Showing papers on "Necking published in 2010"

Characterisation of the Portevin-Le Châtelier effect affecting an austenitic TWIP steel based on digital image correlation

Abstract: When strained in tension, high-manganese austenitic TWIP steels achieve very high strength and elongation before necking. They also present serrated flow within a certain range of temperatures and strain rates. A consequence of this jerky flow is the appearance of strain localisation in the form of narrow deformation bands apparent on the surface of the sample. This phenomenon arises from the dynamic interaction between solute atoms and mobile dislocations, otherwise known as dynamic strain ageing (DSA). In this study, the heterogeneous deformation of a Fe-20wt.%Mn-1.2wt.%C grade has been investigated at room temperature and at different strain rates by means of digital image correlation (DIC) for spatially resolved strain measurements made in situ during tensile tests. Simple tensile tests have also been conducted at different temperatures in order to investigate the evolution of the type of serrations and their influence on the bulk mechanical properties. The results of tests performed at room temperature and at two different strain rates, indicate that the plasticity is entirely governed by the appearance of localised deformation bands, similar to those observed in materials that exhibit the Portevin-Le Chatelier (PLC) effect. However, no critical strain for the onset of the phenomenon could be determined. The origin of these observations is discussed in this paper. It is proposed that the mode of propagation of the bands is dependent on the strain rate and the strain level. Moreover, it is shown that the band propagation is well correlated with the different types of serrations appearing on the stress-strain curve. These results match the characteristics of a classical DSA effect and help to shed light on the remarkable properties achieved by this material. (C) 2010 Elsevier B.V. All rights reserved.

Parametric study on mechanical clinching process for joining aluminum alloy and high-strength steel sheets †

Abstract: The purpose of this study is to investigate the effects of process parameters on the joint characteristics of advanced high-strength steel DP780 and Al5052 alloy sheet in mechanical clinching process. The defects in the clinching joint, such as necking of the upper sheet, cracks in the lower sheet, and no interlocking, occur because of the different ductility between advanced high-strength steel and aluminum alloy. In this study, the effect of the process parameters of the clinching process on the joinability of advanced high-strength steel with Al5052 alloy was investigated using finite element (FE) analysis. From the result, the die radius, die depth, and die groove shape were mainly affected by the joinability of advanced high-strength steel with Al5052 alloy. H-type tension test was performed under the same condition as the FE analysis. In addition, the joint strength was determined by interlocking length as well as neck thickness.

Molecular Stress Relief through a Force-Induced Irreversible Extension in Polymer Contour Length

Abstract: Single-molecule force spectroscopy is used to observe the irreversible extension of a gem-dibromocyclopropane (gDBC)-functionalized polybutadiene under tension, a process akin to polymer necking at a single-molecule level. The extension of close to 28% in the contour length of the polymer backbone occurs at roughly 1.2 nN (tip velocity of 3 μm/s) and is attributed to the force-induced isomerization of the gDBCs into 2,3-dibromoalkenes. The rearrangement represents a possible new mechanism for localized stress relief in polymers and polymer networks under load, and the quantification of the force dependency provides a benchmark value for further studies of mechanically triggered chemistry in bulk polymers.

Mechanical and microstructural stability of P92 steel under uniaxial tension at high temperature

Abstract: 9–12%Cr creep-resistant ferritic-martensitic steels are candidates for structural components of Generation IV nuclear power plants. However, they are sensitive to softening during low-cycle fatigue, creep and creep-fatigue tests, due to the destabilisation of the tempered martensite microstructure, possibly inducing a decrease in further creep resistance. To better identify the softening mechanisms in P92 steel during uniaxial deformation, tensile tests were carried out at 823 K, showing an extended and stable softening stage on true stress–strain curves after some work-hardening. Three phenomena were studied in order to understand this behaviour: mechanical instability (necking), damage and grain size evolution. Examination of fractured and non-fractured tensile specimens (light optical and electron microscopy, macrohardness) suggested that the physical mechanisms responsible for softening are mainly (sub)grain size evolution and diffuse necking. Models were proposed to predict grain growth and beginning of the mechanical instability during homogeneous deformation.

Development of an in-plane biaxial test for forming limit curve (FLC) characterization of metallic sheets

Abstract: The main objective of this work is to propose a new experimental device able to give for a single specimen a good prediction of rheological parameters and formability under static and dynamic conditions (for intermediate strain rates). In this paper, we focus on the characterization of sheet metal forming. The proposed device is a servo-hydraulic testing machine provided with four independent dynamic actuators allowing biaxial tensile tests on cruciform specimens. The formability is evaluated thanks to the classical forming limit diagram (FLD), and one of the difficulties of this study was the design of a dedicated specimen for which the necking phenomenon appears in its central zone. If necking is located in the central zone of the specimen, then the speed ratio between the two axes controls the strain path in this zone and a whole forming limit curve can be covered. Such a specimen is proposed through a numerical and experimental validation procedure. A rigorous procedure for the detection of numerical and experimental forming strains is also presented. Finally, an experimental forming limit curve is determined and validated for an aluminium alloy dedicated to the sheet forming processes (AA5086).

Superelongation and atomic chain formation in nanosized metallic glass.

Abstract: Bulk metallic glasses are brittle and fail with no plastic strain at room temperature once shear bands propagate. How do metallic glasses deform when the size is less than that of shear bands? Here we show that Al(90)Fe(5)Ce(5) metallic glass with a size <20 nm can be extremely elongated to similar to 200%. Remarkably, even an atomic chain was formed after sample necking, which was never observed in metallic glasses. The unexpected ductility may originate from the fast surface diffusion and the absence of shear band formation, and may guide the development of ductile metallic glasses for engineering applications.

Heat sources, energy storage and dissipation in high-strength steels: Experiments and modelling

Abstract: Tensile tests on three high-strength steels exhibiting Luders band propagation are carried out at room temperature and under quasi-static loading conditions. Displacement and temperature fields on the surface of the flat samples are measured by digital image correlation and digital infrared thermography, respectively. The true stress versus true strain curves were calculated from the displacement data, while the thermal data were used to estimate the heat sources using the local heat diffusion equation. Based on these measurements the stored and dissipated energies were estimated up to diffuse necking. A thermodynamically consistent elastic-plastic constitutive model including the von Mises yield criterion, the associated flow rule and two non-linear isotropic hardening variables is applied to describe the behaviour of the high-strength steels. It is shown that this simple model is able to reproduce both the local behaviour, such as the power associated to heat sources, and the global behaviour, such as Luders band propagation and stored and dissipated energies. It is further shown that the ratio of dissipated power to plastic power varies during plastic straining and that this variation is captured reasonably well in the numerical simulations.

A New Model for Void Coalescence by Internal Necking

Abstract: A micromechanical model for predicting the strain increment required to bring a damaged material element from the onset of void coalescence up to final fracture is developed based on simple kinematics arguments. This strain increment controls the unloading slope and the energy dissipated during the final step of material failure. Proper prediction of the final drop of the load carrying capacity is an important ingredient of any ductile fracture model, especially at high stress triaxiality. The model has been motivated and verified by comparison to a large set of finite element void cell calculations.

Formability of Al 5xxx Sheet Metals Using Pulsed Current for Various Heat Treatments

Abstract: Previous studies have shown that the presence of a pulsed electrical current, applied during the deformation process of an aluminum specimen, can significantly improve the formability of the aluminum without heating the metal above its maximum operating temperature range. The research herein extends these findings by examining the effect of electrical pulsing on 5052 and 5083 Aluminum Alloys. Two different parameter sets were used while pulsing three different heat treatments (As Is, 398°C, and 510°C) for each of the two aluminum alloys. For this research, the electrical pulsing is applied to the aluminum while the specimens are deformed, without halting the deformation process (a manufacturing technique known as Electrically-Assisted Manufacturing). The analysis focuses on establishing the effect the electrical pulsing has on the aluminum alloy’s various heat treatments by examining the displacement of the material throughout the testing region of dogbone-shaped specimens. The results from this research show that pulsing significantly increases the maximum achievable elongation of the aluminum (when compared to baseline tests conducted without electrical pulsing). Another beneficial effect produced by electrical pulsing is that the engineering flow stress within the material is considerably reduced. The electrical pulses also cause the aluminum to deform non-uniformly, such that themore » material exhibits a diffuse neck where the minimum deformation occurs near the ends of the specimen (near the clamps) and the maximum deformation occurs near the center of the specimen (where fracture ultimately occurs). This diffuse necking effect is similar to what can be experienced during superplastic deformation. However, when comparing the presence of a diffuse neck in this research, electrical pulsing does not create as significant of a diffuse neck as superplastic deformation. Electrical pulsing has the potential to be more efficient than traditional methods of incremental forming since the deformation process is never interrupted. Overall, with the greater elongation and lower stress, the aluminum can be deformed quicker, easier, and to a greater extent than is currently possible.« less

Characterization and stability of localized bulging/necking in inflated membrane tubes

Abstract: We consider localised bulging/necking in an inflated hyperelastic membrane tube with closed ends. We first show that the initiation pressure for the onset of localised bulging is simply the limiting pressure in uniform inflation when the axial force is held fixed. We then demonstrate analytically how, as inflation continues, the initial bulge grows continually in diameter until it reaches a critical size and then propagates in both directions. The bulging solution before propagation starts is of the solitary-wave type, whereas the propagating bulging solution is of the kink-wave type. The stability, with respect to axially symmetric perturbations, of both the solitary-wave type and the kinkwave type solutions is studied by computing the Evans function using the compound matrix method. It is found that when the inflation is pressure-controlled, the Evans function has a single non-negative real root and this root tends to zero only when the initiation pressure or the propagation pressure is approached. Thus, the kink-wave type solution is probably stable but the solitary-wave type solution is definitely

Multiple necking during the dynamic expansion of hemispherical metallic shells, from experiments to modelling

Abstract: Expansions of hemispheres driven by explosive charge have been conducted for two materials: copper and tantalum. Owing to high speed camera, the time occurrence, the angular position and the number of necks have been identified at the onset of necking. Tantalum and copper have been characterized from quasi-static to dynamic conditions at different temperatures and their behavior has been modelled using three different thermoviscoplastic flow laws (powerlaw, Zerilli–Armstrong, Preston–Tonks–Wallace). From numerical simulations of the expansion process, it has been shown that the onset of necking is located in an axisymmetric layer where plane strain conditions prevail. Since the layer has a small extension in latitude and the shell is thin, the stability problem can be assimilated to that of a plate stretched under dynamic plane strain conditions. A stability analysis is developed based on the concept of an effective rate sensitivity parameter which merges the thermal and the visco-plastic material characteristics. From the evolution of this parameter during loading, a new criterion for the onset of instability is postulated. This criterion, which is different to the usual one based on the evolution of the growth rate of small perturbations, is used to characterize multiple necking observed during the dynamic expansion of copper and tantalum hemispheres. A good agreement with experiments is obtained concerning the number of necks and the time of occurrence of instabilities.

On the dynamics of localization and fragmentation-IV. Expansion of Al 6061-O tubes

Abstract: In this series of papers, we investigate the mechanics and physics of localization and fragmentation in ductile materials. The behavior of ductile metals at strain rates of about 4,000–15,000 per second is considered. The expanding ring experiment is used as the vehicle for examining the material behavior in this range of strain rates. In Parts I–III, we examined the response of rings with cross-sectional aspect ratios in the range of 1–10, exhibiting a transition from diffuse necking to sheet-mode localization. In the present paper we report on experimental observations of high strain-rate expansion of Al 6061-O tubes. Through an innovative use of high-speed imaging aided with a conical mirror we determine the sequence of deformation and failure in the expanding tube. In particular, the conical mirror provides information of surface deformation on the tube. Forming limit diagrams at high strain rates are obtained from post-mortem measurement of the deformed tubes or fragments and are compared with analysis from quasi-static analysis. Numerical simulations are used to explain the experimental observations.

Characterization of necking phenomena in high-speed experiments by using a single camera

Abstract: The purpose of the experiment described herein is the study of material deformation (here a cylinder) induced by explosives. During its expansion, the cylinder (initially 3 mm thick) is thinning until fracture appears. Some tens of microseconds before destruction, strain localizations occur and induce mechanical necking. To characterize the time of first localizations, 25 stereoscopic acquisitions at about 500,000 frames per second are used by resorting to a single ultra-high speed camera. The 3D reconstruction from stereoscopic movies is described. A special calibration procedure is followed, namely, the calibration target is imaged during the experiment itself. To characterize the performance of the present procedure, resolution and optical distortions are estimated. The principle of stereoscopic reconstruction of an object subjected to a high-speed experiment is then developed. This reconstruction is achieved by using a global image correlation code that exploits randommarkings on the object outer surface. The spatial resolution of the estimated surface is evaluated thanks to a realistic image pair synthesis. Last, the time evolution of surface roughness is estimated. It gives access to the onset of necking.

Mixed Mode Stable Tearing of Thin Sheet Al 6061-T6 Specimens: Experimental Measurements and Finite Element Simulations Using a Modified Mohr-Coulomb Fracture Criterion

Abstract: In this investigation, a combined experimental and computational approach with a Modified Mohr Coulomb (MMC) fracture criterion employing post-initiation element softening is used to simulate stable crack propagation under Mode I, Mode III and combined Mode I/III loading conditions. Results from the studies demonstrate that good correlation exists between the measured load-displacement and the numerically predicted response when the stiffness of the specimen fixture is included in the FE model. The numerical results were able to capture most of the experimentally observed features during crack propagation, such as through-thickness slant fracture, necking, tunneling and local specimen twist, thus confirming that the MMC criterion is suitable for predicting in-plane and out-of-plane tearing of sheets. It was found that in order to predict correctly the load-displacement curve as well as the fracture plane, different amount of softening is needed for Mode I and Mode III loading cases. This observation can be justified on the micro-mechanical level, while there is a competition between the mechanisms of dimple and shear fracture.

Application of the hydroforming strain- and stress-limit diagrams to predict necking in metal bellows forming process

Abstract: In order to predict the initiation of necking in metal bellows forming process, a methodology for determination of the forming limit diagram and the forming limit stress diagram is represented in this paper. The methodology is based on the Marciniak and Kuczynski (M–K) model. Comparison between the experimental and theoretical results for hydroforming stress and strain-limit diagrams as predicted by different methods indicates that the present approach is suitable for prediction of necking in tube hydroforming processes. Afterwards, the implementation of the hydroforming strain- and stress-limit diagrams into finite element numerical simulations for the forming of the metal bellows is established. A satisfactory agreement between the finite element method (FEM) and test results is achieved.