Laser-generated, plane-wave, broadband ultrasound sources for metrology

The accurate quantification of ultrasound fields generated by diagnostic and therapeutic transducers is critical for patient safety. This requires hydrophones calibrated to a traceable national measurement standard over the full range of frequencies used. At present, the upper calibration frequency range available to the user community is limited to a frequency of 60 MHz. However, there is often content at frequencies higher than this, e.g., through nonlinear propagation of high-amplitude pulses or tone-bursts for therapeutic applications, and the increasing use of higher frequencies in diagnostic imaging. To reduce the uncertainties and extend the calibrations to higher frequencies, a source of high-pressure, plane-wave and broadband ultrasound fields is required. This is not possible with current piezoelectric transducer technology, therefore laser-generated ultrasound is investigated as an alternative. This consists of an ultrasound wave generated by the pulsed laser excitation of a thin, planar, layer of light absorbing carbon-polymer nanocomposite materials. The work described in this thesis can be divided into three parts. The first part consisted of the fabrication of various nanocomposites in order to study the effect of different polymer types, composite thickness, laser fluence, and concentration of carbon nanotubes, on the ultrasound generated, as well as their stability. This included an investigation into the nonlinear propagation of MPa range laser-generated ultrasound, and the effect of the bandlimited hydrophone response, using a numerical wave solver (k-Wave). In the second part, the effects on the signal of acoustically reflective and matched backings (the substrates onto which the nanocomposite was coated) were studied. It was found experimentally that the backing material can significantly affect the pressure amplitude when the duration of the laser pulse is longer than the acoustic transit time across the thin nanocomposite layer. An analytical model was developed to describe how the signal generated depends on the backing material, absorbing layer thickness, and laser pulse duration. The model agreed well with measurements performed with a variable pulse duration fibre-laser. Finally, in the third part, a laser-generated, plane-wave, broadband ultrasound source device superficially resembling a standard piezoelectric piston source was designed, fabricated, and tested. The source produced quasi-unipolar pressure-pulse of 9 MPa peak-positive pressure with a bandwidth of 100 MHz, and the ultrasound beam is sufficiently planar to reduce uncertainties due to diffraction to negligible levels for hydrophones up to 0.6 mm in diameter

Structural Health Monitoring in Composite Structures: A Comprehensive Review.

This study presents a comprehensive review of the history of research and development of different damage-detection methods in the realm of composite structures. Different fields of engineering, such as mechanical, architectural, civil, and aerospace engineering, benefit excellent mechanical properties of composite materials. Due to their heterogeneous nature, composite materials can suffer from several complex nonlinear damage modes, including impact damage, delamination, matrix crack, fiber breakage, and voids. Therefore, early damage detection of composite structures can help avoid catastrophic events and tragic consequences, such as airplane crashes, further demanding the development of robust structural health monitoring (SHM) algorithms. This study first reviews different non-destructive damage testing techniques, then investigates vibration-based damage-detection methods along with their respective pros and cons, and concludes with a thorough discussion of a nonlinear hybrid method termed the Vibro-Acoustic Modulation technique. Advanced signal processing, machine learning, and deep learning have been widely employed for solving damage-detection problems of composite structures. Therefore, all of these methods have been fully studied. Considering the wide use of a new generation of smart composites in different applications, a section is dedicated to these materials. At the end of this paper, some final remarks and suggestions for future work are presented

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Guided-wave structural health monitoring

Guided-wave (GW) approaches have shown potential in various initial laboratory demonstrations as a solution to structural health monitoring (SHM) for damage prognosis. This thesis starts with an introduction to and a detailed survey of this field. Some critical areas where further research was required and those that were chosen to be addressed herein are highlighted. Those were modeling, design guidelines, signal processing and effects of elevated temperature. Three-dimensional elasticity-based models for GW excitation and sensing by finite dimensional surface-bonded piezoelectric wafer transducers and anisotropic piezocomposites are developed for various configurations in isotropic structures. The validity of these models is extensively examined in numerical simulations and experiments. These models and other ideas are then exploited to furnish a set of design guidelines for the excitation signal and transducers in GW SHM systems. A novel signal processing algorithm based on chirplet matching pursuits and mode identification for pulse-echo GW SHM is proposed. The potential of the algorithm to automatically resolve and identify overlapping, multimodal reflections is discussed and explored with numerical simulations and experiments. Next, the effects of elevated temperature as expected in internal spacecraft structures on GW transduction and propagation are explored based on data from the literature incorporated into the developed models. Results from the model are compared with experiments. The feasibility of damage characterization at elevated temperatures is also investigated. An extension of the modeling effort for GW excitation by finite-dimensional piezoelectric wafer transducers to composite plates is also proposed and verified by numerical simulations. At the end, future directions for research to make this technology more easily deployable in field applications are suggested.Ph.D.Aerospace EngineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/77498/1/Raghavan_PhD_thesis_GWSHM.pd

Micro-/Nano-Fiber Sensors and Optical Integration Devices

The development of micro/nanofiber sensors and associated integrated systems is a major project spanning photonics, engineering, and materials science, and has become a key academic research trend. During the development of miniature optical sensors, different materials and micro/nanostructures have been reasonably designed and functionalized on the ordinary single-mode optical fibers. The combination of various special optical fibers and new micro/nanomaterials has greatly improved the performance of the sensors. In terms of optical integration, micro/nanofibers play roles in independent and movable optical waveguide devices, and can be conveniently integrated into two-dimensional chips to realize the efficient transmission and information exchange of optical signals based on optical evanescent field coupling technology. In terms of systematic integration, the unique optical transmission mode of optical fiber has shown great potential in the array and networking of multiple sensor units.In this book, more than ten research papers were collected and studied, presenting research on optical micro/nanofiber devices and related integrated systems, covering high-performance optical micro/nanofiber sensors, fine characterization technologies for optical micro/nanostructures, weak signal detection technologies in photonic structures, as well as fiber-assisted highly integrated optical detection systems

On the verge : Mechanics in the limit of vanishing strength and stiffness

Material mechanics play a crucial role in a wide variety of scenarios and applications. Here we focused on two central material properties: stiffness and strength. Whereas stiffness characterizes the resistance to deformation for small strains, where the response remains linear, strength describes the resilience of a material to larger deformations and mechanical damage. For conventional materials strength and stiffness are readily described by established mechanical theories. However, many materials in nature, or engineering materials during processing, live in a state where stiffness and/or strength becomes so weak that classical mechanical theories no longer apply. This has been the focal point of this thesis. The exploration of such ultrasoft and/or ultraweak solids faces many challenges, some of which have been addressed in this thesis, including their structure-property relationships and the question howone characterizes these fragile materials where conventional mechanical methods are no longer viable. In chapter 2 we address the challenge of characterizing the mechanical response of solids at the verge of a mechanical instability, where classical approaches fail. We present a new method based on the propagation of infrasonic waves. These waves propagate at low Reynolds numbers, where dissipation is strong. We have not only shown an experimental approach to evaluate wave propagation properties, but also established a theoretical framework to interpret these data and extract quantitative mechanical properties with a unique resolution. In chapter 3 we detail the technical challenges associated with these measurements, performed with the help of optical tweezers to create travelling mechanical waves. When marginal networks are combined with secondary elastic matrices remarkable stiffening is observed. In chapter 4 we present a theoretical model to study the effect of bending rigidity to the mechanics in hybrid materials with simulations. We show how different mechanical regimes arise depending on the bending stiffness and the stiffness of the secondary network. Each of these regimes have different mechanisms that lead to mechanical enhancement of the composite network. Experimental access to these mechanisms is extremely challenging. In chapter 5 we take the first steps to studying these mechanisms experimentally. Here we propose a simple click-chemistry based surface modification method that can help to achieve the complex inter-particle interactions required for establishing hybrid colloidal networks. The second part of this thesis covers hyperweak solids and irreversible deformation. Chapters 6 to 8 deal with colloidal gels that are prototypical examples of hyper weak solids. In chapter 6 we address the structure to dynamics part of the structure-property relation in colloidal gels. We experimentally establish the connection between the intermittent dynamics of individual particles and their local connectivity. We interpret our experimental results with a model that describes single-particle dynamics based on highly cooperative thermal debonding. Our model is in quantitative agreement with experiments and provides a microscopic picture for the structural origin of dynamical heterogeneity and provides a new perspective of the link between structure and the complex mechanics of these heterogeneous solids. Chapter 7 focuses on the dynamics to mechanics part of the structure-property relation by studying fatigue in colloidal gels. Here we combine experiments and computer simulations to show how mechanical loading leads to irreversible strand stretching, which builds slack into the network that softens the solid at small strains and causes strain hardening at larger deformations. We thus find that microscopic plasticity governs fatigue at much larger scales. This sheds new light on fatigue in soft thermal solids and calls for new theoretical descriptions of soft gel mechanics in which local plasticity is taken into account. In chapter 8 we take first steps in investigating the overlooked role of inter-particle friction in colloidal gels. We present a colloidal system with a thermo-responsive trigger for systematically studying the effect of surface properties, grafting density and chain length, on the particle dynamics within colloidal gels. Microscopically, for colloids with a lower grafting density, we observe an increase in the thermal bond angle fluctuations of aggregated colloids. Macroscopically, we observe a clear increase of the linear elastic modulus for gels with increased grafting density and longer chain lengths. These effects are inversely proportional to the magnitude of local bond angle fluctuations. Our model system will allow for further study of the microscopic origins of the complex macroscopic mechanical behavior of hyperweak solids that include bending modes within the network. Fracture and mechanical failure are highly stochastic processes and predicting fracture is highly challenging with conventional theories but crucial to assessing the lifetimes of e.g. buildings, bridges and implants. In chapter 9 we explore new opportunities for predicting fracture in marginal fiber networks. Fracture is the ultimate form of irreversible deformation and, especially in soft materials, characterized with highly non-linear mechanics preempting the moment of failure. We show how machine learning methods can by employed to predict the critical fracture stress solely based on structural and topological input parameters. We show that neural networks, despite their black box behavior, can be used to study the physical mechanisms underlying fracture. By varying the input parameters for our fracture stress predictions we found three parameters for which we can achieve the same prediction quality as for all tested input parameters combined. In the last chapter, the general discussion, we discuss how our results relate to each other and how they fit in a broader context. Furthermore we suggest and describe experiments that can help advance our knowledge of hypersoft and hyperweak materials in the future.</p