Increase In Prestrain Research Articles

The influence of pre-strain on the microstructure and recovery properties of cold-rolled TiNiFe alloy tube

Abstract In the present work, the influence of pre-strain on the microstructures and recovery properties of cold-rolled TiNiFe shape memory alloy tube are studied by pre-strain system, phase transformation testing system, electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). It is demonstrated that the pre-strain has significant effects on the recovery properties, transformation temperatures, and microstructures of TiNiFe alloy. With the increase of pre-strain, the kernel average misorientation (KAM) value and the dislocation pill-up increase, causing the recovery rate of the TiNiFe alloy decreases linearly, and maximum of recovery strain is 8%. And the reverse martensite transformation start temperature (TRs) increases from -133°C to -105°C and TRf increases from -124°C to -97°C, as the increase of pre-strain.

Characterization of Viscoelastic Properties Considering the Nonrelaxation for Filled Rubber

The influence of nonrelaxation on the Payne effect of carbon black-filled rubber is studied. The prestrain is introduced in the Kraus model in the form of exponential growth. Combined with studies of temperature correlations, an explicit model for predicting the Payne effect at different prestrains and temperatures is developed. Dynamic mechanical analyses are performed to determine model parameters and validate the proposed model. To further verify the proposed model, the heat buildup of rubber columns under dynamic tensile load is tested and simulated. The comparison between simulated and measured data shows that the simulation considering nonrelaxation is more accurate than without considering. With the increase of prestrain, the accuracy of considering nonrelaxation becomes more obvious.

Experiment for Measuring Mechanical Properties of High-Strength Steel Sheets under Cyclic Loading by V-Shaped Double-Shear-Zone Specimens.

The simple shear test shows significant advantages when measuring the hardening and shear properties of thin sheet metal at large strains. However, previous shear tests had an end effect caused by local stress concentration and a boundary effect caused by deformation overflow, resulting in non-uniform strain distribution in the shear zone. Therefore, a unique V-shaped double-shear-zone specimen is proposed to measure the Bauschinger effect under cyclic shear loading conditions in this paper. Simple shear experiments and three different types of cycle shear experiments are conducted to analyze the uniformity of deformation in the shear zone and the effect of pre-strain and the number of cyclic loads on the Bauschinger effect of Q890 high-strength steel sheets. The results indicate that the proposed V-shaped double-shear-zone specimen can still maintain uniform shear deformation in forward/reverse cyclic loading experiments, even at large strains. Q890 high-strength steel exhibits a significant Bauschinger effect, which is more pronounced with the increase in shear pre-strain and loading cycles. The results of this paper provide a new approach for studying the hardening characteristics under large strain and the mechanical properties under cyclic shear loading for metal sheets.

Supertoughened Polylactide via the Addition of Low Content Poly(ε-caprolactone) and Tensile Deformation above the Glass Transition Temperature

Polylactide (PLA) is considered as an ideal alternative of petroleum-based plastics due to its high tensile strength and biodegradability. However, the low impact strength limits its extensive application in the engineering field. In this work, a supertough binary PLA blend was fabricated through pre-stretching above the glass transition temperature to a certain pre-strain. The biodegradable and biocompatible poly(ε-caprolactone) (PCL) was used as a toughening modifier for the PLA/PCL 95/5 (wt/wt) blend. It was found that the impact strength increased with the increase in pre-strain. The highest notched Izod impact strength reached 251 kJ/m2 at the pre-strain of 400%, nearly 132 times of that for neat PLA (1.9 kJ/m2). To our knowledge, such fabulous impact toughness has not been referred in literature. Even at the pre-strain of 150%, the blend exhibited a sufficiently high notched Izod impact strength of 149 kJ/m2. Small-angle X-ray scattering demonstrated a typical structural feature of combination of PLA shish-kebabs and oriented PCL lamellar stacks due to strain-induced crystallization. The wide-angle X-ray diffraction indicated that PLA crystals had high orientation at the pre-strain of 200% and above. The crystallinity of the PLA matrix increased with the increase in pre-strain, which was consistent with the increasing trend of impact strength. The high orientation of shish-kebabs and increased crystallinity of PLA matrix were the key factors for drastically toughening PLA. This work provides a feasible and effective method to manufacture competitive PLA materials, which can be extended to improve impact toughness of other brittle glassy polymers.

Effect of pre-strain on microstructure and stamping ability of TA2 titanium alloy and H260 low-alloy steel

Applying pre-strain can not only change the microstructure and the mechanical property of metal material but also affect sheet metal stamping ability. In this paper, taking H260 low-alloy steel and TA2 titanium alloy as experimental materials, pre-strain ranging from 0 to 0.16 was applied using the one-step and the multi-step method respectively. Microstructure observation, tensile test, bending test, and cup bulging test were conducted on the pre-strained samples. Results show that in the pre-strain loading stage, the slipping and twinning deformation occurs in the TA2 alloy sample while dislocation sliding mainly happens in H260 low-alloy. With pre-strain increasing, the product of ultimate tensile strength and elongation rate gradually decreases, and the true ultimate tensile stress and accumulating elongation change rarely. The “product of yielding strength and elongation rate” can be enhanced by applying a pre-strain of 0.04 on the condition that plasticity is still at a high level. In the pre-strain range of 0 - 0.16, a bending test with a 30.8 °-opening angle can be completed on H260 steel. The springback angle changes from negative to positive with an increase in pre-strain. The bending ability of TA2 titanium alloy is very poor, for a crack can easily occur even in the original sample. The cupping value decreases gradually with an increase in pre-strain. The cupping value of TA2 titanium alloy is higher than that of H260 low alloy steel for the constant pre-strain. A quite large difference exists between the two experimental materials. Pre-strain loading pass has no significant effect on the microstructure evolution, mechanical properties, and stamping properties.

Tension-compression asymmetry and Bauschinger-like effect of AZ31 magnesium alloy bars processed by ambient extrusion

Ambient extrusion was used to introduce different pre-strains in billets of AZ31 magnesium alloy. The effect of the pre-strain on tension-compression asymmetry and Bauschinger-like effect were studied. During ambient extrusion, the experimental material experienced {101‾2} twinning, {101‾2} detwinning and {101‾1} twinning sequentially. During tension and compression tests, the dominating deformation mechanisms are different, thus the tensile and compressive yield strength are improved with different trends. The extruded billets showed the strongest tension-compression asymmetry and Bauschinger-like effect when the pre-strain was ∼11%. With further increase of pre-strain, the plastic deformation became more homogeneous and caused a higher strengthening effect on {101‾2} twinning. As a result, the tension-compression asymmetry and Bauschinger-like effect got gradually weakened.

The influence of particle chain-magnetic field spatial location, frequency, dynamic strain amplitude and the prestrain on the mechanical performance of anisotropic magneto-rheological elastomer

Although there are literatures to characterize the properties of anisotropic magneto-rheological elastomer (MRE), more attention is paid when the particle chain is parallel to the applied magnetic field. However, in prospective of modeling and application design, mechanical characterization of anisotropic MRE under other particle chain-magnetic field spatial locations is needed. Herein, mechanical properties of anisotropic MRE with four kinds of particle chain-magnetic field spatial locations under varies frequencies, strain amplitudes and prestrains are tested. It shows that even the particle chain is perpendicular to the magnetic field, there exists an obvious MR effect. Besides the attraction of adjacent magnetized particles, the Maxwell stress tensor also contribute to the MR effect. Furthermore, an obvious strain amplitude dependent viscoelastic behavior is exhibited for anisotropic MRE. Moreover, the MR effect and the loss factor decrease as the increase of prestrain. The investigation contributes to the designing, modeling and applications of anisotropic MRE.

Effect of bending process on the fatigue behaviour of high strength steel

Integrating forming in the fatigue analyses of cold-formed High Strength Steel parts significantly increases predictive accuracy. In this study, the effect of cold-bending is modelled through a coupled forming-fatigue simulation of a bent specimen, made from a 12 mm thick S500MC grade. Different strain hardening models were studied, where a Voce law led to an increase in prestrain of about 10% and a decrease in fatigue life of 20% compared to the reference hardening behaviour. Three bending radii were investigated and a 25% decrease in radius led to a 30% increase in plastic strain. Fatigue tests were performed and a local strain approach resulted in a 76% correspondence with experimental data.

Storage life prediction under pre-strained thermally-accelerated aging of HTPB coating using the change of crosslinking density

In order to predict the storage life of a certain type of HTPB (hydroyl-terminated polybutadiene) coating at 25 °C and analyze the influence of pre-strain on the storage life, the accelerated aging tests of HTPB coating at 40 °C, 50 °C, 60 °C, 70 °C with the pre-strain of 0%, 3%, 6%, 9%, respectively were carried out. The variation regularity of the change of crosslinking density was analyzed and the aging model of HTPB coating under pre-strained thermally-accelerated aging was proposed. The storage life of HTPB coating at 25 °C was estimated by using the Berthelot equation as the end point of the aging life with a 30% decrease in maximum elongation. The results showed that the change of crosslinking density of HTPB coating increased with the increase of aging temperature and aging time, and decreased with the increase of pre-strain. Under 0% pre-strain, the relationship between the change of crosslinking density of HTPB coating and the aging time can be described by the logarithmic model with the confidence probability greater than 99%.The stress relaxation phenomenon existed under 3%, 6% and 9% pre-strained aging. The aging model considering chemical aging and pre-strain was established with the confidence probability greater than 90%. The storage life of HTPB coating was 15.2935 years at 25 °C under 0% pre-strain, which was reduced by 13.9007%, 75.6949% and 89.7859% under 3%, 6% and 9% pre-strain, respectively. The existence of pre-strain has a serious impact on the storage life of HTPB coating, therefore, the pre-strain should be avoided as much as possible during the actual storage.

Hydrogen embrittlement of a cold-rolled Al-containing medium-Mn steel: Effect of pre-strain

It is often unavoidable that automotive steel sheet parts might undergo plastic deformation during forming and service life. The present research explored the effect of pre-strain up to 15% on hydrogen embrittlement (HE) behavior of a cold-rolled and intercritically annealed Al-containing medium-Mn steel by electrochemical hydrogen charging and permeation testing, thermal hydrogen desorption spectrometry and slow strain rate tensile testing. The results show that the primary microstructural changes after pre-strain are the significant increase in dislocation density and the transformation of a slight part of retained austenite into martensite, which cause a decrease in hydrogen diffusion coefficient and a significant increase in absorbed reversible hydrogen concentration. Therefore, the HE susceptibility increases notably with an increase in pre-strain. It is thus concluded that pre-straining not only enhance the strength level but also remarkably increase the HE susceptibility of medium-Mn steel.

Effect of Prestrain on Hydrogen-Induced Delayed Cracking for Medium Mn Steels

Medium Mn steels are a class of the new-generation ultra-high-strength materials used in automotives. However, despite excellent ductility, they may suffer from delayed cracking and thus cause serious concerns. In this study, several medium Mn steels were tested with different prestrain and hydrogen charging conditions. The interaction and synergistic effects of prestrain and hydrogen content on hydrogen-induced delayed cracking behavior are investigated. The threshold stress of hydrogen-induced cracking (HIC) decreased during dynamic hydrogen charging under a constant load. In the process of dynamic hydrogen charging, for M7B and M10B steels, the normalized stress intensity factor σ/σb and the corresponding threshold stress σHIC decreased sharply as prestrain increased. This is because the volume fraction of retained austenite decreased with an increase in prestrain. Similarly, σHIC was reduced and the critical hydrogen content dropped drastically with increasing prestrain. For M7C, the influence of prestrain on threshold stress and hydrogen concentration was less than that of M7B. This is because the different treatment processes leads to a different stability of the retained austenite. By observing the SEM fractographs, the fracture surface of medium Mn steels showed different fracture characteristics, such as dimple fractures and intergranular and transgranular modes.

Influence of Thermally-Accelerated Aging on the Dynamic Mechanical Properties of HTPB Coating and Crosslinking Density-Modified Model for the Payne Effect.

Hydroxyl terminated polybutadiene (HTPB) coating is widely used in a solid rocket motor, but an aging phenomenon exists during long-term storage, which causes irreversible damage to the performance of this HTPB coating. In order to study the effect of aging on the dynamic mechanical properties of the HTPB coating, the thermally-accelerated aging test was carried out. The variation of maximum elongation and crosslinking density with aging time was obtained, and a good linear relationship between maximum elongation and crosslinking density was found by correlation analysis. The changing regularity of dynamic mechanical properties with aging time was analyzed. It was found that with the increase of aging time, Tg of HTPB coating increased, Tα, tan β and tan α decreased, and the functional relationships between the loss factor parameters and crosslinking density were constructed. The storage modulus and loss modulus of HTPB coating increased with the increase of aging time, and decreased with the increase of pre-strain. The aging enhanced the Payne effect of HTPB coating, while the pre-strain had a weakening effect. In view of the Payne effect of HTPB coating, the crosslinking density was introduced into Kraus model as aging evaluation parameter, and the crosslinking density modified models with and without pre-strain were established. The proposed models can effectively solve the problem that the Kraus model has a poor fitting effect under the condition of small strain (generally less than 1%) and on the loss modulus, which have improved the correlations between the fitting results and the test results.

Analysis of dynamic viscoelastic properties of chloroprene rubber considering pre-strain effect

As the vibration damping component of the bridge, the rubber bearings are not only supporting the weight of the beam but also bearing the small vibration load of various moving loads.The dynamic characteristics of rubber are not only dependent on frequency and temperature, but also closely related to large deformation under static behavior. To investigate pre-strain effect, static and dynamic compression experiments were carried out on a cylindrical chloroprene rubber specimen, and the storage modulus and loss modulus under different pre-strain were obtained, the test results show that the storage modulus increases with the increase of pre-strain, while the loss modulus decreases. An empirical formula to describe the relationship between storage modulus, loss modulus and pre-strain.

Ratcheting assessment of pressurized pipelines under cyclic axial loading: Experimental and numerical investigations

The present study aimed to study ratcheting strains of pressurized stainless steel (UNS S32750) pipes with cyclic axial loading conditions. To this aim, some experiments were performed for the tubes involving cyclic axial loading under internal pressure. All specimens were analyzed with a finite element code. Ohno-Wang hardening model was employed in numerical analysis to calculate the ratcheting of the tubes. By using this hardening model, all predicted ratcheting results were consistent with those of the experimental ones. In this study, the effect of pre-compressed strain, internal pressure and stress amplitude were evaluated experimentally and the numerical results were compared with the results. Finally, the tubes collapsed at about the same strain level as it did under monotonic loading. The number of cycles to collapse and the ratcheting strain rate of the pressurized pipelines depend on the different parameters such as cyclic loading parameters, pre-strain and internal pressure. The results show that an increase of pre-strain, internal pressure and cyclic loading parameters leads to increase ratcheting strain rate of pipelines in both axial and circumferential directions.

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.

Prediction of Post-deformation Recrystallization Kinetics in AISI 321 Austenitic Stainless Steel Using Double-Stage Hot Compression

In the present research, a series of double-stage hot compression tests were conducted in the temperature range of 950-1150 °C, strain rates in the range of 0.01-1 s−1, strain in the range of 0.1-0.3, and inter-pass time range of 3-100 s to evaluate the kinetics of the post-deformation recrystallization in AISI 321 austenitic stainless steel. An Avrami-type kinetics equation has been determined based on the obtained experimental data. Results show that the extent of post-deformation recrystallization increases with increase in inter-pass time, deformation temperature, pre-strain, and strain rate. The calculated activation energy for recrystallization (174 kJ/mol) is lower than the activation energy for static recrystallization of austenitic stainless steels. It is concluded that the both static and metadynamic recrystallization occurred during the inter-pass period. Also, the predicted softening fraction using the proposed kinetics equation agreed well with those obtained from experimental results.

Damage evolution in high density polyethylene under tensile, compressive, creep and fatigue loading conditions

Influence of various loading histories on mechanical properties and damage evolution of polyethylene (PE) is quantified through a two-stage test method. The first-stage tests are to introduce damage by subjecting the specimens to different prestrain levels under tensile, compressive, creep and fatigue loading condition. Two months later, the second-stage tests apply monotonic tensile loading at a crosshead speed of 0.01 mm/min to characterize the mechanical properties for specimens that have had damage generated in the first-stage tests. Experimental results suggest that yield stress and loading stiffness decrease and residual plastic strain increases with the increase of prestrain introduced in the first-stage tests. Damage evolution law, based on the degradation of loading stiffness has been established as a function of prestrain and residual plastic strain. Results show that damage variable expressed as a function of prestrain strongly depends on loading conditions applied in the first-stage tests. However, damage evolution described by the relationship between damage variable and plastic strain is found to be independent on the loading condition, suggesting that a unified, plastic strain controlled damage evolution law can be established for PE materials.

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.

Springback prediction and elasticity modulus variation

Abstract Angular precision of air bending is directly related to the precision of the calculated punch stroke, which is computed in order to achieve the required sheet profile before the springback. The calculated springback accuracy depends on the elasticity modulus which, as it is well-known, decreases with increase of plastic prestrain. During the bending process, the strain varies along the plate profile from the punch tip to the contact point of the plate with the die, but also in each cross-section along the thickness. In this contribution a formula is established allowing to calculate the springback taking into account the variation of the elasticity modulus. Experimental results have shown that for many materials reliable results are obtained by applying the developed formula and that for some materials, the latter has to be corrected by a single coefficient regardless of the thickness-tooling-angle combination.

Effect of Prestrain on Hydrogen Embrittlement Susceptibility of EH 36 Steels Using In Situ Slow-Strain-Rate Testing

Hydrogen can provide pure and clean energy; however, to use it as an energy source, facilities such as hydrogen carriers and recharging stations need to be constructed. Structural steels are affected by hydrogen embrittlement (HE), and their susceptibility to this needs to be investigated prior to their use in construction. Most structural steels are normally fabricated using thermomechanical controlled processing, which produces a large dislocation density to increase strength. This study investigated the prestrain effect on HE susceptibility of EH 36 steels using thermal desorption spectroscopy (TDS) and in situ slow-strain-rate testing. Hydrogen was electrochemically charged into specimens, and the reversible hydrogen content and that relating to trap sites were measured using TDS. With an increase in prestrain, there was increase in the diffusible hydrogen content; furthermore, with hydrogen charging, there was a drastic reduction in total elongation with an increase in prestrain. In addition, there was an increase in HE susceptibility with an increase in prestrain compared to when an air condition was employed. Specifically, there was an abrupt increase in HE sensitivity at a prestrain value between 10 and 15%; strain hardening was more dominant below a prestrain value of 10%; and HE was more dominant above a prestrain value of 15% for EH 36 steels.