Showing papers on "Necking published in 2017"

Chemical vapor treatment of ABS parts built by FDM: Analysis of surface finish and mechanical strength

Abstract: The present study investigates the simultaneous effect of part building orientation (along the X, Y, and Z axis) and raster angle (0°, 30°, 60°, and 90°) on surface roughness, tensile strength, flexural strength, consumption of model, support material, and building time of acrylonitrile butadiene styrene (ABS) test specimens fabricated by fused deposition modeling (FDM) process. Mechanical properties and surface roughness show a strong anisotropic behavior for the parts. For parts built with the X or Y orientations and 30° or 60° raster angle, pulling of fiber and a small amount of necking along with tearing are observed, which are responsible for higher strength. Post-built treatment of the parts with cold vapors of dimethyl ketone resulted in an immense improvement in surface finish. Exposing the parts in cold vapors turns the surfaces to a soft/mushy-like state due to the weakening of the secondary bonds, and the minor flow of polymer layers fills the cavity region between the adjacent layers and helps in improving the surface finish after the treatment.

The tensile behaviors and fracture characteristics of stainless steel clad plates with different interfacial status

Abstract: The mechanical properties and fracture characteristics of stainless steel clad plates by vacuum hot rolling at different rolling temperature of 1100 °C (S1), 1200 °C (S2) and 1300 °C (S3) are investigated in detail. The carburized layer thickness and grain size is gradually increased with the increasing rolling temperature. The sufficient alloy element diffusion, recovery and recrystallization at the bonding temperature of 1300 °C lead to the highest interfacial shear strength and the lowest longitudinal tensile strength among the three clad plates. However, the tensile ductility is increased with the increasing rolling temperature, which is attributed to the increased interfacial bonding strength. The strong interface can effectively delay the formation of interfacial delamination crack and premature localized necking, resulting into a prolong uniform plastic deformation stage with a low stress triaxiality. In addition, there are many intergranular tunnel cracks with the length of 50–150 µm presented in the carburized layer due to Cr 23 C 6 carbides on the grain boundary, which can effectively toughen the stainless steel clad plates.

Microstructure, mechanical behaviour and fracture of pure tungsten wire after different heat treatments

Abstract: Plastic deformation of tungsten wire is an effective source of toughening tungsten fibre-reinforced tungsten composites (Wf/W) and other tungsten fibre-reinforced composites. To provide a reference for optimization of those composites, unconstrained pure tungsten wire is studied after various heat treatments in terms of microstructure, mechanical behaviour and fracture mode. Recrystallization is already observed at a relatively low temperature of 1273 K due to the large driving force caused by a high dislocation density. Annealing for 30 min at 1900 K also leads to recrystallization, but causes a rather different microstructure. As-fabricated wire and wire recrystallized at 1273 K for 3 h show fine grains with a high aspect ratio and a substantial plastic deformability: a clearly defined tensile strength, high plastic work, similar necking shape, and the characteristic knife-edge-necking of individual grains on the fracture surface. While the wire recrystallized at 1900 K displays large, almost equiaxed grains with low aspect ratios as well as distinct brittle properties. Therefore, it is suggested that a high aspect ratio of the grains is important for the ductile behaviour of tungsten wire and that embrittlement is caused by the loss of the preferable elongated grain structure rather than by recrystallization. In addition, a detailed evaluation of the plastic deformation behaviour during tensile test gives guidance to the design and optimization of tungsten fibre-reinforced composites.

Evaluation of the VDA 238-100 Tight Radius Bending Test using Digital Image Correlation Strain Measurement

Abstract: The VDA238-100 standard for tight radius bending (v-bending) of sheet materials has received widespread acceptance with automotive suppliers and material producers to characterize local formability However, the test fixture and tooling in the v-bend test standard is not amenable to direct strain measurement and the operator cannot terminate the test at the onset of crack initiation as the outer bend surface is not visible Consequently, fracture is identified using a load threshold and the bend angle estimated from an analytical formula based upon the punch displacement and tooling geometry Bend angles are not directly transferable and must be interpreted relative to the sheet thickness and bend radius unlike a strain measurement By obtaining an in-situ strain measurement on the surface using digital image correlation (DIC), the plane strain fracture limit can be accurately identified at the onset of cracking and remove ambiguity in translating the bend angles to practical forming operations and simulations A novel inverted VDA test frame was developed to incorporate DIC strain measurement during the bend test and a variety of advanced high strength sheet materials were evaluated It was observed that the VDA bend test creates a homogeneous strain state of plane strain across the width of the sample along with a proportional strain path to fracture without necking that is ideal for fracture characterization A correlation is developed to relate the bend angle with the major strain for the materials considered and accounts for the sheet thickness and bend radius A comparison of the bend angle obtained using the formula in the VDA standard based on the punch displacement was in very good agreement with manual measurements and an algorithm to measure the bend angle using DIC analysis was developed

Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components

Abstract: Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.

Microstructure evolution and fracture mechanism of a novel 9Cr tempered martensite ferritic steel during short-term creep

Abstract: In this work, the microstructure evolution and fracture mechanism of a novel 9% chromium tempered martensite ferritic steel G115 were investigated over the temperature range of 625–675 °C using uniaxial creep tests. The creep curves consist of a primary transient stage followed by an apparent secondary stage, and an accelerated tertiary creep regime. The relationship between the minimum creep rate and the applied stress followed Norton's power law. Based on the EBSD analysis, there were no obvious textural features formed after creep deformation, and with the increase in creep time, the number of subgrains slightly increased, and then sharply increased, indicating dynamic recrystallization (DRX) occurs after creep deformation. In addition, three types of precipitates can be observed after creep deformation: W-rich Laves phase, Nb-rich MX, and Cu-rich precipitates. The Nb-rich MX with a square shape and Cu-rich precipitates with an ellipsoidal shape remain very stable. However, the W-rich Laves phases distributed mainly on the grain boundaries have rod-like, chain-like, and bulky shape, which are coarsened significantly. Representative fractographs of the G115 steel after creep deformation exhibit significant necking with an elliptical shape. A dense array of deep and equiaxed dimples appear in the central region under the tested creep conditions. Ductile fracturing is the dominant fracture mechanism during short-term creep deformation.

Evolution of plastic deformation in heavily deformed and recrystallized tungsten of ITER specification studied by TEM

Abstract: Microstructure induced by plastic deformation at 600 °C in as-fabricated and recrystallized tungsten grades of ITER specification is studied by means of transmission electron microscopy (TEM). Both grades are produced by AG Plansee and being used by fusion community as research material for plasma facing and high heat flux components. Reference microstructure was investigated by a combination of TEM, scanning electron microscopy and nano-indentation techniques. TEM was applied to investigate the necking region in the tensile samples deformed up to ~ 30% of stain. The deformation-induced microstructure was characterized and compared for the two grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near high angle grain boundaries. The obtained evolution of the dislocation density was compared with the prediction of the previously developed thermo-mechanical model for the plastic deformation of polycrystalline tungsten and a good agreement was found without modification of the original parameter set.