Hemispherical Punch Research Articles

Hole-flanging behaviour of dual-phase steel using conventional and two-stage incremental forming processes

ABSTRACT The present study aimed to analyse the hole-flanging behaviour of dual -phase steel using conventional and incremental forming processes. The sheet with different hole diameters (40, 30 and 20 mm) are prepared and tested. During conventional flanging operation, the effect of two different punch geometries (flat bottom and hemispherical) are used to study the hole-flanging ability. Whereas, in incremental forming operation, different tool paths are generated, i.e. single (90°) and two stage (45° + 90°). The results shows that the holes flanged using conventional process failed to flange the lower diameter hole which is attributed due to the higher tensile circumferential stresses acted along its circumference. Whereas, for hole flanged using incremental technique has shown similar observations except in case of two-stage strategy. The implementation two-stage strategy helps in successful formation of the flanges for lower diameter holes compared to the remaining one. Additionally, finite element simulations are performed to study various critical parameters such as strain path and thickness. The maximum value of strains and thickness reduction is found near the hole edge. The highest thinning (65%) is found in single-stage process and least (38.04%) in conventional flanging with hemispherical punch.

Effect of impact-corrosion coupling damage on fatigue properties of 2198-T8 aluminum-lithium alloy

The peeling of aircraft surface paint due to foreign object impact can expose the aluminum–lithium alloy substrate to atmospheric corrosion, resulting in the fatigue performance being affected by the combined effect of impact damage and corrosion damage. In this work, the effects of impact damage and corrosion damage on the fatigue life of 2198-T8 aluminum–lithium alloy sheet were investigated. The strain field distribution of specimens with coupled impact and corrosion damage under cyclic loading was analyzed by Digital Image Correlation (DIC) technique, and the coupling relationship between impact and corrosion damage was analyzed based on the crack propagation path and fracture morphology. The results show that surface corrosion pits depth distribution of impact dents follows the same trend as residual stress distribution. For the coupling damage of the hemispherical punch, the coupled effect between impact damage and corrosion damage is most evident at low impact energy. At higher impact energies, the impact damage dominates the coupling damage. As the salt spray corrosion time increases, the fatigue life decrease rate gradually slows with increasing impact energy, and the impact damage has a greater effect on the fatigue life in the early corrosion stage. Compared to the salt spray corrosion environment at 35 °C, the fatigue life of the specimen at 25 °C increased by a maximum of 50.48 % as the impact energy increased from 10 J to 25 J.

Study of single point incremental forming limits of Al 1050/Mg-AZ31B two-layer sheets fabricated by roll bonding technique: Finite element simulation and experiment

In this work, for the first time, the two-layer Aluminum (Al 1050) and magnesium (AZ31B) sheet were used in the single point incremental forming (SPIF) process. Due to the high strength-to-weight ratio of the Al 1050/Mg-AZ31B sample, today it is used in significant industries such as automotive and aerospace. The finite element simulation process was carried out using the Abaqus Software considering five approaches to determine forming limits. These approaches were forming limit diagram criterion (FLDcrt), effective strain rate (ESR), second derivation of thinning (SDT), Major strain rate (MSR) and Thickness strain rate (TSR) methods. The results showed that the failure prediction in the FLDcrt method was more accurate than other methods. The thickness distribution process was carried out both by simulation and experiment to validate the simulation results. The results show a good agreement between simulation and experimental results. Finally, the results obtained from the SPIF process were compared with the FLD results of the conventional hemispherical punch test, and it was observed that the FLD was higher in the SPIF process. Moreover, the layer arrangement, which affects formability, was investigated. The results showed that the FLD with Mg-AZ31B/Al 1050 arrangement fails in higher strains compared to Al 1050/Mg-AZ31B. After validating the simulation results, the parameter effect of the step-down and tool diameter on the two-layer formability was investigated by the design of experiment. Considering the greater importance of the step-down and tool diameter effect on the created stresses and forming time compared to other parameters, their effect will be investigated. The results showed an optimal condition for the tool diameter, but an opposite relationship with formability was obtained for the step-down.

Strain Evolution during Stretch Forming with a Hemispherical Punch in 5052 Aluminum Alloy Sheets

Strain Evolution during Stretch Forming with a Hemispherical Punch in 5052 Aluminum Alloy Sheets

Measurement device for tear defects during preforming of non-woven fabrics made of recycled carbon fibres

Non-woven fabrics, known for their mechanical and acoustic properties, have gained attention in many industries, including the automotive industry. Use of recycled carbon fibres (rCF) in non-woven structures is a cost-effective and sustainable solution. However, the deformability and tear characteristics of these fabrics during preforming are not clearly understood. This study introduces a novel measuring instrument that accurately detects and quantifies the local mass loss in non-woven fabrics during preforming on a hemispherical punch. The instrument demonstrated satisfactory accuracy in identifying and quantifying tear defects, highlighting the high sensitivity of non-woven rCF materials to tear damage during formation. The tear damage was dispersed across different angles, indicating widespread susceptibility to tearing throughout the material.

Numerical and analytical analyses of the formability and fracture of AA7075-O aluminum sheets in hemispherical punch tests

In this study, numerical simulations and experimental validations of the simulation of hemispherical punch tests for AA7075-O aluminum sheets with various sample widths are presented. It is convincingly believed in literature that a finite element (FE) based forming limit diagram (FLD) can be predicted very well using various time-dependent criteria. This matter has been investigated and securitized deeply and the FE-based FLD was predicted using three time-dependent criteria and compared with the experimental FLD. Results show that two of the three criteria could predict the relatively appropriate shape of the FLD while one of them has failed to match. Aside from using FE simulation to predict the FLD, an analytical method, namely, Marciniak-Kuczynski (M-K) was also used to make a better evaluation about the capability of the finite element method (FEM) to predict FLD. Among the FE-based FLDs, the one predicted using the Martinez-Donaire et al., 2014 was found to be close to the one predicted using the M-K method. Although, the main objective of this paper is to investigate the capability of the FEM to predict an FLD for AA7075-O aluminum sheets, the fracture behavior for each case was predicted using a unified fracture model. Several computer runs were executed to perform the tests and reproduce the experimental results. The aim of this study was to discuss the simulation of hemispherical punch tests and analyze the numerical results and compare them with the experimental and analytical methods. It was observed that the experimental FLD can be reproduced using various time-dependent criteria in a relatively appropriate manner but with limitations especially at the equibiaxial stress state. These limitations were scrutinized and discussed in detail by comparing the results with experimental and analytical calculations.

Inverse Identification of Drucker–Prager Cap Model for Ti-6Al-4V Powder Compaction Considering the Shear Stress State

Numerical simulation is an important method to investigate powder-compacting processes. The Drucker–Prager cap constitutive model is often utilized in the numerical simulation of powder compaction. The model contains a number of parameters and it requires a series of mechanical experiments to determine the parameters. The inverse identification methods are time-saving alternatives, but most procedures use a flat punch during the powder-compacting process. It does not reflect the densification behavior under a shearing stress state. Here, an inverse identification approach for the Drucker–Prager cap model parameters is developed by using a hemispherical punch for the powder-compacting experiment. The error between the numerical and experimental displacement–load curves was minimized to identify the Drucker–Prager cap model of titanium alloy powder. The identified model was then verified by powder-compacting experiments with the flat punch. The displacement–load curves acquired by numerical simulation were compared to the displacement–load curves obtained through experiments. The two curves are found to be in good agreement. Meanwhile, the relative density distribution of the powders is similar to the experimental results.

Effect of External Compression on the Thermal Runaway of Lithium-Ion Battery Cells during Crush Tests: Insights for Improved Safety Assessment

To gain better understanding of the safety behavior of lithium-ion batteries under mechanical stress, crush tests are performed and reported in literature and in standards. However, many of these tests are conducted without the use of a cell clamping device, whereas external pressure is applied to the cell in a battery module in applications such as in an electric vehicle. The objective of this manuscript is to determine the effect of differing external compression on the thermal runaway of battery cells. Therefore, in this study, crush tests are performed with a hemispherical punch in a battery cell test chamber on commercially available 5 Ah pouch cells in a clamping device at four different normal stresses. The results are compared to cells that are free to expand with gas evolution. It is shown that applying compression to the cells not only results in a greater reproducibility of the experiments but that it also affects the thermal runaway process itself. With decreasing clamping stresses, the reaction time of the thermal runaway is increased by up to 19%, and the mass ejection is decreased by up to 10%, which, in turn, strongly influences the measurable gas concentrations by up to 80%. Based on this, a defined clamping compression was selected to obtain comparable results for different cell formats.

Determining the influence and correlation for parameters of flexible forming using the random forest method

Single-point incremental forming (SPIF) enables the forming of fix-clamped sheet metal by moving a relatively small geometrically simple tool along the trajectory, producing the desired shape of the final product. Excessive thinning of the sheet results in fracture, determining the limit of formability. This characteristic of the forming process can be improved by upgrading the basic SPIF process to two-step forming, whereby a more even distribution of the sheet thickness can be achieved by pre-bulging with a hemispherical punch. This study focused on analysing the SPIF process and a hybrid two-step forming consisting of sequential bulging and SPIF. The analysis focused on the output parameters of sheet metal thinning and maximum forming force components and was conducted with Abaqus simulation software. An innovative new approach for influence analysis of technological, material and geometrical input parameters and correlation analysis between the mentioned parameters was performed using the random forest (RF) method, which allows the determination of individual parameter influence by analysing tree-shaped models obtained through the training process. The analysis results show a significant influence of the workpiece wall angle and part depth on thinning and initial sheet thickness on values of the forming force components. The results also show a great correlation between the parameters of the bulging depth and the part depth after SPIF and the significant influence of the appropriate choice of the backing plate geometry for the target product geometry.

Strain evolution during hole expansion testing of 800 MPa tensile strength hot-rolled steels

One limitation of the standardized ISO 16630 hole expansion test is that it provides only one result: limiting hole expansion ratio (HER). In practice, steels with similar HER values can have different cut edge forming behavior due to possible differences in strain localization tendencies. Digital image correlation (DIC) strain measurement during formability testing allows more in-detail analysis of strain-state near the cut edge. In this paper, strain evolution during hole expansion testing was investigated for three 800 MPa tensile strength grade hot-rolled strip steels. The steels were selected to have differences in microstructures and anisotropies of mechanical properties. Two different hole expansion test methods with DIC strain measurement were utilized to investigate different edge loading scenarios: in-plane stretching with a flat-top punch and more out-of-plane stretching with a hemispherical punch. Test holes were prepared according to ISO 16630 standard. In order to examine strain evolution and localization during testing, strains were measured with circle-shape sections around the hole in various distances from the cut edge. Strain localization behavior was investigated in different sheet directions and the effect on the hole expansion ratio was evaluated. Results show considerable differences in the cut edge forming behavior between the investigated materials.

Development of a Detailed 3D Finite Element Model for a Lithium-Ion Battery Subject to Abuse Loading

<div class="section abstract"><div class="htmlview paragraph">Lithium-ion batteries (LIBs) have been used as the main power source for Electric vehicles (EVs) in recent years. The mechanical behavior of LIBs subject to crush loading is crucial in assessing and improving the impact safety of battery systems and EVs. In this work, a detailed 3D finite element model for a commercial vehicle battery was built, in order to better understand battery failure behavior under various loading conditions. The model included the major components of a prismatic battery jellyroll, i.e., cathodes, anodes, and separators. The models for these components were validated against the corresponding material coupon tests (e.g., tension and compression). Then the components were integrated into the cell level model for simulation of jellyroll loading and damage behavior under three types of compressive indenter loading: (1) Flat-end punch, (2) Hemispherical punch and (3) Round-edge wedge. The comparisons showed reasonable agreement between modeling and experiments. With the validated numerical model, parametric studies were further performed to analyze the effect of separator anisotropy, to highlight its important role in the overall structural response of LIBs.</div></div>

The effects of friction stir welding on microstructure and formability of 7075-T6 sheet

The present research aims to investigate the effects of friction stir welding (FSW) on the microstructure and mechanical properties of 7075-T6 aluminum alloy thin sheets. The welding has been performed perpendicular to the rolling direction of the sheets at three rotational speeds of 600, 1000, and 1600 rpm. From mechanical and microstructural points of view, a suitable welding condition has been recognized at 1000 rpm and welding speed of 50 mm/min. It is observed that with increasing the welding rotational speed more precipitates of the base metal are being dissolved in the metallic matrix and their distribution is more uniform. This phenomenon is recognized as the main mechanism of mechanical properties decline at 1600 rpm. Surprisingly 15,000 h after the welding process, the natural aging could not recover the precipitations within the welding zone of the specimen welded by 1600 rpm rotational speed. The effects of FSW on the three-dimensional forming behavior of the joints have been examined by the hemispherical punch stretching (HPS) method. Forming limit diagrams (FLD) of the weldments and the starting material have been measured and compared to each other. The formability examinations have demonstrated a 40% decline in the forming limit of the welded specimens in comparison to the base metal. The main reasons for the smaller formability of the welded specimens are comprehensively discussed.

Polar effective plastic strain - forming limit diagram of DP 440 steel sheet with application to two-step forming test

In this work, experimental forming limit curve (FLC) and polar effective plastic strain -forming limit diagram (PEPS-FLD) are developed in order to predict the formability of dual phase (DP) steel grade 440. The Forming Limit Diagrams (FLD) of the examined steel are first obtained using the Nakajima stretch-forming test with a decreased hemispherical punch diameter of 50 mm in combination with Auto-Grid Vialux strain measurement system. To achieve a PEPS-FLD, an experimental FLC data must be completely projected onto the polar space using the results of FLC, Hill's 48 anisotropic yield criterion, and PEPS algorithm through plasticity calculation. Finally, a two-step forming test is conducted both experiments and simulation to demonstrate the practical applicability of the PEPS-FLD. It really would appear that the PEPS-FLD could predict the formability in non-linear strain path and can be utilized to enhance formability by creating non-linear strain path of DP440 more accurately than FLD.

Investigations on sheet metal forming of hybrid parts in different stress states

Conventional processes are being pushed to their limits by growing demands in terms of sustainability and diversity of variants. Hybrid components, which are produced by a combination of two or more manufacturing processes, are a suitable way of meeting these challenges. The combination of sheet metal forming processes with additive manufacturing offers the potential to link personalized components with standardized parts. Knowing that the additively manufactured components influence the forming process, it is essential to understand the interactions in detail. Therefore, this work will compare the influence of several additively manufactured elements (AME) for deep drawing with two different punch geometries. The approach used combines experimental and numerical investigations to improve process insight in relation to sheet metal forming of hybrid components. The results show that the AMEs amplify existing stresses and strains in dependence of the present load. Sections subjected to low loads, as it is found in the bottom of the cup manufactured with a cylindrical punch, are hardly affected, whereas stronger loaded areas, e.g. the center of the parts manufactured with a hemispherical punch, are affected all the more.

Comparative study of Li-ion 21700 cylindrical cell under mechanical deformation

Lithium-ion batteries (LIBs) are an important component of the automotive industry in terms of electrification. Regarding their safety, the researchers perform different scenarios in which they follow closely and carefully their behavior under the action of induced requests. The most common mechanical abuse tests are three-point bending, simple bending, deformation with hemispherical punch head and nail penetration. In this article, the behavior of the 21700 lithium-ion cell is debated and the results are compared. First, experimental tests are performed and the results are validated with FEM model, to check the veracity of the experiment. Also, the interpreted results can be used in future researches regarding safety of batteries.

Experimental study and machine learning model to predict formability of magnesium alloy sheet.

Background: Magnesium alloy is not only light in weight but also possesses moderate strength. Magnesium AZ31-H24 alloy sheet has many applications in the automotive and aerospace industries. Experimental stretch forming tests are performed on this sheet to measure the material's formability by constructing forming limit diagrams. Methods: Several tests of Nakazima were carried out on rectangular samples at 24, 250, 350°C and 0.01, 0.001 mm/s using a hemispherical punch. The work done to predict the formability of magnesium alloys has not been recorded in recent literature on machine learning models. Hence, the researchers of this article choose to explore the same and build three models to predict the formability of magnesium alloy through Random Forest algorithm, Extreme Gradient Boosting, and Multiple linear Regression. Results: The Random Forest showed high accuracy of 96% in prediction. Conclusions: It is concluded that the need for physical experiments can be greatly minimized in formability studies by using machine learning concepts.

A Critical Evaluation of Forming Limit Curves Regarding Layout of Bending Processes

Forming limit curves (FLCs) are important to predict failure against biaxial deformation in sheet metal forming. Particularly crucial is the detection and evaluation of the stable/instable transition which indicates necking and ultimately leads to rupture. In the past, it has been observed that specific processes, in particular free-form bending processes, might not be predicted well by conventionally evaluated FLCs, i.e. the Nakajima experiment, where tapered sheet metal specimens are stretched over a hemispherical punch until material failure. In the present study, FLCs are evaluated from such Nakajima tests and from notched tension tests (NTT) highlighting large differences in between both results. The differences in between the evaluation methodologies are discussed with respect to contact and bending strain in the forming zone. The FLCs of three tested sheet metal materials are compared to fracture strain resulting from an incremental bending process with free forming zone demonstrating an increased failure prediction accuracy by the NTT FLCs than by the conventional ones. In the light of these results, it is therefore encouraged to critically assess the application of FLCs obtained from NTTs to various free-form bending processes.

Determination of Limiting Dome Height (LDH) values for Inconel 718 alloy sheet using FEA and Hemispherical Punch Method

In this work, initially the chemical composition of Inconel 718 alloy sheet was obtained using X-ray fluorescence technique (XRF). The material properties such as yield strength, ultimate tensile strength, percentage of elongation, normal anisotropy, planar anisotropy, strain hardening exponent, and the strength coefficient were determined in longitudinal, diagonal, and transverse rolling direction using a uniaxial tensile test. The Limiting Dome Height (LDH) values are obtained by simulation and experimentation based on the hemispherical punch method. In simulation work, Barlat-89 yield criterion was used to obtain the Limiting Dome Height (LDH) values and strain distribution along with the specimen in Abaqus 6.1 software. The experimental LDH values of Inconel 718 alloy sheet and simulated results obtained from Abaqus 6.1 software were in close agreement. The approach presented in this work can be applied to obtain the LDH test values of interesting material focused by researchers. With the help of experiments, a limit curve was established which ensures the safe working region of Inconel 718 sheet. Scanning Electron Microscope (SEM) analysis of 100 &120 mm width specimens indicated smooth surface and ductile fracture. The examination of 140 &160 mm width specimens showed rough surface and shear-ductile failure. Energy Dispersive Spectrum (EDS) analysis of a fractured surface confirms the constituents of the sheet present before failure.

A coupled thermal–electrical–mechanical analysis for lithium-ion battery

A lithium-ion battery simulation model is developed for a fully coupled thermal–electrical–mechanical analysis. The model is calibrated with the hemispherical punch test for its mechanical response and an external short-circuit test for its electrical property. Realistic physical property and representative geometry are used to model each battery component. Randles circuit is used to represent the electric property of the battery. A piecewise linear plasticity model is applied to model the deformation and failure of each cell material. The simulation result of the external short-circuit test suggests that most heat is generated at the layer of cathode material near the current-collecting tabs. The punch test of a discharging battery is simulated; the result suggests the mechanical deformation can cause extra Ohm heating at the deformed location. The mechanically generated heat caused by impact also plays a role in the rise of the cell temperature.

Characterization of in-situ material properties of pouch lithium-ion batteries in tension from three-point bending tests

To understand the mechanisms of deformation and failure of lithium-ion batteries in case of a crash, it is necessary to accurately characterize their constitutive properties. Previous studies in the field mainly focused on the homogenized compression properties of the cells, which are dominant in load cases such as local indentations. However, in complex practical loadings, such as bending, tension properties play an equally important role. Such complex loads can lead to rupture of the electrode tabs and external short-circuits, which can cause catastrophic outcomes. In the current literature, tensile properties are characterized using specimens extracted from the cell, outside of their operational environment, which leads to unrealistic values. In this study, for the first time, an analytical characterization method is developed to extract the homogenized tensile properties of a cell from bending tests in in-situ conditions. In the next step, this data is used, along with an elaborative procedure, to calibrate a fully uncoupled anisotropic material model for the pouch cells. The material model is utilized to build a single homogenized finite element pouch cell model which is the first of its kind to be validated in all major loading cases; flat, hemispherical, and cylindrical punch indentations and specifically three-point bending, and in-plane compression. This material characterization and the modeling approach provide a universal tool in predicting the load-displacement, shape of deformation, buckling wavelength, and the trend of failure in complex crash scenarios for the safety assessments of Lithium-ion batteries.