Showing papers on "Necking published in 2018"

Forming limit diagrams by including the M–K model in finite element simulation considering the effect of bending:

Abstract: Forming limit diagram is often used as a criterion to predict necking initiation in sheet metal forming processes. In this study, the forming limit diagram was obtained through the inclusion of the...

On the mechanical response and microstructure evolution of NiCoCr single crystalline medium entropy alloys

Abstract: Unusual strain hardening response and ductility of NiCoCr equiatomic alloy were investigated through microstructural analysis of [111], [110] and [123] single crystals deformed under tension. Nano-twinning prevailed at, as early as, 4% strain along the [110] orientation, providing a steady work hardening, and thereby a significant ductility. While single slip dominated in the [123] orientation at the early stages of deformation, multiple slip and nanotwinning was prominent in the [111] orientation. Significant dislocation storage capability and resistance to necking due to nanotwinning provided unprecedented ductility to NiCoCr medium entropy alloys, making it superior than quinary variants, and conventional low and medium stacking fault energy steels.

In-situ investigation of the anisotropic mechanical properties of laser direct metal deposition Ti6Al4V alloy

Abstract: This study compares the microstructure and tensile properties of Ti6Al4V components fabricated by laser direct metal deposition (LDMD) additive manufacturing (AM) in the transverse and longitudinal directions. The results show anisotropic tensile properties with the transverse direction having high tensile and fracture strengths and the longitudinal direction having a high elongation and reduction of cross section. The anisotropic mechanical properties are attributed to the anisotropic microstructural distribution. The transverse tensile specimen is composed of short columnar prior-β grains which grow perpendicular to the tensile direction, and have a lamellar structure. Along the β grain boundary, αGB and large α colonies were identified. However, the longitudinal specimen shows that the long β structure is parallel to the tensile axis and that the microstructure is composed of basket-woven α phases with shorter α plates and smaller colony sizes compared with those in the transverse specimen. The fracture mechanism induced by the anisotropic microstructure along the transverse and longitudinal directions was compared by examining the fracture process in real-time using uniaxial in-situ scanning electron microscopy (SEM) tensile testing. The results show that shear fracture, which is caused by the vertical β grain boundaries and large α colonies with long α plates, occurs in the transverse specimen. The shear mode is the main reason behind the enhanced tensile strength and fracture strength due to the high resistance to microcrack propagation. However, in the longitudinal specimens, symmetric necking behavior due to the fine α grains resulted in uniform deformation of the grains on both sides of the grain boundaries, inducing greater elongation.

Hot tensile behavior of cold-rolled Inconel 718 alloy at 650 °C: The role of δ phase

Abstract: Cold-worked Inconel 718 alloy has successful applications in turbine engines due to its elevated strength. Otherwise, cold working will bring about the undesired ductility degradation accompanied with strength improving. Meanwhile, the susceptibility to δ precipitation is increased with modified morphology. To address these issues, this work aims expressing the effect of cold-rolling on hot ductility and the role of δ phase regarding differential morphologies in tensile behavior of Inconel 718 alloy at 650 °C. The results revealed that the ductility degradation can be attributed to the interactions of necking, microstructure instability and oxidation process tailored by cold-rolling. Nevertheless, appropriate cold-rolling is proved to enhance the resistance to crack propagation. The needlelike δ phase in strain-free structure tightly hindered the migration of horizontal GBs to enhance the strength but promote the intergranular brittle cracking, resulting in the degradation of ductility. In contrast, the δ phase with modified granular morphology in cold-rolled structures showed good deformation compatibility with horizontal GBs, playing a positive role in delaying necking occurrence and impeding crack propagation. In summary, the granular δ precipitation endowed cold-rolled Inconel 718 alloy with a superior combination of strength and ductility at 650 °C.

Deformation and fracture mechanisms in WE43 magnesium-rare earth alloy fabricated by direct-chill casting and rolling

Abstract: This paper examines deformation behavior of WE43 alloy in direct-chill as-cast (as-cast-WE43) and rolled heat-treated T6 (WE43-T6) conditions with an emphasis on fracture mechanisms. Unlike many Mg allows, as-cast-WE43 and WE43-T6 exhibit no tension/compression asymmetry in their yield stress. WE43-T6 material shows more anisotropy in yield stress than as-cast-WE43, which is attributed to their respected initial crystallographic textures. Both WE43-T6 and as-cast-WE43 exhibit some anisotropy in strain hardening due to texture evolution and deformation twinning. Both materials show a small elongation to fracture of approximately 6% in tension. In contrast, strain to fracture in compression is large. Crystallographic texture evolves substantially in compression, where crystals are slowly reorienting their crystallographic c -axis parallel to the loading direction with plastic strain. Both materials fracture by a typical shear fracture in compression. Fractographic analysis of fractured surfaces in compression for WE43-T6 reveals evidence of transgranular facets that are much larger than grain size with minor content of microvoid coalescence. Although elongation to fracture in tension is small with no necking, detailed analysis of fracture surfaces reveals evidence of ductile microvoid coalescence. However, the intergranular fracture character, especially in the central high stress triaxiality region of the samples, limits the ductility of the material.

Breathable and Skin-Mountable Strain Sensor with Tunable Stretchability, Sensitivity, and Linearity via Surface Strain Delocalization for Versatile Skin Activities’ Recognition

Abstract: Metal film/elastomer-based strain sensors usually exhibit small rupture strain (<5%) because of the strain localization and necking effect of the metal film under tension. To achieve both high stretchability and wide linear region is still challenging for metal film-based strain sensors. Here, we propose a low-cost yet effective strategy for fabricating ultrathin, breathable, and skin-mountable strain sensors with high sensitivity (gauge factor from 7.2 to 474.8), high stretchability (up to 140%), and good linearity by regulating the surface strain delocalization in the metal film on elastomer substrate. On the basis of this phenomenon of strain delocalization, the sensitivity and linearity are further enhanced based on a novel diffraction-induced Au film with gradient thickness. Meanwhile, by means of the strain redistribution and Poisson effect, a novel biaxial strain sensor is designed for recognition of complex human motion. On the basis of the enhanced stretchability, linearity, skin-mountable, and breathable properties, the low-cost metal film-based strain sensors can be broadened as disposable wearables for human motion detection, emotional expression recognition, human interaction, and virtual reality.

Investigation on microstructure and mechanical properties of Al/Al-Zn-Mg–Cu laminated composite fabricated by accumulative roll bonding (ARB) process

Abstract: In this study, an Al/Al-Zn-Mg-Cu laminated composite was successfully produced for the first time by using Al 1050 and a super high strength 7xxx aluminum alloy sheets through accumulative roll bonding (ARB) process at an elevated temperature. In this process after sandwich preparation, in per cycle applied 50% reduction in total thickness. The microstructural investigations after different ARB cycles showed that ARB process led to a fine distribution of the secondary phases and also a relatively remarkable decrease in their particle size in the Al-Zn-Mg-Cu (AZMC) layer. Furthermore, it was observed that all the layers of the ARBed sheets remained relatively straight without any necking or fracture during the plastic deformation in the first two cycles. Also, the macrostructure showed elongated and ultra-fine grains after the fourth cycle in the AZMC and pure Al (PA) layers, respectively. In addition, computer simulation demonstrated strain and stress gradients in the ARBed sheet. The mechanical examinations displayed a significant improvement of elongation and ultimate tensile strength (UTS) values and also microhardness values of the AZMC layer with increasing the ARB cycles. However, the microhardness variations were so slight (

Investigation of short-term creep deformation mechanisms in MarBN steel at elevated temperatures

Abstract: This paper reports the short-term creep behavior at elevated temperatures of a MarBN steel variant. Creep tests were performed at 3 different temperatures (625oC, 650oC and 675oC) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 hours. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ.mol-1, suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.) and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.

Ductility and formability of three high-Mn TWIP steels in quasi-static and high-speed tensile and Erichsen tests

Abstract: The ductility and formability properties of three high-Mn TWIP steels were investigated under quasi-static and high-speed deformation conditions. The ductility was evaluated from conventional and Hopkinson split-bar tensile tests at 1250 s−1 and the stretch formability was evaluated using Erichsen tests made with a special high-speed electro-hydraulic forming machine at about 1000 s−1. The data were related to microstructural features revealed using electron backscatter diffraction and X-ray diffraction. Furthermore, the stacking fault energy (SFE) was estimated using a thermodynamic approach. It was found that the 0.6C-22Mn and 0.2C-21Mn-0.23N steels (compositions in wt%) with SFEs of 23–24 mJ/m2 exhibited good elongation and a large Erichsen index at both low and high strain rates. These were attributed to intensive mechanical twinning though partly replaced by dislocation slip in deformation bands in the high-speed tests. However, it was noticed that the high-speed stretching failure of these TWIP steels occurred in the uniform elongation range without diffuse necking. In the austenitic - ferritic 21Mn-3Al-3Si steel strain-induced martensite was formed, but the ferrite phase seemed to impair formability.

Identification of Post-necking Tensile Stress–Strain Behavior of Steel Sheet: An Experimental Investigation Using Digital Image Correlation Technique

Abstract: The stress–strain behavior of sheet metal is commonly evaluated by tensile test. However, the true stress–strain curve is restricted up to uniform elongation of the material. Usually, after the uniform elongation of the material the true stress–strain is obtained by extrapolation. The present work demonstrates a procedure to find out the true tensile stress–strain curve of the steel sheet after necking using digital image correlation (DIC) technique. Hill’s normal anisotropic yield criteria and local strains measured by DIC technique are used to correct the local stress and strain states at the diffuse necked area. The proposed procedure is shown to successfully determine the true tensile stress–strain curve of ferritic and dual-phase steel sheets after necking/uniform elongation.

Plastic deformation and failure mechanisms in nano-scale notched metallic glass specimens under tensile loading

Abstract: In this work, numerical simulations using molecular dynamics and non-local plasticity based finite element analysis are carried out on tensile loading of nano-scale double edge notched metallic glass specimens. The effect of acuteness of notches as well as the metallic glass chemical composition or internal material length scale on the plastic deformation response of the specimens are studied. Both MD and FE simulations, in spite of the fundamental differences in their nature, indicate near-identical deformation features. Results show two distinct transitions in the notch tip deformation behavior as the acuity is increased, first from single shear band dominant plastic flow localization to ligament necking, and then to double shear banding in notches that are very sharp. Specimens with moderately blunt notches and composition showing wider shear bands or higher material length scale characterizing the interaction stress associated with flow defects display profuse plastic deformation and failure by ligament necking. These results are rationalized from the role of the interaction stress and development of the notch root plastic zones. (C) 2017 Elsevier Ltd. All rights reserved.

In Situ Local Measurement of Austenite Mechanical Stability and Transformation Behavior in Third-Generation Advanced High-Strength Steels

Abstract: Austenite mechanical stability, i.e., retained austenite volume fraction (RAVF) variation with strain, and transformation behavior were investigated for two third-generation advanced high-strength steels (3GAHSS) under quasi-static uniaxial tension: a 1200 grade, two-phase medium Mn (10 wt pct) TRIP steel, and a 980 grade, three-phase TRIP steel produced with a quenching and partitioning heat treatment. The medium Mn (10 wt pct) TRIP steel deforms inhomogeneously via propagative instabilities (Luders and Portevin Le Châtelier-like bands), while the 980 grade TRIP steel deforms homogenously up to necking. The dramatically different deformation behaviors of these steels required the development of a new in situ experimental technique that couples volumetric synchrotron X-ray diffraction measurement of RAVF with surface strain measurement using stereo digital image correlation over the beam impingement area. Measurement results with the new technique are compared to those from a more conventional approach wherein strains are measured over the entire gage region, while RAVF measurement is the same as that in the new technique. A determination is made as to the appropriateness of the different measurement techniques in measuring the transformation behaviors for steels with homogeneous and inhomogeneous deformation behaviors. Extension of the new in situ technique to the measurement of austenite transformation under different deformation modes and to higher strain rates is discussed.

The forming limit curve for multiphase advanced high strength steels based on crystal plasticity finite element modeling

Abstract: A computationally efficient rate independent crystal plasticity finite element (CPFE) model was used to predict the forming limit curve (FLC) for a multiphase advanced high strength steel (AHSS). The CPFE model accounts for mechanical properties of the steel phases based on their individual plastic deformation and slip systems. The macroscopic behavior of the polycrystalline aggregate was predicted based on the volume-averaged response of the representative phases, and their volume fraction in the steel sheet. In addition to the random texture distribution assumption for each grain, the volume fractions of various phases were also assumed to be randomly distributed at each integration point. The microstructural inhomogeneity of the material, as well as a geometrical inhomogeneity in the form of a groove region in the specimen, based on the Marciniak-Kuczynski (MK) theory, were considered in the calculation of the FLC using the CPFE model (MK-CPFE). The validity of predicted FLC for quenched and partitioned QP980 steel was confirmed by comparing the results with experimental measurements. The FLC calculated by CPFE model showed that the presence of microstructural inhomogeneity allows for a more realistic prediction of the localized necking phenomenon. Subsequently, the FLC was used to compute the limit strains for the T-shape part stamping process. The limit strains and the punch force-stroke relationship for the T-shape part predicted by the multiphase CPFE model were in good agreement with experimental results.