Actual Strain Research Articles

On the Grain Refinement Mechanisms in SPD – Applying High Resolution Electron Microscopy and the LEDS Approach

The present work is an attempt to contribute the current understanding of the operating mechanisms responsible for the development of ultrafine grains during severe plastic deformation (SPD) in aluminium. Equal channel angular pressing (ECAP) at room temperature has been applied to a commercial Al-Mg-Si alloy. Route A was used in all pressings and the first two passes were studied in most detail. Advanced characterization methods such as FEG-SEM with a state-of-the-art microdiffraction unit has been applied when characterizing samples carefully prepared from different positions along certain flow paths in the process shear zone. Intrinsic strain measurements are done in parallel in order to describe the actual strain tensor in each position studied. Detailed information on phenomena involved in the grain break-up mechanisms has been obtained by high resolution EBSD data collected through the deformation zone. The microstructural development seems to be dominated by deformation banding and may be explained in terms of the LEDS theory.

Extended overstress model with overstress tensor

Introduction The elastoplastic stress is first defined as the stress which evolves as the actual strain rate is induced in an imaginary quasi-static process of elastoplastic deformation, while internal variables evolve with the viscoplastic strain rate calculated by the viscoplastic constitutive equation. Further, the novel variable “overstress tensor” reaching the current stress from the elastoplastic stress is defined. Thus, the overstress model [1] is extended so as to describe also the tangential viscoplastic strain rate induced by the overstress tensor component tangential to the yield surface, extending the tangential inelastic strain rate [2]. Further, the viscoplastic strain rate due to the change of stress inside the yield surface is incorporated by adopting the concept of the subloading surface [3].

연속섬유에 의하여 보강된 철근콘크리트 보의 전단강도 평가

초록 · 키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 최대 섬유량3. 섬유에 의하여 보강된 철근콘크리트 보의 최대 섬유비 평가식 및 평가식의 분석4. 기존 실험 결과와 최대 섬유의 양 및 전단강도의 비교5. 결론참고문헌요약

Tissue Engineering of Human Heart Valve Leaflets: A Novel Bioreactor for a Strain-Based Conditioning Approach

Current mechanical conditioning approaches for heart valve tissue engineering concentrate on mimicking the opening and closing behavior of the leaflets, either or not in combination with tissue straining. This study describes a novel approach by mimicking only the diastolic phase of the cardiac cycle, resulting in tissue straining. A novel, yet simplified, bioreactor system was developed for this purpose by applying a dynamic pressure difference over a closed tissue engineered valve, thereby inducing dynamic strains within the leaflets. Besides the use of dynamic strains, the developing leaflet tissues were exposed to pre-strain induced by the use of a stented geometry. To demonstrate the feasibility of this strain-based conditioning approach, human heart valve leaflets were engineered and their mechanical behavior evaluated. The actual dynamic strain magnitude in the leaflets over time was estimated using numerical analyses. Preliminary results showed superior tissue formation and non-linear tissue-like mechanical properties in the strained valves when compared to non-loaded tissue strips. In conclusion, the strain-based conditioning approach, using both pre-strain and dynamic strains, offers new possibilities for bioreactor design and optimization of tissue properties towards a tissue-engineered aortic human heart valve replacement.

An Embedded FBG Sensor for Dynamic Strain Measurement for a Clamped-Clamped Composite Structure

A comparison of strain measurement results, from an embedded fibre-optic Bragg grating (FBG) sensor and surface mounted strain gauge, at different vibration frequency ranges and using a clamped-clamped glass fibre composite beam, is presented. It is shown that the FBG sensor is able to precisely measure the peaks at the first-two natural frequency modes compared with the spectrum captured from the strain gauge. The results also demonstrate that the strains measured from the FBG sensor agreed well with the strain gauge at frequencies below 100 Hz. Beyond this value, the actual strain on the beam surface was less than 3µe, and the data extracted from the strain gauge are no longer valid. For a clamped-clamped structure, the longitudinal strain of the beam correlates to its vibration amplitude and excitation frequency. Increasing the frequency results in decreasing the longitudinal strain of the beam and erroneous measurements from the strain gauge resulted. This study provides important information on the feasibility of using embedded FBG sensors as vibration monitoring devices to measure mechanical performance of composite structures.

Testing and Analysis of the Inner Stress in Adhesive Coating Layer Using Strain Gauges and Finite Element Method

The residual stress in epoxy adhesive layer deposited on metal and other substrate at room temperature is studied. With embedded strain gauges in arranged depth of epoxy layer, the strain changes in the adhesive layer induced by the curing procedure and the changes of ambient temperature were measured to evaluate the changes of residual stress in place during a period after the curing procedure finished. The actual strain in epoxy adhesive layer from curing is used to estimate the residual stresses in it. While taking the strain obtained from the surface of the adhesive layer as free strain, the residual stress can be calculated and presented a strongly cyclic variation with a period of 24 h. The inner stress is also analyzed using the finite element method.

Characterization of the Flexcell™ Uniflex™ cyclic strain culture system with U937 macrophage-like cells

Mechanical forces alter many cell functions in a variety of cell types. It has been recognized that stimulation of cells in culture may be more representative of some physiologic conditions. Although there are commercially available systems for the study of cells cultured in a mechanical environment, very little has been documented on the validation techniques for these devices. In this study, Flexcell's™ recently introduced Uniflex™ cyclic strain system was programmed to apply 10% longitudinal sinusoidal strain (0.25 Hz) for 48 h to U937 cells cultured on Uniflex™ plates. Image analysis was employed to characterize the actual strain field. For a chosen amplitude of 10% the applied strain was highly reproducible and relatively uniform (10.6±0.2%) in a central rectangular region of the membrane (dimensions of 9.2±2×13.6±0.8 mm 2). The strain increased the release of IL-6, esterase and acid phosphatase activity ( p<0.05) from adherent U937 cells. Cells also displayed altered morphology, aligning and lengthening with the direction of strain, whereas static cells maintained a round appearance showing no preferred orientation. These data indicate that cyclic mechanical strain applied by the Uniflex™ strain system modulates U937 cell function leading to selective increases in enzymatic activities as well as orientation in a favored direction.

Deformation analysis of lap-shear testing of solder joints

A combined numerical and experimental study of lap-shear testing of solder joints was carried out. The primary objective was to provide a quantitative evaluation of shear deformation experienced by the solder. The numerical analysis was based on finite element modeling of deformation in the substrate–solder assembly. The calculated shear response, utilizing both the commonly adopted far-field measurements and the actual shear strain in solder, was found to differ significantly. The geometric and material parameters during lap shear were explored to provide physical insight into the problem. The nature of deformation of the substrate was seen to greatly influence the shear behavior of the solder. The “transmission” of shear strain into the solder is very ineffective when the solder is in the elastic state, resulting in smaller shear strains in the solder than the nominal values. This effect is also true even when the solder is deforming plastically. Experimental measurements of solder shear using direct optical measurements confirmed the numerical findings. Subtraction of the average deformation of the Cu substrate provides a reasonable, albeit not complete, approximation of the solder strain.

Investigating the Non-Linear Viscoelastic Behavior of Filled Rubber Compounds Through Fourier Transform Rheometry

Abstract Fourier transform (FT) rheometry was used to investigate the non-linear viscoelastic behavior of a series of carbon black filled rubber compounds with various filler levels. Using a purposely modified commercial dynamic rheometer, i.e. the Rubber Process Analyzer RPA 2000® (Alpha Technologies), special strain sweep tests protocols were designed and performed in order to capture the actual strain and torque signals up to 500% deformation at constant frequency and temperature. FT yielded the main component and harmonics of strain and torque signals. Results show that the quality of the applied strain signal somewhat deteriorates with increasing stiffness of filled compounds, but remains excellent in the high strain region, where the non-linear viscoelastic response of the materials is investigated. Above a filler volume fraction of around 12–13%, tested materials no longer exhibit a linear viscoelastic response, at least in the strain window investigated, and the FT rheometry results are more complex than what was observed with pure gum samples. This means that most practical rubber compounds are intrinsically non-linear. By essence, FT rheometry is a valid technique for both the linear and the non-linear domains and, as shown, provides original information about complex polymer systems such as filled rubber compounds.

Effect of biaxial elastic constraints onH−Hinteractions in ultrathin vanadium

The hydrogen (H) solubility isotherms and the relative partial enthalpy- and entropy of H in a $\mathrm{Mo}\mathrm{V}∕\mathrm{V}(001)$ superlattice in the range $0\ensuremath{\leqslant}[\mathrm{H}∕\mathrm{V}]\ensuremath{\leqslant}1$ [atomic ratio] are addressed. The difference in solubility between MoV and V assures that H only resides in the $22\mathrm{\AA{}}$ thick V layers, and the use of MoV instead of Mo increases the range of reversibility of the uptake, for the actual strain state. The biaxial elastic constraints, imposed by the H-free layers and the substrate, are shown to have a big influence on the $\mathrm{H}\ensuremath{-}\mathrm{H}$ interaction. At low H content, the mean $\mathrm{H}\ensuremath{-}\mathrm{H}$ interaction is close to zero, while for $[\mathrm{H}∕\mathrm{V}]⪆0.25$, the interaction becomes attractive. We show that for a film constrained to expand in one dimension, the elastic dipole interaction is zero when the H-induced local strain field is isotropic, whereas for an anisotropic local strain field the interaction energy can be attractive, if the strain axes are parallel aligned with the direction of the expansion. Based on the symmetries of the respective local strain fields, we interpret the onset of the attractive $\mathrm{H}\ensuremath{-}\mathrm{H}$ interaction at $[\mathrm{H}∕\mathrm{V}]\ensuremath{\approx}0.25$ as a self-induced change of H site occupancy, from tetrahedral to octahedral sites.

703 カプセル型高温ひずみゲージを用いての鋳鉄材の高温ひずみ測定に関する検討(GS-3 材料計測,研究発表講演)

When the capsule type strain gage is used for strain measurement of cast iron, the shearing force in the spot weld is produced from the difference of the amount of the deformation of the capsule gage and cast iron. And, the actual strain caused in cast iron cannot be accurately measured because this shearing force deforms the spot weld. In this research, the shearing force caused in the spot weld was analyzed taking this deformation into consideration. As a result, it was clarified that a constant ratio existed between the measured strain by gage and the actual strain caused in cast iron. Therefore, the actual strain in cast iron can be estimated from the capsule gage output value and a constant ratio.

An evaluation of the lap-shear test for Sn-rich solder/Cu couples: Experiments and simulation

The lap-shear technique is commonly used to evaluate the shear, creep, and thermal fatigue behavior of solder joints. We have conducted a parametric experimental and modeling study, on the effect of testing and geometrical parameters on solder/copper joint response in lap-shear. It was shown that the farfield applied strain is quite different from the actual solder strain (measured optically). Subtraction of the deformation of the Cu substrate provides a reasonable approximation of the solder strain in the elastic regime, but not in the plastic regime. Solder joint thickness has a profound effect on joint response. The solder response moves progressively closer to “true” shear response with increasing joint thickness. Numerical modeling using finite-element analyses were performed to rationalize the experimental findings. The same lap-shear configuration was used in the simulation. The input response for solder was based on the experimental tensile test result on bulk specimens. The calculated shear response, using both the commonly adopted far-field measure and the actual shear strain in solder, was found to be consistent with the trends observed in the lap-shear experiments. The geometric features were further explored to provide physical insight into the problem. Deformation of the substrate was found to greatly influence the shear behavior of the solder.

The Effect of Overshooting the Target Strain on Estimating Viscoelastic Properties From Stress Relaxation Experiments

Tendon's mechanical behaviors have frequently been quantified using the quasi-linear viscoelastic (QLV) model. The QLV parameters are typically estimated by fitting the model to a single-step stress relaxation experiment. Unfortunately, overshoot of the target strain occurs to some degree in most experiments. This has never been formally investigated even though failing to measure, minimize, or compensate for overshoot may cause large errors in the estimation of parameters. Therefore, the objective of this study was to investigate the effect of overshoot on the estimation of QLV parameters. A simulated experiment was first performed to quantify the effect of different amounts of overshoot on the estimated QLV parameters. Experimental data from tendon was then used to determine if the errors associated with overshoot could be reduced when a direct fit is used (i.e., the actual strain history was used in the curve fit). We found that both the elastic and viscous QLV parameters were incorrectly estimated if overshoot was not properly accounted for in the fit. Furthermore, the errors associated with overshoot were partially reduced when overshoot was accounted for using a direct fit. A slow ramp rate is recommended to limit the amount of overshoot and a direct fit is recommended to limit the errors associated with overshoot, although other approaches such as adjusting the control system to limit overshoot could also be utilized.

Temperature Compensation of a Fiber Optic Strain Sensor Based on Brillouin Scattering

Brillouin scattering-based fiber optic sensors are useful to measure strain or temperature in a distributed manner. Since the Brillouin frequency of an optical fiber depends on both the strain and temperature, it is very important to know whether the Brillouin frequency shift is caused by the strain change or temperature change. This article presents a temperature compensation technique of a Brillouin scattering-based fiber optic strain sensor. Both the changes of the Brillouin frequency and the Brillouin gain power is observed for the temperature compensation using a BOTDA sensor system. Experimental results showed that the temperature compensated strain values were highly consistent with actual strain values.

Curl Estimation of Resin-Coated Paper

Resin-coated paper (RC paper), which is manufactured by extrusion coating with polyethylene resin, is mainly used as a support for imaging materials such as ink-jet receiving paper and photographic paper. It is important to clarify the relationship between the curl of the RC paper and the constitution of the RC paper or characteristics of the resin. We analyzed the curl of RC paper based on the assumption that thermal stress actually generated in the resin-coated paper is the driving force for curl. We constructed equations for curl evaluation by the use of a two-dimensional composite beam model, assuming that paper and solid polyethylene resin are both elastic.We found that the calculated values corresponded well with the experimental values, which gives support for the validity of our curl model. We also found that the actual thermal strain corresponded to about 13.3% of the thermal shrinkage due to ideal solidification. Applying the model to RC papers of varying constitution, we were able to determine the elastic modulus required for the resin to produce a suitable curl in the imaging products.

Viscoelasticity of human alveolar epithelial cells subjected to stretch.

Alveolar epithelial cells undergo stretching during breathing and mechanical ventilation. Stretch can modify cell viscoelastic properties, which may compromise the balance of forces in the alveolar epithelium. We studied the viscoelasticity of alveolar epithelial cells (A549) subjected to equibiaxial distention with a novel experimental approach. Cells were cultured on flexible substrates and subjected to stepwise deformations of up to 17% with a device built on an inverted microscope. Simultaneously, cell storage (G') and loss (G'') moduli were measured (0.1-100 Hz) with optical magnetic twisting cytometry. G' and G'' increased with strain up to 64 and 30%, respectively, resulting in a decrease in G''/G' (15%). This stretch-induced response was inhibited by disruption of the actin cytoskeleton with latrunculin A. G' increased with frequency following a power law with exponent alpha = 0.197. G'' increased proportionally to G' but exhibited a more marked frequency dependence at high frequencies. Stretching (14%) caused a fall in alpha (13%). At high stretching amplitudes, actual cell strain (14.4%) was lower than the applied substrate strain (17.3%), which could indicate a partial cell detachment. These data suggest that cytoskeletal prestress modulates the elastic and frictional properties of alveolar epithelial cells in a coupled manner, according to soft glassy rheology. Stretch-induced cell stiffening could compromise the balance of forces at the cell-cell and cell-matrix adhesions.

Modeling of deformation and rotation bands and of deformation induced grain boundaries in IF steel aggregate during large plane strain compression

A computation using crystal plasticity modeling of an actual IF steel aggregate plane strain compression deformation, underlines the formation of different deformation bands morphologies and grain splitting occurrence, already experimentally observed by different authors. The model based on dislocation densities as internal variables, developed in the framework of finite deformation and implemented in the Finite Element Method, is able to capture the main characteristics of different inhomogeneities and to analyze their formation and further development with strain, from the determination of the active and latent slip systems, and also from the quantification of their dislocation densities and corresponding glide rates evolutions. The respective boundary conditions and material properties effects are discussed.

Modeling damage and deformation in impact simulations

Abstract— Numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence. Quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters, particularly in cases where the final pristine crater morphology is ambiguous or unknown. In this paper, we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results. We demonstrate this with an example simulation of an impact into a terrestrial, granite target that produces a 10 km‐diameter transient crater. The results of the simulation are shown in terms of damage (a scalar quantity that reflects the totality of fragmentation) and plastic strain, both total plastic strain (the accumulated amount of permanent shear deformation, regardless of the sense of shear) and net plastic strain (the amount of permanent shear deformation where the sense of shear is accounted for). Damage and plastic strain are both greatest close to the impact site and decline with radial distance. However, the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains. Plastic strain in brittle rocks is very heterogeneous; however, continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock (large compared to the spacing between individual zones of sliding). This paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones. Furthermore, we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity.

On factors affecting the extraction of elastic modulus by nanoindentation of organic polymer films

ABSTRACTA detailed study of nanoindentation in Continuous Stiffness Mode (CSM) on a family of aromatic thermosetting polymers is carried out to identify the causes for the large variability in the extracted values of the elastic modulus of organic polymer films.It is shown that the variation of parameters determining the dynamics of the force application such as the CSM frequency, the actual strain or load rate, and the duration of the waiting time segments can lead up to 20% difference in the estimated elastic modulus. The reason for this is related to creep, more specifically to viscoelastic behaviour, typical of organic films. On the other hand, pile-up is shown to have a negligible effect on the extraction of the elastic modulus from indentation depths below 50% of the film thickness, even for films with hardness as low as 0.13GPa. It is also concluded that neither pile-up nor creep phenomena can account for the overestimation of the elastic modulus with nanoindentation as compared to the values extracted with the surface acoustic waves technique.

Optimal shakedown loading for circular plates

Optimization of shakedown loading under constrained residual displacement is considered for elastic and perfectly plastic circular plates. The load variation bounds, which satisfy the optimality criterion in concert with plate-strength and stiffness requirements, are identified. The actual strain fields of the plate depend on the loading history. Thus, the load optimization problem at shakedown is stated as a pair of problems that are executed in parallel: the main load optimization and the verification of the prescribed magnitudes of the bounds on the residual deflections. The problem must be solved by iteration. The Rozen projected gradient method is applied. A mechanical interpretation of the Rozen optimality criterion is given, which permits the simplification of the mathematical model for load optimization and the formulation of the solution algorithm. Numerical examples include circular annular plates with and without a rigid inclusion. The results are valid under the assumption of small displacements.