•A model is proposed to describe shear ductile fracture from shear to the balanced biaxial tension.•The stress invariant-based Drucker yield function is specified for BCC and FCC metals to the ...balanced biaxial tension.•The model is successfully employed to predict ductile fracture of AA6082 in wide loading conditions.•The model could also be used to describe ductile fracture under low and negative stress triaxiality with improved flexibility.
A ductile fracture model is proposed to describe shear fracture of sheet metals from shear to balanced biaxial tension via uniaxial and plane strain tension. The fracture criterion models plastic damage as strain-induced void nucleation, triaxiality-governed void enlargement, Lode-controlled void torsion, and shear-restrained coalescence of voids. Its flexibility is investigated by a parameter study of the ductile fracture model proposed. The fracture model is employed to describe ductile fracture behavior of an aluminum alloy AA6082 T6 (thickness: 1.0mm). Dogbone specimens are strained to characterize the strain hardening properties, while another four different specimens are tested to characterize fracture behavior in shear, uniaxial tension, plane strain tension and balanced biaxial tension. The loading processes are analyzed numerically with the stress invariant-based Drucker yield function which is for the first time specified for body-centered cubic and face-centered cubic metals. Fracture strains in various loading conditions are measured with a hybrid experimental-numerical approach. The measured fracture strains are then used to calibrate the ductile fracture model proposed. The ductile fracture model calibrated above is employed to predict the onset of ductile fracture for these four specimens. For the purpose of comparison, the predicted fracture strokes of these four loading conditions are compared with those predicted by the modified Mohr–Coulomb model (Bai and Wierzbicki, 2008), and two micromechanism-inspired criteria proposed recently (Lou et al., 2012, 2014). The comparison reveals that the proposed model predicts the fracture behavior in much better agreement compared with experimental results from shear to the balanced biaxial tension. Accordingly, the proposed ductile fracture criterion is recommended for the prediction of ductile fracture in sheet metal forming processes, optimization of forming parameters and design of tools for both solid elements and shell elements. Besides, the ductile fracture model proposed can also be applied in various bulk metal forming processes in case that the model is calibrated by proper sets of experiments.
The transfer mechanism of a toner pattern printed on a soft film using a laser printer to a metal surface in compression was investigated by experiment and computer simulations. The toner pattern can ...be transferred successfully to the workpiece surface under the conditions that the entire soft film continues to be squeezed out smoothly from the workpiece edge until the transfer process is completed. The results indicate that the average tool pressure needed for the complete transfer of the toner pattern to the workpiece surface can be predicted by the average compression pressure calculated with slab method
Abstract
The experimental results under complex loading in view of theory of processes are explained in this issue. The opportunity for reducing of external efforts at metal forming is shown here. ...These results and investigations can be expanded for anisotropic material and for spatial processes of material forming.
•Ti2AlNb alloy was fabricated by hybrid point-forging and laser-deposition.•Random equiaxial grains were produced by interlayer forging and laser re-heating.•Continuous epitaxial solidification was ...disrupted by random substrate grains.•Recrystallization kinetics was facilitated by finer initial grains.
Combining interlayer point-forging with laser-deposition (PF-LD) provides a new idea to in-situ modify coarse-grained structure and strong solidification texture during high-thermal additive manufacturing process. In this study, a well-designed step-experiment with one to three-pass PF-LD was conducted on columnar-grained Ti2AlNb substrate to elucidate a novel insight of solid–liquid interaction effect. The results revealed that the combination of localized micro-forging and laser re-heating was capable to produce randomly-distributed equiaxial structure via interlayer recrystallization behavior, at the same time it in turn had a positive effect on solidification grain refinement. The essential contribution of these fine-grained microstructures further created an acceleration effect to interlayer recrystallization behavior and promoted the full-equiaxial grain morphology of PF-LD Ti2AlNb part.
In the present work, the friction and galling-prevention properties of a lubricating grease with the addition of Bi2S3 particles have been investigated. To achieve this, commercial lithium greases ...containing Bi2S3, hexagonal boron nitride, and graphite were employed to lubricate steel–aluminum contacts under different tribological configurations, namely reciprocating sliding tests with varying normal loads and sliding speeds and load scanner tests at 25, 200 and 300 °C. The addition of Bi2S3 enhanced the tribological response of the grease in reciprocating tests and provided protection against galling up to temperatures of 200 °C. However, its performance severely deteriorated at 300 °C. Thus, Bi2S3 showed promising properties as an additive for lubricating greases operating under a wide range of normal loads, sliding speeds, and moderate temperatures.
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•Bi2S3 provides a stable COF in reciprocating tests with different loads and speeds.•The wear rate of the Bi2S3 grease-lubricated pairs is lower than graphite and hBN.•Bi2S3 provides excellent galling protection up to temperatures of 200 °C.•Bi2S3 shows promising properties for being used in Al forming operations.
•Measured monotonic tension, compression, and load reversals data for three steels are used to adjust parameters of the FE-EPSC model.•Geometrical shape changes after U-draw/bending forming are ...predicted using the FE-EPSC multi-level simulation framework.•Accounting for backstress fields is revealed as critical for the accurate predictions of part geometries upon U-draw/bending.•Hardening and carrying over residual stress fields are essential for predicting part shape changes involving pre-strained sheets.•Increase in the amount of springback with sheet strength is determined by simulating U-draw/bending of DP 590 and DP 1180.
This work is concerned with predicting geometrical shape changes in sheet metal forming using a multi-level simulation framework that considers the directionality of deformation mechanisms acting at the single-crystal level and microstructural evolution. The multi-level model is an elasto-plastic self-consistent (EPSC) homogenization of single-crystal behavior giving the constitutive response at each finite element (FE) material point. Numerical solution of a boundary value problem over geometry is then obtained using continuum finite elements at the macro-level. First, a set of model parameters for the evolution of slip resistance of ferrite and martensite and backstress are established by fitting a comprehensive set of mechanical data for dual-phase (DP) steels 590, 780, and 1180 using one-element model. Next, the potential of the FE-EPSC modeling framework is illustrated by carrying out a set of hat-shaped draw-bending simulations of the steel sheets. The evolution of geometry after hat-shaped draw-bending and springback is predicted and verified with experimental measurements for as-received DP 780. In doing so, the role of accounting for backstress is revealed as critical for the accurate prediction of the part geometry. The same process simulation involving a pre-strained sheet of DP 780 is compared with a corresponding experiment to reveal the role of strain hardening and residual stress on the subsequent part shape changes after the hat-shaped draw-bending test and springback. Finally, the same process simulations involving DP 590 and DP 1180 are performed to confirm the effect of strength on the geometrical shape changes of the sheets after springback.
This paper aims to develop a lightweight convolutional neural network,
WearNet
, to realise automatic scratch detection for components in contact sliding such as those in metal forming. To this end, ...a large surface scratch dataset obtained from cylinder-on-flat sliding tests was used to train the
WearNet
with appropriate training parameters such as learning rate, gradient algorithm and mini-batch size. A comprehensive investigation on the network response and decision mechanism was also conducted to show the capability of the developed
WearNet
. It was found that compared with the existing networks,
WearNet
can realise an excellent classification accuracy of 94.16% with a much smaller model size and faster detection speed. Besides,
WearNet
outperformed other state-of-the-art networks when a public image database was used for network evaluation. The application of
WearNet
in an embedded system further demonstrated such advantages in the detection of surface scratches in sheet metal forming processes.
Forming is widely used due to its high efficiency in material utilization and its high production rate in general. Most forming processes control the geometry of final products through a set of ...tooling. The increasing demands on lightweight products have challenged the performances and functionalities of tooling. This paper provides a systematic review of recent advancements related to tooling performance and functionalities, including tool materials, tooling fabrication processes (e.g., machining, heat treatment, coating, surface texturing, and additive manufacturing), sensing, and data analytics. Finally, recommendations on future research directions for metal forming tooling are provided.
•An Eulerian formulation based on microstructural vectors is used to model orthotropic inelasticity in sheet metal.•A new functional form for inelastic spin is proposed.•A single material constant ...controlling inelastic spin is calibrated by measured data.•The Eulerian model is insensitive to arbitrariness of a reference configuration.•Simple shear is used to show that a common Lagrangian model predicts unphysical dependence on the reference configuration.
This paper develops a new functional form for the inelastic spin that influences the orientation of microstructural vectors during large inelastic deformations in a recently developed Eulerian formulation of sheet metal. For the problems under consideration, a single constant determines the inelastic spin, and the results of the New model are compared to those of a common Lagrangian sheet metal model. It is shown that the same value of this constant causes the New model to predict good results relative to measured data for uniaxial stress at angles relative to the RD, equibiaxial stress and the R-value distribution for AA6022-T4 sheet metal. In contrast, the Lagrangian model predicts zero inelastic spin, which is inconsistent with the measured data. The example of large deformation simple shear is used to further examine the influence of inelastic spin predicted by the New model and the Lagrangian model. The results show that the predictions of the New model, which is insensitive to arbitrariness of a reference configuration, are reasonable, while those of the Lagrangian model exhibit an unphysical dependence on the reference configuration.
This paper is concerned with modeling of anisotropic fracture forming limit diagram considering non-directionality of the equi-biaxial fracture strain. A new anisotropic ductile fracture criterion is ...developed based on the Lou–Huh ductile fracture criterion (Lou et al., 2012). In an attempt to predict the forming severity of advanced high-strength steel (AHSS) sheets, the proposed fracture criterion is converted into a Fracture Forming Limit Diagram (FFLD) and anisotropic fracture locus considering the sheet metal orientation. Tensile tests of the DP980 steel sheet with the thickness of 1.2 mm are conducted using various specimen geometries including pure shear, dog-bone, and flat grooved specimens. With Digital Image Correlation (DIC) method, equivalent plastic strain distribution on the specimen surface is computed until surface crack initiates. The fracture predictability of the proposed fracture criterion is evaluated with the experimental results which cover a wide range of stress states in various loading directions. By comparing fracture strains obtained from the experiments with the ones predicted from the proposed fracture criterion, it is clearly confirmed that the fracture criterion proposed is capable of predicting the equivalent plastic strain at the onset of fracture with great accuracy over a wide range of stress states while keeping non-directionality of the equi-biaxial fracture strain.
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