Different Levels Of Strain Research Articles

Combined Effect of Pre-Strain at Elevated Temperature and Pre-Aging on Aging Strength and Microstructures of AA6082

With the increasingly serious energy and environmental issues, aluminum alloys are becoming increasingly desirable for transportation facilities due to their high specific strength. Their low formability results in the difficulty of forming complex-shaped panels and bodies. Hot stamping is a new technology developed to address this formability challenge by applying plastic deformation to the heated aluminum alloy sheet at the solution heat treatment temperature. The stamped panels gain high strength via the sequent aging treatment. Different panels, even different regions on the same panel, are subjected to different forming strains. The present paper is focused on the experimentally investigating the effects of the pre-strain at elevated temperature and pre-aging on the mechanical behavior and microstructures of AA6082 after the natural aging, paint-bake treatment, and artificial aging. Sample sheets of AA6082-T6 were pre-strained by uniaxial tension at different levels of strain, and then subjected to the pre-aging, natural aging, paint-bake treatment or artificial aging, respectively. The yield strength, tensile strength and elongation of post-aged samples were gained via the standard uniaxial tensile tests. The results show that both of the pre-strain at elevated temperature and pre-aging suppress the natural aging of AA6082. Pre-strain at elevated temperature induces weak bake softening, and has a detrimental effect on the artificial aging strengthening, while the sequent pre-aging could compensate the strength decrease.

Evolution of Nb oxide nanoprecipitates in Cu during reactive mechanical alloying

Abstract

Effect of strain level on strength evaluation of date palm fiber-reinforced sand

Conventional researches on the behavior of fiber-reinforced and unreinforced soils often investigated the failure point. In this study, a concept is proposed in the comparison of the fiber-reinforced with unreinforced sand, by estimating the strength and strength ratio at different levels of strain. A comprehensive program of laboratory drained triaxial compression test was performed on compacted sand specimens, with and without date palm fiber. The fiber inclusion used in triaxial test specimens was form 0.25%-1.0% of the sand dry weight. The effect of the fiber inclusion and confining pressure at 0.5%, 1.0%, 1.5%, 3.0%, 6.0%, 9.0%, 12%, and 15% of the imposed strain levels on the specimen were considered and described. The results showed that, the trend and magnitude of the strength ratio is different for various strain levels. It also implies that, using failure strength from peak point or the strength corresponding to the axial strain of approximately 15% for evaluating the enhancement of strength or strength ratio, due to the reinforcement, may cause hazard and uncertainty in practical design. Therefore, it is necessary to consider the strength of fiber-reinforced specimen at the imposed strain level, compared to the unreinforced specimen.

A Supervised Artificial Neural Network-Assisted Modeling of Magnetorheological Elastomers in Tension–Compression Mode

Modeling of highly sophisticated behavior of magnetorheological elastomers (MREs) is an essential step toward optimally designing and effectively controlling the smart material-based devices. While modeling MREs in shear mode has been widely carried out by employing continuum mechanics, mathematical techniques, and phenomenological approaches, the correct determination of dynamic behavior of MREs in tension–compression mode has been addressed in only a few studies due to inherent complexities mainly arising from the computational demandingness of the process. This article addresses the functionality of artificial neural network (ANN) for prediction of MRE’s dynamic behavior in tension–compression mode under different levels of strain, frequency, and magnetic flux density. A multilayer perceptron-based feed-forward neural network with backpropagation training technique was used with various structures to identify an optimal configuration. A neural network structure with 20 neurons in the hidden layer was adopted, which revealed the mean square error (MSE) magnitude of 7.1 kPa with $R^{2}$ values above 0.97. Afterward, the predicting capacity of the model was evaluated using experimental data sets. The obtained results are suggestive of reasonably acceptable performance of the proposed ANN model, which holds the capacity for a close mapping of the predicted tension–compression stress values to those of experimental ones. Further development of the proposed ANN model serves as a promising approach to deal with the modeling and controlling of engineering devices equipped with tension–compression MREs.

Modeling shear-induced crystallization in startup flow: The case of segmented copolymers

We propose to extend the well-known Doi–Edwards theory to model the viscosity divergence caused by shear-induced crystallization occurring upon startup flow experiments in segmented block copolymers ( M w ¯ = 50 kg mo l − 1 ). Unlike models such as Convective Constraint Release in which the chain relaxation is accelerated by the shear, we propose to increase the final relaxation time of the polymer within the memory function to take into consideration the progressive association of the hard-segments. To this aim, we make use of the Avrami equation in which we parametrize the crystallization rate to depend on the shear rate in a linear way. In this context and contrary to what is generally proposed for supramolecular polymers, we consider the hard-segments aggregation as an irreversible process (infinite lifetime) due to the crystalline nature and the high-functionality of the resulting topological nodes. The exponential trend of the Avrami process is then validated by stress relaxation experiments performed at different levels of strain. Moreover, the growth mode parameter, usually assigned to the geometry of the crystallites, is found to be independent of the shear rate and in excellent agreement with a previous work in which we investigated the quiescent crystallization of the same system. We finally apply our model to fit data measured at different temperatures, well-confirming the development of hard-segments’ crystallites from the very first stages of the isothermal shearing process.We propose to extend the well-known Doi–Edwards theory to model the viscosity divergence caused by shear-induced crystallization occurring upon startup flow experiments in segmented block copolymers ( M w ¯ = 50 kg mo l − 1 ). Unlike models such as Convective Constraint Release in which the chain relaxation is accelerated by the shear, we propose to increase the final relaxation time of the polymer within the memory function to take into consideration the progressive association of the hard-segments. To this aim, we make use of the Avrami equation in which we parametrize the crystallization rate to depend on the shear rate in a linear way. In this context and contrary to what is generally proposed for supramolecular polymers, we consider the hard-segments aggregation as an irreversible process (infinite lifetime) due to the crystalline nature and the high-functionality of the resulting topological nodes. The exponential trend of the Avrami process is then validated by stress relaxation experime...

Lean Practices in SMEs towards Improvement in Production Performance: A Research

Practicing Lean manufacturing in organizations of any type brings various advantages for example, reduced waste, improved operating efficiency, desired performance evaluation etc. Meanwhile, implementing the Lean strategies is a stimulating task. In literature, numerous frameworks and policies are developed and proposed, suitable for a specific organization and for any kind of SMEs, still the implementation exercise remains burdensome. SMEs are nowadays in the zone of highly practiced Lean implementation as they form one of topmost strength of economies. Here, in this article an effort is made to gather and highlight the recent Lean practices adopted/developed with several modifications to suit the working environment in SMEs. For a quick glance at the Lean methods proposed in different production/manufacturing industries even with some amount of success, information is provided in Tabular form. One highlighting outcome from this review is that, currently, in developing countries implementing Lean methods in SMEs is a biggest opportunity while in underdeveloped countries, the adaptation is very rarely reported. Use of machine learning techniques like artificial neural networks has just begun in this field of interest, hence exploring at a larger scale is required. Simulation methods are a few to name and its practice may or may not be worth requiring different levels of strain

Effect of strain on exciton dynamics in monolayer WS2**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0202302), the National Natural Science Foundation of China (Grant Nos. 61527817 and 61875236), the Initiative Postdocs Supporting Program of China (Grant No. BX201600013), the General Financial Grant from the China Postdoctoral Science Foundation (Grant No. 2017M610756), the Overseas

The exciton dynamics in a WS2 monolayer with strain are studied by transient absorption measurements. We measure the differential transmission signal from monolayer WS2 as a function of the probe wavelength at different levels of strain applied to the sample. The differential transmission spectrum has a positive maximum value at about 614 nm and shows no significant strain dependence. By time-resolving the differential transmission signal, we find that the strain has a minimal effect on the exciton formation process. However, the exciton lifetime is significantly reduced by strain. These results provide useful information for applications of WS2 in flexible electronic and optoelectronic devices where strain is inevitable.

Field-based investigation of fault architecture: A case study from the Lenghu fold-and-thrust belt, Qaidam Basin, NE Tibetan Plateau

AbstractThe fault zone architecture of a thrust fault zone is critical for understanding the strain accommodation and structural evolution in contractional systems. The fault architecture is also important for understanding fluid-flow behavior both along and/or across thrust fault zones and for evaluating potential fault-related compartmentalization. Because mesoscale (1–100 m) structural features are normally beyond seismic resolution, high-resolution outcrop in situ mapping (5–10 cm resolution) was employed to study the deformation features of a thrust fault zone located in the Qaidam Basin, northeastern Tibetan Plateau. The excellent exposure of outcrops enables the detailed investigation of the Lenghu thrust fault zone and its architecture. The Lenghu thrust fault, a seismically resolvable fault with up to ∼800 m of throw, exhibits a large variation of fault architecture and strain distribution along the fault zone. Multiple structural domains with different levels of strain were observed and are associated with the fault throw distribution across the fault. Based on previously proposed models and high-resolution outcrop mapping, an updated fault zone model was constructed to characterize the structural features and evolution of the Lenghu thrust. The possible parameters that impact fault architecture and strain distribution, including fault throw, bed thickness, lithology, and mechanical heterogeneity, were evaluated. Fault throw distributions and linkages control the strain distribution across a thrust fault zone, with local folding processes contributing important elements in Lenghu, especially where more incompetent beds dominate the stratigraphy. Mechanical heterogeneity, induced by different layer stacking patterns, controls the details of the fault architecture in the thrust zone. The variations in bed thicknesses and mechanical property contrasts are likely to control the initial fault dips and fault/fracture density. Large fault throws are associated with wide strain accommodation and damage zones, although the relationship between the development and width of the fault zone and the throw accumulation remains to be assessed. By presenting the high-resolution mapping of fault architecture, this study provides an insight into the subseismic fault zone geometry and strain distributions possible in thrust faults and reviews their application to assessments of fault zone behavior.

Damage modeling and critical damage evaluation of AA2014 cast alloy embedded with fly ash composite under upsetting

Abstract: Upsetting is one of the primary metal forming techniques and needs to be applied to reduce the cross section of the billets for further processing. Ductile fracture in metal upsetting process is an issue and depends on the material used and processing conditions. The current work focus is on the critical damage evaluation of AA2014 cast alloy embedded with fly ash composite. AA2014 alloy was heated to a pouring temperature of 750 ℃ and fly ash of 3% and 6% were added separately with AA2014 matrix material to prepare three sets of cast alloys namely pure AA2014, AA2014 + 3% fly ash, and AA2014 + 6% fly ash. After manufacturing three sets of rods from stir casting process, three sets of billets were machined to a height and diameter of 24 mm. Three sets of billets were compressed between rigid dies to different levels of strain until the initiation of fracture. Ductile fracture criterions were modeled to evaluate the critical damage value and fracture loci were constructed for different compositions.

A study of reproducibility of kinesiology tape applications: review, reliability and validity

BackgroundLiterature addressing the mechanical properties of kinesiology tape is quite scarce. There are no studies which focus on the mechanical characteristics of kinesiology tape, its mechanical properties, nor its adherence following the ISO international standard test methods for tape elongation.MethodsThis study quantified the mechanical characteristics of 380 samples of kinesiology tape from 19 different brands and in 4 different colors using a dynamometer. Mechanical testing was controlled by UNE EN ISO 13934-1.ResultsSignificant differences were found between tape brands in terms of grammage, maximum force tenacity, work, pre-elongation and percentage elongation (P < .001). Regarding kinesiology tape color, statistically significant differences were found between tape brands in terms of grammage, maximum force and tenacity (P < .001), work and pre-elongation (P < .05). When adherence was studied, statistically significant differences were found between tape brands in terms of maximum force and work (P < .001).ConclusionsThe different kinesiology tapes presented different behaviors with regard to rupture and removal when applied to skin in dry state, wet state and after being submerged in artificial acidic sweat solution. Therefore, different kinesiology tape brands will produce different levels of strain even though the same elongation is used. Depending on the characteristics (body dimensions) and properties (skin elongation) of each subject in the sample, bandages with different elongations must be applied to achieve the same strain in all of the tapes and therefore produce the same effect. The absence of these data at this time limits the reliability of previous clinical studies, makes comparing their findings impossible and presents new challenges for research in this field.

Studies on change of strain developed in different wood samples due to change in relative humidity

In this paper the dimensional change of different varieties of wood blocks due to change in relative humidity (RH) is presented. The dimensional change of wood due to variation of RH is measured in terms of strain. For this purpose, a strain measuring system is developed in the laboratory. The developed system is used to measure temperature, RH and strain of wood samples due to change in RH. Seventeen wood blocks of different thickness and different orientations are studied. Out of these, strain sensitivity of three samples covering the measured range of values is presented in this paper. The strain variation of the wood samples is tested in different humidity levels generated by the method of binary salt solutions at constant temperature 25 °C. The results obtained show different levels of strain developed in different species of wood. The change of strain is found to depend on the types of wood, thickness of the samples and orientations.

Staggered Nitinol Wire Actuator Array for High Linear Displacement and Force-to-Mass Ratio.

We present the design and performance of a unique Nitinol (NiTi) actuator design for high linear displacement and force generation through joule heating. The device is comprised of a staggered linear array of NiTi in wire form that, as a shape memory alloy, can achieve linear displacement through material phase change when heated. This change allows the crystal lattice within the material to displace/adjust. The design results in strain levels of 20.4% that are comparable to those of biological muscles and provides potential for additional strain. Three- to seven-staggered NiTi wires are tested to demonstrate the different levels of strain that are achieved with a range of wires in a staggered array. In addition, we measure and compare force generated to the mass of each wire to show system force-to-mass ratio. The effective force to mass for the system is greater than 5500 combined with a seven-wire staggered array. The device shows that a lightweight, high-strain actuator can be developed, and our research demonstrates its potential use in prosthetic actuation.

Effect of Aging on Hardness in the Ring Compression Tests Using Experimental and Finite Element Investigations

Abstract The heterogeneity of hardness distribution in a ring compression test was investigated in the present research. Rings of size 24 mm o.d. and 12 mm hole were machined to a height of 8 mm. All the samples were solutionized at a temperature of 502°C. The solutionized samples were grouped into three sets and each group was aged for 9, 13, and 18 h, respectively. Each group of samples was upset between rigid dies to different levels of strain. The ring compression tests were modeled using a finite element software package and were upset to identical strain levels as done in the experiments for all the aging conditions. The hardness at different zones in the upset ring was measured using Vickers indenter and the hardness was also predicted from the relation between effective strain distribution and Vickers hardness. The predicted results were in close agreement with the experimental results. It can be explained that the hardness in the upset ring is not uniform throughout the ring specimen and will vary near the surface, along the neutral plane, at the bulge head of the ring, and also with aging condition.

Evolution of microstructure and its influence on tensile properties in thermo-mechanically controlled processed (TMCP) quench and partition (Q&P) steel

Controlled hot rolling is performed on low carbon Q&P steel and is subsequently followed by a direct Q&P (DQP) treatment and a separate Q&P treatment (SQP). Two different levels of strain viz. 0.51 and 1.1 are accomplished in the thermomechanical controlled process (TMCP) prior to the Q&P treatment. The microstructures of the steels processed through both the DQP and SQP methods with different thickness reductions, contain primarily small lath packets with fine martensite laths and thin films of inter-lath austenite. Comparatively higher martensite volume fractions and fine lath packets is noticed in the DQP method. Tendency of higher retained austenite fraction is observed in the SQP method. Presence of a high fractions of high angle grain boundaries (HAGB) in the martensite laths indicates a fully recrystallized prior austenite grains in both steels. In general, prior thermo-mechanical treatment improves the partition kinetics which is supported by the high value of calculated carbon content of the austenite, Cϒ, in the DQP steels. Combination of TMCP and Q&P process has resulted in remarkable increase in strength with adequate ductility compared to a simple Q&P treatment alone. Maximum strength of about 1398MPa with a total elongation of 14% is achieved in the 1.1DQP steel. The study suggests that performing a TMCP prior to Q&P promotes grain refinement and formation of high fractions of HAGBs that are beneficial to improve the tensile properties.

Simulations on Monitoring and Evaluation of Plasticity-Driven Material Damage Based on Second Harmonic of S₀ Mode Lamb Waves in Metallic Plates.

In this study, a numerical approach—the discontinuous Meshless Local Petrov-Galerkin-Eshelby Method (MLPGEM)—was adopted to simulate and measure material plasticity in an Al 7075-T651 plate. The plate was modeled in two dimensions by assemblies of small particles that interact with each other through bonding stiffness. The material plasticity of the model loaded to produce different levels of strain is evaluated with the Lamb waves of S0 mode. A tone burst at the center frequency of 200 kHz was used as excitation. Second-order nonlinear wave was extracted from the spectrogram of a signal receiving point. Tensile-driven plastic deformation and cumulative second harmonic generation of S0 mode were observed in the simulation. Simulated measurement of the acoustic nonlinearity increased monotonically with the level of tensile-driven plastic strain captured by MLPGEM, whereas achieving this state by other numerical methods is comparatively more difficult. This result indicates that the second harmonics of S0 mode can be employed to monitor and evaluate the material or structural early-stage damage induced by plasticity.

Higher education reforms: Latin America in comparative perspective

This article introduces a special issue of EPAA/AAPE devoted to recent higher education reforms in Latin America. The last two decades have seen much policy development in higher education in the region, examined and discussed by scholars in each country, but dialog with the international literature on higher education reform, or an explicit comparative focus, have been mostly absent from these works. By way of presentation of the papers included in this issue, we first provide an overview of major policy changes in higher education in the Latin American region since the 1990s. We then turn to the six works in this special issue to describe the theories and methods supporting them. Next, we illustrate how general analytic categories can be derived from single or multi country case studies to illuminate themes capable of cutting across the particulars of national contexts, with their unique traditions, policy paths, and politics. Our three common threads are, first, the types of drivers for reform, that is, how policy change originates, either bottom-up from the institutions, or top-down from the government, and various possibilities in between. Second, understanding challenges to institutional autonomy in a continuum of intensity of state intended intervention in higher education. Third, explaining different levels of strain between public and private sectors in higher education based on conditions of competition for economic resources. While the papers in this special issue do not cover all countries, nor all issues on which policy has been crafted in the last two decades across the region, the collection of articles herein account for topics of enduring importance: faculty work in Ecuador, financial aid in Colombia, public policy decentralization in Argentina, quality assurance models in Colombia and Uruguay, the emerge of new institutions and universities in Argentina and Uruguay, and social justice, access, and inclusion in higher education, in Argentina, Bolivia, Brazil, Chile, Colombia, and Ecuador. The articles presented in this special issue provide much insight onto higher education policy in Latin America and, additionally, offer ample opportunity to develop social science knowledge on the basis of strong comparative work.

Thermal and mechanical stability of retained austenite surrounded by martensite with different degrees of tempering

The mechanical and thermal stability of austenite in multiphase advanced high strength steels are influenced by the surrounding microstructure. The mechanisms underlying and the relations between thermal and mechanical stability are still dubious due to the difficulty of isolating other factors influencing austenite stability. In this work, martensite/austenite microstructures were created with the only significant difference being the degree of tempering of the martensite matrix. Hence, the effect of tempering in martensite is isolated from other factors influencing the stability of austenite. The thermal stability during heating of retained austenite was evaluated by monitoring phase fractions as a function of controlled temperature employing both dilatometry and magnetometry measurements. The mechanical stability was studied by performing interrupted tensile tests and determining the remaining austenite fraction at different levels of strain. The thermal stability of this remaining austenite after interrupted tests was studied by subsequent reheating of strained specimens. The results are evidence for the first time that thermal recovery of martensite during reheating assists austenite decomposition through shrinkage and softening of martensite caused by a reduction of dislocation density and carbon content in solid solution. This softening of martensite also leads to a subsequent reduction of austenite mechanical stability. Additionally, remaining austenite after pre-straining at room temperature was thermally less stable than before pre-straining, demonstrating that plastic deformation has opposing effects on thermal and mechanical stability.

Dynamical properties of the brain tissue under oscillatory shear stresses at large strain range

In this experimental work, we study the viscoelastic behaviour of in vitro brain tissue, particularly the white matter, under oscillatory shear strain. The selective vulnerability of this tissue is the anisotropic mechanical properties of theirs different regions lead to a sensitivity to the angular shear rate and magnitude of strain. For this aim, shear storage modulus (G′) and loss modulus (G″) were measured over a range of frequencies (1 to 100 Hz), for different levels of strain (1 %, to 50 %). The mechanical responses of the brain matter samples showed a viscoelastic behaviour that depend on the correlated strain level and frequency range and old age sample. The samples have been showed evolution behaviour by increasing then decreasing the strain level. Also, the stiffness anisotropy of brain matter was showed between regions and species.

Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δ as Cathode Material for Solid Oxide Fuel Cells.

Strontium-doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3-δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single-crystal NdGaO3, SrTiO3, and GdScO3 substrates, leading to different levels of strain in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total-reflection X-ray fluorescence (TXRF) and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD), and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Effect of aging on hardness in the upsetting tests using experimental and finite element investigations

This paper investigates the hardness distribution in cold upsetting operation of the aged billets. Three sets of cylindrical billets were solutionized at 504 ℃ and then each set of billets were aged at 150 ℃ for a period of 9 h, 13 h, and 18 h, respectively. The billets were upset to different levels of strain and flow stress equation was evaluated for all the aging conditions. The flow stress equation was given as input to the finite element software and simulated to the same level of strain as in the case of experiments. An attempt to predict metal hardness was made by correlating the equivalent strain with the hardness. The hardness measurements predicted were in good agreement with the experimental results. The demand for producing billets with more hardness on the surface and softness inside encourages this investigation to measure hardness experimentally and theoretically.