Prestrain Direction Research Articles

Effect of pre-straining direction on the microstructure, mechanical properties and microstructural response of AA2099-T83 Al-Li alloy

Understanding the microstructural evolution and anisotropic behavior of Al-Li alloys after pre-deformation in different directions is crucial for optimizing their forming performance. This study investigates the microstructural evolution in AA2099 Al-Li alloy rolled sheets after pre-deformation in the extrusion direction (ED) and transverse direction (TD), and the resulting changes in mechanical properties during subsequent tensile deformation. A crystal plasticity finite element model (CPFEM) was established to quantitatively analyze the link between pre-deformation direction and the microstructural response during the hardening stages of tensile deformation. The results showed that the differences in mechanical properties due to pre-deformation direction are mainly attributed to the variations in volume fraction and diameter of the T1 precipitates. The contributions of T1 phase to yield strength are estimated to be 239 MPa (ED) and 223 MPa (TD), respectively. An increase in T1 (Al2CuLi) phase volume fraction and a decrease in phase diameter lead to higher average von Mises stress and average maximum principal strain values during the hardening stages.

Influence of changing loading directions on damage in sheet metal forming

The impact of the stress state on damage evolution, fracture behavior, and product performance is well understood for proportional loading. However, many complex sheet forming operations involve non-proportional loading, which affect the material's hardening and fracture characteristics. This study investigates the influence of a loading direction change on damage evolution in a dual phase steel DP800. Specimens are pre-strained by tensile tests and subsequently loaded in either the same or orthogonal direction to the initial pre-strain direction by additional tensile tests and bending tests. Damage quantification by scanning electron microscopy reveals lower damage evolution after an orthogonal change of loading direction in contrast to monotonic loading directions. The load paths, defined as a history of triaxiality and Lode parameter during loading, are identified numerically under consideration of kinematic hardening. Since kinematic hardening leads to higher triaxialities after orthogonal changes, the load path is not the dominant influence on damage. A possible explanation for the experimental results is the void characteristics after tensile load. After the pre-straining in tensile test, voids are oriented orthogonally to the tensile direction and located between hard martensitic phases. The influence of this morphology on subsequent void growth is illustrated by a simulation verifying that an orthogonal change of loading direction results in void shrinkage, while monotonic loading directions lead to further void growth.

On the determination of viscoelastic properties of EPP foam in dependence on pre-strain and loading direction

Understanding the viscoelastic properties of EPP foams is of crucial importance for their use in industrial applications. However, mechanical testing of polymeric foams is challenging due to the difficulties that arise from their morphology. For this reason, here currently available DMA methods using a single drive and a multi-drive rheometer as well as multiple loading directions are investigated in detail. Experimental challenges are highlighted and discussed with regard to foam-specific properties. Special focus is laid on the influence of compressive pre-strain. Based on the results, guidelines to perform reproducible DMA tests on the investigated type of material are derived. In addition, the results give deeper insight into the deformation behavior of bead foams at small strain levels and reveal an early onset of the transition from linear to plateau regime.

Influence of pre-strain on tensile response of extra deep drawing (EDD) steel under varying strain rates and crash performance

This study explores the impact of pre-strain on the uniaxial tensile stress-strain behavior of extra deep drawing (EDD) steel at varying strain rates. The EDD steel blanks were subjected to a uniaxial pre-strain of 15 % in either the transverse or rolling direction. Subsequently, uniaxial tensile tests were conducted at different strain rates, both orthogonal and parallel to the initial pre-straining direction. The study also investigates the alterations in yield stress (YS), ultimate tensile strength (UTS), uniform elongation (UEL), total elongation (TEL), and strain energy absorption (SED, represented by the area under the tensile stress-strain curve) at diverse strain rates after pre-straining. The experimental findings indicate that pre-straining is advantageous for enhancing YS, UTS, and SED. However, this improvement comes at the expense of UEL or TEL. A square crush tube, which shares similarities with automotive components, was examined to assess the crash performance of several automotive structural parts during crash events. A validated constitutive model was used to predict the square tube's energy absorption and crush performance under high-velocity conditions for different pre-straining scenarios.

Quantitative evaluation of fracture toughness deterioration due to Pre-strain

Previous studies presented the fracture toughness evaluation values decrease due to pre-strain by local compression. However, it was not possible to distinguish whether the cause was fracture toughness deterioration or poor crack front shape straightness due to a bowing crack front at low strain. The authors applied pre-strain without affecting the crack front shape and conducted fracture toughness tests showing that 1% pre-strain reduces fracture toughness by 50%. Furthermore, we investigated the effect of the pre-strain direction on fracture toughness, and proposed a method for converting longitudinal pre-strain to thickness direction strain and demonstrated its effectiveness.

Measurement of two-dimensional residual stress in nanocrystalline superelastic NiTi fabricated with pre-strain laser shock peening

The measurement of residual stress is a challenging issue in industrial fields. In this work, focused ion beam (FIB) and digital image correlation (DIC) are combined together to measure the two-dimensional residual stress in nanocrystalline NiTi plates processed with pre-strain laser shock peening (LSP). A four-point bending experiment verifies the accuracy of this measurement method. The pre-strain LSP-treated surfaces are found to have significant compressive residual stress along the pre-strain direction and tensile residual stress along the vertical to pre-strain direction, which verifies pre-strain LSP as an efficient approach to create gradient residual stress layers in nanocrystalline NiTi plates. The FIB-DIC method has also proven to be an attractive tool to measure the two-dimensional residual stress in phase transition nanocrystalline materials.

Mechanisms controlling ductility loss from abrupt Strain Path Changes in a low carbon steel

The effect of Strain Path Changes (SPCs) on the mechanical properties and crystal-level features of deformation for a single phase, ferritic steel has been investigated. SPCs were applied via a two-step deformation process, which included pre-straining via cold rolling, followed by uniaxial tension. The pre-strain magnitude and direction, as well as the tensile direction, varied between the specimens. The role of texture and micromechanics were examined in-situ, via Synchrotron X-ray Diffraction (SXRD), and ex-situ, via Electron Backscatter Diffraction (EBSD). Abrupt strain paths (i.e. strain paths where the pre-strain and the subsequent loading directions differ; here they are orthogonal) result in a significant ductility reduction, becoming more prevalent for high pre-strain magnitudes. The macroscopic response, as well as the texture configuration were greatly dependent on the pre-strain direction but were insensitive to the direction of uniaxial tension. Increasing pre-strain magnitudes resulted in a stagnation of lattice strain hardening rates in all macroscopic directions and in a significant increase in the Geometrically Necessary Dislocation (GND) densities. This was vastly increased for specimens rolled perpendicular to the as-received prior rolling direction. No correlation was found between the GND density and the grain orientation, eliminating this as a controlling ductility factor for BCC ferrite. Instead, the initial texture and the texture developed in a subsequent pre-strain influences the density of dislocations accumulated in all grains, and ultimately determines ductility.

Investigation on Plastic Flow Behaviors of FCC Polycrystalline Aluminum under Pre-Cyclic Tension-Compression Loading: Experiments and Crystal Plasticity Modeling.

The plastic flow behaviors of FCC polycrystalline aluminum after pre-cyclic tension-compression deformation are mainly investigated in tension–torsion stress space by the physically based crystal plasticity model introducing a back-stress. A global finite element model (GFEM) constructed of sufficient grains was established to simulate the same-size thin-walled tube specimen constrained and loaded as the experiments of yield surfaces. The computational results showed that the shape of subsequent yield surfaces and the plastic flow directions directly depended on the given offset strain levels and the applied re-loading paths under different pre-cyclic deformations. The angle deviation between the plastic flow direction and the theoretical orthogonal direction further indicated that there was a large difference between them in the inverse pre-straining direction, but the difference was negligible in the pre-straining direction. From the influence of the anisotropic evolution of the subsequent yield surfaces on plastic flow, we found that the plastic normality rule followed the smooth yield locus; conversely, the significant non-associated flow was attributed to the distorted yield locus. Furthermore, it was also demonstrated that the anisotropic evolution and the plastic flow trend of the subsequent yield surfaces obtained by experiments can be better reproduced by the crystal plasticity model.

Mechanisms Controlling Ductility Loss from Abrupt Strain Path Changes in a Low Carbon Steel

The effect of Strain Path Changes (SPCs) on the mechanical properties and crystal-level features of deformation for a single phase, ferritic steel has been investigated. SPCs were applied via a two-step deformation process, which included pre-straining via cold rolling, followed by uniaxial tension. The pre-strain magnitude and direction, as well as the tensile direction, varied between the specimens. The role of texture and micromechanics were examined in-situ, via Synchrotron X-Ray Diffraction (SXRD), and ex-situ, via Electron Backscatter Diffraction (EBSD). Abrupt strain paths (i.e. strain paths where the pre-strain and the subsequent loading directions differ; here they are orthogonal) result in a significant ductility reduction, becoming more prevalent for high pre-strain magnitudes. The macroscopic response, as well as the texture configuration were greatly dependent on the pre-strain direction but were insensitive to the direction of uniaxial tension. Increasing pre-strain magnitudes resulted in a stagnation of lattice strain hardening rates in all macroscopic directions and in a significant increase in the Geometrically Necessary Dislocation (GND) densities. This was vastly increased for specimens rolled perpendicular to the as-received prior rolling direction. No correlation was found between the GND density and the grain orientation, eliminating this as a controlling ductility factor for BCC ferrite. Instead, the initial texture, the texture developed in a subsequent pre-strain influences the density of dislocations accumulated in all grains, and ultimately determines ductility.

Effects of strain paths on the fracture forming limit of high strength structural steel sheets

The cruciform two-axial tensile specimen was designed and the two-axial tensile test scheme for different strain paths was developed. The effect of strain paths on the forming limit of high strength structural steel plates was obtained by the DIC method. The results show that for the unidirectional alternating loading test scheme, the equivalent strain of the ultimate strain is less than the sum of the equivalent strains from each step. When the main strain direction of the specimen pre-strain is perpendicular to the final strain path, the forming limit decreases with the increase of the pre-strain.

Notch fatigue performance of DP600 steel under different pre-straining paths

This paper presents the influence of different pre-straining paths on the notch fatigue performance of Dual Phase 600 steel. Initially, pre-straining is done in rolling (or transverse) direction. Then tensile and notch fatigue specimens are fabricated in parallel to the initial pre-straining direction i.e. in rolling (or transverse) direction. In another case, the tensile and notch fatigue specimens are fabricated in orthogonal to the initial pre-straining direction i.e. initial pre-straining in rolling (or transverse) direction, and followed by testing in transverse (or rolling) direction. Tensile and notch fatigue performance has been evaluated for the as-rolled and uniaxial pre-strained specimens having different pre-straining paths. A huge reduction in notch fatigue life is noticed as compared to fatigue performance on smooth samples due to the existence of stress concentration at the notch root. Pre-straining has no significant effect on notch fatigue performance, while a considerable improvement of high cycle fatigue performance is observed for smooth specimens. Finite element analysis suggests that stress concentration at the notch root increases and plastically deformed zone reduces with the pre-straining. At the same time, tensile yield stress and endurance limit rise with pre-straining for the smooth specimens. An increase in stress concentration due to pre-straining nullifies the enhancement of tensile yield stress and endurance limit for the notched specimen.

Low cycle fatigue performance of DP600 steel under various pre-straining paths

The present work concerns the assessment of low cycle fatigue performance of dual phase steel subjected to change in uniaxial pre-strain path. An uniaxial pre-strain of 12.5% in the rolling or transverse direction were imposed on the dual phase steel blanks, thereafter fully reversed strain controlled fatigue tests were conducted parallel or orthogonal to the initial pre-straining direction. As received specimens depict mild cyclic hardening in the initial cycles followed by gradual cyclic softening until failure. However, all pre-strained materials exhibit continuous cyclic softening with higher magnitude and rate throughout the life as compared to the as received specimen. Relaxation of mean stresses were observed for the fatigue tests with pre-straining. Rotation of principal axis due to orthogonal reloading (i.e. low cycle fatigue) after initial pre-straining introduces non-proportionality, thereby leading to significant reduction of fatigue lives. In-grain misorientation and lattice rotation analyses are noticed during electron backscattered diffraction investigation, and which is due to the non-proportional loading effect.

Influence of pre-straining path on high cycle fatigue performance of DP 600 steel

An evaluation of tensile and fatigue performances of dual phase steel subjected to change in the pre-strain path are presented in this work. Initially material is loaded up to a certain amount of pre-strain in the rolling or transverse direction, and after that specimen for tensile and fatigue tests are fabricated in the parallel or orthogonal to the pre-straining directions. Irrespective of pre-straining direction, all the pre-strained specimens show an increase in yield stress and simultaneous reduction in uniform elongation of the material, as well as improvement of high cycle fatigue life. The material exhibits better fatigue resistance for those specimens where fatigue testing is done orthogonal to the initial pre-straining direction.

Establishment of damage estimation rule for brittle fracture after cyclic plastic prestrain in steel

Material damage caused by complicated prestrain like cyclic prestrain and various direction prestrain. It is so important to understand the mechanism of material damage for high-precision lifetime evaluation of steel structure. Material damage depends on not only equivalent plastic strain but the order of pestrain loaded. Also, material damage can be expressed by effective damage strain from back stress uploading and total dislocation density calculated from conventional mechanism based on strain gradient plasticity (CMSGP). These ideas can be extended to multiaxial problems and material damage can be estimated by these parameters as well as uniaxial problems.

Influence of pre-strain direction and hydrogen injection for fracture behavior in aluminum alloy

Influence of pre-strain direction and hydrogen injection for fracture behavior in aluminum alloy

Formability analysis of pre-strained AA5754-O sheet metal using Yld96 plasticity theory: Role of amount and direction of uni-axial pre-strain

Automotive industries are very much interested in formability of different pre-strained aluminum alloy sheets in the context of multistage stamping to fabricate complex components. In the present work, different uni-axial pre-strains of 6.4% and 12.2% were induced in AA5754-O aluminum alloy both along rolling direction (RD) and transverse direction (TD). The true stress-strain response, limiting dome height (LDH) and strain based forming limit diagram (ε-FLD) of as received and all pre-strained materials were evaluated experimentally. The anisotropy constitutive material model was developed using the Yld96 plasticity theory in-conjunction with the Hollomon isotropic hardening law to predict the yield strength evolution of the pre-strained materials. Also, it was found that the limiting strains in ε-FLD shifted significantly depending on the amount and direction of uni-axial pre-strain. Hence, the limiting strains of the as-received materials were transposed into stress space to estimate the stress based forming limit diagram (σ-FLD) using the anisotropy constitutive material model. Further, the dynamic shifts of ε-FLDs of four different pre-strained materials were predicted by successfully decoupling the σ-FLD of as-received materials within root mean square error of 0.008. Finite element models of both uni-axial pre-straining and subsequent LDH tests were developed, and the forming behavior of the pre-strained materials were predicted implementing the Yld96 plasticity model and estimated σ-FLD. It was found that LDH was significantly influenced by the amount of pre-strain, and the maximum thinning location shifted close to pole in the case of 12.2% pre-strained materials. However, the effect of uni-axial pre-strain direction on both LDH and maximum thinning location in AA5754-O material was very negligible.

Irradiation-initiated plastic deformation in prestrained single-crystal copper

With large-scale molecular dynamics simulations, we investigate the response of elastically prestrained single-crystal Cu to irradiation as regards the effects of prestrain magnitude and direction, as well as PKA (primary knock-on atom) energy. Under uniaxial tension, irradiation induces such defects as Frenkel pairs, stacking faults, twins, dislocations, and voids. Given the high dislocation concentration, twins and quad-stacking faults form through overlapping of different stacking faults. Voids nucleate via liquid cavitation, and dislocations around void play a lesser role in the void nucleation and growth. Dislocation density increases with increasing prestrain and PKA energy. At a given prestrain, there exists a critical PKA energy for dislocation activation, which decreases with increasing prestrain and depends on crystallographic direction of the applied prestrain.

Controlled fabrication of hierarchically microstructured surfaces via surface wrinkling combined with template replication

In this paper, we present a simple method by combining surface wrinkling and template replication to create a series of hierarchical structures on polydimethylsiloxane (PDMS) elastomer. The primary stable lined patterns are formed by duplicating commercialized compact disk and digital versatile disk with PDMS. The secondary microscale patterns are from surface wrinkling, which is elicited by oxygen plasma (OP) treatment of the prestrained PDMS stamp followed with the prestrain release. By systematically varying the OP exposure duration, the prestrain, and the angle (θ) between the primary pattern orientation and the prestrain direction, we obtain highly ordered well-organized composite patterns from different patterning techniques and with different length scales and mechanical stabilities.

421 予ひずみ方向による低炭素鋼の時効硬化異方性(OS4-(5),OS4 オーガナイズドセッション《材料・組織と加工》)

421 予ひずみ方向による低炭素鋼の時効硬化異方性(OS4-(5),OS4 オーガナイズドセッション《材料・組織と加工》)

Dependence of compressive deformation on pre-strain and loading direction in an extruded magnesium alloy: Texture, twinning and de-twinning

This study was aimed at identifying compressive deformation behavior of an extruded AM30 magnesium alloy after different amounts of pre-strain along the extrusion direction (ED), with focus on the effect of re-loading direction, texture evolution, and deformation mechanisms. Compressive loading in both ED and transverse direction (TD) resulted in a similar sigmoidal true stress–true strain behavior due to the presence of two sets of basal textures with c-axes aligned nearly parallel to the plate normal direction which facilitated the occurrence of {101¯2} extension twinning. In the two-step ED–ED compression, the compressive yield strength (YS) linearly increased, while the ultimate compressive strength (UCS) and hardening capacity linearly decreased with increasing pre-strain. The disappearance of twin boundaries or the coalescence of twins via twin growth was observed, but the twin boundaries were more visible and the intensity of cumulative textures with c-axes rotating towards the anti-compression direction was lower, in comparison with the one-step continuous compression at an equivalent strain level. In the two-step ED–TD compression, both YS and UCS increased, and hardening capacity decreased with increasing pre-strain. After a 4.0% pre-strain along the ED plus 4.0% re-compression along the TD, two seemingly opposite phenomena, i.e., the formation of new twins and de-twinning were observed to be co-existent due to the presence of multiple sets of textures after the first-step pre-straining. Both the reversal of textures and microstructural examinations corroborated the occurrence of de-twinning or twin narrowing/shortening as a result of the change of deformation path in the second-step compression.