Dynamic Mechanical Stability Research Articles

Full-scale polymer relaxation induced by single-chain confinement enhances mechanical stability of nanocomposites

Polymer nanocomposites with tuning functions are exciting candidates for various applications, and most current research has focused on static mechanical reinforcement. Actually, under service conditions of complex dynamic interference, stable dynamic mechanical properties with high energy dissipation become more critical. However, nanocomposites often exhibit a trade-off between static and dynamic mechanics, because of their contradictory underlying physics between chain crosslinking and chain relaxation. Here, we report a general strategy for constructing ultra-stable dynamic mechanical complex fluid nanocomposites with high energy dissipation by infusing complex fluids into the nanoconfined space. The key is to tailor full-scale polymer dynamics across an exceptionally broad timescale by single-chain confinement. These materials exhibit stable storage modulus (100 ~ 102 MPa) with high energy dissipation (loss factor > 0.4) over a broad frequency range (10−1 ~ 107 Hz)/temperature range (−35 ~ 85°C). In the loss factor > 0.4 region, their dynamic mechanical stability (rate of modulus change versus temperature (k)) is 10 times higher than that of conventional polymer nanocomposites.

An insight into physicochemical properties of hexagonal lyotropic liquid crystal templates from amphiphiles for mesoporous structural administration

Hexagonal lyotropic liquid crystals (HLLC) provide an ideal template for the fabrication of nanofiltration membranes without the flux-selectivity trade-off problem. Amphiphiles, the core components of the columnar micelles in the HLLC system, play a key role in regulating the nanostructure, their effects on dimensional stability and physicochemical properties of the system however have not been explored well. In this research, we compared the dimensional stability and physicochemical properties of HLLC templates prepared from amphiphiles with various head groups and tail lengths. The amphiphiles with charged head groups (dodecyl trimethylammonium bromide, DTAB, and hexadecyl trimethylammonium bromide, CTAB) enable more monomers to be introduced and a much smaller (∼ 1/2) pore size than that with uncharged ones (polyethylene glycol hexadecyl ether, Brij56). The tail length however mainly regulates the width of water channels with CTAB being 23.27 Å and DTAB about 17.06 Å when the head groups are identical. Although the HLLC systems with CTAB and DTAB present a little bit higher phase transition temperature, the dynamic mechanical stability is much better than that with Brij56 at room temperature, facilitating the reorientation process under an electric force with the order parameter reaching to about 0.9 and the retention of the aligned nanostructure with the absence of the force at room temperature. The study enables a better understanding of the effects of amphiphiles on HLLC templates and their tunability for advanced nanofiltration membranes.

Deviating from the Recommended Torque on Set Screws Can Reduce the Stability and Fatigue Life of Pedicle Screw Fixation Devices.

Background and Objectives: Using an appropriate torque to tighten set screws ensures the long-term stability of spinal posterior fixation devices. However, the recommended torque often varies between different devices and some devices do not state a recommended torque level. The purpose of this study is to evaluate the effect of set screw torque on the overall construct stability and fatigue life. Materials and Methods: Two commercial pedicle screw systems with different designs for the contact interface between the set screw and rod (Group A: plane contact, Group B: line contact) were assembled using torque wrenches provided with the devices to insert the set screws and tighten to the device specifications. The axial gipping capacity and dynamic mechanical stability of each bilateral construct were assessed in accordance with ASTM F1798 and ASTM F1717. Results: Increasing or decreasing the torque on the set screw by 1 Nm from the recommended level did not have a significant effect on the axial gripping capacity or fatigue strength of Group A (p > 0.05). For Group B, over-tightening the set screw by 1 Nm did cause a significant reduction in the fatigue strength. Conclusions: Excessive torque can damage the rod surface and cause premature failure. When insertion using a manual driver is preferred, a plane contact interface between the set screw and rod can reduce damage to the rod surface when the set screw is over-torqued.

Li plating on alloy with superior electro-mechanical stability for high energy density anode-free batteries

Anode-free batteries possess high energy density and avoid the use of reactive Li during battery fabrication, and thus are highly desirable for high energy density batteries. However, they encounter fast failure due to the inferior electrochemical reversibility. One main reason is the fast inactive Li accumulation caused by the coupling of side reactions and mechanics during complicated Li plating/stripping processes, among which, mechanical stability of plated Li on current collector is crucial but often ignored. To achieve good electro-mechanical stability, we construct a stable lithium-metal alloy interphase for Li plating, which sharps the Li deposition morphology and enhances the connection with deposited Li. The Li-metal alloy possesses reduced Li nucleation energy barrier and strong bonding with metallic Li, which enables dense and chunky Li deposits with firm connection with the substrate in sharp contrast the fully peeling off of metallic Li for bare Cu after 50 Li plating/stripping cycles. A Li-Sn alloy-modified Cu anode displays stable Li plating/stripping cycling with average Coulombic efficiency of 94.1% for 400 cycles at 1 mA cm–2 and 1 mAh cm–2, significantly outperforming the bare Cu electrode. As a demonstration, in an anode-free cell with Li rich Mn-based cathode, 16.7% increase in capacity and 14.1% increase in capacity retention for 20 cycles are achieved using alloy-modified anode. This sheds new insights on the improvement of dynamic mechanical stability between Li deposits and current collector by regulating Li plating and its connection with the current collector and guides the design of high-performance initial-Li-free anode.

All-Ceramic and Elastic Aerogels with Nanofibrous-Granular Binary Synergistic Structure for Thermal Superinsulation.

High-performance thermally insulating ceramic materials with robust mechanical properties, high-temperature resistance, and excellent thermal insulation characteristics are highly desirable for thermal management systems under extreme conditions. However, the large-scale application of traditional ceramic granular aerogels is still limited by their brittleness and stiff nature, while ceramic fibrous aerogels often display high thermal conductivity. To meet the above requirements, in this study, ceramic nanofibrous-granular composite aerogels with lamellar multiarch cellular structure and leaf-like fibrous-granular binary networks are designed and fabricated. The resulting composite aerogels possess ultralow weight, superelasticity with recoverable compression strain up to 80%, and large mechanical strength. Furthermore, excellent fatigue resistance with 1.2% plastic deformation after 1000 cyclic compressions, temperature-invariant dynamic mechanical stability from -100 to 500 °C, and an operational temperature range from -196 to 1100 °C are successfully achieved in the proposed composites. The nanosized silica granular aerogels are assembled into a leaf-like shape and wrapped around the fibrous cell walls, endowing low thermal conductivity (0.024 W m-1 K-1) as well as favorable high-temperature thermal superinsulation properties. Benefiting from the favorable compatibility, the present strategy for nanofiber-granular composite ceramic aerogels provides a dominant route to produce thermally insulated and mechanically robust composite cellular materials for use in harsh environments.

Mechanically and Electronically Robust Transparent Organohydrogel Fibers.

Stretchable conductive fibers are key elements for next-generation flexible electronics. Most existing conductive fibers are electron-based, opaque, relatively rigid, and show a significant increase in resistance during stretching. Accordingly, soft, stretchable, and transparent ion-conductive hydrogel fibers have attracted significant attention. However, hydrogel fibers are difficult to manufacture and easy to dry and freeze, which significantly hinders their wide range of applications. Herein, organohydrogel fibers are designed to address these challenges. First, a newly designed hybrid crosslinking strategy continuously wet-spins hydrogel fibers, which are transformed into organohydrogel fibers by simple solvent replacement. The organohydrogel fibers show excellent antifreezing (< -80 °C) capability, stability (>5 months), transparency, and stretchability. The predominantly covalently crosslinked network ensures the fibers have a high dynamic mechanical stability with negligible hysteresis and creep, from which previous conductive fibers usually suffer. Accordingly, strain sensors made from the organohydrogel fibers accurately capture high-frequency (4 Hz) and high-speed (24 cm s-1 ) motion and exhibit little drift for 1000 stretch-release cycles, and are powerful for detecting rapid cyclic motions such as engine valves and are difficult to reach by previously reported conductive fibers. The organohydrogel fibers also demonstrate potential as wearable anisotropic sensors, data gloves, soft electrodes, and optical fibers.

Piezoresistive nanocomposite films for foot strike data monitoring

It is important to understand the human gait especially for the patients under rehabilitation or in old age. Foot strike data is considered as the most important parameter to analyse gait. Piezoresponsive sensors are mostly used for this purpose. In this study, carbon nanofiber (CNF) based thermoplastic polyurethane (TPU) piezoresistive, nanocomposite films (15%, 20% and 25% w/w) were prepared, characterized and tested to study their suitability for such applications. Scanning electron microscope (SEM) studies on the cross section of nanocomposite films revealed that the CNF is well connected inside the TPU matrix. This well connected conducting network resulted in low bulk resistivity (98 ± 3.2 Ω-cm) and impedance (∼250 Ω). The compressive test results indicate more than threefold increase in strength due to CNF reinforcement. All the films showed excellent thickness recovery under cyclic loading while, 25% (w/w) CNF dispersed nanocomposite films’ showed more than 99% recovery after 100 cycles of cyclic compression. The samples also showed good dynamic mechanical stability from 1 to 10 Hz at body temperature (37 °C). The change in resistance under cyclic compression up to 70% strain was found to be linear with high correlation (R2 = 0.99). The gauge factor (up to 1.40 ± 0.03) shows little variation at different strain regions. A cyclic load setup was designed and fabricated to mimic the cyclic foot strike pattern of human gait. The nanocomposite films were tested at three different cyclic strain levels using the setup and the responses were monitored for 2 h using cathode ray oscilloscope (CRO). It has been observed that with the increase in CNF concentrations, the change in voltage goes up to 911.8 mV. The consistency of voltage response under cyclic compressive loading demonstrated in this work shows tremendous potential for these nanocomposite films to be successfully used for piezoresponsive applications of human gait monitoring.

Cellulose nanocrystals extracted from rice husks as a reinforcing material in gelatin hydrogels for use in controlled drug delivery systems

Hydrogels with remarkable sensitivity toward changes in pH were prepared using gelatin reinforced with cellulose nanocrystals (CNCs). Glutaraldehyde was used as a crosslinker because of its high chemical reactivity toward the NH2 group on gelatin. CNC ratios of 0%, 5%, 10%, 15%, 20%, and 25% were chosen to study the effects of CNCs on the dynamic mechanical properties and swelling behavior of gelatin-based hydrogels. Crosslinking between gelatin monomers was confirmed by the presence of a CN stretching group at 1630cm−1 in the FTIR spectrum of gelatin hydrogels. The overall crystallinity and dynamic mechanical properties of gelatin hydrogels increased as the CNC content increased. The increase in the overall crystallinity improved the storage modulus of the CNC-gelatin hydrogel from 122Pa to 468Pa by the addition of 25% CNC. From the swelling test, CNC-gelatin hydrogels showed excellent pH sensitivity with a maximum swelling ratio at pH 3. The ability of the CNC-gelatin hydrogel to respond to different pH values along with its high dynamic mechanical stability suggested that CNC-gelatin hydrogels are promising candidates as drug carriers. Theophylline was used in this research as a model drug to further evaluate the potential of these CNC-gelatin hydrogels to act as drug carriers. Drug loading efficiency and drug release profiles of the CNC-gelatin hydrogels were studied. The findings suggest that gelatin hydrogels reinforced with 15% CNC are the best potential candidates for controlled drug delivery system.

X-ray Computerized Tomography Observation of the Interfacial Structure of Liquid Marbles

Abstract We have demonstrated that the liquid/solid interfacial structure of liquid marbles is determined by the surface–interface interaction between the encapsulating particles and liquid droplet. X-ray computerized tomography (X-ray CT) confirmed that the decrease of the solid-over-liquid spreading coefficient (SS/L) allowed the particles to be submerged in the liquid. Moreover, the difference in interfacial structure had a strong effect on the dynamic mechanical stability of liquid marbles. Two liquid marbles only merged upon contact when at least one liquid marble had a small SS/L value. X-ray CT observations also revealed that the liquid at the solid/liquid interface did not interdiffuse easily.

Characterization of the dynamic mechanical stability of liquid-filled lenses

In this paper, the dynamic mechanical stability of the liquid-filled lenses was studied, in which acoustic excitation was used as broad band perturbation sources and the resultant response of the lens was characterized using non-contact laser Doppler vibrometer. To the best of our knowledge, it's the first time that the mechanical stability of liquid-filled lenses was experimentally reported. Both experimental results and theoretical analysis demonstrate that the resonance of the lens will shift to higher frequency while the vibration velocity as well as its magnitude will be reduced accordingly when the pressure in the lens cavity is increased to shorten the focal length. All of these results will provide useful references to help researchers design their own liquid-filled lenses for various applications.

Predicting the onset of dynamic instability of a cylindrical plate under axial flow conditions

The dynamic mechanical stability of a single cylindrical plate under flow conditions is considered herein. Numerous plate-type research reactors such as the Advanced Test Reactor (ATR) located at the Idaho National Laboratory (INL) comprise fuel elements which contain arrays of concentrically aligned cylindrical plates. Several of these reactors are licensed to operate at extreme heat fluxes; as a corollary their hydraulic designs require large flow rates sufficient to remove this heat. These flow rates may reach superficial velocities upwards of 15m/s through individual flow channels. Given that fuel plates typically found in such research reactors are relatively long (∼1.2573m), wide (∼0.1397m), and extremely thin (∼0.00127m) concern is drawn toward the susceptibility of flow induced vibration (FIV). In an attempt to gain a more comprehensive understanding toward the dynamic mechanical limit of stability of cylindrical plates, a FIV model was developed using semi-numerical methods.The FIV model was developed in two separate modules; a plate stability module, and a flow module. These modules were then coupled together to produce a FIV model. In this study, a set of test cases are presented on the plate stability module under free vibration conditions, comparing well against known available information from previous studies. Results are similarly presented on the flow module and compared against a RELAP5-3D model. Lastly, results of these coupled modules are presented and discussion is given toward the relationship between plate natural frequency, geometry, and plate membrane pressures.

Effect of thermal aging on the optical, dynamic mechanical, and morphological properties of phenylmethylsiloxane-modified epoxy for use as an LED encapsulant

We investigated the thermal stability of the diglycidyl ether of bisphenol A (DGEBA)–methylhexahydrophtalic anhydride (MHHPA) modified with phenylmethylsiloxane-modified epoxy (PMSE) and the effects of this hybrid on the performance of light emitting diodes (LEDs). The optical and dynamic mechanical properties of the materials, and the effects of these properties on the light output from the encapsulated LEDs, were studied following long-term thermal aging at 150°C. The DGEBA–PMSE hybrids were highly transparent and its optical stability was shown in the stable light output of the LED, which was 19% higher than that of LEDs encapsulated by DGEBA–MHHPA after 30 days. Molecular rearrangement occurred in the nanoscale domain of the hybrids after thermal aging, which caused the dynamic mechanical effect on the LED performance, as demonstrated by TEM. We observed the hybrid modified using 0.2 equivalents of PMSE possessed both optical and dynamic mechanical stability, and resulted in the most stable LED performance.

Effect on Dynamic Mechanical Stability and Interfragmentary Movement of Angle-Stable Locking of Intramedullary Nails in Unstable Distal Tibia Fractures: A Biomechanical Study

Unstable distal tibia fractures are challenging injuries that require surgery. Increasingly, intramedullary nails are being used. However, fracture site anatomy may cause distal-fragment stabilization and fixation problems and lead to malunion/nonunion. We studied the influence of angle-stable nail locking on fracture gap movement and other biomechanical parameters. Eight pairs of fresh human cadaver tibiae were used. The bone mineral density (BMD) was determined. All tibiae were nailed with a Synthes Expert tibial nail. Within each pair, one tibia was randomized to receive conventional locking screws; the other, angle-stable screws with sleeves. A 7-mm osteotomy was created 10 mm above the upper distal locking screw, to simulate an AO 42-A3 fracture. Biomechanical testing involved nondestructive mediolateral and anteroposterior pure bending, followed by cyclic combined axial and torsional loading to catastrophic failure. The neutral zone was determined. Fracture gap movement was monitored with 3-D motion tracking. The angle-stable locked constructs had a significantly smaller mediolateral neutral zone (mean: 0.04 degree; p=0.039) and significantly smaller fracture gap angulation (p=0.043). The number of cycles to failure did not differ significantly between the locking configurations. BMD was a significant covariate affecting the number of cycles to failure (p=0.008). However, over the first 20,000 cycles, there was no significant correlation in the angle-stable construct. Angle-stable locking of the Expert tibial nail was associated with a significant reduction in the mediolateral neutral zone and in fracture gap movement. Angle-stable fixation also reduced the influence of BMD over the first 20,000 cycles.

Stability of surface tension self-assembled 3D MOEMS

The thermal and mechanical stability of self-assembled MOEMS components consisting of 3D mirrors have been evaluated. The thermal–mechanical constants of the hinge material (AZ4562 photoresist) used to power the surface tension self-assembly process have been measured and used to predict the stability of assemblies. The Young's moduli and thermal expansion coefficients showed constant values of average 6.78 × 10 9 N/m 2 and 4.08 × 10 −6 °C −1 under various thermal conditions. The angle variation of all the components tested under thermal cycling was less than 0.006° °C −1 up to 110 °C. The use of mechanical limiters enhanced the thermal stability to near 0 up to 150 °C. During dynamic mechanical stability measurement, for a maximum external acceleration of >50 g, no device failure was observed.

Dynamic Mechanical Properties of Passive Single Cardiac Fibers From the Crab Cancer Magister

ABSTRACT I determined the dynamic mechanical properties of single relaxed cardiac fibers from the Dungeness crab Cancer magister. Single fibers were mechanically isolated, chemically skinned and subjected to small-amplitude sinusoidal length perturbations over a wide range of strain rates and sarcomere lengths to characterize their viscoelastic behavior. The observed mechanical properties, together with transcardiac pressure recordings and ultrastructural measurements, were related to the overall function of the heart. Single fibers, often longer than 1mm, could be mechanically dissected from the heart of Cancer magister. They typically ranged from 20 to 100 μm in diameter and were surrounded by a 100–400nm thick extracellular matrix. In situ, under normal physiological loads, the heart of Cancer magister generated transcardiac pressures of about 1000Pa and beat at 1Hz, while the sarcomere lengths of fibers changed by 10 % from about 4.0 to 4.4 μm during contractions. The total stiffness of all fibers increased from approximately 0.01MPa to 1MPa in the sarcomere length range from 3.8 to 6.0 μm and increased two-to threefold with a rise in strain rate from 0.01 to 5rad s−1. In the physiological range of sarcomere length (4.0–4.4 μm) and strain rate (0.5–1.2rad s−1), single cardiac fibers behaved viscoelastically, with average values for the relative energy dissipation ranging from 0.5 to 0.7. The volume fraction of the extracellular matrix correlated positively with the stiffness of single cardiac fibers. On the basis of these results, I propose a dual role for the viscoelastic behavior of Cancer magister cardiac fibers: (1) the viscous energy dissipation confers dynamic mechanical stability at the level of the single fiber, and (2) the storage and return of elastic strain energy saves energy at the level of the whole heart.

Dynamic Mechanical Stability in the Variable-Reluctance and Electrostatic Transducers

Large displacement behavior of the variable-reluctance, and the mathematically equivalent electrostatic, transducing mechanism was analyzed in order to evaluate the importance of mechanical instability as a limiting factor in high-power sound projectors. Previous work by Hunt et al. had shown the existence of static instability. This paper first considers the effect of transients on static stability and then shows that instability can also occur under steady-state dynamic conditions even when the system is statically stable. When the excitation is increased in order to increase the amplitude of the oscillations, the average displacement also increases, which leads to collapse of the gap. This interdependence of the oscillatory motion and the average displacement, intentionally excluded from linearized treatments, is the cause of dynamic instability. Detailed calculations show the conditions for instability for typical values of mechanical Q, polarization, and frequency. Under steady-state dynamic conditions the average displacement can exceed the static limit of 13 the gap length, and the relative oscillatory displacement amplitude can reach 12 or more without collapse.

Dynamic Mechanical Stability in the Variable-Reluctance and Electrostatic Transducers

Large displacement behavior of the variable-reluctance, and the mathematically equivalent electrostatic, transducing mechanism was analyzed in order to evaluate the importance of mechanical instability as a limiting factor in high-power sound projectors. Previous work by Hunt et al. had shown the existence of static instability. This paper first considers the effect of transients on static stability and then shows that instability can also occur under steady-state dynamic conditions, even when the system is statically stable. When the excitation is increased in order to increase the amplitude of the oscillations, the average displacement also increases, which leads to collapse of the gap. This interdependence of the oscillatory motion and the average displacement, intentionally excluded from linearized treatments, is the cause of dynamic instability. Detailed calculations show the conditions for instability for typical values of mechanical Q, polarization, and frequency. Under steady-state dynamic conditions, the average displacement can exceed the static limit of 13 the gap length, and the relative oscillatory displacement amplitude can reach 12 or more without collapse. Although such instability renders the transducer inoperative, the calculations indicate that it will seldom be a serious limitation.