Dynamic Stress Relaxation Research Articles

Deviation from Equilibrium Thermodynamics of an Asphaltene Model Compound during Compression-Expansion Experiments at Fluid-Fluid Interfaces.

Asphaltenes play a crucial role in crude oil behavior, and model compounds are often used to capture, mimic, and predict certain interfacial properties. In previous works, sorption of an asphaltene model compound (C5PeC11) was studied using surface pressure isotherms, where a deviation from the expected thermodynamic behavior of the interface during decane-water and air-water compression experiments was observed but not explained. In this work, the interfacial behavior of C5PeC11 was assessed at the decane-water and the air-water interfaces using a multiscale approach that includes: compression-expansion experiments on rectangular and radial Langmuir troughs, dynamic interfacial stress relaxation, and fluorescence microscopy imaging. Connections between molecular and microscopic phenomena strongly suggest that the nonthermodynamic response can be explained through a dynamic effect whose origin lies in the predominance of intermolecular forces in C5PeC11 molecules over the mechanical compression force applied. When aggregation begins at the air-water interface, stable structures are formed, and the nonthermodynamic phenomenon is not observed in subsequent compressions. However, at the decane-water interface, the initial aggregation is not consolidated due to the effect of the oil phase on the free energy of the interface allowing the high reproducibility of the dynamic effect in subsequent compression cycles. These results highlight the need to probe interfacial systems at various length scales to adequately separate equilibrium thermodynamics from dynamic responses.

Adaptive self-healing electronic epineurium for chronic bidirectional neural interfaces

Realizing a clinical-grade electronic medicine for peripheral nerve disorders is challenging owing to the lack of rational material design that mimics the dynamic mechanical nature of peripheral nerves. Electronic medicine should be soft and stretchable, to feasibly allow autonomous mechanical nerve adaptation. Herein, we report a new type of neural interface platform, an adaptive self-healing electronic epineurium (A-SEE), which can form compressive stress-free and strain-insensitive electronics-nerve interfaces and enable facile biofluid-resistant self-locking owing to dynamic stress relaxation and water-proof self-bonding properties of intrinsically stretchable and self-healable insulating/conducting materials, respectively. Specifically, the A-SEE does not need to be sutured or glued when implanted, thereby significantly reducing complexity and the operation time of microneurosurgery. In addition, the autonomous mechanical adaptability of the A-SEE to peripheral nerves can significantly reduce the mechanical mismatch at electronics-nerve interfaces, which minimizes nerve compression-induced immune responses and device failure. Though a small amount of Ag leaked from the A-SEE is observed in vivo (17.03 ppm after 32 weeks of implantation), we successfully achieved a bidirectional neural signal recording and stimulation in a rat sciatic nerve model for 14 weeks. In view of our materials strategy and in vivo feasibility, the mechanically adaptive self-healing neural interface would be considered a new implantable platform for a wide range application of electronic medicine for neurological disorders in the human nervous system.

Unified perspective on structural heterogeneity of a LaCe-based metallic glass from versatile dynamic stimuli

Dynamic mechanical relaxation, stress relaxation and creep were performed in a La56.16Ce14.04Ni19.8Al10 metallic glass with pronounced β relaxation features. The evolution of the microstructural heterogeneity with temperature is indirectly revealed via dynamic heterogeneity and analyzed in terms of several independent theoretical approaches. An apparent activation energy of β relaxation is obtained by assuming the Arrhenius relationship, which is calibrated by either the activation enthalpy of stress relaxation or the activation energy of creep. It is found that similar deformation mechanisms accommodate the dynamics of the LaCe-based metallic glass under different types of external stimuli applied in mechanical experiments.

Study on Closing Spring Fatigue Characteristics of High Voltage Circuit Breaker

One of the most causing closing fault of high voltage circuit breaker is closing spring failure. In order to avoid such closing fault, this paper analyzed the relationship between energy of closing spring and its load, as well as the experiment carried out to get the minimum energy when closing. The time-varying static stress relaxation curve of closing spring force was obtained by analogy the real working condition of high voltage circuit breaker which closing spring is compressed for a long time, the operation time-varying dynamic stress relaxation curve of closing spring force was also obtained by carrying mechanical endurance test. In the end, established the model of closing spring storage life and fatigue life, closing spring life prediction and fault early warning of high voltage circuit breaker was then realized.

Material characterisation of additively manufactured elastomers at different strain rates and build orientations

Material jetting, particularly PolyJet technology, is an additive manufacturing (AM) process which has introduced novel flexible elastomers used in bio-inspired soft robots, compliant structures and dampers. Finite Element Analysis (FEA) is a key tool for the development of such applications, which requires comprehensive material characterisation utilising advanced material models. However, in contrast to conventional rubbers, PolyJet elastomers have been less explored leading to a few material models with various limitations in fidelity. Therefore, one aim of this study was to characterise the mechanical response of the latest PolyJet elastomers, Agilus30 (A30) and Tango+ (T+), under large strain tension-compression and time-dependent high-frequency/relaxation loadings. Another aim was to calibrate a visco-hyperelastic material model to accurately predict these responses. Tensile, compressive, cyclic, dynamic mechanical analysis (DMA) and stress relaxation tests were carried out on pristine A30 and T+ samples. Quasi-static tension-compression tests were used to calibrate a 3-term Ogden hyperelastic model. Stress relaxation and DMA results were combined to determine the constants of a 5-term Prony series across a large window of relaxation time (10 μs–100 s). A numerical time-stepping scheme was employed to predict the visco-hyperelastic response of the 3D-printed elastomers at large strains and different strain rates. In addition, the anisotropy in the elastomers, which stemmed from build orientation, was explored. Highly nonlinear stress-strain relationships were observed in both elastomers, with a strong dependency on strain rate. Relaxation tests revealed that A30 and T+ elastomers relax to 50 % and 70 % of their peak stress values respectively in less than 20 s. The effect of orientation on the loading response was most pronounced with prints along the Z-direction, particularly at large strains and lower strain rates. Moreover, the visco-hyperelastic material model accurately predicted the large strain and time-dependent behaviour of both elastomers. Our findings will allow for the development of more accurate computational models of 3D-printed elastomers, which can be utilised for computer-aided design in novel applications requiring flexible or rate-sensitive AM materials.

Comparing the rheological and reinforcing effects of graphene oxide on glassy and semicrystalline polymers

In this work, a comparative study was performed on nanocomposites, based on three different polymeric matrices, namely polylactic‐acid (PLA), polystyrene (PS), and linear low‐density polyethyelene (mLLDPE), produced by metallocene catalyst. The reinforcing agent was graphene oxide (GO) at one percentage content (1 wt%) and the nanocomposites were prepared by the melt‐mixing procedure, whereas the matrix type employed was found to be decisive in the thermomechanical properties improvement. The thermomechanical performance of these material systems was studied by scanning electron microscopy, differential scanning calorimetry, dynamic mechanical analysis, tensile testing, and creep and stress relaxation. The mechanical and rheological performance was comparatively studied, and an attempt was made to relate the macroscopic response with the nanocomposites micromorphology. Furthermore, the tensile response was analyzed by a viscoplastic model, introduced earlier, and the model parameters involved provide additional evidence about the nanocomposites micromorphology. The findings of the comparative study are encouraging, regarding the potential of PLA and mLLDPE as matrices for graphene‐based nanocomposites at higher GO loadings. POLYM. ENG. SCI., 59:1933–1947, 2019. © 2019 Society of Plastics Engineers

Facile design of heat-triggered shape memory ethylene-vinyl acetate copolymer/nitrile-butadiene thermoplastic vulcanizates via zinc dimethacrylate induced interfacial compatibilization

Design of shape memory polymers (SMPs) based on thermoplastic vulcanizates (TPVs) is considered a challenge because, in most cases, the phase interface of TPV is not powerful enough to perform the shape memory behavior. In this work, a facile and effective strategy was designed to achieve rapidly reconfigurable shape fixity and recovery for a heat-triggered shape memory polymer (HSMP), which was derived from a typical sea-island structured ethylene-vinyl acetate copolymer/nitrile-butadiene rubber (EVA/NBR) TPVs via Zinc Dimethacrylate (ZDMA) induced interfacial compatibilization. The EVA phase served as the heat-control switched phase to fix the deformation status of rubber particles via the ZDMA-enhanced interface. With the incorporation of ZDMA, the average diameter of NBR particles was reduced to 3–5 μm, which markedly enlarged EVA/NBR interfacial contact surfaces. The enhanced interface improved stress delivery efficiency between EVA and NBR phases, which further improved the shape recovery (SR) and shape fixing (SF) ability of TPVs. This novel HSMP exhibited the surprising shape memory property (shape fixities ~ 95%, shape recoveries> 95%, and fast recovery speed < 20 s at the switching temperature of 95 °C). The dynamic mechanical behavior and stress relaxation behavior of EVA/NBR/ZDMA TPVs were characterized, and the results demonstrated that the high modulus value and the low stress relaxation ratio supported the improvement of SF and SR ratios of the EVA/NBR/ZDMA TPVs. This novel HSMP is expected to open up a wide range of potential applications in smart devices.

A transparent, highly stretchable, self-healing polyurethane based on disulfide bonds

Polymers with self-healing ability have attracted great research efforts recently. A transparent self-healing polyurethane (PU) based on disulfide bonds is reported in this work. The crystalline structures, self-healing properties of the polyurethane were investigated by differential scanning calorimetry (DSC), X-ray diffraction measurements (XRD), dynamic mechanical analysis (DMA), stress relaxation and tensile tests. The as-prepared self-healing polyurethane was transparent due to low crystallinity and exhibited excellent self-healing properties, stretchability and healing repeatability. The healing efficiency and elongation at break at a moderate healing temperature were over 90% and 800%, respectively. Meanwhile, the self-healing polyurethane exhibited an excellent healing repeatability and service reliablility. The self-healing mechanism was analyzed through surface analysis of the scratches and shape memory effect. The system was composed of a polyurethane network, which was responsible for the autonomous crack closure due to its shape memory characteristic, and an intrinsic self-healing component through dynamic disulfide bond exchange. The self-healing polyurethane might have potential applications in the fields of flexible electronics, including artificial skins, soft robotics, etc.

Effects of Extractives on Dimensional Stability, Dynamic Mechanical Properties, Creep, and Stress Relaxation of Rice Straw/High-Density Polyethylene Composites.

The removal of rice straw extractives increases the interphase adhesion between rice straw and the high-density polyethylene (HDPE) matrix, while eradicating the inner defects of rice straw/HDPE composites. This study investigated the effect of rice straw extractives removal on the dimensional stability (water uptake and thermal expansion), dynamic mechanical properties, creep, and stress relaxation of rice straw/HDPE composites. Cold water (CW), hot water (HW), and 1% alkaline solution (AL) extraction methods were utilized to remove rice straw extractives. Extracted and unextracted rice straws were mixed with HDPE, maleated polyethylene (MAPE), and Polyethylene wax to prepare composites via extrusion. Removal of rice straw extractives significantly improved the dimensional stability, dynamic mechanical properties, and creep and stress relaxation of rice straw/HDPE composites, with the exception of the thickness swelling of the AL/HDPE and the thermal expansion of the rice straw/HDPE composites. HW/HDPE exhibited the best comprehensive performance.

Compressive mechanical characterization of non-human primate spinal cord white matter

The goal of developing computational models of spinal cord injury (SCI) is to better understand the human injury condition. However, finite element models of human SCI have used rodent spinal cord tissue properties due to a lack of experimental data. Central nervous system tissues in non human primates (NHP) closely resemble that of humans and therefore, it is expected that material constitutive models obtained from NHPs will increase the fidelity and the accuracy of human SCI models. Human SCI most often results from compressive loading and spinal cord white matter properties affect FE predicted patterns of injury; therefore, the objectives of this study were to characterize the unconfined compressive response of NHP spinal cord white matter and present an experimentally derived, finite element tractable constitutive model for the tissue. Cervical spinal cords were harvested from nine male adult NHPs (Macaca mulatta). White matter biopsy samples (3 mm in diameter) were taken from both lateral columns of the spinal cord and were divided into four strain rate groups for unconfined dynamic compression and stress relaxation (post-mortem <1-hour). The NHP spinal cord white matter compressive response was sensitive to strain rate and showed substantial stress relaxation confirming the viscoelastic behavior of the material. An Ogden 1st order model best captured the non-linear behavior of NHP white matter in a quasi-linear viscoelastic material model with 4-term Prony series. This study is the first to characterize NHP spinal cord white matter at high (>10/sec) strain rates typical of traumatic injury. The finite element derived material constitutive model of this study will increase the fidelity of SCI computational models and provide important insights for transferring pre-clinical findings to clinical treatments. Statement of SignificanceSpinal cord injury (SCI) finite element (FE) models provide an important tool to bridge the gap between animal studies and human injury, assess injury prevention technologies (e.g. helmets, seatbelts), and provide insight into the mechanisms of injury. Although, FE model outcomes depend on the assumed material constitutive model, there is limited experimental data for fresh spinal cords and all was obtained from rodent, porcine or bovine tissues. Central nervous system tissues in non human primates (NHP) more closely resemble humans. This study characterizes fresh NHP spinal cord material properties at high strains rates and large deformations typical of SCI for the first time. A constitutive model was defined that can be readily implemented in finite strain FE analysis of SCI.

The structural relaxation in vanadium phosphate glass studied by DMA

The mechanism of structure evolution and microscopic hidden flow in glasses before yield and glass-to-liquid transition(GLT) remains unclear. In this work, the dynamic mechanical relaxation and stress relaxation spectra of vanadium-phosphate binary glass (V2O5-P2O5) when approaching the glass transition temperature T g were investigated by using the dynamic mechanical analysis (DMA). The β and α relaxation behaviors, occurring respectively at T β~ 483 K and T p~543 K and quite similar to those observed in the metal glass systems, were observed, which indicated the occurrence of the slowing-down microscopic localized flow in the investigated vanadium-phosphate glasses. This was further verified by the experimental observation and theoretical prediction of the linear logarithmic relation between the peak height of the β relaxation and the relaxation rate, i.e. ln( Δ ) µ −ln( v ). Furthermore, three stages during the structural evolution of the β and α relaxation in vanadium-phosphate glasses were observed from the temperature dependent curve of the non-exponential factor of β KWW obtained by the Kohlrausch-Williams-Watts (KWW) fitting: (I) β KWW β KWW β KWW>0.5, the region of macroscopic flow.

Ensuring combination of strength, ductility and toughness in medium-manganese steel through optimization of nano-scale metastable austenite

Medium-Mn steel was innovatively designed in terms of alloying elements for medium to heavy steel plates to obtain combination of high strength, good ductility, and excellent low temperature toughness. Martensite was the dominant microstructure in the directly quenched steel plate because of high hardenability obtained by the addition of 5.6wt% Mn. Three intercritical annealing processes were adopted to ensure transformation-induced-plasticity (TRIP) effect through optimization of the volume fraction, morphology, and C and Mn-enriched reversed austenite. On annealing, the dislocation density of martensite plate was decreased because of recovery and recrystallization, and metastable austenite film nucleated at the interface. Both thermal and mechanical stability of austenite decreased with increase in annealing temperature. The high yield strength of 840MPa, good elongation after fracture of 24.3%, and excellent toughness at −60°C of 130.3J was obtained at intermediate annealing temperature of 650°C, and the volume fraction of reversed austenite at room temperature and −80°C was 22% and 17%, respectively. The reversed austenite exhibited different stability with increased tensile strain, leading to improved ductility and delayed necking. Moreover, the crack initiation energy and crack propagation energy were increased via dynamic stress partitioning and relaxation together with the transformation of metastable austenite. Thus, significant degree of TRIP effect induced by moderate stability of austenite played an important role in governing mechanical properties.

Mechanical responses of filled thermoplastic elastomers

In this paper, several mechanical responses of thermoplastic polyurethane (TPU) filled with nano-scale attapulgite (AT) particles, including cyclic loading-unloading behavior, dynamic mechanical behavior and stress relaxation have been investigated. With the addition of AT, it is noticed that the strength, modulus and stress relaxation time of TPU increased significantly compared to that of unfilled materials. It is also observed that, whether filled or unfilled TPU, pronounced inelastic mechanical features such as Mullins effect, residual strain and hysteresis, occurred mainly during the first load loop, but became more notable with AT increase.

The Dynamic Mechanical Analysis of Highly Filled Rice Husk Biochar/High-Density Polyethylene Composites.

In this study, rice husk biochar/high-density polyethylene (HDPE) composites were prepared via melt mixing followed by extrusion. Effects of biochar content and testing temperature on the dynamic mechanical analysis (DMA) of the composites were studied. Morphological analysis of the rice husk biochar and composites were evaluated by scanning electron microscopy (SEM). The results showed that biochar had a positive effect on dynamic viscoelasticity, creep resistance and stress relaxation properties of the composites, but the creep resistance and stress relaxation of the composites decreased with the increase of temperature. SEM analysis showed that HDPE components were embedded in the holes of the rice husk biochar, and it is believed that strong interaction was achieved.

Gelation mechanism of polysaccharides from Auricularia auricula-judae

In the current study, dynamic frequency sweep and stress relaxation tests were performed to elucidate the possible gelation mechanisms of a unique thermal stable polysaccharides from Auricularia auricula-judae (AP). Addition of NaCl, CaCl2 and Ca-chelating agent (EDTA) to AP confirmed that electrostatic interaction was not the deciding force in forming the network of AP dispersion and the intrinsic calcium of AP played a role of inhibition in AP network formation. Addition of the hydrogen bond breaking agent, urea, highlighted the important role of hydrogen bonding in the formation of gel networks. The results suggested that carboxyl group of the uronic acid on AP chain are involved in the formation of inter/intra hydrogen bonding which maintain AP gel network. Quantitative effect of concentration, pH, urea and glucuronic acid on the network structure of AP dispersion were also be evaluated by entanglement network numbers (ENN) based on stress relaxation.

Engineering the Dynamic Properties of Protein Networks through Sequence Variation.

The dynamic behavior of macromolecular networks dominates the mechanical properties of soft materials and influences biological processes at multiple length scales. In hydrogels prepared from self-assembling artificial proteins, stress relaxation and energy dissipation arise from the transient character of physical network junctions. Here we show that subtle changes in sequence can be used to program the relaxation behavior of end-linked networks of engineered coiled-coil proteins. Single-site substitutions in the coiled-coil domains caused shifts in relaxation time over 5 orders of magnitude as demonstrated by dynamic oscillatory shear rheometry and stress relaxation measurements. Networks with multiple relaxation time scales were also engineered. This work demonstrates how time-dependent mechanical responses of macromolecular materials can be encoded in genetic information.

Anomalous TDDB Statistics of Gate Dielectrics Caused by Charging-Induced Dynamic Stress Relaxation Under Constant–Voltage Stress

Anomalous Time Dependent Dielectric Breakdown (TDDB) statistics of thick gate dielectrics, i.e., too large stress field/voltage dependence and nonlinear Weibull plot of TDDB lifetime, have been observed. Through the analysis of behaviors under the TDDB stress and also the comparison with thin gate dielectrics, it has been revealed that just the intrinsic charging of injected carriers to initial and stress-generated defects induces the dynamic stress relaxation under the constant–voltage stress, resulting in anomalous TDDB statistics. This charging-induced dynamic stress relaxation (CiDSR) effect reduces the validity of well-known Weibull statistics and prevents us from accurately predicting TDDB lifetimes utilizing various conventional scaling procedures, such as area, failure rate, and temperature scaling. Impact of the CiDSR effect increases with the thickness of gate dielectrics and the development of an appropriate lifetime prediction method is urgent not only for Si devices but also for various compound devices having thick gate dielectrics, such as GaN and SiC power devices.

Role of Strain Rate in the Pathogenesis of Ventilator-Induced Lung Edema.

Lungs behave as viscoelastic polymers. Harms of mechanical ventilation could then depend on not only amplitude (strain) but also velocity (strain rate) of lung deformation. Herein, we tested this hypothesis. Laboratory investigation. Animal unit. Thirty healthy piglets. Two groups of animals were ventilated for 54 hours with matched lung strains (ratio between tidal volume and functional residual capacity) but different lung strain rates (ratio between strain and inspiratory time). Individual strains ranged between 0.6 and 3.5 in both groups. Piglets ventilated with low strain rates had an inspiratory-to-expiratory time ratio of 1:2-1:3. Those ventilated with high strain rates had much lower inspiratory-to-expiratory time ratios (down to 1:9). Respiratory rate was always 15 breaths/min. Lung viscoelastic behavior, with ventilator setting required per protocol, was "quantified" as dynamic respiratory system hysteresis (pressure-volume loop [in Joules]) and stress relaxation (airway pressure drop during an end-inspiratory pause [in cm H2O]). Primary outcome was the occurrence of pulmonary edema within 54 hours. On average, the two study groups were ventilated with well-matched strains (2.1 ± 0.9 vs 2.1 ± 0.9; p = 0.864) but different strain rates (1.8 ± 0.8 vs 4.6 ± 1.5 s; p < 0.001), dynamic respiratory system hysteresis (0.6 ± 0.3 vs 1.4 ± 0.8 J; p = 0.001), and stress relaxation (3.1 ± 0.9 vs 5.0 ± 2.3 cm H2O; p = 0.008). The prevalence of pulmonary edema was 20% among piglets ventilated with low strain rates and 73% among those ventilated with high strain rates (p = 0.010). High strain rate is a risk factor for ventilator-induced pulmonary edema, possibly because it amplifies lung viscoelastic behavior.

A noble additive cum compatibilizer for dispersion of nanoclay into ethylene octene elastomer

This paper introduces a poly(ethylene-co-octene)-poly(ethylene-co-vinyl acetate) double network hybrid as a noble additive cum compatibilizer for poly(ethylene-co-octene) (POE). The addition of only 0.5 mass% of the hybrid into POE has raised the net crystallinity from 29% to 59% and consequently improved both static (tensile strength, modulus and elongation at break) and dynamic (stress relaxation and hysteresis) properties of the virgin POE. As compatibilizer for organically modified montmorillonite (OMt) most of the properties further improved at a minimal loading of both OMt (0.5–1.0mass%) and compatibilizer (1–2mass%) owing to the establishment of a dominant intercalated structure. Although the die swell is slightly increased in the presence of both double network hybrid and OMt, the extruded profiles exhibit remarkable improvement in appearance compared to the virgin POE.

Nanocomposite hydrogels: Fracture toughness and energy dissipation mechanisms

In this study, fracture toughness of nanocomposite hydrogels is quantified, and active mechanisms for dissipation of energy of nanocomposite hydrogels are ascertained. Poly(N,N-dimethylacrylamide) nanocomposite hydrogels are prepared by in situ free radical polymerization with the incorporation of Laponite, a hectorite synthetic clay. Transmission electron microscopy proves exfoliation of clay platelets that serve as multifunctional crosslinkers in the created physical network. Extraordinary high fracture energies of up to 6800 J m−2 are determined by the pure shear test approach, which shows that these soft and stretchable hydrogels are insensitive to notches. In contrast to single- and double-network hydrogels, dynamic mechanic analysis and stress relaxation experiments clarify that significant viscoelastic dissipation occurs during deformation of nanocomposite hydrogels. Similar to double-network hydrogels, crack tip blunting and plastic deformation also contribute to the observed massive fracture energies. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1763–1773