Showing papers on "Necking published in 2023"

Necking mechanism under various sintering process parameters – A review

Abstract: The process of sintering involves applying pressure and heat to the materials without melting them in order to fuse the particles together into a solid mass. The fusion between the particle are also known as interparticles necking which plays an important role in producing high-density products. Increasing in necking size between particles will allow the formation of smaller pore sizes which help to produce stronger and higher hardness materials. The necking also plays an essential role in producing high porosity products that are commonly used for medical applications which still required high tensile value and hardness. For this, proper process parameters were required to produce larger necking growth. To get a better understanding of this matter, the effect of powder and processing parameters will be reviewed in this article. The parameters for different processes such as conventional sintering, microwave sintering, selective laser melting, and others will be discussed in this paper as well.

Macro-scale modeling of finite strain viscoplasticity in irradiated F/M steels: a continuum thermodynamic framework

Abstract: Abstract The European reduced activation ferritic/martensitic steel Eurofer97 in irradiated states displays significant nonlinear material behavior involving irradiation hardening, loss of strain hardening, and uniform elongation, as well as irradiation-induced embrittlement. Nonlinear behavior of irradiated steel modeled on the continuum scale will help to estimate the maximum operating range of the irradiated components beyond the onset of localized plastic flow. In this work, a thermodynamic framework for modeling irradiation-influenced deformation is established based on irradiation defect density and a thermodynamically consistent finite strain formulation of an existing viscoplastic model using the Dual Variables concept is presented. The model is implemented in ABAQUS allowing the simulation of tensile tests conducted on irradiated and unirradiated materials which shows the model’s ability to capture the post-yield and post-necking behavior observed in experiments up to ductile failure.

Determining Localized Necking in Polycrystalline Sheet Metals Using the Bifurcation Phenomenon in Strain Evolution

Abstract: The forming limit is an important failure criterion for polycrystalline sheet metals when approving the forming process. Recent developments in strain measurement technology, e.g., digital image correlation (DIC), enable the strain evolution to be captured continuously and accurately. This new technology would improve the forming limit measurements if the onset of the necking detection method was developed accordingly. This paper proposes a new method based on the bifurcation phenomenon in strain evolution to detect the onset of localized necking through DIC measurements. This detection method was inspired by a physical understanding and experimental observations of the necking phenomenon. The method eliminates the derivative calculation from the traditional method, while it can directly determine the onset of localized necking through strain evolution curves. The robustness and accuracy of the method are also investigated through experiments. Imperfection and non-defect analyses, based on non-associated and associated flow rules, were utilized and compared to the determined results. The detection method provides satisfactory forming limit results and can be used as an alternative method to determine the forming limit diagram (FLD).

Size Effect on the Post-Necking Behaviour of Dual-Phase 800 Steel: Modelling and Experiment

Abstract: This work investigated the feasibility of using a miniaturised non-standard tensile specimen to predict the post-necking behaviour of the materials manufactured via a rapid alloy prototyping (RAP) approach. The experimental work focused on the determination of the Lankford coefficients (r-value) of dual-phase 800 (DP800) steel and the digital image correlation (DIC) for some cases, which were used to help calibrate the damage model parameters of DP800 steel. The three-dimensional numerical simulations focused on the influence of the size effect (aspect ratio, AR) on the post-necking behaviour, such as the strain/stress/triaxiality evolutions, fracture angles, and necking mode transitions. The modelling showed that although a good correlation can be found between the predicted and experimentally observed ultimate tensile strength (UTS) and total elongation. The standard tensile specimen with a gauge length of 80 mm exhibited a fracture angle of ∼55°, whereas the smaller miniaturised non-standard specimens with low ARs exhibited fractures perpendicular to the loading direction. This shows that care must be taken when comparing the post-necking behaviour of small-scale tensile tests, such as those completed as a part of a RAP approach, to the post-necking behaviours of standard full-size test specimens. However, the modelling work showed that this behaviour is well represented, demonstrating a transition between the fracture angles of the samples between 2.5 and 5. This provides more confidence in understanding the post-necking behaviour of small-scale tensile tests.

Shear testing of clinch joints at different temperatures: Explanation of the failure sequence

Abstract: Clinching is a reliable, high-speed, cost-effective, and environmentally friendly mechanical joining process that enables sheet metals to be joined in large scale productions including whiteware, ventilation and air conditioning systems and automotive applications. Currently, the mechanical behavior and durability of clinch joints under different thermal conditions is poorly understood. This study evaluates the mechanical performance of clinched joints at room temperature, 100 °C and 250 °C experimentally using uniaxial shear tests. Results revealed that all clinched specimens had a similar shaped load displacement curve. Four stages were observed: an initial linear behavior with low sensitivity to temperature; a force plateau caused by necking (the force was only moderately reduced for higher temperatures); a phase where the neck began to crack, resulting in a letdown of the load which showed high thermal dependence, and a final phase of tearing completely through the neck, with joint separation. The ultimate tensile shear strength of the joints was relatively unchanged by increases in temperature. At higher temperatures the stiffness and energy absorbed by the joint decreases. This work shows the results of thermal shear testing for clinched joints, and describes the mechanisms whereby the joint progressively collapses under load.

Profilometry‐Based Indentation Plastometry for Evaluating Bulk Tensile Properties of Aluminum‐Silicon Carbide Composites

Abstract: The need for large samples with specific geometries and the destructive nature of conventional tensile testing pose a challenge in the rapid mechanical characterization of metal matrix composites (MMCs). Herein, the efficacy of a high‐throughput profilometry‐based indentation plastometry (PIP) technique for evaluating bulk tensile properties of SiC‐reinforced aluminum MMC with minimum sample volume and preparation is investigated. Plastic properties, namely yield strength (YS), ultimate tensile strength (UTS), and elongation up to necking (εn) in aluminum composites reinforced with 0, 17.5, and 25 vol% of SiC from PIP, are compared with uniaxial tensile tests. While PIP estimations of YS for all composites and UTS for Al‐17.5 vol% SiC are accurate within 3–6%, those of UTS in 25 vol% and εn in all composites show significant deviation from tensile test data. These deviations are attributed to the PIP overestimation of strength due to local SiC crowding beneath the indenter and the limitation of the Voce plasticity‐based FEM simulation in capturing brittle behavior of high vol% reinforcement. Herein, the high‐throughput PIP technique that can be reliably extended to MMCs with low volume (≈17.5%) of SiC reinforcements is established, thus harboring potential for advancement in the nondestructive testing of MMCs.

Evolution of Poisson’s Ratio in the Tension Process of Low-Carbon Hot-Rolled Steel with Discontinuous Yielding

Abstract: Low-carbon hot-rolled steel generally undergoes a deformation process composed of four phases, i.e., elastic deformation, discontinuous yielding, work hardening, and macroscopic plastic-strain localization in a tension test. The evolution of the Poisson’s ratio in terms of the average Poisson’s ratio and the local Poisson’s ratio in the deformation process from the non-load state to the onset point of specimen necking was investigated. The main results are as follows: (1) the average Poisson’s ratio cannot accurately represent the local Poisson’s ratio in the discontinuous-yielding phase; (2) the Poisson’s ratio varied significantly within a plastic band in the discontinuous-yielding phase, and the maximum Poisson’s ratio was reached within the plastic band; and (3) the strain rate greatly increased the Poisson’s ratio.

An improved approach for forming limit prediction of strongly anisotropic thin-walled tube combining M-K model and ductile fracture criterion

Abstract: Accurate prediction of the forming limit at necking of strongly anisotropic thin-walled tube is an urgent requirement in tube hydroforming. M-K model is one of the most widely used prediction models. However, the failure criterion in M-K model is based on the ratio of major true strain increments in the groove and uniform regions, which sometimes results in overestimation of the limit strains. To address this issue, a combined model of M-K model and ductile fracture criterion (DFC) for tube was proposed in this paper, in which the failure condition of the groove region is determined by the DFC. The characteristics of the M-K+DFC model were analyzed by combining with the DF2012 phenomenological DFC. To validate the M-K+DF2012 model, the limit strains at necking of an AA6061 tube with strong in-plane anisotropy were tested through tube controllable biaxial loading experiments. The results show that the forming limit curve (FLC) predicted by the M-K+DFC model does not exceed the FLC calculated by the M-K model and the fracture forming limit curve (FFLC) determined by the corresponding DFC. The M-K+DF2012 model provides a reasonable forming limit prediction of the AA6061 tube, and the shortcoming of the M-K model mentioned above is overcome. Meanwhile, a reasonable constitutive model considering the in-plane anisotropy is very important in the forming limit prediction of a strongly anisotropic thin-walled tube.

Investigation on Constitutive and Ductile Damage Mechanism of High-Density Polyethylene under Tensile and Bending Conditions

Abstract: High-density polyethylene (HDPE) is one of the most commonly used materials for manufacturing pipelines. In this paper, the deformation behavior of HDPE materials under uniaxial tensile stress was investigated by employing uniaxial tensile tests and macroscale/mesoscale morphology analysis. The experimental results indicated that the stress–strain relationship of HDPE is nonlinear, and the ductile fracture characteristics of necking and local failure appear in the sample after the engineering strain reaches 35%. Therefore, for the first time, a hyperelastoplastic constitutive model that describes the elasticity by the Marlow model and combined the isotropic plasticity-ductility damage correction is established, and a corresponding numerical approach is proposed. The simulation scheme was implemented in finite-element software, and the mechanical responses of HDPE under uniaxial tensile and bending loads were predicted. Obtained calculations were relatively consistent with experimental observations. Hence, the proposed model and approach have a good simulation effect on the mechanical properties of HDPE and can be employed for the process prediction of such materials. This study is beneficial to the in-depth understanding of the hyperelastoplasticity and failure mechanism of HDPE and provides ideas for the deformation and failure study of other polymer materials.

A one-dimensional model for axisymmetric deformations of an inflated hyperelastic tube of finite wall thickness

Abstract: We derive a one-dimensional (1d) model for the analysis of bulging or necking in an inflated hyperelastic tube of {\it finite wall thickness} from the three-dimensional finite elasticity theory by applying the dimension reduction methodology proposed by Audoly and Hutchinson (J. Mech. Phys. Solids, 97, 2016). The 1d model makes it much easier to characterize fully nonlinear axisymmetric deformations of a thick-walled tube using simple numerical schemes such as the finite difference method. The new model recovers the diffuse interface model for analyzing bulging in a membrane tube and the 1d model for investigating necking in a stretched solid cylinder as two limiting cases. It is consistent with, but significantly refines, the exact linear and weakly nonlinear bifurcation analyses. Comparisons with finite element simulations show that for the bulging problem, the 1d model is capable of describing the entire bulging process accurately, from initiation, growth, to propagation. The 1d model provides a stepping stone from which similar 1d models can be derived and used to study other effects such as anisotropy and electric loading, and other phenomena such as rupture.

Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS)

Abstract: We report a new method to shape double-network (DN) hydrogels into customized microscale 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermo-reversible sol-gel κ-carrageenan with a suitable crosslinker, and photo-initiator/absorbers are optimized. A new TOPS system is utilized to photo-polymerize the primary acrylamide network into a 3D structure above the sol-gel transition of κ-carrageenan (80°C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37μm) and vertical (180μm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400% respectively under tension, and simultaneously exhibit high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.

On the utilization of radial extrusion to characterize fracture forming limits. Part II: testing and modelling

Abstract: Abstract. This second part of the paper is focused on double-action radial extrusion testing and modelling to characterize material formability and failure in the bulk-to-sheet material flow transitions that are commonly found in metal forming. Results show that three-dimensional to plane-stress evolutions at the radially extruded flanges lead to different modes of fracture (by tension and by shear) that may or may not be preceded by necking. The use of double-action radial extrusion as a formability test also reveals adequate to characterize the failure limits of very ductile metallic materials, which cannot be easily determined by conventional upset compression tests, and to facilitate the identification of the instant of cracking and of the corresponding fracture strains by combination of the force vs. time evolutions with the in-plane strains obtained from digital image correlation.

Corrugation Reinforced Architectured Materials by Direct Laser Hardening: A Study of Geometrically Induced Work Hardening in Steel

Abstract: Improving the strength‐to‐ductility trade‐off remains the prime driving force for the development of advanced high‐strength steel. Traditionally research breakthroughs are focused on the microstructure and relative phase composition. Herein, laser hardening is applied to ductile ferritic steel to introduce straight and corrugated martensitic reinforcements, effectively generating architectured steel sheets. Tensile behavior of laser‐architectured samples is studied both using finite‐element method simulation and mechanical testing to reveal the effect of laser‐induced corrugations on strength and necking strain. The results show that with the same reinforced volume fraction of 24%, an increase in corrugation height/period leads to a gain in necking strain with a loss in yield strength and ultimate tensile stress. This beneficial effect on necking strain is due to the corrugation unbending process which introduces so‐called geometric work hardening during tension. Extended simulations are carried out on various corrugation heights/periods and the evolution trends of ultimate tensile strength and necking change with different reinforced volumes. This study proposes a perspective on corrugation‐reinforced architectured materials. Corrugation parameters can be chosen to tailor the mechanical behavior of laser‐architectured materials.

Investigation of the relationship between the microstructural evolution mechanism and mechanical properties in hot rolled Fe-0.2C-6Mn-3Al steel

Abstract: The objective of the present research is to investigate the influence of the microstructural evolution mechanism on mechanical properties during deformation. For this purpose, hot rolled Fe-0.22C-6.12Mn-3.08Al steel is subjected to intercritical annealing at 740 °C for 10 min. The annealed specimen contains a mixture of lath-like α-ferrite and reverted γ, whose volume fraction is approximately 38.7%. During deformation, once the critical stress caused by the severe dislocation pile-ups is reached, the strain/stress-induced phase transformation occurs. First, grain boundaries of γ/α serve as the preferential nucleation sites for fresh α′-martensite. Subsequently, fresh α′-martensite nucleates inside reverted γ. In the meantime, the interfacial α′-martensite grows continuously into the reverted γ interior. During deformation (prestrain at 0.0%, 5.1% and 7.8%), the misorientation (MO) in α-ferrite+α′-martensite continuously rises from 0.63° to 0.67°; nevertheless, the MO in reverted γ increases from 0.65° to 0.73° and then rapidly decreases to 0.69°. Hence, the phase transformation of reverted γ is accompanied by obvious stress softening; thus, effective work hardening occurs in the subsequent deformation region due to α-ferrite and fresh α′-martensite. This phenomenon greatly enhances the strength and delays necking, e.g., the annealed specimen possesses outstanding mechanical properties (992 MPa, 47.6% and 47.2 GPa × %). Once necking occurs, the residual reverted γ is rapidly consumed in the necking region. Hence, the hardness in the necking region (5.31 ± 0.20 GPa) is much higher than that in the uniform deformation region (4.53 ± 0.17 GPa) due to the abundant hard fresh α′-martensite.

Shape Memory Alloy Thin Film Auxetic Structures

Abstract: Auxetic structures provide an interesting approach to solving engineering problems due to their negative Poisson's ratio, which allows for elongation perpendicular to applied stresses, opposite to a conventional structure's necking behavior. Thus, they can function well in applications requiring compacting the device into a small volume during the deployment (e.g., implants inserted with catheters) or stretchability with area coverage (e.g., stretchable electronics). Fabricating them with shape memory alloys (SMAs) expands the possibilities. The high strains experienced by auxetic structures may become reversible compared to ordinary metals due to superelastic or shape memory effect. This work studies four different auxetic microstructures using thin film SMAs that are capable of surviving strains up to 57.4%. Since these structures are fabricated by layer deposition and lithography, other components, such as microelectronics, can be seamlessly integrated into the fabrication process. These auxetic thin films are investigated for their mechanical behavior under tension for their stretchability and stability. Under tension, thin films are known to show wrinkling instabilities. In two of four designs, the large auxetic behavior leads to wrinkling, while the other two display stable, non‐wrinkling behavior. These designs can be candidates for stretchable electronics, wearable medical devices (e.g., biosensors), or implants (e.g., stents).

A rheology and processing study on controlling material and process defects in polymer melt extrusion film casting using polymer blends

Abstract: The primary objective of this research paper is to control the material and process defects in polymer melt extrusion film casting (EFC) process for linear chain architecture polyethylene (PE) resins through polymer blending methodology. Extrusion film casting is a well-known industrially important manufacturing process that is used to manufacture thousands of tons of polymer/plastic films/sheets and coated products. In this research, the necking defect in an EFC process has been studied experimentally for a linear low density polyethylene (LLDPE) resin and attempts have been made to control its necking by blending in a long chain branched (LCB) low density polyethylene (LDPE) resin. The blending methodology is based on the understanding that a LDPE resin displays enhanced resistance to necking as compared to the LLDPE resin. It is found that added LDPE resin enhances necking resistance for the primary LLDPE resin. Further, as the LDPE concentration increases in the blend formulation, the necking is further reduced as compared to pure LLDPE. Analogous to past studies on EFC of linear and long chain branched architecture containing PEs, it is observed that as the LDPE is increased in the blend formulations, the formulations displayed enhanced melt elasticity and extensional strain hardening in rheological studies. It is concluded from this study that polyethylene resins having linear chain architecture can be made amenable to enhanced resistance to necking using appropriate amount of a long chain branched resins. Finally, process defects such as the draw resonance onset could be shifted to higher draw ratios as the LDPE level is increased in the LLDPE-LDPE blend formulation.

Energy Storage and Dissipation in Consecutive Tensile Load-Unload Cycles of Gum Metal

Abstract: Multifunctional β-titanium alloy Gum Metal, characterized by a relatively low elastic modulus, superelastic-like behavior and high strength, was subjected to cyclic tensile loadings. The characteristics of macroscopic scale energy storage and dissipation in the consecutive loading–unloading cycles were studied. Various kinds of energy components related to the alloy deformation process were determined experimentally and analyzed using thermodynamic relations. The values of the entire work needed to deform the alloy Wext, the work used for recoverable deformation Wrec consisting of the elastic deformation energy Wel , the superelastic-like energy Wpt , and the energy of thermoelastic effect Eth , were derived from the Gum Metal stress and temperature vs. strain curves. The irrecoverable mechanical energy Wir expended on plastic deformation, the dissipation energy Q, and finally the stored energy Es were estimated. The stored energy represents a change in the internal energy of the deformed material and is an essential measure of cold-worked state. The Es value turned out to be not large for the Gum Metal, which confirms the alloy low hardening property. The energy components determined for each of the 24 loading cycles enabled us to analyze various stages of the Gum Metal deformation process, including necking and damage.