Showing papers on "Stress relaxation published in 2022"

Crack-resistant σ/FCC interfaces in the Fe40Mn40Co10Cr10 high entropy alloy with the dispersed σ-phase

Abstract: We report the role of morphology and size of the σ-phase on mechanical properties of the Fe40Mn40Co10Cr10 non-equiatomic high entropy alloy. The dispersed and fine σ-precipitates formed after cold-rolling and annealing at 700 °C for 1 h lead to an increase in strength without severe ductility loss compared to the material without the σ-phase. However, the coarsened and connected σ-phase formed after prolonged annealing at 700 °C for 100 h results in early failure due to the activation of microcracks along σ/σ interfaces. The σ/FCC interface could act as strong obstacles for dislocation motion and could effectively relieve the stress concentration by activating deformation twins or dislocations. The σ/FCC interfaces are excellent in terms of hardening, accommodation of plastic deformation, and stress relaxation leading to the observed crack resistance. Therefore, we suggest that increasing σ/FCC interfaces by controlling size and dispersion of the σ-phase is essential to develop HEAs with an excellent strength-ductility combination and damage tolerance.

Fast Dynamic Siloxane Exchange Mechanism for Reshapable Vitrimer Composites

Abstract: To develop siloxane-containing vitrimers with fast dynamic characteristics, different mechanistic pathways have been investigated using a range of catalysts. In particular, one siloxane exchange pathway has been found to show a fast dynamic behavior in a useful temperature range (180-220 °C) for its application in vitrimers. The mechanism is found to involve 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) as an organic catalyst in the presence of hydroxyl groups. Using this new mechanistic approach, vitrimers with ultrafast stress-relaxation characteristics (relaxation times below 10 s) have been prepared with a readily available epoxy resin and siloxane-amine hardener. Subsequently, the low viscosity siloxane-containing vitrimer resin enabled the preparation of glass fiber-reinforced vitrimer composites using an industrially relevant vacuum-assisted resin infusion technique. The resulting composite was successfully thermoformed into a new shape, which makes it possible to envision a second life for such highly engineered materials.

On the Influence of Alloy Composition on the Additive Manufacturability of Ni-Based Superalloys

Abstract: Abstract The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys—some of existing (heritage) type but also other newly-designed ones—are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found—solidification cracks and solid-state ductility dip cracks—to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000 °C and measurements of tensile ductility at 800 °C are used to interpret the effects of alloy composition. A model for solid-state cracking is proposed, based on an incapacity to relax the thermal stress arising from constrained differential thermal contraction; its development is supported by experimental measurements using a constrained bar cooling test. A modified solidification cracking criterion is proposed based upon solidification range but including also a contribution from the stress relaxation effect. This work provides fundamental insights into the role of composition on the additive manufacturability of these materials.

Visualizing the failure of solid electrolyte under GPa-level interface stress induced by lithium eruption

Abstract: Solid electrolytes hold the promise for enabling high-performance lithium (Li) metal batteries, but suffer from Li-filament penetration issues. The mechanism of this rate-dependent failure, especially the impact of the electrochemo-mechanical attack from Li deposition, remains elusive. Herein, we reveal the Li deposition dynamics and associated failure mechanism of solid electrolyte by visualizing the Li|Li7La3Zr2O12 (LLZO) interface evolution via in situ transmission electron microscopy (TEM). Under a strong mechanical constraint and low charging rate, the Li-deposition-induced stress enables the single-crystal Li to laterally expand on LLZO. However, upon Li "eruption", the rapidly built-up local stress, reaching at least GPa level, can even crack single-crystal LLZO particles without apparent defects. In comparison, Li vertical growth by weakening the mechanical constraint can boost the local current density up to A·cm-2 level without damaging LLZO. Our results demonstrate that the crack initiation at the Li|LLZO interface depends strongly on not only the local current density but also the way and efficiency of mass/stress release. Finally, potential strategies enabling fast Li transport and stress relaxation at the interface are proposed for promoting the rate capability of solid electrolytes.

The nonlinear rheology of complex yield stress foods

Abstract: Many foods have a yield stress that allows them to retain a desired shape at rest, but transition into a viscous fluid when being served or consumed. The determination of the yield stress of the food dictates how the foods are formed and packaged, how they are served, and how they are perceived when being eaten. Oscillatory shearing provides an ideal test protocol to map the rheology across a range of time and flow strength scales. We couple oscillatory shearing and an iterative recovery procedure to show that the yielding process is a continuous transition for two common yield stress foods. We show that unrecoverable processes from oscillatory tests are equivalent to the steady shear flow behavior. We show that this yielding behavior can be well approximated by a recently published model that treats yield stress materials as continuous viscoelastic fluids with a rate-dependent relaxation time and has parameters that can be obtained from the linear viscoelastic oscillatory frequency sweep and the steady shear flow curve.

Effect of cyclic straining with various rates on stress softening/hysteresis and structural evolution of filled rubber: A time-resolved SANS study

Abstract: The effect of cyclic straining on stress softening/hysteresis and structural evolution silica filled deuterated silicone rubber (SFdSR) during cyclic deformation has been investigated. In situ rheo-small-angle neutron scattering (Rheo-SANS) measurements were conducted with engineering strain rates (έ) spanning from 0.001 to 0.1 s−1. Moreover, superimposed time-resolved SANS technique is utilized. The non-linear behaviors all exhibit strong έ and cycle-number (No.C) dependency. Rheo-SANS suggests that the orientation of bound rubber responses sensitively to the strain, έ, and No.C, which presents an accumulative characteristic, associated with cyclic stress relaxation of the engineering stress-nominal strain curves. All the results consistently suggest that some substantial permanent changes occur within bound rubber, filler network, and rubber matrix after deforming the samples of virgin state. Evolutions would drastically develop during the initial couple of cycles, then accumulate and reach to a newly established equilibrium under constant έ, as long as the maximum strain is not exceeded.

Tuning Dual-Dynamic Network Materials through Polymer Architectural Features

Abstract: Dynamic materials are known for their self-healing, adhesive, and shape memory applications. Interpenetrating networks (IPNs) are types of materials that can hold dual-dynamic crosslinkers to show complementary chemical and mechanical properties. There have been a number of research studies exploring the dynamic chemistries involved in IPN materials. Not only the bond type but also the polymer network architecture play an important role in governing IPN material properties. In this study, we show that network architectural features are as much as important as studying the dynamic chemistries using an IPN system with quadrupole hydrogen (H) bonding and thiol-Michael (TM) bonding. This work varied network types, chain lengths, dynamic bond compositions, crosslink densities, and crosslink distributions within a system to explore their effects on the thermomechanical properties. The synergetic effects of H and TM bonds revealed excellent stress relaxation and self-healing at room temperature and elevated temperatures. Increment of chain length and crosslink density enhanced the strength of the materials to as high as 3.5 MPa, while the crosslink distribution boosted the creep resistance under an applied force. Furthermore, complementary H and TM bonding assisted in improving the adhesive properties in these materials to hold up to 2 kg weight with the adhered wood strips.

Microscopic dynamics underlying the stress relaxation of arrested soft materials

Abstract: Significance The linear viscoelasticity of soft materials is governed by the microscopic thermal fluctuations of the underlying constituents in the system, which are expected to give rise to mono-exponential stress relaxation processes. However, many soft materials, such as glasses and gels, instead exhibit a broad distribution of stress relaxation times, for which the microscopic origin remains elusive. Here, we investigate the microscopic fluctuations inside an arrested gel and reveal the presence of two distinct microscopic relaxation mechanisms: quiescent relaxations governed by the buildup of internal stresses during arrest, and perturbation-induced avalanche relaxation events governed by mechanical deformations in the system. We show that both nonlinear relaxation mechanisms are essential components of nonexponential stress relaxation in arrested soft materials.

Stress relaxation amplitude of hydrogels determines migration, proliferation, and morphology of cells in 3-D culture

Abstract: The viscoelastic behavior of hydrogel matrices sensitively influences the cell behavior in 3-D culture and biofabricated tissue model systems. Previous reports have demonstrated that cells tend to adhere, spread, migrate and proliferate better in hydrogels with pronounced stress relaxation. However, it is currently unknown if cells respond more sensitively to the amplitude of stress relaxation, or to the relaxation time constant. To test this, we compare the behavior of fibroblasts cultured for up to 10 days in alginate and oxidized alginate hydrogels with similar Young's moduli but diverging stress relaxation behavior. We find that fibroblasts elongate, migrate and proliferate better in hydrogels that display a higher stress relaxation amplitude. By contrast, the cells' response to the relaxation time constant was less pronounced and less consistent. Together, these data suggest that it is foremost the stress relaxation amplitude of the matrix that determines the ability of cells to locally penetrate and structurally remodel the matrix on a molecular level, which subsequently leads to better spreading, faster migration, and higher cell proliferation. We conclude that the stress relaxation amplitude is a central design parameter for optimizing cell behavior in 3-D hydrogels.

Isotropy‐Induced Stress Relaxation and Strong‐Tolerance for High‐Rate and Long‐Duration Sodium Storage by Amorphous Structure Engineering

Abstract: High‐performance conversion‐reaction‐based electrodes have received enormous attention due to their high theoretical capacities, yet the poor structural stability and large stress accumulation induced from volume change are identified as major limiting factors for further utilization. Although viable strategies are exerted to acquire durable storage capacity, the controllable technology of inner stress is still required to explore. Herein, the amorphous structured GeS2 with 2D porous sheet morphology is designed for sodium‐ion batteries (SIBs). The differences in reaction mechanism and structure reversibility between amorphous and crystalline states are systematically investigated and unraveled. Notably, the amorphous matrix can ensure evenly distributed intermediate interface and alleviated aggregation of active materials, promoting the complete conversion reaction with decreased hysteresis. Furthermore, it provides more isotropic Na‐ion diffusion channels and active sites, effectively boosting the reaction kinetics. Meanwhile, it can fully relieve the volume expansion and restrain the inner stress of GeS2, resulting in excellent structural tolerance. As expected, it expresses prominent rate capability and cyclic life (512.8 mA h g−1 at a superhigh rate of 10 A g−1 after 1000 cycles) for SIBs. Therefore, amorphization engineering is a reliable strategy to obtain the robust framework and unimpeded diffusion channels for developing high‐performance conversion‐reaction‐based electrode materials.

Critical Effects of Branch Numbers at the Cross-Link Point on the Relaxation Behaviors of Transesterification Vitrimers

Abstract: Vitrimers are functional cross-linked materials, exhibiting reprocessability, recyclability, and healability, and thus these are expected for application as sustainable materials. The functionalities of vitrimers are attributable to their associative bond exchange mechanism that is activated at a certain high temperature. The construction of a tuning method for the bond exchange properties must be useful for coming practical application of the vitrimer concept. Here, we prepare transesterification-based vitrimers via the thiol–epoxy click reaction to elucidate the essential effects of the branch numbers (f) at the cross-link point on their bond exchange properties, where f can be readily tuned via the functionality of the starting materials. The temperature-ramp creep and stress–relaxation tests then demonstrate that the vitrimer properties, such as the softening and stress–relaxation behaviors, vary depending on f. The experimental results derive some empirical relationships between f and the relaxation time and between f and activation energy of the bond exchange. In addition, the relaxation behavior of the vitrimer network with mixed f is investigated in the final section, showing the relaxation rate can be determined by the harmonic mean of relaxation time weighted by the mole fraction of the network components having different f. Overall, this study demonstrates that the design of a proper f is crucial to obtain the desired properties of vitrimers.

Measurement and modeling of creep property of high-strength concrete considering stress relaxation effect

Abstract: Measurement of concrete creep under constant sustained stress conditions is usually a long process. The external load applied on the specimen may relax gradually during this long process due to the time-dependent properties of concrete and test set-up, which would lead to an improper assessment of concrete creep without considering the stress relaxation effect. This study developed a theoretical method based on the linear superposition principle to derive the specific creep of concrete from the compressive creep test results where a certain amount of stress has relaxed. Three types of high-strength concretes were cast for the creep tests. In addition, the workability, mechanical properties, and shrinkage of the three concretes were measured. Then the application of the existing models including the ACI model, CEB-FIP model, GL2000 model, and B4 model to predict the creep and shrinkage of high-strength concrete was discussed. The results reveal that the specific creep of concrete used in this study without considering the stress relaxation effect decreases by 4.3%∼11.0% when 6.7%∼13.1% of the applied stresses relax during the creep tests. Both creep and shrinkage of high-strength concrete exposed to drying decrease with increasing curing age. It is found that the CEB-FIP model can predict the shrinkage of high-strength concrete well and the B4 model can give a predicted result relatively close to the measured creep of high-strength concrete.

Star‐Polymer–DNA Gels Showing Highly Predictable and Tunable Mechanical Responses

Abstract: Dynamically crosslinked gels are appealing materials for applications that require time-dependent mechanical responses. DNA duplexes are ideal crosslinkers for building such gels because of their excellent sequence addressability and flexible tunability in bond energy. However, the mechanical responses of most DNA gels are complicated and unpredictable. Here, a DNA gel with a highly homogeneous gel network and well predictable mechanical behaviors is demonstrated by using a pair of star-polymer-DNA precursors with presimulated DNA sequences showing the two-state transition. The melting curve analysis of the DNA gels reveals the good correspondence between the thermodynamic potentials of the DNA crosslinkers and the presimulated values by DNA calculators. Stress-relaxation tests and dissociation kinetics measurements show that the macroscopic relaxation time of the DNA gels is approximately equal to the lifetime of the DNA crosslinkers over 4 orders of magnitude from 0.1-2000 s. Furthermore, a series of durability tests find the DNA gels are hysteresis-less and self-healable after the applications of repeated temperature and mechanical stimuli. These results demonstrate the great potential of star-polymer-DNA precursors for building gels with predictable and tunable viscoelastic properties, suitable for applications such as stress-response extracellular matrices, injectable solids, and soft robotics.

Residual stress control of 316 L stainless steel using pulsed-wave laser additive manufacturing

Abstract: In this paper, 316 L stainless steel were fabricated by direct laser deposition (DED) using both continuous-wave (CW) and pulsed-wave (PW) laser modes. Effects of laser modes on residual stress, geometric dislocation density (GND) and solid phase transformation with a given heat energy input were investigated. Results showed that PW mode tends to generate less concentrated, lower residual stress than that CW mode does, which the decline range was reached about 42% at the joint of the deposition and the substrate and approximately 75% on the deposition respectively. This is determined by the process characteristics of PW mode. Firstly, the relatively small temperature gradient in PW mode directly causes small thermal stress. Secondly, the characteristic of periodic temperature history and an improved cooling rate of molten pool in PW mode leads to more thermal stress relaxation. The released thermal stress can be used as a driving force for the formation of dislocations at grain boundaries. In addition, solid phase transformation does not occur with PW mode, which has little contribution to the reduction of residual stress.