Local Stress Relaxation Research Articles

Monitoring of Biological Changes in Electromechanical Reshaping of Cartilage Using Imaging Modalities.

Electromechanical reshaping (EMR) is a promising surgical technique used to reshape cartilage by direct current and mechanical deformation. It causes local stress relaxation and permanent alterations in the shape of cartilage. The major advantages of EMR are its minimally invasive nature and nonthermal electrochemical mechanism of action. The purpose of this study is to validate that EMR does not cause thermal damage and to observe structural changes in post-EMR cartilage using several imaging modalities. Three imaging modality metrics were used to validate the performance of EMR by identifying structural deformation during cartilage reshaping: infrared thermography was used to sense the temperature of the flat cartilages (16.7°C at 6 V), optical coherence tomography (OCT) was used to examine the change in the cartilage by gauging deformation in the tissue matrix during EMR, and scanning electron microscopy (SEM) was used to show that EMR-treated cartilage is irregularly arranged and the thickness of collagen fibers varies, which affects the change in shape of the cartilage. In conclusion, the three imaging modalities reveal the nonthermal and electromechanical mechanisms of EMR and demonstrate that use of an EMR device is feasible for reshaping cartilage in a minimally invasive manner.

Defect-induced carbon nanoscroll formation

Carbon nanoscrolls (CNSs), a graphitic structure formed by rolling of a graphene plane, must overcome an energy barrier in order to form. This energy barrier is the result of competing interactions between the van der Waals force, which wants to cause overlap of the graphene, and the torsional or bending force, which resists bending of the graphene plane. In this study we used molecular dynamics simulation to examine the effect of vacancies on the CNS formation dynamics. We found that the energy barrier, which must be overcome to obtain a stable scroll structure, can be lowered or completely eliminated by introducing vacancies. Individual vacancy and divacancy configurations are studied and found to reduce the torsional bending energy by allowing local stress relaxation around the defect site. A structural transition diagram is created in which we show the energy barrier height for a range of vacancy concentrations and CNS rolling widths. These results provide the theoretical backing for a new method of fabricating CNSs using a focused ion beam and shed new light on the self-rolling phenomenon in graphene.

Wavelet based approach to signal activity detection and phase picking: Application to acoustic emission

Locating the sources of elastic waves during rapid local stress relaxation in solids under load is a central element in acoustic emission non-destructive testing, seismology, etc. The location problem relies heavily on the accuracy of arrival time detection. To increase the reliability of real time signal detection and to ensure precise phase picking of transient waveforms of a low amplitude, we propose a novel Wavelet transform-based algorithm. Benefiting strongly from the neighboring concepts in the wavelet theory, the shortcomings of conventional amplitude threshold-based and Short Term Average/Long Term Average methods are addressed. The proposed method was validated in a variety of acoustic emission tests, demonstrating the excellent temporal localization of the picked phases even for the signals with very low signal-to-noise ratio.

Guided wrinkling with nanoimprinted SU-8 surfaces

The wrinkling of SU-8 may be useful for microelectromechanical systems and optical applications, but highly regular and periodic wrinkles are the most convenient for such applications. This paper reports wrinkles generated on SU-8 by exposure to 172 nm-wavelength ultraviolet under ozone. To identify the relevant processing parameters, the impact of the SU-8 layer thickness and the effects of the postexposure bake temperature and UV-ozone treatment time are investigated. With 5 μm-thick SU-8 layers, distinct wrinkles are obtained with an ultraviolet-ozone treatment of 3 min and at a postexposure bake temperature of at least 140 °C. To guide the wrinkling, topography is induced on the SU-8 surface via capillary force lithography. By using a stamp with equidistant cavities possessing different widths, the structure size dependence is investigated. Owing to local stress relaxation, single or double wrinkles form along the elevated lines depending upon the line width. Within the gaps between the lines, wrinkles oriented perpendicular to the lines develop induced by the nonrelaxed compressive stress. Wrinkling represents a simple alternative for covering the surface of SU-8 with largely self-forming regular structures that may act as transmissive optical elements.

A combinatorial X-ray sub-micron diffraction study of microstructure, residual stress and phase stability in TiAlN coatings

The understanding of the relationship between structure and properties of thin coatings is one of the key requirements for their further improvement. Especially for metastable TiAlN, commonly used as a protective coating for cutting applications, detailed knowledge about the apparent microstructure, the resulting residual stresses and the accompanied decomposition behavior at elevated temperatures are of vital importance, as the subsequent high-temperature formation of the thermally stable wurtzite phase results in deteriorated mechanical properties. In this study, the recently introduced cross-sectional scanning X-ray diffraction method is applied as a combinatorial tool to investigate the depth evolution of the local microstructure and residual stresses and their effect on phase stability of a TiAlN coating synthesized by cathodic arc evaporation at stepwise increased bias voltages. The decomposition behavior of the coating during annealing is obviously given by the initial microstructure and stress state, which control the local stress relaxation as well as the formation of precipitates. Thus, the phase transformation is favored in the coating regions that developed under moderate ion-irradiation conditions, characterized by low stress and well-developed columnar grains, and retarded under the presence of high compressive stress. The observed non-linear dependency of phase decomposition on the initial microstructure and stress is interpreted by ab initio calculations.

In‐depth analysis of pH‐dependent mechanisms of electromechanical reshaping of rabbit nasal septal cartilage

Electromechanical reshaping (EMR) involves reshaping cartilage by mechanical deformation and delivering electric current to the area around the bend axis, causing local stress relaxation and permanent shape change. The mechanism of EMR is currently unclear, although preliminary studies suggest that voltage and application time are directly related to the concentration and diffusion of acid-base products within the treated tissue with little heat generation. This study aims to characterize local tissue pH changes following EMR and to demonstrate that local tissue pH changes are correlated with tissue damage and shape change. Ex vivo animal study involving EMR of rabbit nasal septal cartilage and biochemical estimation of tissue pH changes. The magnitude and diffusion of acid-base chemical products in control (0V, 2 minutes), shape change (4V, 4 minutes; 6V, 1, 2, 4 minutes; 8V, 1, 2 minutes), and tissue damage (8V, 4, 5 minutes; 10V, 4, 5 minutes) parameters following EMR are approximated by analyzing local pH changes after pH indicator application. There is a direct relationship between total charge transfer and extent of acid-base product diffusion (P <0.05). A "pH transition zone" is seen surrounding the bend apex above 8V, 2 minutes. Colorimetric analysis suggests that small local pH changes (10(-8) hydrogen ions) are at least partly implicated in clinically efficacious EMR. These results provide additional insight into the translational applications of EMR, particularly the relationship among pH changes, shape change, and tissue injury, and are integral in optimizing this promising technology for clinical use.

Remarks on residual stress measurement by hole-drilling and electronic speckle pattern interferometry.

Hole drilling is the most widespread method for measuring residual stress. It is based on the principle that drilling a hole in the material causes a local stress relaxation; the initial residual stress can be calculated by measuring strain in correspondence with each drill depth. Recently optical techniques were introduced to measure strain; in this case, the accuracy of the final results depends, among other factors, on the proper choice of the area of analysis. Deformations are in fact analyzed within an annulus determined by two parameters: the internal and the external radius. In this paper, the influence of the choice of the area of analysis was analysed. A known stress field was introduced on a Ti grade 5 sample and then the stress was measured in correspondence with different values of the internal and the external radius of analysis; results were finally compared with the expected theoretical value.

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by ΔμH(σ) = −1.124σ kJ/molH for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.

Feasibility of Local Stress Relaxation by Laser Annealing and X‐ray Measurement

ABSTRACTMost manufacturing processes generate residual stresses inside the materials and components. Stress relief heat treatments can be used to reduce the magnitude of the residual stresses: by uniformly heating a structure to a sufficiently high temperature, and then uniformly cooling it, residual stresses can be relaxed. This process generally requires the insertion of the entire component in a large furnace, for a fairly long period of time.In this paper, the possibility of using a high power laser source to locally relieve residual stresses is studied; this has the advantage of calibrating and confining the relief process just to the laser affected area. An experimental set‐up was tested on an aluminium specimen, and the preliminary results are reported in this paper.

In-situ TEM Observation Of Electron Irradiation Induced Shape Transition Of Elongated Gold Nanoparticles Embedded In Silica

Elongated Au nanoparticles (NPs) embedded in silica matrix were fabricated by 100 MeV Ag ion irradiation of 3 MeV Au ion implanted SiO2/Si(100) substrates, annealed at 1050 o C. Electron-beam-induced shape evolution of elongated Au NPs embedded in SiO2 has been studied by high resolution transmission electron microscopy. Electron beam irradiation resulted in a decrease in the aspect ratio of Au NPs from ~ 1.4 to 1 with increase in irradiation time. The observed ellipsoidal-to-spherical shape transition of Au NPs has been ascribed mainly to the cumulative effects of electron beam induced heating, softening of silica matrix and radiation enhanced diffusion of knock-on displaced O and Si atoms, resulting in local stress relaxation. Copyright © 2013 VBRI press.

Real-time transmission Mueller polarimetry on hydrogenated polymorphous silicon under current injection

We report on the use of an innovative optical characterization technique—real-time Mueller polarimetric imaging in transmission—for the characterization of thin-film silicon solar cells. In this work, we used this technique to monitor the evolution of optical retardance induced by the mechanical stresses in hydrogenated amorphous and polymorphous silicon (a-Si : H and pm-Si : H) p–i–n (PIN) solar cells. Under current injection of 200 mA cm−2, the retardance of the pm-Si : H PIN solar cells decreased, while that of the a-Si : H PIN solar cells showed no significant change. After the current injection, the pm-Si : H PIN solar cells showed dramatic macroscopic changes on a scale of tens of micrometres, such as local peel-off and delamination from the substrate. Our results demonstrate that current injection introduces local stress relaxation, which can be efficiently monitored prior to irreversible damage from a decrease in the retardance of the pm-Si : H PIN solar cells.

Dynamics of energy exchange in model samples subjected to elastic and electromagnetic impacts

The effects of elastic and electromagnetic (EM) fields are studied as an additional factor of energy exchange in the process of the deformation of a heterogeneous medium. The threshold value of initiating energy, Ktr(kp), relative to the current relaxation process is quantitatively estimated. It is shown that the estimated energy impacts below the threshold can initiate relaxation of local structural stresses and, thus, reduce the risk of a macrofracture. In a seismically active region, a similar scenario of initiation of dynamic development is considered in the local zones of potential sources of earthquakes. The possibility to determine the location, the time, and the intensity of the initiating EM impact is considered. From the experiments, the coefficient of electromechanical conversion is calculated.

Newtonian flow of heterogeneous synthetic gabbros at high strain: Grain sliding, ductile failure, and contrasting local mechanisms and interactions

We combined experimental and numerical approaches, in order to constrain the rheology of ultramylonitic rocks characteristic of gabbroic high-temperature shear zones. Synthetic samples containing 70 vol% fine-grained anorthite matrix (<5 μm) and 30 vol% coarse diopside inclusions (<55 μm) were deformed in torsion at T = 1150 °C and P = 370 MPa. At modest shear stresses (<40 MPa) the data evidenced linear-viscous flow with a stress exponent n ≈ 1. The outermost rim of the samples exhibited substantial cavitation coalescence leading to ductile damage and ultimately to ductile failure at shear strains between γ = 2–3.5. The mechanical data and the microstructures indicate dominant grain boundary sliding (GBS) and diffusional mass transfer. However, extensive crystal slip plasticity (CSP), dislocation creep and dynamic recrystallization operated within and in the vicinity of the diopside inclusions. For instance, the inclusions presented an extremely fine-grained interfacial layer, resulting from their intense peripheral recrystallization. The latter crystal plasticity mechanism require differential stresses exceeding by far the overall flow stress supported by our specimens, which indicates substantial local stress enhancement related to the heterogeneous nature of the two-phase rock. In order to characterize the local mechanical responses we performed finite element numerical modelling of the shear deformation process, considering elastoviscoplastic behavior based on the constitutive laws for GBS and CSP of the constituent phases. We emphasized the effects of inclusion shapes and interactions. We show that even at relatively low concentrations (25 vol%) and for overall Newtonian flow, the important strength contrasts and the interactions between neighbouring and irregular inclusions rapidly induce significant local stress amplifications, allowing for twinning and dislocation creep. On the other hand, the simulations suggest that the presence of fine-grained recrystallized interfacial layers deforming by GBS allows for very efficient local stress relaxation. As a result, we suggest that whilst the overall material sustains fairly low stresses and presents GBS related Newtonian flow, locally the diopside inclusions and the surrounding matrix experience highly fluctuating stresses in relation with cycles of dislocation creep, peripheral recrystallization and interfacial GBS. On the one hand, from a stochastic point of view, the cyclic nature of the process may ensure a stable overall flow stress. On the other hand, at the local scale the sequential mechanism allows for continuous size reduction of the stronger inclusions, a process that we call “ductile abrasion”.

Variation of adhesive force at the interface of Pd and SrTiO3 as a consequence of residual stresses

Initially, Pd thin films were deposited over a hard substrate using electron beam physical vapour deposition. The growth and the surface roughness of the films were analysed and their effects upon the conventional indentation test were discussed. Afterwards, an experimental method is described which can measure the critical fracture force in thin films using oscillating indentation. Initially, repetitive contacts at a single point with the purpose of identifying the fracture time provide the fracture force versus fracture time plot. Non-linear curve fitting of the data reveals the theoretical fracture force by a single indentation, which is called the critical fracture force. Arguments are put forward to show the relation between piling up height and applied force. Discrepancies were observed in the plot of the ratio between total indentation depth and piling up height versus applied force when higher loads than a critical fracture force were applied. Discrepancies appear as a result of indenting the substrate. A nanoscratch test facilitated the possibility of measuring adhesion strength and adhesion energy of the films considering the measured critical fracture force as the maximum applied force. The relation between residual compressive stresses, adhesion strength, plastic deformation and piling up area was discussed using dislocation theories. Indentation with high applied loads leaves behind large plastic deformation and reduces the accuracy and reliability of the test results. Hence, lower loads (in the order of nanonewtons) were applied using atomic force microscopy in the friction mode. A pulling off force was mapped in each thickness of Pd films. The results confirm that the area around a hillock exhibits a higher pulling off force due to the local stress relaxation as a consequence of hillock formation. By repeating the mapping process over different areas with various applying forces, the plot of the pulling off force versus applied load was drawn representing discrepancies in the results at higher loads. This phenomenon is associated with the plastic deformation in the films.

Evolution of stress and morphology in thermal barrier coatings

Residual stress in the thermally grown oxide (TGO) in thermal barrier coatings (TBCs) was measured by photoluminescence piezospectroscopy (PLPS) and stress maps created to track local stress changes as a function of thermal cycling. The local stress images were observed to be correlated with morphological features on the metal surface that were purposely introduced during specimen preparation. Local stress relaxation and morphological evolution with thermal cycling were studied using the stress maps combined by post-mortem SEM examination. It was found that the morphology in the specimen having an initial polished surface was quite stable, while that in the specimen with a rough surface was unstable. The average residual stress in the specimen with the unstable morphology decreased with thermal cycling and it eventually failed along TGO/YSZ interface. The specimen with stable morphology maintained a high TGO stress throughout the thermal cycling process and failed along TGO/bond coat interface. The rough surface was also found to give rise to the formation of transition alumina (θ–Al 2 O 3 ) in the TGO which was correlated with a reduced TGO stress.

Computational simulation of the relaxation of local stresses due to foreign object damage under cyclic loading

In this study, the phenomenon of residual stress relaxation from foreign object damage (FOD) is numerically simulated using a hybrid explicit—implicit finite-element method. The effects of cycle fatigue loadings on stress relaxation were studied. FOD is first simulated by firing a 3mm cube impacting onto a plate made of titanium alloy Ti-6Al-4V at 200m/s. The FOD impact produces two distinct stress concentrations: one is compressive directly beneath the impact site; the other is tensile around the outer edge of the impact. The plate was then assumed to be subjected to a cyclic fatigue loading. The stress relaxation was investigated under a range of stress ratios and maximum applied stresses. Two different material models were considered for the simulations, namely an elastic—perfectly plastic model and a non-linear kinematic hardening model.

Transient Binding and Dissipation in Cross-Linked Actin Networks

In contrast with entangled actin solutions, transiently cross-linked actin networks can provide highly elastic properties while still allowing for local rearrangements in the microstructure-on biological relevant time scales. Here, we show that thermal unbinding of transient cross-links entails local stress relaxation and energy dissipation in an intermediate elasticity dominated frequency regime. We quantify the viscoelastic response of an isotropically cross-linked actin network by experimentally tuning the off rate of the transiently cross-linking molecules, their density, and the solvent viscosity. We reproduce the measured frequency response by a semiphenomenological model that is predicated on microscopic unbinding events.

Modelling of local strain and stress relaxation in bronze processedNb3Sn wires

Recently, the need for mechanical modelling ofNb3Sn wires has increased, since these wires are used in large projects such as the InternationalThermonuclear Experimental Reactor (ITER). The finite element method makes it possibleto solve multi-axial stress and strain distributions in complicated wire geometries. However,Nb3Sn conductors possess features that make building the mechanical model challenging andcomplicate the use of the finite element method. This paper focuses on two problems. First,detailed modelling of geometries with large filament amount can produce numericalproblems that are too large for computers. A simple approach to circumvent this problem ispresented, and its validity investigated. Second, the uncertainty in modelling thermalstress, caused by stress relaxation and other high-temperature phenomena, istreated. To investigate the credibility of the models, the computed results arecompared with critical current and stress–strain data measured for a bronze processedNb3Sn wire in an axial tension test. Computed results agree well withthe measured ones, and show that the mechanical strain state ofNb3Sn in filaments is very sensitive to the degree of stress relaxation.

Direct imaging of quantum wires nucleated at diatomic steps

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place preferentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step.

Stress corrosion constitutive laws as a possible mechanism of intermediate-term and short-term seismic quiescence

SUMMARY Three types of seismic quiescence are recognized in the earthquake cycle. Post-seismic quiescence occurs due to the stress drop caused by a previous major earthquake, and results in long-term seismic gaps; infermediate-term quiescence occurs due to a small stress relaxation in the volume around the next mainshock area; and short-term quiescence during foreshock sequences occurs due to slipweakening or dilatancy hardening concentrated on the nucleation point. In this paper a single quantitative model for intermediate-term and short-term quiescence is developed from (a) observation of subcritical crack growth due to stress corrosion, and (b) a general model-for subcritical damage development where a fractal population of fractures results, irrespective of the underlying mechanism. In the former the stress intensity K of a single dominant macrocrack is the appropriate constitutive variable, while the latter more general formulation relies on a mean potential strain energy release rate (G) proportional to (a) the square of the applied effective stress and (b) the mean fracture length. Stress corrosion provides an important concrete example as well as a useful analogy for interpreting the general theory. We then consider the effect of a stress decrease in the intermediate-term on seismic event rates using the approporiate constitutive laws for both variables. Simple calculations for a material of stress corrosion index n = 30 show that a 45 per cent reduction in event rates is consistent with only a 2 per cent reduction in K, and a 90 per cent reduction results from only a 7 per cent decrease in K. A similar order of magnitude of quiescence can be predicted theoretically by considering the effect of similar small changes in (G) for a fractal pupulation of faults or cracks averaged over a range of length scales. Such intermediate-term quiescence occurs in the model when K or (C) decreases because of the reduction in applied stress, during a phase of strain softening late in the earthquake cycle. Such a decrease in K or (G) moves the system temporarily further from the failure condition K = K, (or (G) = (G),) and is therefore stable. This temporary stability is consistent with the relatively long duration of intermediate-term quiescence (months or years). Observed intermediate-term seismic quiescences are relatively easily explained in the model by stable, regional decreases in stress in a volume much larger than the mainshock area, but prior to the period when concentrated accelerating crack growth in the nucleation zone dominates the strain softening. The general mechanism is compared to the Kaiser effect as a possible alternative explanation for intermediate-term seismic quiescence. The two models are not necessarily mutually exclusive, though the Kaiser effect involves local stress and strain relaxation, and predicts more abrupt changes in event rate when the stress decreases. Short-term quiescence is also a feature of the general model, and is predicted when a concomitant decrease in seismic b-value occurs, as the larger events near the nucleation zone begin to dominate the stress relaxation, thereby inhibiting fracture