Influence Of Pre-strain Research Articles

Influence of pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel

In the present work, influences of room temperature tensile pre-strain on cryogenic tensile properties of 316LN austenitic stainless steel (SS) were investigated. The tensile properties of the 316LN SS with various tensile pre-straining amounts of 15%, 25% and 35% were measured at both room temperature (RT) and liquid helium temperature (4.2 K). Results indicated that both 0.2% proof strength (Rp0.2) and ultimate tensile strength (Rm) at both RT and 4.2 K of the pre-strained 316LN SS significantly increased compared to those of the as-received material. The Rp0.2 and Rm at 4.2 K for 35% pre-strained specimens increased around 86% and 18%, respectively, compared to those of the as-received material. However, the elongation at fracture (A) at both RT and 4.2 K decreased for the pre-strained 316LN SS compared to those of the as-received material. The fractured morphologies of the pre-strained 316LN SS were investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Dimples in the surface fractured at both RT and 4.2 K of the as-received and pre-strained 316LN exhibited a ductile fracture mode. Moreover, fractographic results indicated that the pre-strain treatment considerably decreased the ductility at both RT and 4.2 K. TEM analyses revealed that pre-strain treatment at RT led to dislocation accumulation and stress-induced martensite phase occurred at 4.2 K, which interpreted the cryogenic tensile properties of the pre-strained 316LN SS.

Influence of pre-strain on microstructural characteristics and tensile deformation behaviour of a cold-rolled Al-containing medium Mn steel

Automobile steel sheet parts often experience unavoidable plastic deformation during cold stamping forming and service life. In this study, we explored the microstructure variation and tensile deformation behaviour of a cold-rolled Al-containing medium Mn steel (0.25C–8.67Mn–0.54Si–2.69Al, wt%) subjected to different degrees of pre-straining by tensile testing, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and X-ray diffraction. The cold-rolled steel sheet processed by intercritical annealing at 725 °C for 60 min exhibited a superior combination of strength and ductility [the product of ultimate tensile strength (UTS) and total elongation (TEL), UTS × TEL] as high as 69 GPa% due to the TWIP (twinning-induced plasticity) and TRIP (transformation-induced plasticity) plasticity-enhancing mechanism during tensile deformation. The tensile behaviour, which originally exhibited notable Luders plus Portevin–Le Châtelier (PLC) banding phenomenon, was significantly changed by pre-straining. The strength increased, while the TEL and UTS × TEL decreased linearly with an increase in the pre-strain. A pre-strain equalling to the Luders strain, i.e. 3%, could cause an elimination of the Luders bands, while it had almost no influence on the PLC bands. The increase in the dislocation density, the enhanced stability of retained austenite and the strain-induced martensite and mechanical twins caused the reduction and elimination of the Luders bands with increasing amounts of pre-strain.

Influence of uniaxial tensile pre-strain on forming limit curve by using biaxial tensile test

This paper focused on the effect of pre-strain on forming limit curves (FLC) of 5754-O aluminum alloy sheet through utilizing biaxial tensile approach. Based on Swift model and Yld2000-2d yield criterion, the dimensions of cruciform specimen was optimized through applying finite element method for increasing the strain at specimen center. After that, with the recommended specimen size, the cruciform specimen was tested under various stroke ratios to experimentally characterize the limit strains under different pre-strain levels. Subsequently, the biaxial tensile tests were simulated by Abaqus to obtain the limit strains and validate the material models. It can be observed in both experiments and simulations that the pre-strained uniaxial tension followed by plane tension or equi-biaxial tension can improve the formability of sheet metals. Besides, the strain path change affects the trend of first derivative of strain rate difference between neighboring points with respect to time. An early increase occurred and then fell back to the stable value, the steady evolution continued until to a new increase reaching the critical value. The M–K prediction approach was simulated to verify the influence of pre-strain on FLC. It can be found that the early increase peaks of the major strain incremental ratio rose with the amplitude of pre-strain. Finally, the phenomenon of pseudolocalization caused by the strain path change was explained through evolution of stress state inside the groove.

Influence of Prestrain on the Mechanical Properties and Constitutive Model of Hydroxyl-Terminated Polybutadiene Films as Used for Coatings in Solid Rocket Motors

Low-field 1H NMR tests and uniaxial tensile tests of hydroxyl-terminated polybutadiene (HTPB) films, subjected to a prestrain before the NMR and tensile tests related to the use of the HTPB as coatings for solid rocket motors, were carried out. The mechanisms of the variations of crosslinking density, tensile strength and maximum elongation of the HTPB films with prestrain were analyzed, and a Gaussian function model of the variation of the crosslinking density with prestrain was obtained. By introducing crosslinking density and prestrain as correction coefficients, two prestrain constitutive models of the stress-strain relationship of HTPB films related to the Mooney-Rivlin and Ogden hyperelastic constitutive models, respectively, were established and the parameters of the models were obtained based on nonlinear least squares fitting. The accuracy and effectiveness of the prestrain constitutive models were determined with that based on the Ogden model proven to be of important engineering value.

Influence of Pre-strain on the Strain Rate Response of Ni-Ti Shape Memory Alloy

The presence of Shape Memory Alloy (SMA) in civil engineering fields offered a lot of advantages to the industries. The superiority of SMA is when the material has the ability to remember its previous shape after it has deformed and it will return back to its original defined shape when it is subjected to heat. Nickel-Titanium (Ni-Ti) is known as the finest alloy among SMA groups. In this research, the influence of pre-strain on the mechanical behavior of Ni-Ti shape memory alloy bars were examined in room temperature with concerns of a different strain rates, which are 5.5x10-4 s-1 and 1x10-3 s-1. Each of the test piece was pre-strained at level 5ξ, 10ξ and 15ξ. A quasi-static tensile test was carried out on heat treated Ni-Ti by using a Universal Testing Machine to observe the Ni-Ti stress-strain response. The results show tensile strength of Ni-Ti increased after applying pre-strain and heat treatment at 500°C. However, it is observed tensile strength of Ni-Ti started to drop after pre-strained at 15ξ. So, it indicates that strain ageing of Ni-Ti bars at a large strain becomes practically irrelevant, even though Ni-Ti is a super-alloy. Furthermore, Ni-Ti is a strain rate dependent material where the strength increases at a higher rate. Thus, it was found Ni-Ti exhibited yield strength differently where the yield strength increases as the strain rate decreases.

Buckling-controlled two-way shape memory effect in a ring-shaped bilayer

Shape memory polymers (SMPs) usually have a one-way shape memory effect. In this paper, an easy-operating method to realize a two-way shape memory effect was demonstrated in a ring-shaped bilayer structure where the two layers are SMPs with different thermal transition temperatures. By designing specific thermomechanical processes, the mismatched deformation between the two layers leads to a morphology change of ring-shaped bilayer structures from a smooth ring to a gear-like buckling shape under cooling and a reversible recovery to the smooth shape under heating. Such a morphology change is ascribed to occurrence and recovery of thermoelastic buckling. This method was validated by finite element simulation. We experimentally investigated the influence of pre-strain on buckling, and it was found that both the buckling occurrence and recovery temperature vary with pre-strain. Furthermore, considering a ring-shaped SMP–SMP bilayer structure, finite element analysis was conducted to study the influence of film thickness and modulus ratio of two layers on buckling behavior. The results showed that the critical buckling wavelength was greatly influenced by film thickness and modulus ratio. We made a theoretical analysis that accorded well with the numerical results.

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 finite-element model and experimental investigation of the influence of pre-straining of wire on the sensitivity of binary crack sensors

Steel girder bridges make up a significant percentage of all bridges. Many of the steel girder bridges are aging and approaching their designed service lives. Crack formation and its propagation over time is one of the main deficiencies of aging steel girders and may result in unusable or unsafe service conditions. The existing distributed crack detection methods such as fiber optics sensors are costly to deploy and maintain. A new cost effective binary sensor has been developed, has the potential to be installed on steel girder bridges at a fraction of the cost, and is sensitive enough to detect the presence of a crack opening with a width of 0.2 mm. The crack sensor is a closed electrical circuit and comprised of copper wire and epoxy. When a crack forms in the steel girder, the strain will be transferred to the wire through the adhesive. As the crack on the girder widens over time, strains in the wire increase until it reaches its ultimate tensile strain. The wire then fractures and creates an open circuit. This can be detected by monitoring the electrical continuity of the sensor. One of the main challenges in developing the binary sensor is to select appropriate materials for both wire and adhesive. The final tensile strain as well as the bonding stiffness between the wire and the epoxy have important impacts on the performance of the sensor. In this work, pre-straining the sensor wire was found to be effective in minimizing the width of the detected crack opening. The average was reduced from 0.36 to 0.13 mm and the standard deviation reduced from 0.16 to 0.03. In addition, microbond test was carried on to estimate the interfacial-bonding stiffness between wire and epoxy. The interfacial stress was found to be approximately 2.0 MPa. These parameters were used in a finite-element model of the sensor to predict the behaviour of the binary sensor and, consequently, to optimize the installation position of the sensor on a girder of a bridge.

Enhanced elevated-temperature mechanical properties of Al-Mn-Mg containing TiC nano-particles by pre-strain and concurrent precipitation

The influence of pre-strain on the tension and creep behavior of Al-Mn-Mg containing nano-sized TiC particles at elevated temperatures was investigated. Introduction of TiC nano-particles and the application of pre-strain significantly increase the strength at the ambient temperature and at 473 K, even though the strengthening effect diminishes at 573 K due to the more activated dislocation climb. Small pre-strain (von Mises strain of 0.2) could increase the yield strength at 573 K from 72 MPa to 96 MPa. Larger pre-strain promotes the precipitation during hot deformation at 573 K, and concurrent precipitation suppresses work softening by the pinning effect, leading to a significantly enhanced ductility and creep rupture life.

Influence of pre-strain on the gaseous hydrogen embrittlement resistance of a high-entropy alloy

The effect of pre-strain on the resistance to gaseous hydrogen embrittlement of CoCrFeMnNi high-entropy alloy (HEA) was investigated through mechanical testing and thermal desorption analysis. The results reveal that prior plastic deformation does not affect either the hydrogen contents or the excellent hydrogen embrittlement resistance of the HEA.

High temperature grain shrinkage under different pre-strains: a phase-field-crystal study

In this work, we use the phase-field-crystal method to study high temperature grain shrinkage. A circular grain embedded in a symmetric tilt planar grain boundary (GB) is constructed as the simulation system. Misorientation angle of the circular GB has influence on the specific evolution process. Difference between low and high misorientation angle systems is explored. In low misorientation angle system, grain shrinkage is first enabled by dislocation migration. Then dislocation rearrangement process in trijunction areas triggers the further shrinkage of inner grain. The free energy density (FED) curve has a rising stage during the overall decline process. For high misorientation angle system, dissociation and recombination reaction of dislocations is the primary way to shrink inner grain. The FED curve monotonically declines. Additionally, we apply pre-strain to simulation system. The influence of pre-strain on grain shrinkage in low and high misorientation angle systems is also investigated. When pre-strain is relatively small, the evolution process has no difference with unstrained situation, but grain shrinkage is impeded. Further increasing pre-strain, dislocations are emitted from circular GB. Grain shrinkage is accelerated and the inner grain eventually disappears prior to the grain disappearance in unstrained system. There exists a critical pre-strain to control the emission of dislocations.

Effect of the amount and temperature of prestrain on tensile and low-cycle fatigue properties of Fe-17Mn-0.5C TRIP/TWIP steel

A high-Mn austenitic steel represents excellent combination of tensile strength and ductility, but shows very low yield strength. To increase the yield strength, pre-deformation is applied before use. However, the other properties such as fatigue resistance should also remain high. In this study the influence of pre-strain on tensile and low cycle fatigue (LCF) resistance were investigated. The amount of pre-strain (ε=0, 0.2 and 0.4) and pre-straining temperature (203K∼490K) were varied, which resulted in the variation of deformation-induced twin boundaries and fraction of martensite. Tensile tests and fully-reversed strain-controlled fatigue tests were conducted on plate-type samples. As the amount of pre-strain increased, yield and tensile strength were increased, but ductility and LCF life were decreased. The LCF properties of samples at a fixed amount of pre-strain were analyzed in the context of the fractions of martensite and mechanical twin boundaries. The presence of a small amount of ε-martensite increased yield stress and fatigue resistance by fostering deflected-and-branched propagation of fatigue cracks, whereas a high fraction of ε-martensite degraded the mechanical properties. In contrast, the presence of a high fraction of mechanical twin boundaries enhanced both tensile and fatigue properties under the conditions used in this work. The sizes of dislocation cells decreased and spacing of fatigue striations narrowed as the fraction of mechanical twin boundaries increased. Results suggest that a microstructure composed of ~20% ε-martensite and numerous mechanical twin boundaries can represent superior tensile and fatigue properties; this microstructure was obtained by step pre-straining (SP) process, where pre-strain was imposed twice at different temperatures.

Influence of Plastic Pre-Strain on Low-Temperature Gas Carburization of 316L Austenitic Stainless Steel

In this paper, the influence of pre-strain on low-temperature gas carburization of 316L austenitic stainless steel was investigated. A group of flat specimens were uniaxial tensile to several levels of pre-strain including 5%, 10%, 15%, 20% and 25% engineering strain. Then, the pre-strained specimens was treated by low-temperature gas carburization at 470 °C for 30 h. In order to elucidate the effect of pre-strain on low-temperature gas carburization, optical microscopy (OM), X-ray diffractometer (XRD), scanning electron probe micro-analyzer (EPMA), microhardness tester and residual stress analyzer were used. Meanwhile, dislocation density of the pre-strained specimens was semi-quantitatively measured by means of X-ray diffraction analysis and the role of dislocation density on carbon diffusion during low-temperature gas carburization was discussed. The results show as follow: (1) the thicknesses of the carburized layers are independent of the pre-strain degree. (2) dislocation density increases with the increasing pre-strain, but almost has no effect on carbon diffusion at the given carburizing temperature. (3) an outstanding surface with hardness (≈ 1150 HV0.1) and compressive residual stress (≈1900 MPa) is introduced by low-temperature gas carburization, and the strengthening results of carburization are unaffected by pre-strain.

Comparison of Mechanical Properties Between PE80 and PE100 Pipe Materials

Mechanical properties, including yield stress, relaxation behavior, moduli (elastic modulus at the strain of 0.5% and strain hardening modulus at strains above 70%), viscous stress, and quasi-static stress, are compared between polyethylene (PE) pipes that are made of PE80 and PE100 resins. The mechanical properties are measured using D-split tensile test on modified notched pipe ring specimens. The comparison includes the influence of strain rate (by the change of crosshead speed) on the yield strength and influence of pre-strain on the relaxation behavior and the modulus values. A two-stage approach is used to characterize the influence of pre-strain on the moduli, to ensure that viscous recovery from the first-stage of the test, to introduce the pre-strain, does not affect the modulus measurement from the second-stage test. The results show that elastic modulus, yield stress, strain hardening modulus, viscous stress, and quasi-static stress for PE100 are higher than those for PE80, but PE80 shows higher resistance to stress relaxation. The results also show that with the increase in the pre-strain level, the elastic modulus drops but the strain hardening modulus remains relatively constant.

Influence of pre-strain on the aging hardening effect of the Mg-9.02Zn-1.68Y alloy

The solid-solution state Mg-7.68Zn-1.68Y alloy was pre-compressed before the isothermal aging treatment at 200°C, then the pre-induced twinning and its influence on the aging microstructure and aging hardening effect were investigated using EBSD, TEM and SEM. Results showed that: about 15% (area fraction) lenticular (101¯2) tensile twinning were induced into the matrix when the alloy was compressed by 3% pre-strain; and when the amount of pre-strain increased, area fraction of twinning would increase, average size of twinning would decrease, morphology and structure of twinning would change: when the pre-strain reached 15%, about 28% (area fraction) of narrow double twinning (101¯1)-(101¯2)would be induced, while the anamorphosis of double twinning were mostly inter-crossing. In addition, twinning could become an effective substrate for nucleation. And different from precipitates in the matrix, which is rod-shaped β′1-MgZn2 growing along a certain direction, precipitates in the twin boundaries were granular icosahedral quasi-crystal phase with 5-times rotational symmetric diffraction pattern. And when the pre-strain was in the range of 3–15%, aging hardening effect increased with pre-strain; when pre-strain were 10–15%, the peak hardness arrived 110HV, 28% improved compared to that of the not-pre-deformed one (86HV); when the pre-strain exceeded 20%, the aging hardening effect would decrease.

Influence of Pre-Strain and Simulated Paint-Bake on Mechanical Properties of High Strength Aluminum Alloy AA7020

In modern car bodies lightweight structures are used to achieve a reduction of energy consumption and CO2-emissions. One of the most important lightweight materials is aluminum and its alloys. Current state of the art in automotive industry is utilizing precipitation hardenable aluminum alloys of the 6000 series whereas the crash-relevant strength is achieved by artificial ageing during the paint drying process. Due to newly developed coatings which provide faster curing at lower temperatures, post-forming ageing of the 6xxx alloys to satisfactory strength levels becomes more difficult. The aim of this study is to investigate the feasibility of employing high strength aluminum alloys of the 7000 series in order to reduce artificial ageing time and temperature while keeping required strength. Within this contribution, the influence of pre-straining and subsequent heat treatment on mechanical properties will be presented.

Effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels

The present work investigated the effect of cathodic hydrogen-charging current density on mechanical properties of prestrained high strength steels. This was done by tensile tests on both hydrogen-charged and -uncharged prestrained specimens at a cross-head displacement speed of 0.03mm/min. The influence of prestrain on hydrogen behavior was also studied using an electrochemical permeation technique. The results show that the relationship between ultimate tensile strength of the hydrogen-charged specimens (UTS-H) and prestrain depends on current density. The UTS-H decreases with increasing current density independent on prestrain. With an increase in prestrain the diffusion coefficient of hydrogen gradually decreases, which is attributed to increasing dislocations density acted as hydrogen traps. SEM fractograph reveals that hydrogen charging causes a change from ductile to brittle failure.

Prestrain-dependent viscosity of a highly filled elastomer: experiments and modeling

Highly filled elastomers exhibit a complex microstructure made up of rigid fillers bounded by a thin layer polymeric matrix. The interactions between the fillers and the binder amplify locally the applied strains and induce a nonlinear viscoelastic behavior. The aim here is to analyze the influence of prestrain on the viscoelastic behavior. This paper proposes a prestrain-dependent viscoelastic constitutive model. The model is a superposition of three relaxation spectra, each corresponding to a family of polymer chains, and can be regarded in either its continuous or discrete expression. More specifically, one of these relaxation spectra is modified to assure the prestrain sensitivity. The parameters of the discrete model are identified from relaxation and DMA experiments performed on a solid propellant, and the obtained predictions match closely the experiments. The novelty of the analysis proposed in this paper is threefold. On the one hand, we report a new series of experimental measures, performed for a large range of frequencies for the DMA experiment and relaxation times for the relaxation experiment, and, on the other hand, we propose a constitutive law compatible with the principles of thermodynamics, which predicts closely the measurements. Finally, the analysis is performed comparing both relaxation and DMA experiments using the spectrum of relaxation times. A peculiarity of the present discussion is the novel identification method used, which identifies directly the relaxation times. This technique leads to models with smaller and optimum numbers of parameters than classical methods based on a logarithmic distribution of relaxation times.

Influence of Pre-strain on the Mechanical Properties of A6111-T4P Sheet with Bake Hardening

A6111 is an aluminum alloy, which exhibits good formability and excellent bake hardening property. This study aimed to reveal the influence of strain path, pre-strain orientation (α) as well as pre-strain level on the mechanical properties of A6111-T4P sheet under bake treatment through uniaxial tension test. (0–5)% pre-strain, 150–170 °C bake temperature and 20–30 min bake time were considered in the study by referring to the actual production process. The results show that both pre-strain level and strain path play significant roles in improving the material properties. In the condition that tensile orientation (β) parallel with pre-strain orientation (β = α), the yield strength can be remarkably improved, and much higher parameter of n in Hockett–Sherby model can be obtained than those when tensile orientation non-parallel with pre-strain orientation (α ≠ β). In addition, when the pre-strain level, paint bake process were settled and β = α, the curves obtained in five tension orientations are similar in the plastic deformation stage.

Use of stress based forming limit diagram to predict formability in two-stage forming of tailor welded blanks

In the present work, laboratory scale two-stage stretch forming processes were designed and fabricated where an interstitial free (IF) steel was first pre-strained in biaxial tensile mode, and subsequently it was deformed in a limiting dome height (LDH) test set up in tension–tension and tension–compression modes. Tailor welded blanks (TWB) of dissimilar materials were fabricated by laser welding of dual phase (DP) and IF steels, and these TWBs were subjected to the above two-stage stretch forming processes to characterize the influence of pre-strain, strain path and weld line orientation on formability. A mathematical framework was developed to estimate stress based forming limit diagram (σ-FLD) from strain based forming limit diagram (ε-FLD) using Hill-48 and Barlat-89 anisotropic plasticity theories incorporating three different strain hardening laws. The forming behaviour of pre-strained IF steel material and TWBs was predicted by finite element analysis (FEA) implementing both the ε-FLD and σ-FLD. It was found that the LDH was over-predicted by ε-FLD compared to σ-FLD in case of bi-axial tensile pre-strained IF steel specimens. However, the ε-FLD under-predicted the LDH of circular pre-strained TWBs and similar LDH was predicted by both the FLDs in case of pre-strained longitudinal TWBs. It was observed that the σ-FLD was more robust compared to the ε-FLD, and it was capable to predict formability of TWBs in multi-stage forming processes taking care of deformation histories. Lastly, the predicted thickness distribution, maximum thinning location and load progression curve during the second stage LDH test were validated with experimental results.