Shape memory alloys (SMA) have the unique ability to recover large strains and generate large recovery stresses via a repeatable martensitic transformation. Stress-strain and shape memory effect characteristics are needed in order to develop SMA force actuator design methods. Moreover, constitutive models able to quantitatively predict these characteristics and thus be useful as engineering design tools are also needed. An experimental apparatus designed to characterize the mechanical behavior of SMA was built and utilized. The apparatus is used specifically to gather stress-strain and shape memory effect characteristics from nitinol wire whereby mechanical properties associated with the material are determined. Phenomena such as the R-phase and stress induced martensite serration are investigated. A one-dimensional constitutive model is presented that quantitatively predicts stress-strain and shape memory effect behavior and was developed with the intention of being an engineering design tool for SMA force actuators. Experimental stress-strain and shape memory effect results are compared against that predicted by the model with the intention of verifying the model. The model displays the ability to predict stress-strain behavior that is in good quantitative agreement with experiment. The model also displays the ability to predict hysteric shape memory effect behavior for free, controlled, and restrained recovery cases of selected prestrains that is in good quantitative agreement with experiment. The model is unable to predict shape memory effect behavior such as the R-phase. Demonstrating the ability to experimentally investigate a constitutive model will hopefully inspire further combined experimental and theoretical SMA research. / Master of Science

The design of unmanned underwater vehicle (UUV) was inspired by the form and functionality of Jellyfish. These natural organisms were chosen as bio-inspiration for a multitude of reasons including: efficiency, good room for payload, and a wide range of sizes and morphology. Shape memory alloy (SMA) actuators were selected as the primary source of actuation for the propulsion of the artificial jellyfish node. These actuators offer high power density which enables a compact system size and silent operation which is preferred for surveillance.  SMA wires mimic the form and function of natural muscles; allowing for a wider range of applications than conventional actuators. Commercial SMA wires (100 um in diameter) can exhibit a 4% deformation of the initial actuator length with a blocking stress of over 200 MPa. The deformation of SMA wire is not enough to mimic the bell contraction of jellyfish. In order to resolve this problem, a beam-shape composite actuator using SMA wires as the active component, termed as BISMAC, was designed to provide large curvature. The BISMAC design was inspired by rowing jellyfish bell contraction. Characterization of maximum deformation in underwater conditions was performed for different actuator configurations to analyze the effect of design parameters that include silicone thickness, flexible steel thickness and distance between SMA and flexible steel. A constant cross-section (CC) BISMAC of 16 cm in length was found to achieve deformation with a radius of curvature of 3.5 cm. Under equilibrium conditions, the CC-BISMAC was found to achieve 80% of maximum deformation consuming 7.9 J per cycle driven at 16.2 V/0.98 A and frequency of 0.25 Hz. Using the a developed analytical model, an actuator design was fabricated mimicking the maximum deformation profile of the A. aurita. The optimized AA-BISMAC achieved a maximum curvature of 0.428 1/cm as compared to 0.438 1/cm for the A. aurita with an average squared root error of 0.043 (1/cm), 10.2% of maximum A. aurita curvature.   BISMAC actuators are unidirectional flexible actuators capable of exhibiting high curvature. To extend the application range of these actuators, they were modified to achieve bidirectional deformation. The new bidirectional actuators termed as "BiFlex" actuators had the capability to achieve large deformation in two directions. The FlexLegs consist of six segments which can be actuated individually. Two different sets of legs were constructed to determine the effect of size. The small legs measured 35.8 mm in height and 63.2 mm in width and the large legs were 97.4 mm in height and 165.4 mm in width. The small FlexLegs achieved a maximum deformation of 12 % and 4 % in the x- and y-direction respectively using a power of 0.7 W while producing a maximum force of 0.023 N. They were also able to withstand a load of 1.18 N. The large FlexLegs had a maximum deformation of 57 % and 39 % in the x- and y-direction respectively using a power of 3 W while producing a force of 0.045 N. They were able to withstand a load of 0.25 N. The legs were also able to perform several walking algorithms consisting of stepping, crabbing and yawing. In order to reduce the power consumption and contraction time of SMA wires, a feedback control scheme using wire resistance was developed. The controller required the knowledge of threshold resistance and safe current inputs which were determined experimentally. The overheating effect of SMA wires was analyzed for BioMetal Fiber (BMF) and Flexinol 100 "m diameter wires revealing an increase in resistance as the wires overheated. The controller was first characterized on a SMA wire with bias spring system for a BMF 100 using I_hi=0.5 A and I_low=0.2 A, where hi corresponds to peak current for fast actuation and low corresponds to the safe current which prevents overheating and maintains desired deformation. A contraction of 4.59% was achieved in 0.06 s using the controller and the deformation was maintained for 2 s at low current. The BISMAC actuator was operated using the controller with I_hi=1.1 A and I_low=0.65 A achieving a 67% decrease in contraction time compared to using a constant driving current of I_low=0.2 A and a 60% decrease in energy consumption compared to using constant I_hi=0.5 A while still exceeding the contraction requirements of the Aurelia aurita. Two fundamental parameters at the composition level were associated with the power consumption of SMA: i) martensite to austentite phase transition temperature and ii) thermal hysteresis. Ideally, one would like to reduce both these quantities and for this purpose an equiatomic Ni-Ti alloy was modified with Cu. Replacing nickel with 10 at% copper reduces the thermal hysteresis by 50% or more. For Ni-Ti alloys with nickel content greater than 50 at%, transition temperature decreases linearly at a rate of 100 "C/Ni at%. Given these two power reducing factors, an alloy with composition of Ni40+xTi50-xCu10 was synthesized with x = 0, ±1, ±2, ±3, ±4, ±5. Metal powders were melted in an argon atmosphere using an RF induction furnace to produce ingots. All the synthesized samples were characterized by differential scanning calorimetric (DSC) analysis to reveal martensite to austenite and austenite to martensite transition temperatures during heating and cooling cycles respectively. Scanning electron microscopy (SEM) was conducted to identify the density and microstructure of the fractured samples. The results show the possibility of achieving low power consuming high performance SMAs. Using the BISMAC actuator and feedback control system, a robotic jellyfish called Robojelly that mimics the morphology and kinematics of the Aurelia aurita species was created. A systematic fabrication technique was developed to replicate the essential structural features of A. aurita. Robojelly's body was fabricated from RTV silicone having a total mass of 242 g and bell diameter of 16.4 cm. Robojelly was able to generate enough thrust in static water conditions to propel itself and achieve a proficiency of 0.19 s-1 while the A. aurita achieves a proficiency of around 0.25 s-1. A thrust analysis based on empirical measurements for natural jellyfish was used to compare the performance of the different robotic configurations. The configuration with best performance was a Robojelly with segmented bell and a passive flap structure. Robojelly was found to consume an average power on the order of 17 W with the actuators not having fully reached thermal steady state. A comparative kinematics analysis was conducted between a natural Aurelia aurita and Robojelly. The resistance feedback controller was implemented to tailor the deformation profile of BISMAC actuators embedded in Robojelly. Robojelly's performance was quantified in terms of thrust production and power consumption during vertical swimming experiments. A maximum average instantaneous thrust production of 0.006 N was achieved at a driving current (Ihi) of 1.5 A with 35% duty cycle. Rapid heating of SMA wires was found to reduce power consumption and increase thrust. The bell kinematic analysis revealed resemblance and differences in bell deformation trajectories of the biomimetic and natural jellyfish. The inflexion point of the A. aurita was found to convert an inner bell trajectory into an outer one during contraction which assists the thrust production. A biomimetic robot inspired by Cyanea capillata, termed as "Cyro", was developed to meet the functional demands of underwater surveillance in defense and civilian applications. The design of Cyro required kinematics of large C. capillata which are elusive creatures. Obtaining accurate kinematic data of animals is essential for many biological studies and bio-inspired engineering applications. Many animals such as the C. capillata however, are either too large or too delicate to transport to controlled environments where accurate kinematic data can easily be obtained. Often, in situ recordings are the only means available but are often subject to multi-axis motion and relative magnification changes with time, which lead to large discrepancies in animal kinematics. In Chapter 5, techniques to compensate for magnification and body rotation of animal footage were developed. A background reference point and animal dimensions were used to account for magnification. A linear fit of body points were used to measure body rotation. These techniques help resolve animal kinematics from in situ video footage. The techniques were applied to a large jellyfish, Cyanea capillata, swimming in ocean waters. The bell kinematics were captured by digitizing exumbrella profiles for two full swimming cycles. Magnification was accounted for by tracking a reference point on the ocean floor and by tracking the C. capillata exumbrella arclength in order to have a constant scale through the swimming cycles. A linear fit of the top bell portion was used to find the body angle with respect to the camera coordinate system. Bell margin trajectories over two swimming cycles confirm the accuracy of the correction techniques. The corrected profiles were filtered and interpolated to provide a set of time-dependent points along the bell. The ability to use in situ footage with significant multi-axis motion provides an opportunity to analyze previously impractical footage for gaining a better understanding of large or delicate organisms. The swimming kinematics of the C. capillata were analyzed after extracting the required kinematics from the in situ video. A discrete model of the exumbrella was developed and used to analyze the kinematics. The exumbrella discretization was done using three different methods. The first method consists of analyzing the animal anatomy for structural and mechanical features. The second method consists of analyzing the bell kinematics for areas of highest deformation over time. The third method consists of optimizing node locations that can provide minimal error with comparison to the digitized profiles. Two kinematic models of the C. capillata swimming motion were developed by fitting Fourier series to the discretized segments and angles formed by each segment. The four-segment anatomical kinematic model was used to analyze the bell kinematics of the C. capillata. It was found that the bell does not deform uniformly over time with segments lagging behind others. Hysteresis between contraction and relaxation was also present through most of the exumbrella. The bell margin had the largest hysteresis with an outer path during contraction and inner path during relaxation. The subumbrella volume was approximated based on the exumbrella kinematics and was found to increase during contraction. Cyro was designed to mimic the morphology and swimming mechanism of the natural counterpart. The body of the vehicle consists of a rigid support structure with linear DC motors which actuate eight mechanical arms. The mechanical arms in conjunction with artificial mesoglea create the hydrodynamic force required for propulsion. The full vehicle measures 170 cm in diameter and has a total mass of 76 kg. An analytical model of the mechanical arm kinematics was developed. The analytical and experimental bell kinematics were analyzed and compared to the C. capillata. Cyro reached the water surface untethered and autonomously from a depth of 182 cm in five actuation cycles. It achieved an average velocity of 8.47 cm/s while consuming an average power of 70 W. A thrust stand was developed to calculate the thrust directly from a single bell segment yielding an average thrust of 27.9 N for the whole vehicle. Steady state velocity during Cyro's swimming test was not reached but the measured performance during its last swim cycle resulted in a cost of transport of 10.9 J/kg m and total efficiency of 3%. It was observed that a passive flexible margin or flap, drastically increases the performance of the Robojelly. The effects of flap length and geometry on Robojelly were analyzed using PIV. The flap was defined as the bell section which is located between the flexion point and bell margin. The flexion point was established as the location where the bell undergoes a significant change compliance and therefore in slope. The flap was analyzed in terms of its kinematics and hydrodynamic contribution. An outer trajectory is achieved by the flap margin during contraction while an inner trajectory is achieved during relaxation. The flap kinematics was found to be replicable using a passive flexible structure. Flaps of constant cross section and varying lengths were put on the robotic vehicle to conduct a systematic parametric study. Robojelly's swimming performance was tested with and without a flap. This revealed a thrust increase 1340% with the addition of a flap.  Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) to analyze the change in vortex structures as a function of flap length.  The robot input parameters stayed constant over the different configurations tested thus maintaining a near constant power consumption. Non-dimensional circulation results show a dependence on flap kinematics and geometry. The robot was approximated as a series of pitching panels circularly oriented around its apex. The first circulation peak of the pitching panel approximation revealed a normalized standard deviation of 0.23. A piston apparatus was designed and built to test different flexible margin configurations. This apparatus allow the isolation of the flap parameters and remove the uncertainties coming from the robotic vehicle. / Ph. D.

The invention of shape memory alloys spurred a period of intense interest in the area of heat engines in the late 70's and early 80's. It was believed that these engines could use heat from low temperature sources such as solar heated water, geothermal hot water and rejected heat from conventional engines as a significant source of power. The interest has since dwindled, largely because small prototype devices developed in the laboratory could not be scaled up to produce significant power. It is believed that the scaled-up designs failed because they were dependent on friction as the driving mechanism, which led to large energy losses and slip. This thesis proposes a new chain and sprocket driving mechanism that is independent of friction and should therefore allow for large-scale power generation. This thesis begins by presenting properties and applications of shape memory alloys. The proposed design is then described in detail, followed by a review of the evolution that led to the final design. A brief chapter on thermodynamic modeling and a summary chapter suggesting improvements on the current design follow. / Master of Science

This work presents a novel approach to arranging shape memory alloy (SMA) wires into a functional heat engine. Significant contributions include the design itself, a preliminary analytical model and the realization of a research prototype; thereby, laying a foundation from which to base refinements and seek practical applications. Shape memory alloys are metallic materials that, if deformed when cold, can forcefully recover their original, "memorized" shapes, when heated. The proposed engine consists of a set of SMA wires stretched between two crankshafts, synchronized to rotate in the same direction. Cranks on the first crankshaft are slightly longer than cranks on the second. During operation, the engine is positioned between two distinct thermal reservoirs such that half of its wires are heated while the other half are cooled. Wires on the hot side attempt to contract, driving the engine in the direction that relieves the heat-induced stress. Wires on the cold side soften and stretch as the engine rotates. Because the force generated during heated recovery exceeds that required for cooled deformation, the engine is capable of generating shaft power. Limited experimental measurements of shaft speed were performed. An analytical model of the engine predicts that the maximum output power for the prototype, under test conditions, should be 0.75 W. Thermal efficiency, though not measured or calculated in this work, is expected to be low. Potential applications may include the conversion of waste heat into shaft power. / Master of Science

Published Article / The link from medical scan images through data manipulation to additive manufacturing is well established. Various types of software are used to deliver the required .STL file(s). Often, the data manipulation will require the generation of new shapes around existing geometry, e.g. an implant that will replace missing bone tissue. This paper reports exploratory work undertaken to assess the feasibility of using haptic modelling and "virtual sculpting" software to generate novel designs of vertebrae implants for correction of spinal curvature. .STL data of several vertebrae, originating from CT scans, was imported into the Freeform system from SensAble technologies. It was used to create immutable "bucks" around which the user "sculpted" three-dimensional implant geometries. It must be noted that the designs have not been medically assessed and were for demonstration purposes only. However, the process route followed did prove to be feasible and offered some particular advantages, e.g. a precise fit between the implant and the vertebra and the possibility of enabling the direct intervention of medics in the implant design process.

The purpose of the present study was to assess the feasibility of a laser layer manufacturing technique for realization of shape memory alloy (SMA) components with complex geometry. Pre-placed laser solid freeform fabrication (LSFF) was utilized to produce straight and curvaceous SMA parts from a mixture of 55.2 wt%Ni - 44.8 wt%Ti powder. A pulsed Nd:YAG laser was used; while laser pulse width and frequency were held constant at what are considered their optimal values (4 ms and 50 Hz, respectively), laser energy and scanning speeds were varied across samples to determine appropriate values for fabrication of high quality SMA parts . Different pre-placed powder thicknesses were deposited and then mechanically and physically studied. Optical microscopy, SEM, EDS, and XRD methods, as well as microhardness measurements, were used to examine the microstructural characteristics and hardness of the SMA samples. Also, differential scanning calorimetry (DSC) was performed to determine the transformation temperatures of the fabricated parts. The results confirmed the formation of crack-free solid surfaces in which two types of microstructure exist: solid (non-prose) and dendrite arms. EDS chemical composition analysis confirmed the absence of any impurity or oxidise in the cross section of the samples as well as the presence of only nickel and titanium. XRD spectrum analysis indicated the presence of Ni-Ti intermetallic phases, which are almost Ni-Ti but contain a small amount of Ti2Ni. The XRD results also indicated the presence of austenite and martensite phases, which are exchanged during heating or mechanical deformation. The hardness of these samples varied from 250 to 450 HV0.3. Several tests were carried out to investigate the shape memory effect (SME). It was observed that the fabricated SMAs can recover from the bent condition very quickly (i.e., 1 to 8 seconds) depending on their thickness. In general, the fabricated parts were first bent out of their original shapes then heated, in various ways, above the transformation temperature. To theoretically assess the SME performance of the fabricated SMAs with the proposed geometry two models were developed. The first model was established based upon a lump approach in which the part was exposed to an electrical current. The second model, however, was established based upon a finite element method in which a specific domain at one end of the sample was exposed to a source of heat. It was found that the theoretical outputs from both models were in good agreement with the experimental results.

The purpose of the present study was to assess the feasibility of a laser layer manufacturing technique for realization of shape memory alloy (SMA) components with complex geometry. Pre-placed laser solid freeform fabrication (LSFF) was utilized to produce straight and curvaceous SMA parts from a mixture of 55.2 wt%Ni - 44.8 wt%Ti powder. A pulsed Nd:YAG laser was used; while laser pulse width and frequency were held constant at what are considered their optimal values (4 ms and 50 Hz, respectively), laser energy and scanning speeds were varied across samples to determine appropriate values for fabrication of high quality SMA parts . Different pre-placed powder thicknesses were deposited and then mechanically and physically studied. Optical microscopy, SEM, EDS, and XRD methods, as well as microhardness measurements, were used to examine the microstructural characteristics and hardness of the SMA samples. Also, differential scanning calorimetry (DSC) was performed to determine the transformation temperatures of the fabricated parts. The results confirmed the formation of crack-free solid surfaces in which two types of microstructure exist: solid (non-prose) and dendrite arms. EDS chemical composition analysis confirmed the absence of any impurity or oxidise in the cross section of the samples as well as the presence of only nickel and titanium. XRD spectrum analysis indicated the presence of Ni-Ti intermetallic phases, which are almost Ni-Ti but contain a small amount of Ti2Ni. The XRD results also indicated the presence of austenite and martensite phases, which are exchanged during heating or mechanical deformation. The hardness of these samples varied from 250 to 450 HV0.3. Several tests were carried out to investigate the shape memory effect (SME). It was observed that the fabricated SMAs can recover from the bent condition very quickly (i.e., 1 to 8 seconds) depending on their thickness. In general, the fabricated parts were first bent out of their original shapes then heated, in various ways, above the transformation temperature. To theoretically assess the SME performance of the fabricated SMAs with the proposed geometry two models were developed. The first model was established based upon a lump approach in which the part was exposed to an electrical current. The second model, however, was established based upon a finite element method in which a specific domain at one end of the sample was exposed to a source of heat. It was found that the theoretical outputs from both models were in good agreement with the experimental results.

A series of segmented poly (epsilon-caprolactone) polyurethane-ureas (PCLUUs) were prepared from poly (epsilon-caprolactone) (PCL) diol, different dissociates and chain extenders to improve the recovery stress of shape memory polymers. NMR and FT-IR were used to identify the structure of the synthesized shape memory polyurethane-ureas. Parameters such as soft segment content (molecular weight and content), chain extender and the rigidity of the main chain were investigated to understand the structure-property relationships of the shape memory polymer systems through DSC, DMA, physical property test, etc. Cyclic thermal mechanic tests were applied to measure the shape memory properties which showed that the recovery stress can be improved above 200% simply by modifying the chain extender. Meanwhile, the synthesis process was optimized to be similar to that of Spandex /LYCRA®. Continuous fibers were made from a wet spinning process, which indicated excellent spinnability of the polymer solution. Small angle neutron scattering (SANS) was used to study the morphology of the hard segment at different temperatures and stretch rates and found that the monodisperse rigid cylinder model fit the SANS data quite well. From the cylinder model, the radius of the cylinder increases with the increasing hard segment content. The SANS results revealed phase separation of hard and soft segments into nano scale domains.

Ti-Ni phase transformation behaviour is very sensitive to the thermal and mechanical history of the alloy. Thermomechanical cycling through the full transformation range may degrade the Ti-Ni functional properties (functional fatigue). These repeated transformation cycles cause changes in the SMA phase transformation behaviour due to the formation and accumulation of defects in the alloy microstructure. The main objective of this thesis is to establish the relationships between the changes in Ti-Ni phase transformation behaviour caused by thermomechanical processes, especially in the R-phase range, and the functional properties of the Ti-Ni shape memory alloys (SMAs). Establishing these relationships should allow us to find appropriate thermomechanical processes to substantially improve the Ti-Ni one-way and two-way shape memory effects. To achieve this objective, several experimental techniques are used including measuring variations of the electrical resistivity with temperature, X-ray diffraction, isothermal tension testing, thermal cycling under constant stress, and thermal cycling under zero stress. This study of the phase transformation changes caused by thermomechanical processes has led to a new way of interpreting resistivity curves for calculating the transformation temperatures. Moreover, we have determined how the R-phase influences the functional properties of SMA and, finally, we have substantially improved the properties of one-way and two-way shape memory effects by using thermal processes that avoid permanent deformation of the alloy. / Las transformaciones de fase en aleaciones de NiTi son altamente dependientes de la composición de la aleación, así como de la historia térmica y mecánica previa al uso de la SMA. El objetivo principal de esta tesis es establecer los vínculos existentes entre los cambios producidos en las transformaciones de fase por procesos termomecánicos y las propiedades funcionales en aleaciones con memoria de forma de Ti-Ni, tal que nos permita hallar los procesos termomecánicos más adecuados que proporcionen una mejora substancial en las propiedades funcionales de estas aleaciones. La determinación de estas relaciones, prestando especial atención a la fase-R, nos debe proporcionar las claves para incrementar las propiedades de memoria de forma y doble memoria de forma. Para la consecución de este objetivo se han usado diferentes técnicas de caracterización experimental: variación de la resistividad eléctrica con la temperatura (ER), difractometría de rayos X (DRX), ensayos de tracción isotérmicos, ciclados térmicos a tensión constante y ciclados térmicos a tensión nula. A través del estudio exhaustivo de las transformaciones de fase, se ha aportado una nueva interpretación de las curvas de resistividad para el cálculo de las temperaturas de transformación, se ha determinado la influencia de la fase-R en las propiedades funcionales de las SMA y se han mejorado substancialmente las propiedades de memoria simple y doble memoria de forma mediante procesos térmicos que evitan deformaciones permanentes antes del uso de la aleación. / Les transformacions de fase en aliatges de NiTi són altament dependents de la composició de l'aliatge, així com de la història tèrmica i mecànica prèvia a l'ús de la SMA. L'objectiu principal d'aquesta tesi és establir els vincles existents entre els canvis produïts en les transformacions de fase per processos termomecànics i les propietats funcionals en aliatges amb memòria de forma de Ti-Ni, tal que ens permeti trobar els processos termomecànics més adequats que proporcionin una millora substancial en les propietats funcionals d'aquests aliatges. La determinació d'aquestes relacions, prestant especial atenció a la fase-R, ens ha de proporcionar les claus per incrementar les propietats de memòria de forma i doble memòria de forma. Per a la consecució d'aquest objectiu s'han usat diferents tècniques de caracterització experimental: variació de la resistivitat elèctrica amb la temperatura (ER), difractometria de raigs X (DRX), assaigs de tracció isotèrmics, ciclats tèrmics a tensió constant i ciclats tèrmics a tensió nula. A través de l'estudi exhaustiu de les transformacions de fase, s'ha aportat una nova interpretació de les corbes de resistivitat per al càlcul de les temperatures de transformació, s'ha determinat la influència de la fase-R en les propietats funcionals de les SMA i s'han millorat substancialment les propietats de memòria simple i doble memòria de forma mitjançant processos tèrmics que eviten deformacions permanents abans de l'ús de l'aliatge.

The objective of this work is to characterize and understand structure- mechanical property relationships in (meth)acrylate networks. The networks are synthesized from mono-functional (meth)acrylates with systematically varying sidegroup structure and multi-functional crosslinkers with varying mole fraction and functionality. Fundamental trends are established between the network chemical structure, crosslink density, glass transition temperature, rubbery modulus, failure strain, and toughness. The glass transition temperature of the networks ranged from -29 to 112 °C, and the rubbery modulus ranged from 2.8 to 129.5 MPa. At low crosslink density (Er < 10 MPa) network chemistry has a profound effect on network toughness. At high crosslink densities (Er > 10 MPa), network chemistry has little influence on material toughness. The characteristic ratio of the mono-functional (meth)acrylates components is unable to predict trends in thermoset toughness as a function of chemical structure, as is accomplished for thermoplastics. The cohesive energy density is a better tool for prediction of network mechanical properties. Due to superior mechanical properties, networks with phenyl ring sidegroups are further investigated to understand the effect of phenyl ring distance on toughness. This work provides a fundamental basis for designing (meth)acrylate shape memory polymer networks with specific failure strain, toughness, glass transition temperature, and rubbery modulus.