The rapid growth of plastic waste from food packaging around the world demands renewable substitutes, such as natural fibers and biocomposites. Wood fibers are natural fibers extracted from trees and are commonly used in packaging. In order for renewable alternatives to compete against plastics and other non-renewable materials, a better understanding of the mechanical properties of single fibers at the micro-scale are necessary. A great deal of previous research into the mechanical properties of single wood fibers has focused on their tensile behavior, however, little work has been published about their compressive behavior. It is difficult to measure the compressive strength of single fibers directly due to fiber buckling. The purpose of this study is to investigate how post-buckling of single wood pulp fibers affects the mechanical properties of fibers in tension. Two alternative hypotheses were tested through experiments in The Odqvist Laboratory for Experimental Mechanics at KTH. The major part of the thesis process has been invested in developing components called grippers, and testing methods for the Single Fiber Testing System, in order to be able to perform the experiments. The existing grippers were tested and alternative grippers were developed, as well as an alternative testing method without grippers, called the Paper frame method (PFM). PFM was used in the final experimental work to test the hypotheses. The main finding from this study is that there is not enough evidence to suggest that the tensile strength or tensile stiffness of single wood fibers are significantly reduced by post-buckling damage. This finding is mostly relevant in the research and development of fibrous material with larger distances between individual fibers, such as low-density fiber network materials. The main findings from the single fiber testing methods development were that the existing grippers cannot prevent fiber slippage. Furthermore, the alternative gripper 22A with its arc design generates higher grip force than previous grippers but lacks surface friction in the contact region in order to prevent fiber slippage. PFM has an experimental success rate of over 80 % for trained users and easy usage for the operator. The testing equipment Single Fiber Testing System displays several systematic errors occurring in the post-processing process of tests with cyclic loads. / Den snabba tillväxten av plastavfall från livsmedelsförpackningar runt om i världen kräver förnybara alternativ, såsom förpackningar gjorda av naturfibrer och biokompositer. Träfibrer är naturliga fibrer som utvinns från trä och används ofta i förpackningar. För att dessa förnybara alternativ ska kunna konkurrera mot plast och andra icke-förnybara material krävs en bättre förståelse av de mekaniska egenskaperna hos enskilda fibrer på mikronivå. Det finns en omfattande forskning om de mekaniska egenskaperna i drag hos enskilda träfibrer. Däremot existerar det lite publicerad forskning om träfibrers kompressionsegenskaper. Kompressionsegenskaperna är svåra att mäta direkt på grund av fiberknäckning. Syftet med denna studie är att undersöka hur skadan som uppstår efter knäckning av enskilda träfibrer påverkar de mekaniska egenskaperna hos fibrer i drag. Två alternativa hypoteser testades genom experiment i Odqvistlaboratoriet för experimentell mekanik vid KTH. Huvuddelen av examensarbetet har investerats i att utveckla grepparmar och testmetoder för testmaskinen Single Fiber Testing System, för att kunna utföra experiment. De befintliga grepparmarna testades och nya grepparmar utvecklades, och även en alternativ testmetod utan grepparmar som kallas Paper frame method (PFM) utvecklades. PFM användes i det sista experimentella arbetet för att pröva hypoteserna. Huvudslutsatsen från denna studie är att det inte finns tillräckligt med bevis för att stödja hypotesen att enskilda träfibrers draghållfasthet eller dragstyvhet reduceras av skada som uppstår efter knäckning. Detta resultat är mest relevant för forskning och utveckling av fibernätverks material med större avstånd mellan fibrerna, såsom fibermaterial med låg densitet. Huvudslutsatserna från utvecklingen av testmetoder var att de befintliga grepparmarna inte kunde förhindra fiberglidning. Den alternativa grepparmen 22A med sin bågkonstruktion genererade högre greppkraft än tidigare grepparmar men saknar rätt beläggning i kontaktområdet för att förhindra glidning av fiber. PFM har en hög test framgångsgrad med över 80 % för erfarna användare och den är enkel att arbeta med. Testmaskinen Single Fiber Testing System visar flera systematiska fel som blir märkbar under dataanalys av tester med cykliska belastningar.

<p dir="ltr">Structural instability, traditionally deemed undesirable in engineering, can be leveraged for beneficial outcomes through intelligent design. One notable instance is elastic buckling, often leading to structures with two stable equilibria (bistable). Connecting bistable elements to form multistable mechanical metamaterials can enable the discretization and offer tunability of mechanical properties without the need for continuous energy input.<i> </i>In this work, we study the physics of these multistable metamaterials and utilize their state and property alterations along with snap-through instabilities resulting from state change for engineering applications. These materials hold potential for diverse applications, including mechanical and thermo-mechanical defrosting, energy absorption, energy harvesting, and mechanical storage and computation.</p><p dir="ltr">Focusing on defrosting, we find that the energy-efficient mechanical method using embedded bistable structures in heat exchanger fins significantly outperforms the thermal methods. The combination of manufacturing methods, material choice, boundary conditions, and actuation methodologies is systematically investigated to enhance defrosting performance. A purely mechanical strategy is effective against solid, glaze-like ice accumulations; however, performance is substantially diminished for low-density frost. To address this limitation, we study frost formation on the angular shape morphing fins and subsequently introduce a thermo-mechanical defrosting strategy. This hybrid approach focuses on the partial phase transition of low-density frost to solid ice through thermal methods, followed by mechanical defrosting. We experimentally validate this approach on a multistable heat exchanger fin pack.</p><p dir="ltr">Recent advancements have led to a new paradigm of reusable energy-absorbing materials, known as Phase Transforming Cellular Materials (PXCM) that utilize multiple negative stiffness elements connected in series. We explore the feasibility of this multistable metamaterial as frequency up-conversion material and utilize these phase transformations for energy harvesting. We experimentally demonstrate the energy-harvesting capabilities of a phase-transforming unit-cell-spring configuration and investigate the potential of multicell PXCM as an energy harvesting material.</p><p dir="ltr">The evolution towards intelligent matter, or physical intelligence, in the context of mechanical metamaterials can be characterized into four distinct stages: static, responsive, adaptive, and intelligent mechanical metamaterials. In the pursuit of designing intelligent mechanical metamaterials, there has been a resurgence in the field of mechanical computing. We utilize multistable metamaterials to develop mechanical storage systems that encode memory via bistable state changes and decode it through a global stiffness readout. We establish upper bounds for maximum memory capacity in elastic bit blocks and propose an optimal stiffness distribution for unique and identifiable global states. Through both parallel and series configurations, we realize various logic gates, thereby enabling in-memory computation. We further extend this framework by incorporating viscoelastic mechano-bits, which mimic the decay of neuronal action potentials. This allows for temporal stiffness modulation and results in increased memory storage via non-abelian behavior, for which we define a fundamental time limit of detectability. Additionally, we investigate information entropy in both elastic and viscoelastic systems, showing that temporal neural coding schemes can extend the system’s entropy beyond conventional limits. This is experimentally validated and shown to not only enhance memory storage but also augment computational capabilities.</p><p dir="ltr">The work in this thesis establishes multistability as a key design principle for developing responsive, adaptive, and intelligent materials, opening new avenues for future research in the field of multistable metamaterials.</p>

<p dir="ltr">Origami has emerged as a design paradigm to realize morphing structures with rich kinematic and mechanical properties. Biological examples augment the potential folding design space by suggesting intriguing routes for achieving and expanding crease patterns which traditional origami laws are unable to capture. Specifically, spring origami theory exploits the material system architecture and energy storage mechanism of the earwig wing featuring one of the highest folding ratios in the animal kingdom (1:18), minimal energy required for deployment and collapse of the wing, and bistability locking the wing in closed, and open configurations for crawling through tunnels, and flight, respectively. The central mechanism responsible for bistability in the wing features a non-developable crease pattern with a non-zero Gaussian curvature. Reconfiguring, or even flattening a structure with such an intrinsic property requires stretching or tearing; soft, rubbery material found in the creases of the central mechanism allows for stretching enabling shape transformations between open and closed states without tearing. In the first part of this thesis, such characteristics are transferred to a synthetic bistable soft robotic gripper leveraging the shape adaptability and conformability exhibited by the biological organism to minimize actuation energy. This is achieved by integrating soft, flexible material in the bioinspired gripper that allows kinematically driven geometries to grasp and manipulate objects without continuous actuation. Secondly, the stiffening effect from spring origami is utilized in a bioinspired wing for an aerial--aquatic robot. Transitions between air and sea in multimodal robots is challenging, however, a structurally efficient and multifunctional membrane is developed to increase locomotive capabilities and longer flights. This is motivated by the flying fish's locomotive modules and origami design principles for deployment and folding. Additionally, to keep the wing in a stiff state while gliding, spring origami bistable units are integrated into the membrane inducing self-stiffening and a global curvature reducing energy expenditure while generating lift. While the previous examples present solutions to adaptive manipulation and membrane multifunctionality, once programmed, their shapes are fixed. In the third application, a class of multistable self-folding origami architectures that are reprogrammable post fabrication are presented. This is achieved by encoding prestrain in bilayer creases with anisotropic shrinkage that change shape and induce a local curvature in the creases in response to external stimuli. The topology of the energy landscapes can thus be tuned as a function of the stimulation time and adaptable post fabrication. The proposed method and model allows for converting flat sheets with arranged facets and prestrained mountain-valley creases into self-folding multistable structures. Lasty, encoding crease prestrain is leveraged to manufacture a biomimetic earwig wing featuring the complex crease pattern, structural stability, and rapid closure of the biological counterpart. The presented method provides a route for encoding prestrain in self-folding origami, the multistability of which is adaptable after fabrication.</p>

<p>    </p> <p>Additive manufacturing (or 3D printing) is one of the manufacturing processes which is currently being explored for its applicability under space boundary conditions, also known as in-space manufacturing. The space boundary conditions specifically affect material properties which in turn affect the printability of materials in space. Printing of soft materials in space is a novel application and the intent of this research was to print the softest of materials: edible materials, as a case study. 3D food printing is a novel food delivery method of using food products to either reproduce as a more aesthetically pleasing product or to print more nutrient-diverse foods. Launch of payload carrier and the boundary conditions of low Earth orbit including a vacuum environment, microgravity, temperature fluctuations, etc. These conditions make printing difficult, and my thesis is to overcome the boundary conditions (except microgravity) using a 3D soft material printer operating within a CubeSat. A CubeSat is a small satellite usually launched as an auxiliary payload used for basic Earth observation and radio communication. The printer must be able to survive launch and operation conditions, print within a simulated space environment, and adhere to the American Society for Testing and Materials (ASTM) specific definition of additive manufacturing. The 3D soft material printer was designed, fabricated, and tested using space and CubeSat boundary conditions for determining optimal design. Testing conditions including: (1) printing under Earth conditions showing it follows ASTM standards, (2) surviving NASA standards for vibration testing for microsatellites under launch conditions, (3) completing a print under a vacuum setting. The results of the testing would prove a small microsatellite could print in the vacuum of space and survive launch parameters. Further work would provide insight into the design of food printers being readily available in smaller sizes and its operability in microgravity condition. </p>

<p dir="ltr">We investigate the crystal plasticity and shock response of single crystal and polycrystalline pentaerythritol tetranitrate (PETN) using mesoscale finite element simulations. The model includes the Mie-Grüneisen Equation of State and a single crystal plasticity model. Simulations with single crystals with different orientations are tested using our plasticity model under shock compression to explore shear stress and slip. Parameters regarding the Mie-Grüneisen Equation of State are also verified in various orientations from 0.50 to 1.75 km/s. A polycrystalline PETN sample with varying grain sizes and orientations are subjected to shock loading with impact velocities ranging from 0.25 to 0.75 km/s. We study how differences in shock orientation affect slip and stress in PETN at different shock strengths.</p>

In the absence of a fusion neutron source, research on the structural integrity of materials in the fusion environment relies on current fission data and simulation methods. Through investigation of the Fe-Cr system, this detailed study explores the challenges and limitations in the use of currently available radiation sources for fusion materials research. An investigation of ion-irradiated Fe12%Cr using nanoindentation with a cube corner, Berkovich and spherical tip, and micro-cantilever testing with two different geometries, highlighted that the measurement of irradiation hardening was largely dependent on the type of test used. Selected methods were used for the comparison of Fe6%Cr irradiated by ions and neutrons to a dose of 1.7dpa at a temperature of 288&deg;C. Micro-cantilever tests of the Fe6%Cr alloy with beam depths of 400 to 7000nm, identified that size effects may significantly obscure irradiation hardening and that these effects are dependent on radiation conditions. Irradiation hardening in the neutron-irradiated alloy was approximately double that of the ion-irradiated alloy and exhibited increased work hardening. Similar differences in hardening were observed in an Fe5%Cr alloy after ion-irradiation to a dose of 0.6dpa at 400&deg;C and doses rates of 6 x 10^-4dpa/s and 3 x 10^-5dpa/s. Identified by APT, it was shown that increased irradiation hardening was likely to be caused by the enhanced segregation of Cr observed in the alloy irradiated with the lower dose rate. These observations have significant implications for future fusion materials research in terms of the simulation of fusion relevant radiation conditions and micro-mechanical testing.

A need to find better achievement has always been required in the space industrythrough time. Advanced technologies are provided to accomplish goals for humanityfor space explorer and space missions, to apprehend answers and widen knowledges. These are the goals of improvement, and in this thesis, is to strive and demandto understand and improve the mass of a space nozzle, utilized in an upperstage of space mission, with an expander cycle engine. The study is carried out by creating design of experiment using Latin HypercubeSampling (LHS) with a consideration to number of design and simulation expense.A surrogate model based optimization with Multidisciplinary Design Optimization(MDO) method for two different approaches, Analytical Target Cascading (ATC) and Multidisciplinary Feasible (MDF) are used for comparison and emend the conclusion. In the optimization, three different limitations are being investigated, designspace limit, industrial limit and industrial limit with tolerance. Optimized results have shown an incompatibility between two optimization approaches, ATC and MDF which are expected to be similar, but for the two limitations, design space limit and industrial limit appear to be less agreeable. The ATC formalist in this case dictates by the main objective, where the children/subproblems only focus to find a solution that satisfies the main objective and its constraint. For the MDF, the main objective function is described as a single function and solved subject to all the constraints. Furthermore, the problem is not divided into subproblems as in the ATC. Surrogate model based optimization, its solution influences by the accuracy ofthe model, and this is being investigated with another DoE. A DoE of the full factorial analysis is created and selected to study in a region near the optimal solution.In such region, the result has evidently shown to be quite accurate for almost allthe surrogate models, except for max temperature, damage and strain at the hottestregion, with the largest common impact on inner wall thickness of the space nozzle. Results of the new structure of the space nozzle have shown an improvement of mass by ≈ 50%, ≈ 15% and ≈ -4%, for the three different limitations, design spacelimit, industrial limit and industrial limit with tolerance, relative to a reference value,and ≈ 10%, ≈ 35% and ≈ 25% cheaper to manufacture accordingly to the defined producibility model.

<div>The mechanical behaviors of different kinds of biological tissues, including muscle tissues, cortical bones, cancellous bones and skulls, were studied under various loading conditions to investigate their strain-rate sensitivities and loading-direction dependencies. Specifically, the compressive mechanical behaviors of porcine muscle were studied at quasi-static (<1/s) and intermediate (1/s─10^2/s) strain rates. Both the compressive and tensile mechanical behaviors of human muscle were investigated at quasi-static and intermediate strain rates. The effect of strain-rate and loading-direction on the compressive mechanical behaviors of human frontal skulls, with its entire sandwich structure intact, were also studied at quasi-static, intermediate and high (10^2/s─10^3/s) strain rates. The fracture behaviors of porcine cortical bone and cancellous bone were investigated at both quasi-static (0.01mm/s) and dynamic (~6.1 m/s) loading rates, with the entire failure process visualized, in real-time, using the phase contrast imaging technique. Research effort was also focused on studying the dynamic fracture behaviors, in terms of fracture initiation toughness and crack-growth resistance curve (R-curve), of porcine cortical bone in three loading directions: in-plane transverse, out-of-plane transverse and in-plane longitudinal. A hydraulic material testing system (MTS) was used to load all the biological tissues at quasi-static and intermediate loading rates. Experiments at high loading rates were performed on regular or modified Kolsky bars. Tomography of bone specimens was also performed to help understand their microstructures and obtain the basic material properties before mechanical characterizations. Experimental results found that both porcine muscle and human muscle exhibited non-linear and strain-rate dependent mechanical behaviors in the range from quasi-static (10^(-2)/s─1/s) to intermediate (1/s─10^2/s) loading rates. The porcine muscle showed no significant difference in the stress-strain curve between the along-fiber and transverse-to-fiber orientation, while it was found the human muscle was stiffer and stronger along fiber direction in tension than transverse-to fiber direction in compression. The human frontal skulls exhibited a highly loading-direction dependent mechanical behavior: higher ultimate strength, with an increasing ratio of 2, and higher elastic modulus, with an increasing ratio of 3, were found in tangential loading direction when compared with those in the radial direction. A transition from quasi-ductile to brittle compressive mechanical behaviors of human frontal skulls was also observed as loading rate increased from quasi-static to dynamic, as the elastic modulus was increased by factors of 4 and 2.5 in the radial and tangential loading directions, respectively. Experimental results also suggested that the strength in the radial direction was mainly depended on the diploë porosity while the diploë layer ratio played the predominant role in the tangential direction. For the fracture behaviors of bones, straight-through crack paths were observed in both the in-plane longitudinal cortical bone specimens and cancellous bone specimens, while the cracks were highly tortuous in the in-plane transverse cortical bone specimens. Although the extent of toughening mechanisms at dynamic loading rate was comparatively diminished, crack deflections and twists at osteon cement lines were still observed in the transversely oriented cortical bone specimens at not only quasi-static loading rate but also dynamic loading rate. The locations of fracture initiations were found statistical independent on the bone type, while the propagation direction of incipient crack was significantly dependent on the loading direction in cortical bone and largely varied among different types of bones (cortical bone and cancellous bone). In addition, the crack propagation velocities were dependent on crack extension over the entire crack path for all the three loading directions while the initial velocity for in-plane direction was lower than the other two directions. Both the cortical bone and cancellous bone exhibited higher fracture initiation toughness and steeper R-curves at the quasi-static loading rate than the dynamic loading rate. For cortical bone at a dynamic loading rate (5.4 m/s), the R-curves were steepest, and the crack surfaces were most tortuous in the in-plane transverse direction while highly smooth crack paths and slowly growing R-curves were found in the in-plane longitudinal direction, suggesting an overall transition from brittle to ductile-like fracture behaviors as the osteon orientation varies from in-plane longitudinal to out-of-plane transverse, and to in-plane transverse eventually.</div>

<p dir="ltr">Positive displacement machines are used in various industries spanning the power spectrum, from industrial robotics to heavy construction equipment to aviation. These machines should be highly efficient, compact, and reliable. It is very advantageous for designers to use virtual simulations to design and improve the performance of these units as they significantly reduce cost and downtime. The recent trends of electrification and the goal to increase power density force these units to work at higher pressures and higher rotational speeds while maintaining their efficiencies and reliability. This push means that the simulation models need to advance to account for various aspects during the operation of these machines. </p><p dir="ltr">These machines typically have several bodies in relative motion with each other. Quantifying these motions and solving for their effect on the fluid enclosed are vital as they influence the machine's performance. The push towards higher rotational speeds introduces unwanted cavitation and aeration in these units. To model these effects, keeping the design evaluation time low is key for a designer. The lumped parameter approach offers the benefit of computational speed, but a major drawback that comes along with it is that it typically assumes fluid inertia to be negligible. These effects cannot be ignored, as quantifying and making design considerations to negate these effects can be beneficial. Therefore, this thesis addresses these key challenges of cavitation dynamics, body dynamics, and accounting for fluid inertia effects using a lumped parameter formulation.</p><p dir="ltr">To account for dynamics features related to cavitation, this thesis proposes a novel approach combining the two types of cavitation, i.e., gaseous and vaporous, by considering that both vapor and undissolved gas co-occupy a spherical bubble. The size of the spherical bubble is solved using the Rayleigh-Plesset equation, and the transfer of gas through the bubble interface is solved using Henry's Law and diffusion of the dissolved gas in the liquid. These equations are coupled with a novel pressure derivative equation. To account for body dynamics, this thesis introduces a novel approach for solving the positions of the bodies of a hydraulic machine while introducing new methods to solve contact dynamics and the application of Elasto Hydrodynamic Lubrication (EHL) friction at those contact locations. This thesis also proposes strategies to account for fluid inertia effects in a lumped parameter-based approach, taking as a reference an External Gear Machine. This thesis proposes a method to study the effects of fluid inertia on the pressurization and depressurization of the tooth space volumes of these units. The approach is based on considering the fluid inertia in the pressurization grooves and inside the control volumes with a peculiar sub-division. Further, frequency-dependent friction is also modeled to provide realistic damping of the fluid inside these channels.</p><p dir="ltr">To show the validity of the proposed dynamic cavitation model, the instantaneous pressure of a closed fluid volume undergoing expansion/compression is compared with multiple experimental sources, showing an improvement in accuracy compared to existing models. This modeling is then further applied to a gerotor machine and validated with experiments. Integrating this modeling technique with current displacement chamber simulation can further improve the understanding of cavitation in hydraulic systems. Formulations for body dynamics are tested on a prototype Gerotor and Vane unit. For both gerotor and vane units, comparisons of simulation results to experimental results for various dynamic quantities, such as pressure ripple, volumetric, and hydromechanical efficiency for multiple operating conditions, have been done. Extensive validation is performed for the case of gerotors where shaft torque ripple and the motion of the outer gear is experimentally validated. The thesis also comments on the distribution of the different torque loss contributions. The model for fluid inertia effects has been validated by comparing the lumped parameter model with a full three-dimensional Navier Stokes solver. The quantities compared, such as tooth space volume pressures and outlet volumetric flow rate, show a good match between the two approaches for varying operating speeds. A comparison with the experiments supports the modeling approach as well. The thesis also discusses which operating conditions and geometries play a significant role that governs the necessity to model such fluid inertia effects in the first place.</p>

<p dir="ltr">Additive manufacturing (AM) of gun propellants is an emerging and promising field which addresses the limitations of conventional manufacturing techniques. Overall, this thesis is a body of work which serves to bridge the gap between fundamental research and application of additively manufactured gun propellants.</p>