Novel computational model for the failure analysis of composite pipes under bending

Tianyu Wang: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Investigation, Formal analysis, Data curation, Conceptualization. Oleksandr Menshykov: Writing – review & editing, Supervision, Project administration, Methodology, Conceptualization. Marina Menshykova: Writing – review & editing, Supervision, Project administration, Methodology, Conceptualization.Peer reviewe

Analysis of bendable osteochondral allograft treatment and investigations of articular cartilage wear mechanics

Osteoarthritis is a highly prevalent, debilitating disease characterized by the wear and degradation of articular cartilage. While many surgical interventions exist, few are consistently effective and those that are effective are not necessarily suitable for all patients. The objective of this dissertation is to improve patient care through the development of a new surgical technique and through basic science studies which seek to better understand articular cartilage wear initiation. Four studies, which address this objective are summarized below. Osteochondral allograft transplantation provides a safe and effective treatment option for large cartilage defects, but its use is limited partly due to the difficulty of matching articular surface curvature between donor and recipient. We hypothesize that bendable osteochondral allografts may provide better curvature matching for patella transplants in the patellofemoral joint. The finite element study presented in Chapter 2 investigates patellofemoral joint congruence for unbent and bendable osteochondral allografts, at various flexion angles. Finite element models were created for 12 femur-patella osteochondral allograft pairings. Two grooves were cut into the bony substrate of each allograft, allowing the articular layer to bend. Patellofemoral joints with either unbent (OCA) or permanently bent (BOCA) allografts were articulated from 40 to 70 degrees flexion and contact area was calculated. OCAs and BOCAs were then shifted 6 mm distally toward the tibia (S-OCA, S-BOCA) to investigate the influence of proximal-distal alignment on congruence. On average, no significant difference in contact area was found between native patellofemoral joints and either OCAs or BOCAs (p > 0.25), indicating that both types of allografts restored native congruence. This result provides biomechanical support in favor of an emerging surgical procedure. S-BOCAs resulted in a significant increase in contact area relative to the remaining groups (p < 0.02). The fact that bendable osteochondral allografts produced equally good results implies that these bendable allografts may prove useful in future surgical procedures, with the possibility of transplanting them with a small distal shift. Surgeons who are reluctant to use osteochondral allografts for resurfacing patellae based on curvature matching capabilities may be more amenable to adopting bendable osteochondral allografts. The recent development of bendable osteochondral allografts provides the potential for improved osteoarthritis treatment for joints whose current treatment is unsatisfactory. One such joint is the carpometacarpal joint in the thumb. While the current standard of care for carpometacarpal osteoarthritis, ligament reconstruction and tendon interposition, can reduce pain in the joint, it does not restore full joint function and mobility. A proposed alternative includes using an osteochondral allograft harvested from the femoral trochlea in a donor knee, machining grooves in the bone to allow the allograft to bend, and replacing the trapezium with this bent osteochondral allograft [1,2]. Chapter 3 of this dissertation discusses adjustments to the original design of the bendable allograft and the design of a custom surgical tool to perform the proposed surgery. Specification changes of the allograft included an overall size reduction in order to better fit within the carpometacarpal joint, minimum bone thickness requirements to avoid bone cracking during the surgical procedure, and a reduction from three grooves to two grooves, which provided sufficient bending yet avoided fracture of the allograft. The surgical tool was designed to be a custom forceps device, whose primary features included (1) jaws with an angled face to match the angle of allograft bending and (2) insertion holes for the Kirschner wire and compression screws used to anchor the allograft in the bent position. These customizations allow the tool to be used to bend the allograft, fix it in the bent configuration, and place the allograft in its proper position in the hand during anchoring of the bent allograft to the native trapezium. The final two studies presented in this dissertation focus on furthering our current understanding of wear and structure-function relationships of articular cartilage. We hypothesize that cartilage wears due to fatigue failure in reciprocating compression instead of reciprocating friction. Chapter 4 compares reciprocating sliding of immature bovine articular cartilage against glass in two testing configurations: (1) a stationary contact area configuration (SCA), which results in static compression, interstitial fluid depressurization and increasing friction coefficient during reciprocating sliding, and (2) a migrating contact area configuration (MCA), which maintains fluid pressurization and low friction while producing reciprocating compressive loading during reciprocating sliding. Contact stress, sliding duration, and sliding distance were controlled to be similar between test groups. SCA tests exhibited an average friction coefficient of μ=0.084±0.032, while MCA tests exhibited a lower average friction coefficient of μ=0.020±0.008 (p<10^(-4)). Despite the lower friction, MCA cartilage samples exhibited clear surface damage with a significantly greater average surface deviation from a fitted plane after wear testing (R_q=0.125±0.095 mm) than cartilage samples slid in a SCA configuration (R_q=0.044±0.017 mm, p=0.002), which showed minimal signs of wear. Polarized light microscopy confirmed that delamination damage occurred between the superficial and middle zones of the articular cartilage in MCA samples. The greatest wear was observed in the group with lowest friction coefficient, subjected to cyclical instead of static compression, implying that friction is not the primary driver of cartilage wear. Delamination between superficial and middle zones imply the main mode of wear is fatigue failure under cyclical compression, not fatigue or abrasion due to reciprocating frictional sliding. The final study of this dissertation, presented in Chapter 5, investigates the importance of collagen fibril distribution in articular cartilage computational models. Finite element models were created to approximate a bovine humeral head and replicate previous experimental loading conditions [3]. Five different finite element analyses were run, each using a different fibril distribution model. Three of the models used two, four, or eight discrete fibril bundles, while two models used continuous fibril distributions with either isotropic or depth-dependent ellipsoidal distributions. Two primary findings arose from this investigation. The first was the discovery that as the fibril distribution became more isotropic, the strain throughout the tissue decreased, even though the contact area between the articular surface and rigid platen remained relatively equal across distribution models. This suggests that computational models which approximate the collagen fibrils with an isotropic distribution may be underestimating the strain through the depth of the tissue. The second primary finding was that in the discrete distribution model with two fibril bundles, which followed the classically described Benninghoff structure [4], the greatest magnitude of shear strain during compressive loading was observed in the middle zone. However, the highest magnitude of shear strain observed in the isotropic fibril distribution model occurred in the deep zone near the subchondral surface. The observed results suggest that the type of fibril distribution used to model collagen in articular cartilage plays a role in depth-dependent strain magnitude and strain distribution

A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond

Since the first successful synthesis of graphene just over a decade ago, a variety of two-dimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene

옥세틱 구조개발을 통한 소프트 재료의 변형특성 설계 및 유연소자 적용에 대한 연구

학위논문 (박사)-- 서울대학교 대학원 공과대학 재료공학부, 2017. 8. 주영창.As the development of flexible devices has progressed for user's convenience, researches on the rational design of architectures and structures which could provide mechanical functionality to flexible devices, have been actively carried out as well as material development perspective. The control of mechanical properties through structural innovation has the advantage that it provides unprecedented properties beyond the existing material limits and is easy to design predictably. Based on this feature, there is a growing interest in reconfigurable materials that could operate in response to external physical stimuli. Auxetics, which are one of the mechanically reconfigurable materials, is a structure which are able to perform negative Poisson's ratio behavior. Based on the characteristics, auxetics have an excellent expandability and possible to maintain an excellent conformability even on a non-zero Gaussian surface. Therefore, auxetics are an attracting attention as a structural material for a next generation flexible device. In this thesis, the mechanical and electrical performance of flexible devices were improved through proper geometric design of two dimensional auxetic structure, and suggested auxetics as a new paradigm of structural materials for flexible devices. At first, a platform for flexible devices was proposed, which was capable of large displacement in all directions by using a rotational unit auxetic including a self-similar hierarchical structure. Through the finite element analysis, it was proved that even if the hierarchical auxetic was subjected to complicated deformation, not only the tension but also crumpling, the deformation could be concentrated only in the hinges connecting the individual unit. Based on the deformation characteristics, an omnidirectionally and extremely deformable battery was developed. When the hinge was composed of an elastomer having excellent mechanical reliability, it was possible to deform the hinge in an unexpected three dimensional manner beyond the viewpoint of the conventional two dimensional auxetic view. As a result, the degree of freedom of hinge deformation could be increased to infinity. Also, as the level of the hierarchical structure increased, the strain concentrated on the hinge is relaxed even at the same level of strain, thereby improving the mechanical stability and improving the stretchability and be crumpled easily. In addition, it was possible to design the same hierarchical auxetic in a thin plastic substrate through the cutting process. In this case, the sharp cut pattern could cause tearing, which could result in severe mechanical failure. To improve the mechanical reliability at the hinge, a design to prevent the crack propagation was proposed, thereby confirmed the potential ability of applying the hierarchical auxetic to thin sheets. Secondly, a hybrid auxetic composite had been developed which could be predictably design the modulus of elasticity and Poisson's ratio, by embedding two dimensional re-entrant auxetic as a composite scaffold into soft material. This composite could have an anisotropic deformation behavior due to the influence of re-entrant structure, which had a negative Poisson's ratio behavior in in-plane and a positive Poisson's ratio in the normal direction. Especially, the thickness of matrix in the composite could be decreased even more compared to the conventional isotropic materials due to the volume conservation. Using the anisotropic mechanical property of composite, a stretchable capacitive strain sensor having improved sensing property was developed which could stretch up to 50 %. The Gauge factor, which is a ratio of change in capacitance to the stretch, of the conventional capacitive strain sensor was limited to 1 due to the geometric factor. Applying the auxetic composite as a dielectric, sharper capacitance change was achieved even under the same degree of stretch compared to the conventional sensor. In addition to the improvement, the composites could represent a good conformability to such as elbows and knees, thus our composites could suggest a new direction in geometry design for wearable sensors. This thesis suggested auxetics as a new paradigm as a structural material for flexible devices by improving the mechanical and electrical performance of various flexible devices that are currently being developed, by properly customizing the architectural design.Chapter 1. Introduction 1.1. Emergence of soft electronics ......... 1 1.2. Necessity of geometric design for soft electronics .. 4 1.3. Auxetic: A new geometric concept for soft electronics ............ 6 1.4. Thesis objectives ....................................................... 12 1.5. Organization of the thesis ....................................................... 13 Chapter 2. Theoretical Background 2.1. Neo-Hookean solid ..................................... 14 2.2. Theoretical limit in Poissons ratio of istropic material ......... 16 2.3. Auxetic ......... 18 2.3.1. Classification of auxetic ................................................ 18 2.3.2. Mechanical model for re-entrant auxetic ................. 21 2.4. Mechanism of strain sensor ................................................ 23 Chapter 3. Hierarchical Auxetic for Extremely deformable Device Platform 3.1. Introduction ................................................................. 25 3.2. 3-dimensional deformation of soft hierarchical auxetic ............. 29 3.3. Experimental procedure ........................... 37 3.4. FEM for 3-D deformation of hierarchical auxetic .... 41 3.4.1. Tensile deformation model ........................................... 43 3.4.2. Crumple deformation model ........................................... 47 3.4.3. Motif array dependence ........................................... 53 3.5. Experimental realization ............................................... 56 3.4.1. Mechanical reliability confirmation of hinge ............ 56 3.4.2. Omni-directionally deformable batteries ............ 59 3.4.3. Hierarchical auxetic design for thin film application ......... 61 3.6. Summary ................................................................... 65 Chapter 4. Tunable Elastic Property of Soft Materials by Auxetic Composites and Strain Sensing Application 4.1. Introduction .............................................................................. 66 4.2. Experimental procedure ...................................... 71 4.2.1. Fabrication of auxetic composite ...................................... 71 4.2.2. Fabrication and measurement of strain sensor ...................... 75 4.3. Tensile behavior of auxetic composite .... 77 4.4. Auxetic composite design through FEM .... 81 4.4.1 Determination of elastic property of composite element .... 81 4.4.2. Stretch direction dependence of re-entrant auxetic .... 83 4.4.3. Boundary condition for stretch of auxetic composite .... 86 4.4.4. Geometric dependence ............................................ 92 4.4.5. Material dependence ............................................ 101 4.4.6. Thickness dependence of auxetic composite .... 103 4.5. Hybrid auxetic composite for capacitive strain sensor .... 109 4.5.1. Performance of auxetic strain sensor ................................ 110 4.5.2. Capacitance calculation by FEM ................................ 112 4.5.3. Analytic model for predicting gauge factor ........................ 115 4.6. Summary ...................................................................................... 121 Chapter 5. Conclusion 5.1. Summary of results .................................................................... 122 5.2. Future work and suggested research ...................................... 124Docto

An Application of Optimized Bistable Laminates as a Low Velocity, Low Impact Mechanical Deterrent

This research considers the problem of using bistable laminates as a mechanical deterrent to the impending impact of a particle. The structure will be controlled through an algorithm that will utilize piezoelectric devices to activate them in unison with the bistable laminate to successfully deter. A novel experimental setup will be constructed to ensure that the bistable laminate stays fixed when acting as a mechanical deterrent. Piezoelectricity is the main driving force of the bistable laminate to morph and this study will use a Macro Fiber Composite (MFC) actuator that contains piezoelectric ceramic rods in a patch to transfer electrical energy into mechanical action. The bistability of the composite laminate is the ability to morph between two stable forms of the stacked laminate that will act as the moving element to deflect the incoming particle. The bistable mechanism containing the piezoelectric patch and bistable composite will undergo an optimization algorithm to maximize the chances of a successful deflection event. Having greater distance between states increases the chances of ensuring proper contact with the particle. Optimization can be utilized to maximize the total deflection between states of the bistable composite structure while also maximizing the piezoelectric limits. Areas that influence the bistable laminate such as deformation amount, edge lengths, and MFC patch compatibility will be included in the optimization algorithm. The MFC patch will influence the mechanism based on its active lengths and free strain. For this application-based approach, three different sizes of MFC piezoelectric patches will be used. Based on the particle\u27s characteristics, the timing of the bistable composite mechanism with the MFC patch will be rigorously studied to ensure proper deflection or reduction of impact through a Data Acquisition System and High Voltage Amplifier

Interfacial fracture of micro thin film interconnects under monotonic and cyclic loading

The goal of this research was to develop new experimental techniques to quantitatively study the interfacial fracture of micro-contact thin film interconnects used in microelectronic applications under monotonic and cyclic loadings. The micro-contact spring is a new technology that is based on physical vapor deposited thin film cantilevers with a purposely-imposed stress gradient through the thickness of the film. These "springs" have the promise of being the solution to address near-term wafer level probing and long-term high-density chip-to-next level microelectronic packaging challenges, as outlined by the International Technology Roadmap for Semiconductors. The success of this technology is, in part, dependent on the ability to understand the failure mechanism under monotonic and cyclic loadings. This research proposes two experimental methods to understand the interfacial fracture under such monotonic and fatigue loading conditions. To understand interfacial fracture under monotonic loading, a fixtureless superlayer-based delamination test has been developed. Using stress-engineered Cr layer and a release layer with varying width, this test can be used to measure interfacial fracture toughness under a wide range of mode mixity. This test uses common IC fabrication techniques and overcomes the shortcomings of available methods. The developed test has been used to measure the interfacial fracture toughness for Ti/Si interface. It was found that for low mode mixity Ti/Si thin film interfaces, the fracture toughness approaches the work of adhesion which is essentially the Ti-Si bond energy for a given bond density. In addition to the monotonic decohesion test, a fixtureless fatigue test is developed to investigate the interfacial crack propagation. Using a ferromagnetic material deposited on the micro-contact spring, this test employs an external magnetic field to be able to drive the interfacial crack. Fatigue crack growth can be monitored by E-beam lithography patterned metal traces that are 10 to 40nm wide and 1 to a few µm in spacing. The crack initiation and propagation can be monitored through electrical resistance measurement. In the conducted experiments, it is seen that the interfacial delamination does not occur under fatigue loading, and that the micro-contact springs are robust against interfacial fracture for probing and packaging applications.Ph.D.Committee Chair: Sitaraman, Suresh; Committee Member: Degertekin,Levent; Committee Member: McDowell, David; Committee Member: Tummala,Rao; Committee Member: Vandentop, Gilroy; Committee Member: Wang, Zhong Li

Ultra-thin silicon technology for tactile sensors

In order to meet the requirements of high performance flexible electronics in fast growing portable consumer electronics, robotics and new fields such as Internet of Things (IoT), new techniques such as electronics based on nanostructures, molecular electronics and quantum electronics have emerged recently. The importance given to the silicon chips with thickness below 50 μm is particularly interesting as this will advance the 3D IC technology as well as open new directions for high-performance flexible electronics. This doctoral thesis focusses on the development of silicon–based ultra-thin chip (UTC) for the next generation flexible electronics. UTCs, on one hand can provide processing speed at par with state-of-the-art CMOS technology, and on the other provide the mechanical flexibility to allow smooth integration on flexible substrates. These development form the motivation behind the work presented in this thesis. As the thickness of any silicon piece decreases, the flexural rigidity decreases. The flexural rigidity is defined as the force couple required to bend a non-rigid structure to a unit curvature, and therefore the flexibility increases. The new approach presented in this thesis for achieving thin silicon exploits existing and well-established silicon infrastructure, process, and design modules. The thin chips of thicknesses ranging between 15 μm – 30 μm, were obtained from processed bulk wafer using anisotropic chemical etching. The thesis also presents thin wafer transfer using two-step transfer printing approach, packaging by lamination or encapsulation between two flexible layerand methods to get the electrical connections out of the chip. The devices realised on the wafer as part of front-end processing, consisted capacitors and transistors, have been tested to analyse the effect of bending on the electrical characteristics. The capacitance of metal-oxide-semiconductor (MOS) capacitors increases by ~5% during bending and similar shift is observed in flatband and threshold voltages. Similarly, the carrier mobility in the channel region of metal-oxide-semiconductor field effect transistor (MOSFET) increases by 9% in tensile bending and decreases by ~5% in compressive bending. The analytical model developed to capture the effect of banding on device performance showed close matching with the experimental results. In order to employ these devices as tactile sensors, two types of piezoelectric materials are investigated, and used in extended gate configuration with the MOSFET. Firstly, a nanocomposite of Poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE) and barium titanate (BT) was developed. The composite, due to opposite piezo and pyroelectric coefficients of constituents, was able to suppress the sensitivity towards temperature when force and temperature varied together, The sensitivity to force in extended gate configuration was measured to be 630 mV/N, and sensitivity to temperature was 6.57 mV/oC, when it was varied during force application. The process optimisation for sputtering piezoelectric Aluminium Nitride (AlN) was also carried out with many parametric variation. AlN does not require poling to exhibit piezoelectricity and therefore offers an attractive alternative for the piezoelectric layer used in devices such as POSFET (where piezoelectric material is directly deposited over the gate area of MOSFET). The optimised process gave highly orientated columnar structure AlN with piezoelectric coefficient of 5.9 pC/N and when connected in extended gate configuration, a sensitivity (normalised change in drain current per unit force) of 2.65 N-1 was obtained

Development of Analytical Models for Evaluating the Mechanical and Electrochemical Response of Flexible and Stretchable Lithium Ion Battery Materials

Flexible and stretchable batteries have become a highly active area of research in recent years due to a new demand for mechanically compliant energy storage devices for a wide range of flexible applications including wearable and implantable electronics, touch-screens, and smart technology. Lithium ion batteries are leading candidates for flexible and stretchable energy storage devices due to their high energy density and efficiency. Considerable research has been related to developing flexible and stretchable materials, and solid polymer electrolyte lithium ion batteries show promise, offering many mechanical and safety advantages. While much experimental work has been in the development of these batteries, considerably less analytical modeling and numerical work has been a part of the development, which would elucidate experimental observations and provide enhanced understanding of the materials behavior in these new batteries. The work presented in this dissertation includes results of computational modeling and simulation of the mechanical and electrochemical behavior of flexible and stretchable battery materials under normal operating conditions and applied deformations resulting from mechanical loads. Additionally, analytical multiphysics models in the form of series of differential equations were derived to explain experimental observations of changes in battery performance and material properties due to applied loads and deformations. These models can be used to predict materials behavior and to relate key design parameters of flexible and stretchable batteries. The battery materials and designs that are assessed in this work were developed in our lab. Objectives of this work include understanding relationships between mechanical loading and certain key controllable fabrication parameters such as layer interface contact properties to predict the influence on flexible battery performance, and exploring how deformation occurring in the polymer electrolyte due to an applied mechanical load influences electrochemical performance. The effect of loading on other performance parameters, including battery impedance, is further studied, and all analytical work is compared to experimental data. An important aspect of this development is the consideration of nonlinearity in the models. Novel approaches are taken to include and address nonlinearity within the systems considered. While these models can be simplified through linearization, limitations of linear solutions are also discussed.Mechanical Engineering, Department o