FE Simulation Results Research Articles

Numerical Inverse Method for Friction Coefficient Estimation in Tube Hydroforming

Friction plays an important role in forming processes, in fact it influences the material flow and therefore it affects the process and part characteristics. In particular, friction is a very influencing factor in Tube Hydroforming (THF), where high die-part contact pressure and area make the material sliding very difficult. As a consequence, the material hardly flows to the expansion zones and the part formability can be compromised. To obtain sound parts, FEM models allow to study the process and optimize its parameters, but they require the right definition of the friction at tube-die interface. For these reasons, friction represents a key-point in THF processes and its knowledge and prediction are very important even if, nowadays, a comprehensive friction test for THF is not available in literature. With this paper, the Authors want to propose a novel approach to estimate friction for THF processes. In particular it will be described a numerical inverse method that allows to estimate the Coulombian friction coefficient combining experimental test and FE simulation results. The method is based on the effects of friction on the tube final thickness distribution when it is pressurized and compressed by two punches under different lubrication conditions without expansion. In particular, it will be shown how the use of few and fast FE simulations allows to estimate an analytical function that takes into account the process conditions and that can be used in combination with experimental results in order to estimate the friction coefficient in THF processes.

The Effect of Mandrel Speed upon the Residual Stress Distribution around Cold Expanded Hole

In this paper, cold expansion process on aluminum alloy 2A12TA was simulated using ABAQUS finite element package and the effect of different mandrel speeds upon residual stress distribution around cold expanded hole was considered. The results gained by simulation show major difference between the diagrams of residual stress distribution corresponding different mandrel speeds. The results of FE simulation in slowly loaded process was verified by experimental data. As can be seen, when mandrel speed increases the residual stress around cold expanded hole increases as well and this is caused the fatigue life improvement of fasten holes.

The Effect of Mandrel Speed upon the Residual Stress Distribution around Cold Expanded Hole

In this paper, cold expansion process on aluminum alloy 2A12TA was simulated using ABAQUS finite element package and the effect of different mandrel speeds upon residual stress distribution around cold expanded hole was considered. The results gained by simulation show major difference between the diagrams of residual stress distribution corresponding different mandrel speeds. The results of FE simulation in slowly loaded process was verified by experimental data. As can be seen, when mandrel speed increases the residual stress around cold expanded hole increases as well and this is caused the fatigue life improvement of fasten holes.

Location of the Critical Solder Joint of a PBGA under Temperature Cycling Load for SAC and SnPb Solder. Results of Experiments vs. FE-Simulations on System and Board Level.

PBGAs with SnPb and SnAgCu (SAC) solder joints were stressed with temperature cycles on board- and system-level. A significant influence of the different solder materials on the location of the most damaged PBGA solder balls was observed in the experiment. The reason for this experimental finding was investigated and explained by FE–simulation. The simulations of the PBGAs were done on package-, board- and system-level (PCB within a metal housing). For the system level simulation a 2-step sub-model technique described in [1] was used. Through such an approach the transient PCB deformation and the transient temperature field within the ECU-housing can be incorporated into a creep simulation of the PBGA solder joints. The creep results for both SnPb and SnAgCu solder joints from the board- and system-level simulation were compared. The calculated damage factor due to the ECU-housing influence is different for PBGA with SnPb and SAC solder joints. The simulation results were validated step by step with measurements and experiments: warpage of the non-soldered PBGA, mechanical strain and temperature on the mounted PCB, crack length evaluation of all PBGA solder joints.

The Low-Velocity Impact Response of Sandwich Panels

The low velocity impact response of sandwich panels at different energy levels has been investigated by conducting drop-weight impact tests using an instrumented falling-weight impact tower. Impact parameters like maximum impact force and the extent of the damage were evaluated and compared for different types of sandwich panels. Finite elements simulations have been undertaken using the LS-DYNA software; the results of FE simulations have a good agreement with the experiments. It shows that, the impact force increased with thickness of face-sheets and foam core, the extent of the damage increased with the impact energy, sandwich panels with steel face sheet has a good impact resistance in comparison with sandwich panel with aluminum face sheets.

Estimation of fatigue life for plates including pre-treated fastener holes using different multiaxial fatigue criteria

In this paper seven different multiaxial fatigue criteria were used to estimate the fatigue lives of aluminium 7075-T6 plate specimens including a pre-treated fastener hole. To do so, the results of available FE simulations of three different models, called open hole, cold-expanded and clamped-hole were used as basic data in multiaxial fatigue criteria to estimate fatigue lives and they were compared with available experimental fatigue tests results to investigate if these criteria predict the fatigue lives of such specimens reasonably. The comparison of the experimental and estimated fatigue lives showed that for different specimen batches different criteria can give reasonable lives.

Experimental and numerical simulation of carbon manganese steel for cyclic plastic behaviour

The paper deals with finite element modeling of saturated low cycle fatigue and the cyclic hardening phenomena of the materials Sa333 grade 6 carbon steel and SS316 stainless steel. Von Mises’ yield function and Chaboche’s kinematic hardening rules have been used. The recall terms of Chaboche’s kinematic hardening rule is calibrated with a memory function. The evolution law of the memory function is determined from experimental observations. The material constants are identified and derived from experimental results. The FE simulated results are compared with the experimental results from virgin state to saturation state of the materials. Keywords: cyclic plasticity, kinematic hardening, low cyclic fatigue, dilatation

Plastic Deformation Characteristics and Evaluation of Press Formability for Ti-6Al-4V Sheet at Warm Temperature

Titanium alloy sheets have excellent specific strength and corrosion resistance as well as good performance at high temperature. Recently, titanium alloys are widely employed not only aerospace parts but also prosthetics and motorcycle. However, titanium and its alloys are difficult-to-form materials due to limited slip system and plastic anisotropy. Titanium alloy sheets were usually formed by slow forming or hot forming with heating die and specimen. In the sheet metal forming area, FE simulation technique to optimize forming process is widely used. To achieve high accuracy FE simulation results, identification of material properties and deformation characteristics such ad yield behaviors are very important. In this study, the yield locus of Ti-6Al-4V sheet was obtained at warm temperature. The experimental results are compared with the theoretical predictions. Also, the Forming Limit Curves (FLC) was achieved at warm temperature.

Combined kinematic/isotropic hardening behavior study for magnesium alloy sheets to predict ductile fracture of rotational incremental forming

In order to predict the ductile fracture of rotational incremental forming for magnesium alloy sheet , a combination of kinematic and isotropic hardening law is implemented and ev aluated from the histories of ductile fracture value (I) by means of finite element analysis. Here, the criterion for a ductile fracture, as developed by OYANE, [J. Mech. Work. Tech. 4 (1980), pp. 65-81], is carried out via a user material, using finite el ement code. To simulate the effect of the large amount of heat generation at elements in the contact area due to friction energy of the rotational toolspecimen interface on equivalent stress-strain evolution in incremental forming, Johnson -Cook model was applied and also compared with equivalent stress -strain curves obtained by tensile test at elevated temperatures. The FE simulation results of ductile fracture was then compared with the experimental results of 80 mm × 80 mm × 25 mm square shape with 45°, 60°, and 80 mm × 80 mm × 20 mm square shape with 70° wall angles. The trend of FE simulation results were quite in good agreement with experiment results. Finally, the effect of process parameters e. g tool down -step and tool radius on the ductile fracture value and forming limit curve at fracture (F LCF) were investigated using FE simulation results

Low Velocity Impact Behavior of Aluminum Honeycomb Structures

Impact behaviors of aluminum honeycomb sandwich panels (AHSPs) are investigated experimentally by using a drop weight test setup. The specimens of 12.7 mm cell size were tested by impacting at four different initial contact points, namely face center, corner, long edge and short edge of core cells, with two impactors of weights 5.25 kg and 11.9 kg, respectively. Dynamic nonlinear transient analysis was also carried out by a finite element simulation model developed based on continuum damage mechanics to account for nonlinear and elastoplastic behavior. The results revealed that while the impact behaviors of AHSPs were nearly the same for low impact energy, they were different for high impact energy. The peak resistance force of AHSPs was the highest for impact at the face center and the lowest for impact at the short edge of the core cells. The results of FE simulation revealed that the real time deformation produced fracture when the crack initiated and propagated to the honeycomb core from the facesheet.

Estimating Subgrade Reaction Modulus for Transversely Isotropic Rock Medium

Most of the rock medium possesses intrinsic grain orientation or preferred bedding and joint directions, thus requiring the use of at least transverse isotropy to describe its elastic behavior. This paper presents a series of charts, based on extensive finite element parametric studies along with nonlinear regression analysis of FE simulation results, for estimating the subgrade reaction modulus (or initial tangent to the p-y curve) using five elastic constants of a transversely isotropic rock mass. The proper characterization of subgrade reaction modulus is critical for accurate prediction of the elastic lateral deflection of a rock socketed drilled shaft under the applied lateral loads. The sensitivity of the response of a laterally loaded drilled shaft to the degree of anisotropy and orientation of the plane of anisotropy (bedding plane direction of the rock medium) was demonstrated in this paper for an actual lateral load testing case in Ohio. It is highly recommended to use five elastic constants to estimate subgrade reaction modulus of rock medium exhibiting high degree of cross anisotropy.

Preform map for mild steel upsetting and its experimental verification

In upsetting process, an initial block of metal (billet) is compressed between two or more dies to produce a complex part. Geometry of the final product is strongly dependent on the shape of initial work piece as well as on the perform shapes at each of the subsequent forming stages. Design of the optimum preform for near net shape manufacturing is a crucial step in the designing of many upsetted products of mild steel. In this study, the same is realized by using profile map, which is generated using the results of FE simulations of varying geometrical and processing parameters. The map is further verified experimentally using a mild steel specimen. It is shown that preform designed on the basis of profile map results in near net shape manufacturing. Such map offers a powerful tool for near net shape upsetting.

3D mathematical modelling for robotic pick up of textile composites

An area of interest in the automated manufacture of composite components is the prediction in real-time of the deformed shape of a textile reinforcement in 3D space during robotic handling operations. The deformed shape can be used to guide robotic end-effectors to ensure accurate fabric placement and avoid collisions. In this paper, a nonlinear mathematical model using large deflection plate and shell theories is presented. The model is able to predict the 3D deformed shape of limp sheet materials being picked-up by multi robotic grippers for three boundary conditions. The main factors affecting the deformation behaviour of the sheet during the operation are identified and analysed, and the contributions of different energies during deformation are presented in detail. Good agreement is obtained when comparing the solutions of the model with FE simulation results. This study demonstrates the possibility of developing a modelling capability for material on-line response in automatic flexible material handling.

Al6061 합금의 열간 압출공정에서 금형 냉각시스템에 의한 압출재의 결정립 성장 제어

In this study, die cooling system using the nitrogen gas has been applied to hot aluminum extrusion process for refining grains and reducing of grain growth. Computational fluid dynamics(CFD) has been carried out to evaluate die cooling effect by nitrogen gas, and the results of CFD have been used to FEsimulation for the prediction of the extrudate temperature in hot extrusion process. Experimental hot extrusion has been performed to observe microstructure and to measure temperature of extrudate. The results of FESimulation have been good agreement with those of experiment. Finally, process condition of hot extrusion can be established to reduce grain growth of Al6061 through the experiment.

An FE analysis for assessing the effect of short-term exposure to elevated temperature on residual stresses around cold expanded fastener holes in aluminum alloy 7075-T6

A previous experimental study revealed fatigue life reduction in Al 7075-T6 cold expanded fastener holes exposed to 120 °C for 1 h. The obtained experimental evidence indicated a residual stress reduction associated with material softening at elevated temperatures, termed as thermo-mechanical stress relaxation. In order to identify and characterize the potential features of this phenomenon, FE analysis is carried out in this study and a detailed body of evidence is provided for occurrence of a time-independent thermo-mechanical residual stress relaxation around cold expanded fastener holes due to exposure to elevated temperature. The results of FE simulation demonstrate a good agreement with experimental results obtained earlier.

Ductile tearing resistance of metal sheets

The concept of R-curves has been adopted to characterise stable crack extension and predict residual strength of thin-walled structures particularly in the aircraft industry. The present contribution uses results of FE simulations of crack extension in panels by the cohesive model to validate analytical procedures for determining J-integral values at large crack extension from measurable quantities, namely the force vs. displacement records. The numerically determined J-integral is taken as the benchmark for the outcome of the analytical formulas. The geometry dependence of J and CTOD based R-curves is investigated and alternative concepts like CTOA and dissipation rate at crack extension are discussed.

스플라인 이형인발을 위한 중간 다이 단면형상 설계

The cross sectional shape of intermediate die is one of important parameters to improve dimensional accuracy of final product in shaped drawing process. Until now, it has been designed by the experience or trial and error of the expert. In this study, the cross sectional shape of intermediate die for spline shape is determined by the electric fields analysis and scale factor method. The result of the electric fields analysis and scale factor method have been compared with that of the expert method. The effects of cross sectional shape on the dimensional accuracy were investigated by using FE-simulation. And then the multi-stage shaped drawing experiments were performed to verify the results of FE-simulation. As a result, the cross sectional shape from the electric fields analysis and scale factor method had the good dimensional accuracy. These two methods can be used for the method to obtain the cross sectional shape of intermediate die in shaped drawing process.

Analysis of Residual Stress in Laser Welded Aerospace Aluminium T-Joints by Neutron Diffraction and Finite Element Modelling

The residual stress distribution in three laser welded T-joint configurations of aerospace aluminium alloys were measured using neutron diffraction and FE-simulation was conducted to compare with the experimental results. A 2 mm thick AA 6013 T6 sheet (as clip) was welded to a 4.5 mm thick and 400 mm wide AA 6156 T4 base plate (as skin of the airframe). In two samples, the thickness of the plate was reduced in some areas after welding to produce so-called “pockets” with the purpose of the weight reduction to resemble the fabrication practise in aircraft industry. The effect of pocketing process, which produced two different geometries around the clip weld on the residual stress evolution was analysed. In the plain sample (without pockets), residual stresses were predicted using the SYSWELD finite element software. The strain measurements on the base plate were performed at three locations; namely, the middle of the weld length (mid-clip), welding start (run-in) and end (run-out) locations. In all welded plates, slightly higher longitudinal tensile residual stresses were detected at the midclip locations, whereas transverse residual stresses were similar for all locations. In the run-out location, higher longitudinal tensile residual stresses were present than in the run-in location, which was the case in our previous results on other samples. The first results of the SYSWELD FE-simulation of the plain sample were compared with experimental results. The comparison has shown particularly good agreement for the transverse stresses. Although the simulation yields higher longitudinal tensile stresses than the experimental results, the stress distributions were very similar.

Process analysis and die design in 12 cells condenser tube extrusion of Al3003

Abstract Condenser tubes are mainly produced by precision extrusion with a porthole die and are used to allow flow in refrigerant cooling systems in automobiles. To produce condenser tubes, the recent goal has been to develop a tube that has high strength to allow for multi-cellizing and flattening, and to minimize the wall thickness, and to increase the heat transfer area and heat efficiency. This study was designed to produce Al3003 condenser tube for industrial production, including extrusion die design, inspection of die design through the results of FE simulation and performance of trial extrusion. Chamber shape dimension and initial temperatures of tool and die were variables and the possibility of extrusion was estimated from the forming load, welding pressure, and stress analysis of die in this paper. The validity of the simulated results was verified by extrusion experiments on the condenser tubes.

Determination of critical fracture energy, Gfr, from crack tip stretch

Characterization of ductile fracture is a major problem, especially in the nuclear industry where the material behaviour in the upper shelf region is important. Use of the J−Δ a curve to cover the risk of ductile fracture is popular among the designers. But, it is well known that the J−Δ a curve can be geometry dependent, so, transfer of laboratory test data to the components is a problem. Serious attempts have been made in the recent past to identify a unique material function (independent of geometry) to characterize ductile fracture. Those attempts are partly successful, but there are still many aspects to be solved. The present work aims at the same direction as mentioned above. The paper has three distinct parts, namely a theoretical model, FE simulation and discussions on FE-simulated results. In the theoretical model attempts have been made to relate critical fracture energy, G fr, with crack tip deformation (stretch) by using the final stretch theory of Michael Wnuk together with a cohesive traction–separation model. The objective of this model is to extract G fr from local deformation of the crack tip so as to identify it as a crack growth parameter. Next, this model has been successfully implemented in FE simulation of CT specimens and TPBB specimens of ductile materials. G fr is obtained from the local crack tip stretch together with other associated results. Finally, the results obtained from FE simulation are analysed. It is found that G fr is a fracture parameter connected to crack tip deformation and it can simulate ductile crack growth process in a steady-state condition.