Effect Of Strain Reversal Research Articles

Flexural behaviour and efficiency of CFRP-laminate reinforced recycled concrete beams: Optimization using linear weighted sum method

The increased demand for lightweight high-performance composites has led to search for alternative reinforcement to improve the mechanical performance of conventional structures. Likewise, various research initiatives have advocated recycling of construction and demolition wastes and novel technologies to avert their generation. Owing to disadvantages of steel rebar, carbon fibre reinforced polymer (CFRP) was utilized as potential internal reinforcement in recycled concrete beam owing to its lightweight, non-corrosiveness, high-stiffness-to-weight ratio and flexibility. Our study revealed significant improvement in the mechanical performance and efficiency which is controlled by the fibre architecture. The improved mechanical properties was attributed to the Bauschinger strain-reversal effect, made possible by the effective CFRP tensile strength mobilization, its high bonded surface area and interfacial energy as well as the composite action of the multi-layered CFRP reinforcements. The best configuration (N4) revealed by the simplified linear weighted sum optimization method achieved strengthening (load) efficiency of 402.7%, ductility efficiency of 299.7%, fracture toughness efficiency of 567.1% and fracture energy efficiency of 5713.9% compared to the unreinforced control. In addition, CFRP laminate was 3.67–4.9 times more cost-effective than steel rebar in terms of fracture toughness. Therefore, CFRP-reinforced recycled concrete is recommended for cost-effective and sustainable prefabricated concrete structures.

Role of Strain Reversal in Microstructure and Texture of Pure Al during Non-Monotonic Simple Shear Straining

The effect of reverse straining on the microstructure, texture, and hardness of the pure Al during the single cycle of the simple shear extrusion (SSE) process is investigated. Electron backscattering diffraction (EBSD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) are used for microstructural evaluations. Due to the nature of the SSE process, the direction of the shear is reversed in the second half channel. As a result, the mean misorientation angle, dislocation density, and hardness decrease. The grain size increases to some extent in the exit of the channel compared with that in the middle. The structural evolution during the single pass of SSE is described in terms of the dislocation cancelling, “untangling” of the cell walls and disintegration of the “forward” cell structure due to the reverse straining in the second half channel. Some simple shear components replace the first texture component in the middle of the channel. At the exit of the channel, the primary texture components appear somewhat confirming the strain reversal effect in the second half channel.

Effect of strain reversal on the stress-induced martensitic transformation and tensile properties of a metastable β titanium alloy

In this study, the effect of strain reversal on the evolution of microstructure and mechanical properties was investigated. Metastable β titanium alloy samples with single β phase were twisted up to 360° with different strain paths: monotonic torsion (MT) and cyclic forward-reverse torsion (CFRT). The results revealed CFRT with strain reversal accumulated lower dislocation density and less martensitic transformation in comparison with MT under the same accumulative strain. The stress-induced martensitic transformation (SIMT) was retarded by strain reversal and this suppression of martensitic transformation is more pronounced with the decreasing amplitude of strain reversal. It was further observed that CFRT samples with a gradient α″ martensite have a higher mechanical property both in strength and ductility than MT samples, which are due to interactions between α″ martensite and dislocation slip, especially the kinks of α″ martensite.

Through-Scale Analysis of Strain Path Effects in Microalloyed Austenite Subjected to Reverse Rolling

Thermomechanical hot rolling processes are often realized using reverse rolling stands, where the rolled stock is fed forward and backward through the rolling gap. During those processes, material undergoes several strain reversals that significantly alter microstructure evolution of austenite with respect to continuously rolled counterparts. In Nb-microalloyed steels, where precipitation hardening is usually expected, the effects of strain reversal are especially complex. When rolling direction is reversed, both static recrystallization (SRX) kinetics and strain-induced precipitation (SIP) processes are slowed down due to decreasing dislocation density. It affects the competition between driving force for SRX and pinning pressure for SIP and, in turn, changes the non-recrystallization temperature (Tnr), compared to the case where strain path is linear. In the present paper, detailed through-scale analysis of strain path effects in microalloyed austenite will be presented. Physical simulation and detailed microstructural analysis will be employed to study global and local effects in microalloyed austenite after complex deformation histories. Conclusions regarding the influence of strain path changes on the interactions between SRX and SIP will be drawn.

Thermo-hydro-mechanical coupling analysis of a thermo-active diaphragm wall

Thermo-active diaphragm walls that combine load bearing ability with a ground source heat pump (GSHP) are considered to be one of the new technologies in geotechnical engineering. Despite the vast range of potential applications, current thermo-active diaphragm wall designs have very limited use from a geotechnical aspect. This paper investigates the wall–soil interaction behaviour of a thermo-active diaphragm wall by conducting a thermo-hydro-mechanical finite element analysis. The GSHP operates by circulating cold coolant into the thermo-active diaphragm wall during winter. Soil contraction and small changes in the earth pressures acting on the wall are observed. The strain reversal effect makes the soil stiffness increase when the wall moves in the unexcavated side direction, and hence gives different trends for long-term wall movements compared to the linear elastic model. The GSHP operation makes the wall move in a cyclic manner, and the seasonal variation is approximately 0.5–1 mm, caused by two factors: the thermal effects on the deformation of the diaphragm wall itself and the thermally induced volume change of the soil and pore water. In addition, it is found that the change in bending moment of the wall due to the seasonal GSHP cycle is caused mainly by the thermal differential across the wall during the winter, because the seasonal changes in earth pressures acting on the diaphragm wall are very limited.

Modeling the effect of strain reversal on grain refinement and crystallographic texture during simple shear extrusion

This paper is concerned with the modeling of the effect of strain reversal on crystallographic texture and microstructure evolution in simple shear extrusion (SSE) processing of commercial purity copper. To reach this goal, the evolution of microstructure is predicted by a dislocation density-based constitutive model in which the effect of strain reversal is considered. This dislocation-based model is embedded in a crystal plasticity finite element (CPFE) model. In FE models, a representative volume element of the polycrystal at the central region of the billet is subjected to deformation history experienced by the SSE process with backpressure. By comparison the predicted crystallographic texture with the published data, it is concluded that the CPFEM well predicts a simple shear texture after a complete pass. The predicted results show 5–10% increase in the rate of dynamic recovery of the second half of deformation zone in comparison with the first half. The modified dislocation-based model predicts that the cell size of a complete pass is more than that of 0.5 pass sample as much as 27%.

Effect of strain reversal on microstructure and mechanical properties of Ti-6Al-4V alloy under cyclic torsion deformation

Abstract Generally, torsion deformation has been recognized as a method of severe plastic deformation (SPD) to obtain a higher shear strain. The effect of strain reversal on the microstructure and mechanical properties was investigated using commercially Ti-6Al-4V alloy by twisting the rods with different effective strain amplitude, such as 180#x00B0;, 270#x00B0;, 360#x00B0; and 450#x00B0; per half cycle. In this work, the microstructure evolution and fracture morphology were carried out by a scanning electron microscope (SEM) and transmission electron microscope (TEM). An Instron 3382 was used for determining ultimate compression strength (UCS). The results show that UCS is very sensitive to cyclic forward-reverse torsion (CFRT) processing and increase distinctly after pre-CFRT. And the value of UCS reaches maximum at 180#x00B0; per half cycle. Besides, monotonic torsion (MT) by twisting the rods in one direction until the fracture occurs was also compared with CFRT. It is found that the ability of work hardening and accumulation of plastic strain is higher for CFRT as compared to MT, which is due to the latticed dislocation in the interior of the grains under CFRT deformation. Grain refinement and fracture feature were discussed to reveal the deformation mechanism of CFRT and MT. The discussion of microstructure evolution and strengthening mechanisms will provide a view to extrapolation to larger strains, finer structures and larger stresses.

Vickers microhardness and microstructure relationship of Ti-6Al-4V alloy under cyclic forward-reverse torsion and monotonic torsion loading

The effect of strain reversal on the hardening and grain refinement of Ti-6Al-4V alloy due to cyclic forward-reverse torsion (CFRT) has been investigated in this study by twisting the bars with different strain reversal amplitude. The results showed that microhardness and its distribution were very sensitive to CFRT and increased distinctly after pre-CFRT pure shear deformation. It was observed that the ultimate microhardness and homogeneity were superior for CFRT as compared to monotonic torsion (MT), while the hardenability exponent of MT is higher than CFRT. Besides microhardness in the center layer and surface layer of a bar exhibited different evolutionary process with the increasing amplitude of strain reversal. Grain refinement induced by latticed dislocation tangle and work hardening enhanced by strain gradient were also discussed to reveal the evolution of microstructure and microhardness.

On the Bauschinger effect in TiAl alloys

The Bauschinger effect has been investigated at room temperature on a multiphase titanium aluminide alloy based on γ(TiAl). The effect of strain reversal was assessed by comparing uni-directional tensile tests with those performed on specimens made from pre-compressed material. This deformation sequence has the advantage that both the forward and reverse parts of the Bauschinger cycle are in tension, i.e., they are not affected by non-axial deformation, buckling or specimen end constraint. The results suggest that no permanent softening develops upon strain reversal whereas significant transient softening is present. The effect seems to be caused by heterogeneities in the deformed state due to the multiphase constitution of the material.

Effects of strain reversal on elastic and viscoelastic properties of early age cement paste beams

Abstract Characterization of the early age properties of concrete is an ongoing area of research particularly important in understanding longer-term behavior of pre-stressed and conventional concrete members subjected to varying load histories, including strain reversals, such as may occur during placement of a concrete deck atop pre-stressed girders. Models currently in widespread use specifically exclude cases where strain reversals are present. Research presented in this paper was undertaken to refine the understanding of the viscoelastic behavior of cement paste at an early age when subject to strain reversals. A series of small-scale beams were tested in 3-point loading with the center point displaced at irregular time intervals. At some time intervals, beams were displaced negatively, inducing a strain reversal. Results indicate significant load loss in the first few days followed by steady relaxation throughout the length of testing. Results further indicate that load relaxation after reversal is significantly slower than that with same-direction increases in load. Finally, measurements of Young’s modulus of elasticity throughout the testing program indicate that sustained loading leads to an apparent increase in beam stiffness compared to unloaded beams of the same age. The explanation and significance of these phenomena are explored.

Study of the Effect of Cyclic Strain Reversal on the Interactions between SRX and PPT in the Microalloyed Austenite during Hot Rolling Process

In the present work cyclic torsion test was used to simulate hot plate rolling process in order to study the effect of strain reversal on non-recrystallisation temperature using unalloyed and microalloyed austenite model alloys. It was found that the amount of strain reversal directly influences both static recrystallisation and strain-induced precipitation process significantly delaying their kinetics. The proper assessment of the interactions between strain reversal and microstructure evolution plays a crucial role during hot rolling process - as continuous changes in the deformation mode (strain reversal) affect the level of redundant strain (in the areas near the surface of the stock) and lead to strain inhomogeneity across the plate thickness. This complex strain path introduces microstructural inhomogeneity and makes its predictions very difficult. Proper understanding of the effects of strain reversal on microstructure evolution in the austenite will help to optimise the hot rolling process.

Effect of temperature and strain on microstructure refinement in low carbon microalloyed steel during multi-axis deformation process

Multi-axis deformation (MAD) tests were performed on a low carbon microalloyed steel with different process parameters. The effect of temperature and strain on the microstructure evolution were analyzed. The strain reversal effect was observed from the comparison between the microstructure resulting from the MAD process and the one from a uniaxial deformation process. However, with 0.3 strain per pass and the alternating compression time equal to 10 or more during the MAD process, ultra fine structures were obtained at deformation temperature below 800 °C. Due to the microalloyed elements in the tested steel, nanometer sized precipitates were introduced under MAD conditions, which also contributed to the microstructure refinement.

On the Effect of Strain Reversal on Static Recrystallisation and Strain-Induced Precipitation Process Kinetics in Microalloyed Steels

Recent observations show that the strain reversal affects significantly and in a complex way both the static recrystallisation (SRX) and strain-induced precipitation (PPT) kinetics in Nb-microalloyed steel. It is already known that the recrystallisation stagnation is a consequence of the competition between the driving pressure for recrystallisation and the pinning pressure caused by the strain-induced precipitation of Nb (C,N) precipitates. Both of these parameters depend in turn on the local dislocation density. Thus, it is expected that a variation of the local dislocation density due to reversal of the strain will affect at the same time the local driving and the pinning pressures, which will cause the difference in the hardening levels. In the present paper, the influence of strain path change on microstructure evolution and mechanical behaviour in Nb-microalloyed steel (API X-70 grade) was studied. The deformation schedules were designed in order to investigate an effect of strain reversal on both static recrystallisation and strain-induced precipitation process kinetics. Flow curves recorded during deformation of X-70 steel showed clear influence of applied strain path on both static recrystallisation kinetics and strain-induced precipitation process.

A Step Forward in the Understanding of the Strain Reversal Effect on the Recrystallization Kinetics after Hot Working

A reversion of the strain produces a modification of the static recrystallization kinetics. Initially, the reversion increases the recrystallization time, that reaches a maximum at a certain strain, and decreases again for increasing reverse strains. This transient on recrystallization kinetics develops over a strain interval similar to that of the microstructural and stress-strain transients. At strains beyond the transient, the reversion can be regarded as a shift on the strain axis. However, at the authors knowledge there is no formulation able to describe the material behaviour during the transient. The present work introduces an equivalent strain concept based on the substructural dissolution/build-up processes taking place as a result of the strain reversal. This formulation allows including the effect of the strain path on recrystallization models.

The Effect of Strain Path Reversal during Austenite Deformation on Phase Transformation in a Microalloyed Steel Subjected to Accelerated Cooling

In the present study, monotonic and cyclical torsional deformations of an X-70 microalloyed steel were conducted at austenite temperatures below the recrystallisation-stop temperature (T5%). The austenite deformation is followed by accelerated continuous cooling to allow the investigation of the strain reversal effect on the subsequent phase transformation mechanisms. The transformation behaviours were studied by a dilatometry method, and the microstructures of the transformed products have been analysed using electron back scatter diffraction (EBSD). The results of this study shows that although subjected to the same total cumulative strain and the same cooling rate, strain path reversal by cyclical torsion produces lower temperature transformation products involving mainly a displacive mechanism, comparing to simple strain path deformation which leads to higher temperature transformation by a reconstructive mechanism.

Effect of strain reversal on texture and grain refinement in route C equal channel angular pressed copper

Texture development and grain refinement were studied experimentally and by simulations using a recently proposed Taylor-type grain refinement model for oxygen-free high-conductivity copper deformed by two passes of 90° equal channel angular pressing (ECAP) in route C. Route C is a deformation mode which reverses the shear deformation in ECAP, but does not completely reverse the texture. The results demonstrate better reproduction of the experimental strain reversal shear texture than the VPSC model alone, while also reproducing the grain size distribution.

Microstructure development and hardening during high pressure torsion of commercially pure aluminium: Strain reversal experiments and a dislocation based model

The effect of strain reversal on hardening due to high pressure torsion (HPT) was investigated using commercially pure aluminium. Hardening is lower for cyclic HPT (c-HPT) as compared to monotonic HPT (m-HPT). When using a cycle consisting of a rotation of 90° per half cycle, there is only a small increase in hardness if the total amount of turns is increased from 1 to 16. Single reversal HPT (sr-HPT) processing involves torsion in one direction followed by a (smaller) torsion in the opposite direction. It is shown that a small reversal of 0.25 turn (90°) reduces hardness drastically, and that decrease is most marked for the centre region. These behaviours and other effects are interpreted in terms of the average density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs). A model is presented that describes the experimental results well. A key element of the model is the assumption that at the very high strains developed in severe plastic deformation processes such as HPT, the dislocation density reaches a saturation value. The model indicates that the strength/hardness is predominantly due to GNDs and SSDs.

Effect of the strain reversal on austenite–ferrite phase transformation in a Nb-microalloyed steel

Multipass torsion tests were performed on an Nb-microalloyed steel at decreasing temperatures so as to analyze the effect of strain reversal – in the strain accumulation region – on the γ→α transformation kinetics. Strain reversal involved a significant delay in the start of the transformation as well as a microstructural transient compared to monotonic strain conditions. However, after several passes with no reversal performed within the inter-critical temperature region, both types of tests led to similar microstructures.

An analysis of microband orientation in a commercial purity aluminium alloy subjected to forward and reverse torsion using Electron Backscatter Diffraction (EBSD)

High-resolution electron backscatter diffraction has been used to study the effects of strain reversal on the evolution of microbands in commercial purity aluminium alloy AA1200. Deformation was carried out using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300 degrees C and strain rate of 1 s(-1) to a total equivalent tensile strain of 0.5. In both cases, microbands were found in the majority of grains examined with many having microband walls with more than one orientation. For the forward/forward condition, the microband clusters were centred around -20 degrees and +45 degrees to the equivalent tensile stress axis, whereas for material subjected to a strain reversal, the clusters were at -65 degrees and -45 degrees . There was no evidence of microbands that were formed in the forward deformation step in the reversed material, which would suggest that a strain of 0.25 is sufficient to dissolve any microstructure history generated by the first step. Furthermore, the microbands within the strain-reversed material had a reduction in misorientation compared with the lineally strained material, suggesting that these microbands only formed at the onset of the second deformation step. This confirms that microband formation is complex and sensitive to strain path; however, it is still unclear to what extent microband formation is dependent on strain path history compared with the instantaneous deformation mode.

Fracture of prismatic aluminum tubes under reverse straining

The effect of loading history on fracture was thoroughly investigated at the material and structural level. Tests on tensile fracture of initially pre-compressed round specimens were analyzed and two alternative methods were developed to account for the effect of strain reversal on initiation of fracture in uncracked bodies. A complete calibration for plasticity and fracture without and with history effect was performed for 2024-T351 aluminum alloy. In the present approach, it is postulated that fracture occurs when the accumulated plastic strain modified by the effect of stress triaxiality and the loading history reaches a critical value. The newly developed theory then was applied to predict initiation of fracture in prismatic square aluminum tubes subjected to crush loading. Compression tests were performed on small columns with the width to thickness ratio covering the range 10–30. First fracture was observed at different locations depending on the thickness of the tube. The numerically predicted point of fracture initiation and the displacement corresponding to formation of crack agreed well with test results. It is concluded that the effect of loading history must be included in the formulation of fracture criteria and in the application to the failure analysis of structural components where large pre-compression should be expected.