Effect Of Prestrain Research Articles

Material pre-straining effects on fatigue behaviour of S355 structural steel

A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.

Study on displacement cascade and tensile simulation by molecular dynamics: Formation and properties of point defects

The molecular dynamics method is used to investigate the formation and properties of irradiation-induced damage (point defects). Displacement cascade simulations are performed to study the effects of primary knock-on atom (PKA) energy, temperature, vacancy concentration and tensile pre-strain on irradiation-induced damage in [Formula: see text]-Fe. An increase in PKA energy, vacancy concentration and tensile pre-strain can lead to an increase in defect numbers. In contrast, an increase in temperature decreases the defect numbers. After cascade collisions, tensile tests are performed to investigate the effect of point defects on mechanical properties. The yield stress and corresponding strain of irradiated Fe decrease with an increase in the number density of Frenkel pairs. Results show that irradiation accelerates damage of the internal structure, decreases the number of slip bands and increases the instability of the structure during plastic deformation.

Effects of Prestrain and Inner Stress on the Susceptibility of Hot Stamped Boron Steel to Hydrogen Embrittlement

Hot stamped boron steel parts with martensite and bainite combined microstructures are widely used in lightweight manufacturing of automotive bodies. The final products usually experience the manufacturing process, such as trimming or punching, which inevitably introduces inner stress and plastic strain. Hot stamped parts with complex microstructures and manufacturing effects exhibit some special characteristics of hydrogen embrittlement (HE) in service. In this paper, hardened boron steels with a total martensite content and different martensite-bainite combinations, called martensite or bainite dominant microstructures (MDM/BDM), are experimentally obtained by a flexible quenching tool, and the HE susceptibility considering the effects of prestrain and inner (tensile/compressive) stress is investigated based on slow strain rate tensile tests by developing hydrogen charging equipment under constant loads. SEM and EBSD analyses on hydrogen-inducted cracks (HICs) and thermal desorption analysis of hydrogen for different microstructures and strain/stress states are carried out to illustrate the HE mechanism. The HE susceptibility of the as-quenched specimen decreases with increasing volume fraction of bainite, with the fractograph exhibiting intergranular fracture in the MDM and the dimples feature in the BDM. Prestrain weakens the HE susceptibility in the MDM but raises it in the BDM due to the enhancement of hydrogen adsorption, promoting local plasticity in the evolution of the HICs. Inner stresses increase the HE susceptibility in the MDM, which lifts the hydrogen segregation at the grain boundaries to intensify the hydrogen-enhanced decohesion effect. Tensile stress intensifies the sensitivity to HE in the BDM. The promotion of hydrogen adsorption facilitates crack initiation. However, compressive stress has opposite effects on the HE susceptibility due to the improvement of the material ductility by reducing the hydrogen content in the specimen.

Effect of deformation parameters on microstructure of martensitic stainless steel during static recrystallization

In this paper, the static recrystallization behavior of X20Cr13 martensitic stainless steel was studied based on the hot compression test. The experimental results show that when the strain of the first pass is less than the critical strain of dynamic recrystallization, the main static softening mechanism is static recrystallization. The effects of prestrain, initial austenite grain size, deformation temperature and strain rate on the microstructure during static recrystallization were analyzed in detail.

Research on Light-Weighting Mechanism of Pressure Strengthening for Austenitic Stainless Steels Cryogenic Vessels

Abstract Pressure strengthening (PS) technology, also called cold stretching, is widely used to increase the allowable stress and reduce the weight of cryogenic pressure vessels (CPVs) made of austenitic stainless steels (ASSs). However, with taking accounts of the manufacturing process and service environment of CPVs, the main source of the strengthening effect for yielding strength improvement is still unclarified. Therefore, in this work, the effect of the 9% prestrain and low-temperature (−196 °C) on cryogenic mechanical properties of S30408 stainless steel was comparatively investigated by cryogenic tensile test and martensitic transformation measurement. After evaluating the main source of strengthening increments, the effect of prestraining on welded joints was studied by Vickers microhardness test. Then the engineering significance of performing the PS on a cryogenic inner vessel was explored based on finite element analysis (FEA). It revealed that at −196 °C, the strengthening effect of low-temperature plays a leading role for strength increments, though the yield strength can be increased obviously by prestraining at room temperatures. Meanwhile, the performing of 9% prestrain could enhance the distribution uniformity of the Vickers microhardness of welded joints. Furthermore, the prior operation of PS on inner vessels is beneficial to prevent further plastic deformation during the subsequent hydraulic test, avoid thermal insulation degradation of the vacuum jackets and even stress concentrations in roots of pipes, which is also of engineering significance to smooth the subsequent processes such as the assembly with outer containers and other product tests.

Influence of plastic prestraining on the fatigue crack propagation rate of S355MC and S460MC structural steels

AbstractCold forming induces significant plastic deformation at corner locations of structural steel members, resulting in both material strain hardening and the development of residual stresses, which influence the fatigue resistance of the steel members. Reliable fatigue assessment at such critical locations requires knowledge of the influence of pre‐existing plastic deformation on material behavior. The effect of plastic prestraining on the fatigue crack growth resistance of S355MC and S460MC structural steels has been evaluated experimentally. A comparative study, including fatigue crack growth and fracture resistance tests for both reference (non‐prestrained) and prestrained materials, has been performed. The experimental results, which are correlated to examined microstructural features and fracture characteristics of the materials revealed that fatigue crack growth rates of the prestrained material are higher than the reference material in the low Δκ region. Also, the S460MC steel exhibits higher crack growth rates compared to S355MC steel within the examined Δκ range.

Analysis of the pre-straining effects on the limit strains of interstitial-free steel: experiments and elasto-plastic modeling of strain and stress-based forming limit curves

In this work, the limit strains under as-received and pre-strained conditions of an interstitial-free steel sheet were investigated by mechanical tests and numerical modeling. An original testing procedure was proposed to reproduce two equivalent pre-straining amounts under equibiaxial stretching (EB-PS) and uniaxial tension (UT-PS) deformation modes. The limit strains were defined from Nakajima’s hemispherical punch tests using digital image analysis according to the ISO 12004:2 standard. The plastic behavior of the interstitial-free steel sheet was evaluated by uniaxial tensile, disk compression, and hydraulic bulge tests. Moreover, an elasto-plastic framework based on the Marciniak–Kuczynski (M–K) model is proposed to forecast both the strain-based and stress-based forming limits. The parameters of the Yld-2000-2d yield criterion were identified using the plastic anisotropy coefficients with the flow-stress values determined from two distinct amounts of plastic-work per unit volume. For both deformation modes and pre-straining levels defined by the von Mises effective plastic-strain (4.8% and 9.0% EB-PS; 5.0% and 10% UT-PS), the transformed polar effective plastic-strain (PEPS) and forming limit stress (FLSC) results shown to be more strain-path insensitive than the conventional strain-based forming limit curve (FLC). The proposed elasto-plastic M–K model successfully forecasted the linear strain-path limit strains defined using the FLC, PEPS, and FLSC representations. The M–K model also provided quite conservative predictions for the pre-strained PEPS and FLSC. The Yld-2000-2d parameters identified using the flow-stress values defined from an upper-level of the plastic-work per unit volume proved to be the best choice in the present M–K modeling framework.

Effect of Pre-straining and Subsequent Annealing on Microstructure and Mechanical Property of Zr–Sn–Nb–Fe Zirconium Alloy

Zirconium alloy is extensively used in nuclear industries as cladding and core structural material. However, the formability and service behavior are affected due to the anisotropy of mechanical property, which is detrimental to the specific application. In this work, effect of pre-straining and subsequent annealing (PA) on microstructure and mechanical property of Zr–Sn–Nb–Fe zirconium alloy is determined by means of scanning electron microscope equipped with electron backscatters diffraction and digital image correlation technique. It is found that the basal texture intensity and grain size of as-received sample decreases after pre-straining, the work-hardening value (n) increases from 0.126 of as-received sample to 0.152 of PA samples, while the anisotropy of mechanical property decreases gradually with the increase of pre-straining level. In addition, the strength, ductility and work-hardening value (n) of the PA samples tend to be accordant in the transverse direction, rolling direction (RD) and the 45° direction when the pre-straining level reaches 8%. The basal texture is gradually weakened and dispersed along the RD after annealing, whose orientation is favorable for the activation of pyramidal 〈c + a〉 slip to accommodate to the strain in thickness direction, which leads to the lower anisotropy (featured by the r-value). The low r-value and high work-hardening value (n) are favorable for improving formability. Hence, microstructure and texture of zirconium alloy can be modified though appropriate PA process to improve the comprehensive properties.

Effect of pre-strain on uniform formability of metal foils in multi-pass microforming process assisted by resistance heating

Effect of pre-strain on uniform formability of metal foils in multi-pass microforming process assisted by resistance heating

On the interplay of friction and stress relaxation to improve stretch-flangeability of dual phase (DP600) steel

Industrial servo presses have been used to successfully demonstrate improved formability when deforming sheet metals. While the time dependent viscoplastic behavior of material is attributed to the observed formability improvement, much less effort has been devoted to understand and quantify the underlying mechanisms. In this context, the hole expansion test (HET) of a dual phase steel was interrupted at pre-defined punch travel heights to understand the time-dependent effects on stretch-flangeability. The effect of pre-strain, hold time and edge quality on hole expansion ratio (HER) improvement was studied. The present study shows that the HER improves significantly in interrupted HET. This improved HER is due to the combined effects of stress relaxation and friction on deformation behavior. The ductility improvement estimated from uniaxial stress relaxation tests was used to estimate the contribution of stress relaxation and friction, respectively, in HET. This study shows that friction plays a significant role in improving HER at high pre-strain. It was also demonstrated that frictional effects are largely influenced by edge quality.

On the effect of plastic pre-straining on the corrosion behaviour of a duplex stainless steel and how the emergence of slip steps affects the hydrogen evolution reaction kinetics

The effect of increasing pre-straining on the corrosion behaviour of UNS S32304 duplex stainless steel has been studied using potentiodynamic tests in a highly acidic chloride-rich solution. The emergence of slip steps in the austenitic phase has been particularly addressed showing that the hence modified topographies had a significant impact on the kinetics of the Hydrogen Evolution Reaction (HER) that, in turn, tuned a non-monotonic behaviour of the corrosion rate. An attempt is made to link the variation of the Electron Work Function (EWF) of the surface with the electrochemical results by quantifying slip steps using Atomic Force Microscopy.

Deviations in yield and ultimate tensile strength estimation with the Small Punch Test: Numerical analysis of pre-straining and Bauschinger effect influence

There is a growing interest on the Small Punch Test (SPT) and its many applications in a broad range of industries and sectors. The miniature volume of the SPT specimen eases the mechanical characterization of components and structures when standard tests are impracticable. But this miniature test is limited for one of its more constraining requirements: the material isotropy. Since this test subjects the material to triaxial stress and strain fields, isotropy is necessary to compare the resulted data from SPT with the standard tests, which generally show uniaxial stress fields. Another key element of this question is that initially, isotropic material can lose this property due to a cold pre-straining process and the existence of the Bauschinger effect. In this investigation, a numerical study with the finite element method was performed to understand the influence of pre-straining and the Bauschinger effect in yield strength and ultimate tensile strength estimation with the SPT. This study concludes that in the absence of isotropy, generated by a pre-straining, the SPT estimated a mean value of the principal yield strength components of the yield surface. It is also verified that presence of pre-straining in the SPT specimens invalidated their use to calculate the coefficients of the correlation equations for yield strength estimation.

Anisotropy of small punch creep performance of selective laser melted GH4169 at 650 °C

The large temperature gradient and the epitaxial growth of grains during selective laser melting process usually lead to the mechanical anisotropy of metal parts. To evaluate creep anisotropy of selective laser melting-fabricated GH4169 alloy, in this paper creep performance of GH4169 was investigated by means of small punch creep test method. The effect of the small punch loading-induced prestrain on creep properties of the specimens was examined. The results indicate that under the same heat-treatment and the loading conditions, the small punch creep specimens loaded along the building direction have the longer creep life and the larger creep plasticity than the ones loaded along the scanning direction. The finding reveals that loading deformation-induced dynamic recrystallization has a significant effect on improving creep properties by weakening anisotropy properly. The small punch creep properties of the specimens loaded along the building direction superior to that of the specimens loaded along the scanning direction are mainly attributed to the higher deformability of heterogeneous microstructure, the lower recrystallization activation energy and the deformation activation energy.

Effect of Prestrain on the Actuation Characteristics of Dielectric Elastomers

Dielectric elastomers (DEs) represent a class of electroactive polymers that deform due to electrostatic attraction between oppositely charged electrodes under a varying electric field. Over the last couple of decades, DEs have garnered considerable attention due to their much-coveted actuation properties. As far as the precise measurement systems are concerned, however, there is no standard instrument or interface to quantify various related parameters, e.g., actuation stress, strain, voltage and creeping etc. In this communication, we present an in-depth study of dielectric actuation behavior of dielectric rubbers by the state-of-the-art “Dresden Smart Rubber Analyzer” (DSRA), designed and developed in-house. The instrument allowed us to elucidate various factors that could influence the output efficiency of the DEs. Herein, several non-conventional DEs such as hydrogenated nitrile rubber, nitrile rubber with different acrylonitrile contents, were employed as an electro-active matrix. The effect of viscoelastic creeping on the prestrain, molecular architecture of the matrices, e.g., nitrile content of nitrile-butadiene rubber (NBR) etc., are also discussed in detail.

Effects of Prestrain on the Nonlinear Viscoelastic Properties of a Model Sealant

Abstract Elastomeric sealants are nonlinear viscoelastic materials that exhibit the Mullins effect. As a result, the behavior of a sealant depends on the previous history of strains the sample has seen. To deal with this issue, characterization tests like ASTM C1735, Standard Test Method for Measuring the Time Dependent Modulus of Sealants Using Stress Relaxation, subject the specimen to a specific strain history before measuring properties. To gain a better understanding of this history effect, this article studied a model sealant where the nonlinear viscoelastic properties were measured after a variety of strain histories ranging from no exposure to strain before the test to the history specified in ASTM C1735. The results provide insight into how strain history affects properties, supports information for the ASTM characterization test, and adds knowledge on the nature of the Mullins effect.

Abnormal Evolution of Pitting Behavior of Warmly Pre-Strained Austenitic Stainless Steels

The effect of warm pre-strain on the corrosion behavior of 304 and 316 steels in 3.5% NaCl solution was first explored by electrochemical tests, passive film characterization, and structural analysis. The pitting corrosion resistance of 304 steel decreases with the increase of strain level below 20% and increases with the strain at a higher strain level, while the pitting corrosion resistance of 316 steel increases slightly with the increase of strain level below 20% and decreases with the strain at a higher strain level. The random grain boundaries in 304 steel are interrupted with the increase of strain level higher than 20%; this does not occur in 316 steel, which exhibits more dislocations. The coupling effect of microstructure changes induced by warm pre-strain affects the oxide content of the passive film, especially for the chromium oxide, which results in abnormal corrosion behavior. Knowing the corrosion mechanism is of value for academic research and enables the design of advanced stainless steel with high strength and corrosion resistance.

Effect of Nb addition and Pre-strain on Hydrogen Embrittlement of Low-carbon Steels with Ferrite-pearlite Structure

The effect of pre-strain on the hydrogen embrittlement of Nb-free and Nb-added low-carbon steels with ferrite-pearlite structure was investigated in this study. After the steels were electrochemically charged with hydrogen, slow-strain rate tensile (SSRT) tests were conducted on them to examine hydrogen embrittlement behavior. The SSRT test results revealed that the Nb-added steel had a lesser decrease of elongation and reduction of area than the Nb-free steel. The formation of NbC carbide and grain refinement caused by the Nb addition improved resistance to hydrogen embrittlement. The loss of elongation and the reduction of area after hydrogen charging occurs when pre-strain is increased. The pre-strain increases dislocation density and thus increases the amount of reversible hydrogen trap sites associated with hydrogen embrittlement. 10% pre-strained specimens exhibited a significant loss in elongation and reduction of area, regardless of Nb addition. Based on the results of electron back-scatter diffraction, fractographic, and silver decoration analyses for Nb-free and Nb-added steels, the hydrogen embrittlement mechanism in low-carbon steels with different amounts of pre-strain is discussed in terms of dislocation density and hydrogen distribution.

Coupling Effect of Pre-Strain Combined with Isothermal Ageing on Mechanical Properties in a Multilayered Ti-10Mo-1Fe/3Fe Alloy

Coupling Effect of Pre-Strain Combined with Isothermal Ageing on Mechanical Properties in a Multilayered Ti-10Mo-1Fe/3Fe Alloy

The Effect of Pre-strain and Subsequent Electrically Assisted Annealing on the Mechanical Behaviors of Two Different Aluminum Alloys

The effects of pre-strain and subsequent electrically assisted annealing on the mechanical behaviors of two different (Al–Mg and Al–Si–Mg) aluminum alloys during electrically assisted dual stage forming are experimentally investigated. First, a specimen is deformed to a specific pre-strain by uniaxial tension and then automatically unloaded. After that, the pre-strained specimen is subjected to electrically assisted annealing by electric current with a fixed subsecond duration. Finally, the specimen is reloaded until fracture. Experimental results show that application of electric current with a subsecond duration induces electrically assisted annealing to both pre-strained aluminum alloys. The electric current also increases total achievable elongation until fracture during electrically assisted dual stage forming for both aluminum alloys. However, analysis of the stress–strain behavior during reloading and microstructural observations suggest that the quantitative effects of electric current on the post-electrically assisted annealing mechanical behavior and resultant microstructure are strongly dependent on the type of aluminum alloy. With the electric current density of 140 A/mm2, a full recrystallization followed by grain growth occurs in the pre-strained Al–Mg alloy specimens. For the pre-strained Al–Mg–Si alloy specimens, the electric current density of 140 A/mm2 induces both annealing and solid solutioning. Together, our findings indicate that while electrically assisted annealing is effective at improving the productivity of dual stage forming of an aluminum alloy, the composition of the aluminum alloy should be carefully considered in the design of forming process utilizing the concept of electrically assisted annealing since the beneficial effect was limited for the precipitation hardening 6061-T6 aluminum alloy.

Novel Finite Element Analysis of Curved Concrete Box Girders Using Hybrid Box Elements

Horizontally curved concrete bridges are widely used in urban viaducts and overpasses all over the world. A box cross-section is often used in curved concrete girders because of its high resistance to both bending and torsion. This study focuses on the development of a new finite element analysis (FEA) methodology incorporating a novel formulation for curved box sections using orthotropic constitutive models for reinforced concrete, along with a layered shell theory approach. In the new approach, the box section is treated as a frame consisting of curved shell elements modeling webs and flanges and curved beam elements in the web-flange junctions. The use of shell and beam elements in the formulation significantly reduces the number of elements needed to model the box-section girder while maintaining the accuracy of the model. A degenerate superparametric shell element with reduced integration is used to avoid shear-locking, membrane-locking, and zero-energy problems. Prestrain effects are considered in the formulation to account for prestressing forces. The simulation results are compared to the available experimental results on four straight and curved, reinforced and prestressed, concrete box-section girders, with good agreement in terms of the deflections, twist angles, and strains in the prestressed reinforcement. Some critical issues in the analysis of concrete box girders, such as postpeak-strength behaviors, distortion of box section, are also discussed.