Energy absorption mechanisms during crack propagation in metal matrix composites

The stress distributions around individual fibers in a unidirectional boron/aluminum composite material subjected to axial and transverse loadings are being studied utilizing a generalized plane strain finite element analysis. This micromechanics analysis was modified to permit the analysis of longitudinal sections, and also to incorporate crack initiation and propagation. The analysis fully models the elastoplastic response of the aluminum matrix, as well as temperature dependent material properties and thermal stress effects. The micromechanics analysis modifications are described, and numerical results are given for both longitudinal and transverse models loaded into the inelastic range, to first failure. Included are initially cracked fiber models

ADAPTIVE GRAVITY BALANCING ARM SYSTEMS

Upadhyay, Harshal. M.S.M.E., Purdue University, May 2016. Adaptive Gravity-Balancing Arm Systems. Major Professor: Justin Seipel, School of Mechanical Engineering

Multiscale Modeling of Smart Materials under Static and Dynamic Thermo-mechanical Loading

Engineering material systems with tailored capabilities are a topic trend in plethora of research. Polymer based Artificial Muscle, PAM, and Shape memory polymer fiber, SMPF, enable structural engineers to incorporate smartness functionality into their design through programming cycles. Smartness functionality leads to the production of artificial muscles with different load carrying capability. SMPF is another category of smart materials, which are capable of being micro-structurally engineered to isolate vibration at different temperature and frequency conditions. The smartness functionality offers the adjustment between inherent properties of these materials with their industrial applications through modeling techniques. Mixture of phenomenological, numerical, mathematical models provides phenomenological Multiscale model to study effect of thermal fluctuation on mechanical response of polymer based artificial muscle. This model provides an insight to the nature of thermo mechanical response at macroscopic scale as well as the theory behind stress-strain evolution over working temperature. Multiscale modeling techniques is applied to study dynamic response through relating the damping and storage properties of a smart material, SMPF, to the stiffness and damping coefficient of a single degree of freedom system, SDOF. Damping coefficient, c, is related to the loss factor and natural frequency of the system; equivalent stiffness, k, is correlated to the storage modulus and geometry of the specimen

Carbon nanotube forests: synthesis, patterning, milling, mechanics, and applications

Carbon nanotube (CNT) forests are a nano-structured material consisting of vertically oriented carbon nanotubes. The material has many exceptional and unique physical properties that have motivated considerable research in the past 10 years. CNT forests hold promise for diverse applications in the fields of thermal interfaces, mechanical interfaces, electrical interconnects, molecular sensors, battery cathodes, energy storage devices, and low reflectance coatings. This thesis seeks to introduce the reader to nanotechnology and nano-carbon materials, and explain several processes for creating, patterning, shaping, modifying physical properties, testing, and using carbon nanotube forests in novel applications. The thesis is organized so that each chapter describes an individual project or publication and is independent of other chapters.Includes bibliographical reference

Graphene Foam Reinforced Shape Memory Polymer Epoxy Composites

Shape memory polymer (SMP) epoxy has received growing interest due to its facile processing, low density, and high recoverable strain. Despite these positive attributes, SMP epoxy has drawbacks such as slow recovery rate, and inferior mechanical properties. The slow recovery rate restricts the use of SMP epoxy as a functional structure. The aim of the present work is to explore the capabilities of three-dimensional (3D) graphene foam (GrF) and graphene nanoplatelet (GNP) as reinforcements in SMP epoxy to overcome their slow recovery and improve the mechanical properties. GrF and GNP based SMP epoxy composites are fabricated by mold-casting approach and 3D printing techniques, respectively. They are investigated for their thermal, shape recovery, and mechanical behaviors. 0.13 wt.% GrF addition results in 19% increase in the glass transition temperature (Tg) of mold-cast SMP epoxy. GrF-based SMP epoxy composite displays thermal conductivity of 0.296 W mk-1 at 70oC, which is 57% greater than that of SMP epoxy. The addition of GrF results in excellent thermal and electrical conductivity of SMP epoxy by providing a continuous network of graphene for phonon and electron flow, respectively. Thus, thermal and electrical stimulation are employed to actuate shape recovery in GrF-reinforced SMP epoxy composite. Maximum shape recovery ratio is achieved for thermally actuated GrF-based SMP epoxy composite with a 23% improvement in the recovery rate. GrF addition transforms a non-electrically conductive SMP epoxy to an electrically conductive polymer. Moreover, 0.5 wt.% GrF integration enhances tensile strength and elastic modulus of SMP epoxy by 6% and 20%, respectively which is attributed to excellent stress transfer from matrix to GrF reinforcement. Damping behavior of of SMP epoxy -0.5 wt.% GrF is also improved by 180%, respectively. SMP epoxy-GNP composite is successfully 3D printed using a slurry-based extrusion technique. 3D printed composites exhibit complete shape recovery. A mere 0.1 wt.% GNP addition resulted in enhanced tensile strength (30%) and elastic modulus (17%). Damping behavior of 3D printed of SMP epoxy-GNP composite is also improved by 50% (below its Tg) as compared to 3D printed SMP epoxy. This study demonstrates that graphene-based reinforcement endow SMP epoxy with multifunctional capabilities; thereby paving the way for a new generation of advanced shape memory polymer composite, finding potential applications in electro-mechanical systems, micro-robots and morphing wing of an aircraft

Respiratory, postural and spatio-kinetic motor stabilization, internal models, top-down timed motor coordination and expanded cerebello-cerebral circuitry: a review

Human dexterity, bipedality, and song/speech vocalization in Homo are reviewed within a motor evolution perspective in regard to 

(i) brain expansion in cerebello-cerebral circuitry, 
(ii) enhanced predictive internal modeling of body kinematics, body kinetics and action organization, 
(iii) motor mastery due to prolonged practice, 
(iv) task-determined top-down, and accurately timed feedforward motor adjustment of multiple-body/artifact elements, and 
(v) reduction in automatic preflex/spinal reflex mechanisms that would otherwise restrict such top-down processes. 

Dual-task interference and developmental neuroimaging research argues that such internal modeling based motor capabilities are concomitant with the evolution of 
(vi) enhanced attentional, executive function and other high-level cognitive processes, and that 
(vii) these provide dexterity, bipedality and vocalization with effector nonspecific neural resources. 

The possibility is also raised that such neural resources could 
(viii) underlie human internal model based nonmotor cognitions. 
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An Overview on Principles for Energy Efficient Robot Locomotion

Despite enhancements in the development of robotic systems, the energy economy of today's robots lags far behind that of biological systems. This is in particular critical for untethered legged robot locomotion. To elucidate the current stage of energy efficiency in legged robotic systems, this paper provides an overview on recent advancements in development of such platforms. The covered different perspectives include actuation, leg structure, control and locomotion principles. We review various robotic actuators exploiting compliance in series and in parallel with the drive-train to permit energy recycling during locomotion. We discuss the importance of limb segmentation under efficiency aspects and with respect to design, dynamics analysis and control of legged robots. This paper also reviews a number of control approaches allowing for energy efficient locomotion of robots by exploiting the natural dynamics of the system, and by utilizing optimal control approaches targeting locomotion expenditure. To this end, a set of locomotion principles elaborating on models for energetics, dynamics, and of the systems is studied

Passive Variable Compliance for Dynamic Legged Robots

Recent developments in legged robotics have found that constant stiffness passive compliant legs are an effective mechanism for enabling dynamic locomotion. In spite of its success, one of the limitations of this approach is reduced adaptability. The final leg mechanism usually performs optimally for a small range of conditions such as the desired speed, payload, and terrain. For many situations in which a small locomotion system experiences a change in any of these conditions, it is desirable to have a tunable stiffness leg for effective gait control. To date, the mechanical complexities of designing usefully robust tunable passive compliance into legs has precluded their implementation on practical running robots. In this thesis we present an overview of tunable stiffness legs, and introduce a simple leg model that captures the spatial compliance of our tunable leg. We present experimental evidence supporting the advantages of tunable stiffness legs, and implement what we believe is the first autonomous dynamic legged robot capable of automatic leg stiffness adjustment. Finally we discuss design objectives, material considerations, and manufacturing methods that lead to robust passive compliant legs

Musculoskeletal stiffness and Achilles tendon mechanical property changes following exercise-induced muscle damage

This thesis investigated the affect of exercise induced muscle damage (EIMD) on musculoskeletal stiffness (MSS), Achilles tendon (AT) stiffness and AT strain. Furthermore, this thesis determined the reliability of a protocol used to measure MSS with the aim to apply this protocol in the investigation of the EIMD associated changes in MSS, AT stiffness and AT strain. Three studies were conducted as part of this thesis