Showing papers on "Stress relaxation published in 2015"

Residual Stress and Distortion Modeling of Electron Beam Direct Manufacturing Ti-6Al-4V

Abstract: In this work, a finite element model is developed for predicting the thermo-mechanical response of Ti-6Al-4V during electron beam deposition. A three-dimensional thermo-elasto-plastic analysis is performed to model distortion and residual stress in the workpiece and experimental in situ temperature, and distortion measurements are performed during the deposition of a single-bead-wide, 16-layer-high wall built for model validation. Post-process blind hole–drilling residual stress measurements are also performed. Both the in situ distortion and post-process residual stress measurements suggest that stress relaxation occurs during the deposition of Ti-6Al-4V. A method of accounting for such stress relaxation in thermo-elasto-plastic simulations is proposed where both stress and plastic strain are reset to 0, when the temperature exceeds a prescribed stress relaxation temperature. Inverse simulation is used to determine the values of the absorption efficiency and the emissivity of electron beam–deposited, wir...

Soft Material Characterization for Robotic Applications

Abstract: In this article we present mechanical measurements of three representative elastomers used in soft robotic systems: Sylgard 184, Smooth-Sil 950, and EcoFlex 00-30. Our aim is to demonstrate the effects of the nonlinear, time-dependent properties of these materials to facilitate improved dynamic modeling of soft robotic components. We employ uniaxial pull-to-failure tests, cyclic loading tests, and stress relaxation tests to provide a qualitative assessment of nonlinear behavior, batch-to-batch repeatability, and effects of prestraining, cyclic loading, and viscoelastic stress relaxation. Strain gauges composed of the elastomers embedded with a microchannel of conductive liquid (eutectic gallium–indium) are also tested to quantify the interaction between material behaviors and measured strain output. It is found that all of the materials tested exhibit the Mullins effect, where the material properties in the first loading cycle differ from the properties in all subsequent cycles, as well as respon...

Recoverable plasticity in penta-twinned metallic nanowires governed by dislocation nucleation and retraction

Abstract: There has been relatively little study on time-dependent mechanical properties of nanowires, in spite of their importance for the design, fabrication and operation of nanoscale devices. Here we report a dislocation-mediated, time-dependent and fully reversible plastic behaviour in penta-twinned silver nanowires. In situ tensile experiments inside scanning and transmission electron microscopes show that penta-twinned silver nanowires undergo stress relaxation on loading and complete plastic strain recovery on unloading, while the same experiments on single-crystalline silver nanowires do not exhibit such a behaviour. Molecular dynamics simulations reveal that the observed behaviour in penta-twinned nanowires originates from the surface nucleation, propagation and retraction of partial dislocations. More specifically, vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery.

Dynamically Cross-Linked Polydimethylsiloxane Networks with Ambient-Temperature Self-Healing

Abstract: Dynamically cross-linked polymer networks have promising potential to serve as self-healing materials, but this is often limited to occur at elevated temperature. We overcome this limitation by refinement of an earlier approach to synthesize dynamically cross-linked polydimethylsiloxane networks such to show stress relaxation and self-healing at room temperature on short time scales, which we probe by oscillatory shear rheology and piercing experiments. Our studies reveal a direct correlation between the amount of anionic groups in the polymer networks and their stress-relaxation and self-healing rates, allowing these materials to be designed in a rational fashion.

An elasto-plastic self-consistent model with hardening based on dislocation density, twinning and de-twinning: Application to strain path changes in HCP metals

Abstract: In this work, we develop a polycrystal mean-field constitutive model based on an elastic–plastic self-consistent (EPSC) framework. In this model, we incorporate recently developed subgrain models for dislocation density evolution with thermally activated slip, twin activation via statistical stress fluctuations, reoriented twin domains within the grain and associated stress relaxation, twin boundary hardening, and de-twinning. The model is applied to a systematic set of strain path change tests on pure beryllium (Be). Under the applied deformation conditions, Be deforms by multiple slip modes and deformation twinning and thereby provides a challenging test for model validation. With a single set of material parameters, determined using the flow-stress vs. strain responses during monotonic testing, the model predicts well the evolution of texture, lattice strains, and twinning. With further analysis, we demonstrate the significant influence of internal residual stresses on (1) the flow stress drop when reloading from one path to another, (2) deformation twin activation, (3) de-twinning during a reversal strain path change, and (4) the formation of additional twin variants during a cross-loading sequence. The model presented here can, in principle, be applied to other metals, deforming by multiple slip and twinning modes under a wide range of temperature, strain rate, and strain path conditions.

Time-dependent compaction band formation in sandstone

Abstract: Compaction bands in sandstone are laterally extensive planar deformation features that are characterized by lower porosity and permeability than the surrounding host rock. As a result, this form of localization has important implications for both strain partitioning and fluid flow in the Earth's upper crust. To better understand the time dependency of compaction band growth, we performed triaxial deformation experiments on water-saturated Bleurswiller sandstone (initial porosity = 0.24) under constant stress (creep) conditions in the compactant regime. Our experiments show that inelastic strain accumulates at a constant stress in the compactant regime, manifest as compaction bands. While creep in the dilatant regime is characterized by an increase in porosity and, ultimately, an acceleration in axial strain rate to shear failure, compaction creep is characterized by a reduction in porosity and a gradual deceleration in axial strain rate. The global decrease in the rates of axial strain, acoustic emission energy, and porosity change during creep compaction is punctuated at intervals by higher rate excursions, interpreted as the formation of compaction bands. The growth rate of compaction bands formed during creep is lower as the applied differential stress, and hence, background creep strain rate, is decreased. However, the inelastic strain associated with the growth of a compaction band remains constant over strain rates spanning several orders of magnitude (from 10−8 to 10−5 s−1). We find that despite the large differences in strain rate and growth rate (from both creep and constant strain rate experiments), the characteristics (geometry and thickness) of the compaction bands remain essentially the same. Several lines of evidence, notably the similarity between the differential stress dependence of creep strain rate in the dilatant and compactant regimes, suggest that as for dilatant creep, subcritical stress corrosion cracking is the mechanism responsible for compactant creep in our experiments. Our study highlights that stress corrosion is an important mechanism in the time-dependent porosity loss, subsidence, and permeability reduction of sandstone reservoirs.

On rate-dependent polycrystal deformation: the temperature sensitivity of cold dwell fatigue.

Abstract: A temperature and rate-dependent crystal plasticity framework has been used to examine the temperature sensitivity of stress relaxation, creep and load shedding in model Ti-6Al polycrystal behaviour under dwell fatigue conditions. A temperature close to 120°C is found to lead to the strongest stress redistribution and load shedding, resulting from the coupling between crystallographic slip rate and slip system dislocation hardening. For temperatures in excess of about 230°C, grain-level load shedding from soft to hard grains diminishes because of the more rapid stress relaxation, leading ultimately to the diminution of the load shedding and hence, it is argued, the elimination of the dwell debit. Under conditions of cyclic stress dwell, at temperatures between 20°C and 230°C for which load shedding occurs, the rate-dependent accumulation of local slip by ratcheting is shown to lead to the progressive cycle-by-cycle redistribution of stress from soft to hard grains. This phenomenon is termed cyclic load shedding since it also depends on the material's creep response, but develops over and above the well-known dwell load shedding, thus providing an additional rationale for the incubation of facet nucleation.

Piezoresistive and compression resistance relaxation behavior of water blown carbon nanotube/polyurethane composite foam

Abstract: The in-situ bulk polycondensation process in combination with a ball milling dispersion process was used to prepare the water blown multiwall carbon nanotubes (CNT)/polyurethane (PU) composite foam. The mechanical properties, piezoresistive properties, strain sensitivity, stress and resistance relaxation behaviors of the composite foams were investigated. The results show that the CNT/PU composite foam has a better compression strength than the unfilled polyurethane foams and a negative pressure coefficient behavior under uniaxial compression. The resistance response of CNT/PU nanocomposites foam under cyclic compressive loading was quite stable. The nanocomposite foam containing a weight fraction of carbon nanotubes close to the percolation threshold presents the largest strain sensitivity for the resistance. The characteristic of resistance relaxation of CNT/PU composite foam is different from the stress relaxation due to the different relaxation mechanism. During compressive stress relaxation, the CNT/PU foam composites have excellent resistance recoverability while poor stress recoverability.

Cooling rate and stress relaxation in silica melts and glasses via microsecond molecular dynamics.

Abstract: We have conducted extremely long molecular dynamics simulations of glasses to microsecond times, which close the gap between experimental and atomistic simulation time scales by two to three orders of magnitude. Static, thermal, and structural properties of silica glass are reported for glass cooling rates down to 5×10(9) K/s and viscoelastic response in silica melts and glasses are studied over nine decades of time. We present results from relaxation of hydrostatic compressive stress in silica and show that time-temperature superposition holds in these systems for temperatures from 3500 to 1000 K.

On a new method to determine the yield stress in lubricating grease

Abstract: An experimental study using both a controlled stress and a controlled strain rheometer has been undertaken to characterize lubricating grease in shear, creep, stress relaxation, and oscillatory flow, with a main focus on determining the yield stress. The yield stress was examined using a cone–plate and parallel-plate system with smooth and rough surfaces. Clear discrepancies were observed in the yield stress values obtained using different techniques where oscillatory strain sweep measurements seem to be the best choice. This technique is less sensitive to wall slip, shows good reproducibility, and is relatively easy to perform. The method also shows that the yield stress is a function of the imposed frequency and therefore of the time domain. At lower values of shear—that is, in the linear viscoelastic regime—there is no structural breakdown and the rheology of the grease can be described by the Maxwell model where the stress and the strain are almost proportional to each other. Based on this observation, a novel method to determine the yield stress is proposed: “The yield stress can be determined from the point where this linearity no longer applies.” This method is compared to those that are commonly used. The yield stress was found to depend exponentially on temperature and linearly on frequency.

Interactions and Stress Relaxation in Monolayers of Soft Nanoparticles at Fluid-Fluid Interfaces

Abstract: Nanoparticles with grafted layers of ligand molecules behave as soft colloids when they adsorb at fluid-fluid interfaces. The ligand brush can deform and reconfigure, adopting a lens-shaped configuration at the interface. This behavior strongly affects the interactions between soft nanoparticles at fluid-fluid interfaces, which have proven challenging to probe experimentally. We measure the surface pressure for a stable 2D interfacial suspension of nanoparticles grafted with ligands, and extract the interaction potential from these data by comparison to Brownian dynamics simulations. A soft repulsive potential with an exponential form accurately reproduces the measured surface pressure data. A more realistic interaction potential model is also fitted to the data to provide insights into the ligand configuration at the interface. The stress of the 2D interfacial suspension upon step compression exhibits a single relaxation time scale, which is also attributable to ligand reconfiguration.

An assessment of creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) steel

Abstract: Creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb steel have been investigated at 873 K, 923 K and 973 K over a stress range of 80–220 MPa. The absence of clear primary creep regime and prolonged secondary stage of creep deformation have been noticed under lower stress level at 973 K. The variation of minimum creep rate with applied stress obeyed Norton's power law of creep. The apparent stress exponents of 15.2, 12.3 and 5.8, and apparent activation energy of 619 kJ/mole have been estimated for creep deformation of the steel. The apparent stress exponents and activation energy have been rationalised on the basis of threshold stress. The threshold stress values of 137.5 MPa, 83.3 MPa and 29.7 MPa were obtained at 873 K, 923 K and 973 K respectively. The threshold stress compensated true stress exponent of 4 and true activation energy of 244 kJ/mole, and threshold stress normalised by Orowan stress confirms that the lattice diffusion assisted localised climb of dislocation is the rate controlling of creep deformation in the steel. The steel obeyed Monkman and modified Monkman–Grant relationships. Damage tolerance factor of 6 in the steel demonstrates that the microstructural degradation such as coarsening of precipitates and subgrain structure is the dominant creep damaging mechanism in the steel.

Evolution of microstructural and mechanical properties of lanthanum zirconate thermal barrier coatings at high temperature

Abstract: Ceramic thermal barrier coatings (TBCs) are advanced systems for more efficient turbine engines. In this work, thick lanthanum zirconate TBCs were fabricated by Air Plasma Spraying (APS). The coatings showed a lamellar microstructure containing splat boundaries, pores and microcracks. After thermal exposure at 1200 and 1350 °C the partial sintering of the porous microstructure occurred. High-temperature evolution of the mechanical properties was investigated by arranging specific bending tests up to 1500 °C using SiC testing assembly which allowed to calculate the strain by means of curvature measurement. The mechanical properties improved with increasing the testing temperature, due to the inelastic deformation and stress relaxation which counteracted the sintering effects. The elastic modulus increased after thermal aging, but decreased for the aged coatings tested at higher temperature. The thermal expansion coefficient of as-sprayed coatings slightly increased after thermal aging, while a reduction of specific heat capacity was noticed.

Linear oscillation of gas bubbles in a viscoelastic material under ultrasound irradiation

Abstract: Acoustically forced oscillation of spherical gas bubbles in a viscoelastic material is studied through comparisons between experiments and linear theory. An experimental setup has been designed to visualize bubbledynamics in gelatin gels using a high-speed camera. A spherical gas bubble is created by focusing an infrared laser pulse into (gas-supersaturated) gelatin gels. The bubble radius (up to 150 μm) under mechanical equilibrium is controlled by gradual mass transfer of gases across the bubble interface. The linearized bubbledynamics are studied from the observation of spherical bubble oscillation driven by low-intensity, planar ultrasound driven at 28 kHz. It follows from the experiment for an isolated bubble that the frequency response in its volumetric oscillation was shifted to the high frequency side and its peak was suppressed as the gelatin concentration increases. The measurement is fitted to the linearized Rayleigh–Plesset equation coupled with the Voigt constitutive equation that models the behavior of linear viscoelastic solids; the fitting yields good agreement by tuning unknown values of the viscosity and rigidity, indicating that more complex phenomena including shear thinning, stress relaxation, and retardation do not play an important role for the small-amplitude oscillations. Moreover, the cases for bubble-bubble and bubble-wall systems are studied. The observed interaction effect on the linearized dynamics can be explained as well by a set of the Rayleigh–Plesset equations coupled through acoustic radiation among these systems. This suggests that this experimental setup can be applied to validate the model of bubbledynamics with more complex configuration such as a cloud of bubbles in viscoelastic materials.

Fundamentals of Creep in Materials

Abstract: Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 T m , where T m is the absolute melting temperature. The plasticity under these conditions is described for constant stress and constant strain-rate conditions. Several aspects of the curve require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where the creep rate (plastic strain rate), ɛ ˙ = d ɛ /d t is changing with increasing plastic strain or time. The primary creep rate decreases with increasing strain, but with some types of creep, such as solute drag with “3-power creep,” an “inverted” primary occurs where the strain rate increases with strain. Analogously, under constant strain rate conditions, the metal hardens, resulting in increasing flow stresses. Often, in pure metals, the strain rate decreases or the stress increases to a value that is constant over a range of strain. The phenomenon is termed Stage II, secondary, or steady-state creep. Eventually, cavitation and/or cracking increases the apparent strain rate or decreases the flow stress. This regime is termed Stage III, or tertiary, creep and leads to fracture. Sometimes, Stage I leads directly to Stage III and an “inflection” is observed. Thus, care must sometimes be exercised in concluding a mechanical steady state.

Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals

Abstract: A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80-400 nanometers) and grain sizes (50-220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts.

Analysis of flax fibres viscoelastic behaviour at micro and nano scales

Abstract: In glass or carbon fibres reinforced plastics, creep or stress relaxation, arise from the polymeric nature of the matrix. Plant fibres, used in bio-composites, are also polymers. Therefore, the issue of their service life requires studying the viscoelastic behaviour of both the matrix and the fibres. In this study, we investigate, at different length scales, the response of elementary flax fibres to tensile tests, as well as to nano-indentation tests on their secondary cell walls. The results of these experiments are then analysed via linear viscoelastic rheological models and identification procedures. The values of the identified parameters (relaxation time, viscosity and elastic stiffness) are discussed in relation to the microstructure of the flax fibre (cellulose microfibrils, hemicelluloses and pectins). The nano-indentation technique provides much more deterministic results than tension tests on an entire fibre. The scale of the secondary wall cell is then relevant to assess the viscoelastic behaviour of the fibres.

Volmer-Weber growth stages of polycrystalline metal films probed by in situ and real-time optical diagnostics

Abstract: The Volmer-Weber growth of high-mobility metal films is associated with the development of a complex compressive-tensile-compressive stress behavior as the film deposition proceeds through nucleation of islands, coalescence, and formation of a continuous layer. The tensile force maximum has been attributed to the end of the islands coalescence stage, based on ex situ morphological observations. However, microstructural rearrangements are likely to occur in such films during post-deposition, somewhat biasing interpretations solely based on ex situ analysis. Here, by combining two simultaneous in situ and real-time optical sensing techniques, based on surface differential reflectance spectroscopy (SDRS) and change in wafer curvature probed by multibeam optical stress sensor (MOSS), we provide direct evidence that film continuity does coincide with tensile stress maximum during sputter deposition of a series of metal (Ag, Au, and Pd) films on amorphous SiOx. Stress relaxation after growth interruption was testified from MOSS, whose magnitude scaled with adatom mobility, while no change in SDRS signal could be revealed, ruling out possible changes of the surface roughness at the micron scale.

Build-up and two-step relaxation of internal stress in jammed suspensions

Abstract: We perform experiments on jammed suspensions of microgels with different constituent properties to determine their stress relaxation behavior on flow cessation. We observe that the stress relaxes through a two-step process: A rapid initial relaxation where internal stresses are trapped followed by a much slower decay. Trapped internal stresses are related to the solvent viscosity, particle elasticity, and volume fraction through a universal scaling. The second slower relaxation of the internal stress is characterized by a single exponential decay, which is independent of the preshear stress and relatively insensitive to the material properties of the microgel suspension. Particle-scale simulations are used to understand the microscopic mechanisms which drive the amplitude and the kinetics of the stress relaxation as well as the local particle dynamics in each regime. The rapid initial relaxation occurs through ballistic particle motion, where the number of contacts and average compression return to their static values but the asymmetry of the pair distribution function remains as a signature of the internal stress.