic interaction macroscopically: (i) elastobuoyant assembly of particles suspended inside a soft elastic gel, (ii) elastocapillary assembly of particles floating on the surface of soft gels analogous to capillary attraction of objects on the surface of liquids, and (iii) assembly of particles on the surface of thin elastic membranes supported over a viscous liquid. In the second last chapter in this thesis, we presented some results on how soft elastic gel spheres spread on rigid substrates with different surface energies. Our observations indicate that their contact angles are slightly greater than those of equivalent liquid drops on similar substrates. The contact angles of these gel spheres increase as a function of elasticity and decrease when surface energy increases. We derived an expression for the excess elastic tension in the gel spheres at the crack tip by using an approach that is similar to estimating the viscous dissipation at the contact line during spreading of liquids. By using a general constitutive law where the elastic energy is not limited to the square of the strains, the singularity at the crack tip is artificially removed thereby forcing the gel to assume a liquid-like behavior. Our experimental results agreed reasonably well with the model. In the last chapter, we summarized the doctoral research and presented suggestions for future investigations. There are several appendices in this thesis that have interesting observations from partially completed projects that need additional research and analysis in the future