Effect Of Prestrain Research Articles

The distinct influences of pre-strain on low cycle fatigue behavior of CP-Ti along rolling direction at different strain amplitudes

In this paper, the distinct influences of pre-strain on low cycle fatigue (LCF) behavior of CP-Ti at different strain amplitudes are clarified from macroscopic and microscopic aspects. At low strain amplitude, the sample of as-received material (AR) shows pronounced secondary cyclic hardening, however, this phenomenon gradually disappears in pre-strained (PS) sample with the increase of pre-strain. As cyclic stress amplitude and mean stress of PS sample increase with the decrease of strain amplitude, the detrimental effect of pre-strain on LCF life becomes more significant at lower strain amplitude, indicating the pre-strain dependent LCF behavior. At high strain amplitude, due to the more significant cyclic softening and mean stress reduction behavior, cyclic stress amplitude and mean stress rapidly decay to the value of AR sample in the initial few cycles, indicating the pre-strain independent LCF behavior. As the reduction of LCF life is related to the increase of mean stress and stress amplitude, the asymmetry coefficient related model can be applied to predict LCF life of CP-Ti. Further TEM observation shows that at low strain amplitude, dislocation structures of AR sample are mainly composed of planar slip structures and the secondary cyclic hardening is caused by corduroy structure. After pre-strain, both planar slip and wavy slip structures are observed in PS sample, indicating that the irreversibility of the dislocation structures inherited from pre-strain causes the decrease of LCF life. At high strain amplitude, the dislocation structures introduced by pre-strain can be erased completely during subsequent cyclic deformation, consequently, dislocation configurations of AR and PS samples are composed of the same wavy slip structures.

A theoretical and experimental investigation on the stress distribution in the interface of pre-strained SMA wire/polymer composites

The major concern in using the shape memory alloy (SMA) wires embedded in a polymer matrix is the possibility of debonding between SMA and the surrounding polymer due to the SMA phase transition recovery stress. Hence, it is important to examine the distribution of stresses in the SMA wire and the surrounding matrix. In the present study, using the principle of the minimum complementary energy a new analytical model was developed to determine the axial, radial, and shear stresses along the SMA/polymer interface. By considering the Poisson's effect and shear stress in the present analytical model, while ignored in the classical shear-lag model, more accurate results were obtained. Furthermore, by considering the effects of the SMA wire pre-strain, actuating temperature, recovery stress and thermal expansion, a novel relationship was also presented for calculating the maximum interfacial shear stress. To show the effect of the SMA wire pre-strain on the distribution of stresses throughout the SMA/epoxy interface, pull-out tests were conducted at different pre-strain levels in the SAM wire. Results showed applying the pre-strain in the SMA wire followed by a thermal activation enhanced the debonding load between the SMA wire and epoxy matrix and caused a significant variation in the stress distribution.

Effect of Prestraining and Preaging on Properties of Al-Mg-Si Alloys Before and After Plane Baking

The effects of different prestraining and preaging on the mechanical properties of automobile plane Al-1.02%Mg-0.93%Si alloys after 2 weeks of natural aging and plane baking treatment were investigated by Vickers hardness and tensile tests. The results showed that prestraining subsequent preaging simultaneously inhibited natural aging and enhanced the plane baking treatment. Differential scanning calorimetry and transmission electron microscopy analyses revealed the effectiveness of dislocation introduced by prestraining. The interaction between dislocation, quenched-in vacancies, solute atoms and precipitates was discussed, and the influence of these mechanisms on strength and formability was investigated.

Pre-strain effect on twist springback of a 3D P-channel in deep drawing

Abstract With the widely use of high-strength steel sheets in the automotive industry, the twist springback phenomenon of the steel sheets under multi-step forming conditions has received extensive attention. In this work, the Dual Phase steel DP500 is taken as the research object to investigate the complex non-linear elastoplastic behaviors and twist springback under two-step loading paths. The large specimen with a pre-strain of 4% true strain in rolling direction is carried out on a large tensile testing machine, and several specific blanks are extracted from it at different directions for a subsequent P-channel forming. The influence of twist springback associated with the pre-strain is analyzed. The finite element model based on the non-linear elastic model and the homogeneous anisotropic hardening model (HAH) is also established for the springback prediction and stress analysis. The results indicate that the pre-strain has a considerable impact on the twist springback. The non-linear strain path changes resulted from pre-straining not only influence the residual stress but also affect the elastic modulus distribution.

Effect of pre-strain on the electrochemical and stress corrosion cracking behavior of E690 steel in simulated marine atmosphere

Slow strain rate tests and constant strain corrosion tests were employed to study the effect of pre-strain on stress corrosion cracking behavior and initiation mechanism of E690 steel in simulated marine atmosphere. Results revealed that pre-strain can result in enhancement of both anodic and cathodic reactions of E690 steel in simulated marine atmosphere. Intergranular corrosion and microcracks formed after 4 days of exposure in sulfite-containing marine atmosphere at constant pre-strain, which consequently caused detrimental effects on the mechanical properties and subsequent SCC behavior of E690 steel. The presence of sulfite and high-angle grain boundaries of prior austenite grain boundaries (PAGBs) accounted for the intergranular corrosion and SCC initiation along PAGBs.

Effect of Pre-Straining and Natural Aging on the Hardening Response during Artificial Aging of EN AW 6082 and Lead Free EN AW 6023 Aluminium Alloys

In order to study the pre-straining and natural aging effects on the age-hardening response of EN AW 6082 and EN AW 6023 aluminium alloys during artificial aging at 170°C, the pre-straining by 5% was performed immediately after solution treatment of alloys at 550°C and subsequent quenching. The age-hardening response during artificial aging applied after various natural aging time (from 0.1 to 5 000 hours) was investigated using Vickers microhardness measurements and transmission electron microscopy characterization. It was found that pre-straining of quenched alloys state caused a dislocation density increasing in solid solution, which resulted in an immediate microhardness increase of alloys. During the subsequent natural aging of EN AW 6082 alloy, its microhardness increased right after alloy quenching and pre-straining, but only to the values obtained for the unstrained alloy state. On the contrary, the hardness of pre-straining EN AW 6023 alloy that is alloyed by Sn did not increase either after 10 hours of natural aging. This phenomenon is attributed to the effect of Sn on suppression of the strengthening clusters formation. The hardness of alloys increased greatly during artificial aging after pre-straining and natural aging due to accelerating the formation of coherent β″-phase particles. The negative effect of natural aging on the maximum age-hardening response obtained during alloys artificial aging had been observed for most of the pre-strained and naturally aged alloys states, with exception of EN AW 6023 alloy states that were pre-strained and shortly naturally aged (up to 100 hours).

Tensile-compression loading and pre-strain effects on the evolution of stacking fault tetrahedra, dislocation density, and free volume in crystal-amorphous thin film interface: A large-scale molecular dynamics study

Molecular dynamics (MD) simulations are carried out to study the tensile-compression cyclic loading behavior and structural changes in Al (metal)-Cu50Zr50 (metallic glass) thin film interface model (TFIM). The model comprises of 2.88 million atoms, interactions between Al, Cu and Zr atoms are modeled using EAM (Embedded Atom Method) potential. The tensile-compression test cycles are performed at a temperature of 100 K and strain rate of 109 s−1 at two pre-strains (e = 0.033 and e = 0.2). The stress-strain curves reveal that during cycle-I tensile loading, TFIM exhibits yield strength of 266 MPa and peak strength of 760 MPa. Increase in the tensile pre-strain from e = 0.033 to e = 0.2 has no significant effect on the compressive yield stress. The deformation mechanism in Al is by slip by partial dislocation nucleation. Shockley partial dislocation density (2 × 1017–4.5 × 1017 m−2) is observed to be the highest of all the other types of dislocations. Complete SFT (stacking fault tetrahedra) is formed in Al region when strained beyond e = 0.033 and during compression cycles only. The number of SFT increases from 8 during tensile to 28 during the compression cycle indicating that compressive test assists the formation of SFT. Beyond the elastic strain, the collapse of (00120) and (0 2 8 2) polyhedral network is observed in Cu50Zr50 metallic glass region at distance of 25 Å from the interface resulting in the creation of low dense regions.

Three-dimensional finite element analyses of repair process and safety margin assessment

An effective repair process is often needed to correct installation slope and geometrical correction of piping, but the repair method, residual stresses (RS), plastic damage and safety margin are needed to be assessed. The results of previous studies indicate that the RS caused by welding process and external correction force are complex, and there may be large local plastic strain damage after the repair process. Fracture toughness of stainless steels (SS) may be reduced significantly by both plastic pre-strain damage and thermal aging effect during the operation. However, there is little public literature to discuss the synergistic action of plastic pre-strain and thermal aging effect. In this paper, three-dimensional finite element (FE) analyses of RS and plastic strain of the repair process are performed, and the synergistic action of plastic pre-strain and thermal aging is analyzed based on the experimental results. In the case study, the repair process is shown to be effective, and the synergistic action is indistinctive as the fracture toughness at saturation state of thermal aging is not significantly affected by the fact of plastic strain. In the study case, the critical failure mode is still the plastic collapse, and the safety margin is increased after the repair process for the typical crack because work hardening is induced by plastic pre-strain.

An Investigation into Creep Cavity Development in 316H Stainless Steel

Creep-induced cavitation is an important failure mechanism in steel components operating at high temperature. Robust techniques are required to observe and quantify creep cavitation. In this paper, the use of two complementary analysis techniques: small-angle neutron scattering (SANS), and quantitative metallography, using scanning electron microscopy (SEM), is reported. The development of creep cavities that is accumulated under uniaxial load has been studied as a function of creep strain and life fraction, by carrying out interrupted tests on two sets of creep test specimens that are prepared from a Type-316H austenitic stainless steel reactor component. In order to examine the effects of pre-strain on creep damage formation, one set of specimens was subjected to a plastic pre-strain of 8%, and the other set had no pre-strain. Each set of specimens was subjected to different loading and temperature conditions, representative of those of current and future power plant operation. Cavities of up to 300 nm in size are quantified by using SANS, and their size distribution, as a function of determined creep strain. Cavitation increases significantly as creep strain increases throughout creep life. These results are confirmed by quantitative metallography analysis.

Effect of Pre-Strain on Small Punch Creep Test of 316L Stainless Steel at 373K

Creep does not only appear at high temperature, but also appears at low temperature for 316L stainless steel that threatens the safety of equipment. In this work, the creep behavior of as-received and pre-strained 316L stainless steel at 373K was investigated by uniaxial creep (UC) tests and small punch creep (SPC) tests. The parameters of power-law creep model were determined from stress dependence of UC tests. Then, the creep behavior of SPC test was analyzed by finite element (FE) simulation combined with power-law creep model. Comparing with experimental creep deflection, the results of FE simulation can reasonably reflect the creep behavior of as-received and pre-strained small punch specimens. Based on the comparison of as-received specimen and pre-strained specimen from UC test, SPC test and FE simulation, pre-strain significantly restrains creep behavior of 316L austenitic steel at 373K.

Epicardial prestrained confinement and residual stresses: a newly observed heart ventricle confinement interface

The heart epicardial layer, with elastin as the dominant component, has not been well investigated, specifically on how it contributes to ventricular biomechanics. In this study, we revealed and quantitatively assessed the overall status of prestraining and residual stresses exerted by the epicardial layer on the heart left ventricle (LV). During porcine heart wall dissection, we discovered that bi-layered LV surface strips, consisting of an epicardial layer and cardiac muscle, always curled towards the epicardial side due to epicardial residual stresses. We hence developed a curling angle characterization technique to intuitively and qualitatively reveal the location-dependency and direction-dependency of epicardial residual stresses. Moreover, by combining prestrain measurement and biaxial mechanical testing, we were able to quantify the epicardial prestrains and residual stresses on the unpressurized intact LV. To investigate the potential mechanical effect of epicardial prestraining, a finite-element (FE) model has been constructed, and we demonstrate that it is the prestraining of the epicardial layer, not the epicardial layer alone, providing an additional resistance mechanism during LV diastolic expansion and ventricular wall protection by reducing myocardial stress. In short, our study on healthy, native porcine hearts has revealed an important phenomenon-the epicardial layer, rich in elastin, acts like a prestrained 'balloon' that wraps around the heart and functions as an extra confinement and protection interface. The obtained knowledge fills a gap in ventricular biomechanics and will help design novel biomimicking materials or prosthetic devices to target the maintenance/recreation of this ventricle confinement interface.

Combined effects of prior plastic deformation and sensitization on hydrogen embrittlement of 304 austenitic stainless steel

Austenite stainless steels (ASSs) may suffer from both cold deformation and sensitization prior to hydrogen exposure. There is scant data in literature on the combined effect of prior deformation and sensitization on the hydrogen embrittlement (HE) of ASSs. The present study investigated the combined effects of tensile plastic prestrain (PS) and 650 °C sensitization (ST) on the HE of 304 steel by hydrogen pre-charging and tensile testing. The results are explained by terms of pre-existing α′ martensite content. PS higher than 10% can enhance HE significantly by inducing severe α′ transformation prior to hydrogen exposure. Prior ST also enhances HE, but submitting the prestrained and α′-containing 304 steel to short-time ST can diminish the enhancement of HE by prestraining, as ST can cause the reversion of α′ to austenite, reducing pre-existing α′ content. It is inadvisable to make 304 steel be sensitized/welded firstly and deformed subsequently, even if the ST time is short such as what happens during welding, because this treating sequence can induce more α′ than prestraining alone, enhancing HE more significantly. Apparent hydrogen diffusivity can be related quantitatively to pre-existing α′ content, proving directly that α′ platelets can act as diffusion “highways” in ASSs. It is indicated that pre-existing α′ can enhance subsequently the HE of ASSs is because it can lead to a large amount of hydrogen entering the ASSs during hydrogen exposure by acting as diffusion “highways”. HE is enhanced by increasing hydrogen amount rather than by pre-existing α′ itself.

Study on Static Recrystallization Behavior of 38MnVTi Non-Quenched and Tempered Steel

The static recrystallization behavior of 38MnVTi non-quenched and tempered steel within the interval of passes was investigated by double-pass compression experiments using Gleeble-1500 thermal simulation machine. The true strain-stress curves of the steel were obtained, and the effects of deformation temperature, strain rate and pre-strain on the static recrystallization behaviours were studied. Single-pass compression experiments for 38MnVTi non-quenched and tempered steel were finished with the same conditions to the double-pass compression experiments. The static recrystallization dynamics model and the static recrystallization grain size model can be established. The predicted results of the model accord well with experimental ones, which indicate the proposed model, can give an accurate prediction of the static softening behaviour for 38MnVTi non-quenched and tempered steel.

Tailoring mechanical behavior of a fine-grained metastable austenitic stainless steel by pre-straining

A fine-grained AISI 304 austenitic stainless steel (ASS) was fabricated by cryogenic-rolling and cycle annealing. The fine-grained ASS was pre-strained to various strains of 0–0.16, and the corresponding microstructures, kinetics of strain-induced α′-martensite (SIMα′) and work-hardening behavior were investigated based on quantitative characterization, interrupted-tensile tests and physical metallurgy. We find that the yield strength of the ASS can be enhanced by combining grain-refining with pre-straining due to the strengthening effect of grain-refinement, dislocations, shear bands and SIMα′. The formation rate of shear band increases with increasing pre-strain (εp) and the probability for an intersection of shear band to form a SIMα′ embryo is insensitive to the pre-strain, and as a result, the kinetics of SIMα′ is accelerated by pre-straining. The fine-grained ASS with εp ≈ 0–0.16 display a three-stage work-hardening behavior, and the corresponding Stage I, II and III are completed in advance mainly due to their faster kinetics of SIMα′ and severer dynamic recovery caused by pre-straining. The work-hardening rate (Θ) of pre-strained fine-grained ASS decreases faster than that of non-pre-strained counterpart at Stage I, accompanying with dislocations-dominated hardening. The Θ ascend with increasing strain to a peak value at Stage II and then decline continuously to necking at Stage III, following as SIMα′-governed hardening, which can be tailored by grain-refining and pre-straining. Our studies provide the direct experimental evidence for the pre-straining to trim the kinetics of SIMα′ in a fine-grained ASS. These observations help to deepen understanding of the effect of pre-strain on the mechanical behavior of other fine-grained strengthening ASS and transformation-induced-plasticity (TRIP) assisted alloy.

Effect of torsional pre-strain on low cycle fatigue performance of 304 stainless steel

Uniaxial tensile tests and low cycle fatigue tests were performed on 304 stainless steel treated with different torsional pre-strains. The relationship between torsional pre-strain and mechanical properties is established. Furthermore, the changes in the amount of martensite phase transitions are measured during low cycle fatigue tests and after specimen fracture are observed. This is mainly because martensitic transformation is a frequent phenomenon during plastic deformation, which is closely related to the mechanical properties. In this study, it is found that torsional pre-strain increases yield strength and reduces elongation. The ductility of 304 stainless steel decreases with the increase of torsional pre-strain, while its fatigue life does not. As the amplitude of torsional pre-strain increases, the fatigue life of the material changes in s-type. In particular, the fatigue life increases when the torsional pre-strain is between 5% and 8%. From the analysis of the microcosmic distribution of the martensite, it is found that the fatigue life is affected by the combined action of martensite transformation and ductility. The test results suggest that it should skip the relatively dangerous small strain area when improving mechanical properties of 304 stainless steel by way of changing shear pre-strain.

High-performance self-centering steel columns with shape memory alloy bolts: Design procedure and experimental evaluation

Steel moment-resisting frames are popular structural systems used extensively around the world. However, conventional column base connections are vulnerable to large residual deformation after strong earthquakes. By contrast, shape memory alloys (SMAs), which are high-performance metallic materials, can experience large strains and still recover their initial shape through either heating (shape memory effect) or unloading (superelastic effect). The superelastic behavior of SMAs is appealing to the earthquake engineering community because of the material’s excellent self-centering (SC) and energy dissipation capabilities. In this paper, a novel type of steel columns equipped with NiTi SMA bolts was introduced and its potential for achieving earthquake resilience were investigated. Structural details of the column base and mechanical properties of the SMA bolts were described first. Subsequently, an analytical model of the SC column for different limit states and the corresponding design procedure were presented. The seismic behaviors of two steel column specimens were experimentally tested to investigate the effects of the initial prestrain in the SMA bolts and the axial compressive force in the column under cyclic loading. Results showed that the steel columns equipped with SMA bolts exhibited satisfactory and stable flag-shaped hysteresis loops with excellent SC and moderate energy dissipation capabilities. More importantly, SMA bolts with prestrain could still be tightened after removal of lateral force. Therefore, the proposed SC column could achieve seismic resilience design that requires no (or minimal) repair even after strong earthquakes and remains highly functional for aftershocks or future earthquakes. In addition, the analytical model was verified through a comparison with test results obtained at key limit states.

Effects of Aging Temperature, Time, and Pre-Strain on Mechanical Properties of AA7075

Aluminum alloys of the 7xxx series (AA7075) are preferred in the aerospace and automotive industries due to their low densities, high strength, good corrosion resistance properties. Additionally, these alloys show the most effective aging properties among aluminum alloys. For this reason, it is very important to determine the most appropriate aging parameters for microstructural development. Literature review reveals that the effect of pre-strain on springback has not been studied yet. In this study, the effects of aging temperature, time, and pre-strain on mechanical properties are investigated for AA7075. Precipitates present in solid solutions of AA7075 and their effects are examined. Results reveal that MgZn2 precipitation is not observed at aging temperatures of 120 and 160 oC. After the formation of MgZn2 precipitates, microstructure becomes softer when aging continues at a higher temperature or longer period of time. It is clearly seen that pre-strain causes Portevin-Le Chatelier (PLC) effect after aging at 120 and 160 oC for aging times of 30 and 90 minutes.

Effects of prestrain on high-temperature tensile properties in commercial pure titanium

Effects of prestrain on high-temperature tensile properties in commercial pure titanium

Effect of pre-strain on cyclic plastic behaviour of 7050-T6 aluminium alloy

This paper aims at studying the pre-strain effect on cyclic plastic behaviour of the 7050-T6 aluminium alloy. In order to meet this goal, different series of tests with various pre-strain histories are performed under fully-reversed strain-controlled conditions. In a second stage, fracture surfaces are examined by scanning electron microscopy to identify the main fracture damage micro-mechanisms associated with the pre-strain scenarios. Overall, the results show that higher tensile pre-strain levels lead to lower fatigue lives. This reduction of the fatigue life expectancy tends to increase for higher cyclic strain amplitudes.

Effect of pre-strain on microstructure and hydrogen embrittlement of K-TIG welded austenitic stainless steel

The effects of pre-strain on hydrogen embrittlement of 304L K-TIG welded joint were investigated. As pre-strain of joint increased, fracture location of hydrogen-charged joint changed from weld metal to base metal. In low prestrained joints, fracture may be initiated at the sites of clustered dislocations due to hydrogen in weld metal with a high density of pre-existing dislocations. In high prestrained joints, some regions in base metal suffered severe strain-induced α′ martensitic transformation due to Ni depletion bands and cracking occurred at phase boundaries where may provide more sites for the accumulation of hydrogen atoms and dislocations.