Anisotropic behaviour of human gallbladder walls

Inverse estimation of biomechanical parameters of soft tissues from non-invasive measurements has clinical significance in patient-specific modelling and disease diagnosis. In this paper, we propose a fully nonlinear approach to estimate the mechanical properties of the human gallbladder wall muscles from in vivo ultrasound images. The iteration method consists of a forward approach, in which the constitutive equation is based on a modified Hozapfel–Gasser–Ogden law initially developed for arteries. Five constitutive parameters describing the two orthogonal families of fibres and the matrix material are determined by comparing the computed displacements with medical images. The optimisation process is carried out using the MATLAB toolbox, a Python code, and the ABAQUS solver. The proposed method is validated with published artery data and subsequently applied to ten human gallbladder samples. Results show that the human gallbladder wall is anisotropic during the passive refilling phase, and that the peak stress is 1.6 times greater than that calculated using linear mechanics. This discrepancy arises because the wall thickness reduces by 1.6 times during the deformation, which is not predicted by conventional linear elasticity. If the change of wall thickness is accounted for, then the linear model can used to predict the gallbladder stress and its correlation with pain. This work provides further understanding of the nonlinear characteristics of human gallbladder

Lay-up characterization and elastic property determination in composite laminates

This dissertation focuses on two important nondestructive evaluation and materials characterization problems related to composite laminates: ply lay-up characterization and elastic property determination. For ply lay-up characterization, we have developed a shear wave transmission technique to effectively detect ply lay-up errors in composite laminates. The effects of fiber orientation on normal-incident shear waves propagating through a composite laminate have been investigated both theoretically and experimentally. To facilitate rotation, EMATs (electromagnetic acoustic transducers) were used to generate and receive the shear waves. It was found that the transmitted shear waves when the EMAT transmitter and receiver were perpendicular to each other had a great sensitivity to ply lay-up errors. This technique has been successfully demonstrated on both cured and uncured composite laminates. For elastic property determination, we have first applied the simultaneous velocity and thickness imaging technique to map out small changes in ultrasonic velocity (hence elastic constant) when the material thickness was unknown or varied spatially. Applications to several industrial materials have demonstrated the usefulness of this technique for both materials characterization and flaw detection in metals and composite laminates. We have also extended this technique to generate images of sample surface contours and cross-sectional profiles when the velocity was unknown. Next, we have extended the synthetic aperture scanning method using planar transducers in an immersion leaky wave reflection or transmission measurement to allow the use of focused transducers. The complex transducer point approach has been used to model the receiver output voltage and to analyze the transducer beam effects on the result of a synthetic aperture scan. It was found that the large angular beam spread of focused transducers can be used for rapid mapping of the reflection or transmission coefficient and the associated dispersion spectrum. A novel stepwise, targeted procedure has also been developed to allow efficient reconstruction of material elastic property with only minimal use of the highly redundant dispersion spectrum data. Experiments on both isotropic and anisotropic plates showed that this method can be used for rapid evaluation of the elastic behavior of composite laminates and other plate materials with a reasonably good accuracy

Simplification of Mathematical Models for Medical Ultrasound Poroelasticity Imaging

The use of an understanding of mechanical properties of tissues for the purposes of medical diagnosis has been going on since the foundation of the medical field as a science. In recent decades, medical ultrasound elastography techniques have been developed and improved and have helped the medical community improve the state of diagnosis and tracking of various diseases like cancer, and lymphedema. Poroelastography, refers to the extension of ultrasound elastography techniques towards imaging the mechanical properties of tissues that are modeled as poroelastic. Currently, the field of poroelastography is stuck, largely due to the complication in the mathematical models surrounding poroelastic materials. This dissertation focuses on the investigation of the suitability of a simplified equation involving a single saturating exponential (i.e. time constant curve) to describe the local time-dependent strain response of non-homogeneous poroelastic materials placed under creep compression. A new algorithm of measuring how precisely a non-linear equation fits a set of data samples from an experiment, the Resimulation of Noise (RoN) algorithm, was developed and implemented for the time constant curve case. The RoN algorithm was shown to track the precision of the fit in a more intuitive and accurate manner than previously used quality of fit metrics. The RoN algorithm coupled with an in-depth FEM simulation study was conducted to see how well the single exponential time-constant curve fit the localized strain samples of a simulated prismatic phantom with a cylindrical inclusion under different permeability and stiffness contrasts. The study showed that, on average, the single exponential time constant curve was suitable within 10% precision for 90% of the phantom's area so long as a mean-mask filter was applied the localized strain images before attempting the curve-fit. Future work in the field of poroelasticity imaging should center around the use of the single exponential time constant curve. This will require the development of a full understanding of how poroelastic material parameter contrast affects the contrast of the measured time constants. Procedures that will help this endeavor: such as the parallelization of the RoN algorithm as well as the development of novel nonhomogeneous poroelastic phantoms with the aid of 3-D printers are also proposed

Fatigue behaviour of a GFRP laminate by thermographic measurements

Composite materials are widely used to build structural components, thanks to their mechanical properties. Those are generally considered ‘engineering materials’, since they are tailored to meet specific requirements. Due to their use for structural components, it is important to know their mechanical behaviour, especially under cyclic loads. At present, there is a common interest, among researchers, to study the mechanical behaviour of composites, by means of both traditional and innovative techniques, with the final purpose of making previsions regarding their service life. In fact, due to their composite nature, they behave in a different mode compared to homogeneous materials. This study is focused on a glass fibre-reinforced plastic (GFRP); the aim of this work is to study its fatigue behaviour, from both the mechanical and the thermal points of view. The main reason is that there is a lack of knowledge, in the literature, about the fatigue of composites. In this study, a GFR laminate was characterized under static and dynamic loading conditions; during the experimental tests, thermal measurements were carried out by means of an IR-thermal camera. Temperature measurements were done during the static tests, whereas in the dynamic tests the dissipated energy was measured, by using the dissipation method (D-mode). Then, various criteria for fatigue life estimation were applied fitting the experimental data. Since different thermographic techniques have been used to estimate the fatigue behaviour, a final comparison between the experimental data and the predicted fatigue behaviour is proposed and discussed, showing a good agreement

Fast Determination of Soil Behavior in the Capillary Zone Using Simple Laboratory Tests

INE/AUTC 13.1

Inversion strategies for seismic surface waves and time-domain electromagnetic data with application to geotechnical characterization examples.

Geophysical methods are broadly used to map the subsurface. Their ability to investigate large areas in a short time and to reach significant depths with good resolution makes them suitable for a wide range of applications: from hydrological studies, mineral exploration, archaeological investigations to geotechnical characterization. Unfortunately, most of the geophysical methods are ill-posed. Thus, to be able to effectively invert the geophysical data and get meaningful models of the subsurface a priori information needs to be included in the process. This is the basic idea behind the inversion theory. This thesis deals with the inversion of two types of geophysical measurements: the Seismic Surface Waves (SSW) data and the Time Domain Electromagnetic (TDEM) observations. The present work consists of two parts: (1) The first one is about possible implementations of the minimum gradient support stabilizer into a SSW inversion routine and its extension to the laterally constrained case. By means of this novel approach, it is possible to tune the level of sparsity of the reconstructed velocity model, providing a solution with the desirable characteristics (smooth or sharp) in both directions (vertically and laterally). The capabilities of the proposed approach have been tested via applications on synthetic and measured data. (2) The second part of the thesis is about the joint interpretation of SSW and TDEM measurements for an improved geotechnical characterization of an area intended for construction. In this case, the SSW results, together with other ancillary data, are used as prior information for the subsequent inversion of TDEM measurements. In this respect, the SSW results have been translated into pieces of information to be used in the TDEM inversion via a petrophysical relationship. This work is coherent with one of the goals of the United Nations Agenda 2030 for sustainable development, specifically, the item 11b, as geotechnical characterization is one of the essential components for the design of civil engineering works, ensuring the necessary safety and resilience to natural disasters and climate change. However, the field of application of the proposed approaches is very broad as they can also be used, e.g., for groundwater mapping, as well as for the evaluation of aquifer contamination. In this respect, the present work is also in line with items 6.1, 6.3 and 6.4 of the 2030 UN Agenda

Master of Science

thesisDue to the complex failure modes associated with composites, a structural health monitoring system capable of accurately locating the source of strength-reducing events is desirable in order to reduce inspection time and time out of service. Various active and passive inspection techniques exist but most require large footprints and extensive cabling to monitor full scale structures. This work derives various location techniques by coupling modal acoustic emissions with phased array techniques to detect and accurately locate the source of strength-reducing events such as impacts. Phased array techniques provide a method to more accurately track phase points for determining arrival times used to back-calculate the source, as well as providing a method that can incorporate anisotropic wave speeds. To increase accuracy by neglecting local to global material changes, the local velocity profile per component was found and built into the derived location algorithms. The location algorithms were then tested on two full scale composite structures based on strength and stiffness critical design considerations. It was found that with two arrays, each with dimensions of 1 inches in width and 8 inches in length and consisting of four sensors each, events could be accurately located over a 65 ft2 region on the stiffness critical structure with an average error of 10 inches and over a 100 ft2 region on the strength critical structure with an average error of 9 inches