Dome Tests Research Articles

Plane-Stress Deformation Behavior of CoCrFeMnNi High-Entropy Alloy Sheet under Low Temperatures.

High-entropy alloys are promising candidates expected to be applied in transportation equipment serving in extreme environments due to their excellent properties. CoCrFeMnNi high-entropy alloy is a typical representative of them, and its low temperature performance is excellent. In this study, to evaluate the feasibility of forming HEA shells, the deformation behavior of CoCrFeMnNi under a plane-stress state at lower temperatures was thoroughly studied. Firstly, a thin-walled HEA tube was fabricated using hot extrusion and further formed into a thin shell for uniaxial tensile and biaxial bulging tests. Subsequently, uniaxial tensile tests at cryogenic temperatures were conducted. Both the strength and the ductility improves as the temperature decreases from -160 °C to -196 °C. Then, a systematic low-temperature bulging test was performed using isothermal dome tests and the thickness uniformity analysis of the bulged specimens was carried out. In addition, grain microstructural observation using EBSD was characterized analyze the possible deformation mechanism at the cryogenic temperature under the biaxial stress state. This study, for the first time, investigated the biaxial deformation behavior of HEA. Considering the plane-stress state deformation is the dominant type in the thin-walled shell deformation, this study enables us to provide direct guidance for various sheet-forming processes of HEAs.

Sheared edge formability characterization of cold-rolled advanced high strength steels for automotive applications

Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.

Fracture Response in Hot-Stamped Tailor-Welded Blanks of Ductibor® 500-AS and Usibor® 1500-AS: Experiments and Modelling

Hot-stamped tailor-welded blanks (TWBs) comprised of steel sheets with different chemical compositions are of interest within the automotive sector to enhance the crash performance of vehicles via imparting local ductile regions to ultra-high strength structural components. This study examines the fracture behavior of hot-stamped TWBs composed of mono- and multi-gauge laser-welded sheets of Ductibor® 500-AS and Usibor® 1500-AS, which are under consideration for use in crash-safety members. Microstructural and microhardness investigations were conducted across the weld line of the TWBs to address the variations of mechanical properties. A fracture characterization campaign via a series of coupon tests, including uniaxial tension, equibiaxial tension Nakazima dome, and V-bend, was conducted on specimens with a range of orientations of the weld line with respect to the loading direction. Numerical models of the tensile and Nakazima dome tests with and without considering the weld region in the TWBs were developed using the LS-DYNA finite-element method software based on mapping constitutive properties and fracture-limit curves across the weld and in parent metals as functions of local hardness. The fracture characterization tests revealed that weld fracture occurred only in the V-bend tests for which the weld line was parallel to the punch (0° V-bend), while in all of the other mechanical tests considered, fracture was initiated in one of the parent metals. The Ductibor® 500-AS region was the site of fracture initiation in the transversely-welded tensile and centrally-welded Nakazima dome samples, whereas fracture initiation occurred in the Usibor® 1500-AS region for the longitudinally-welded tensile and 45° and 90° V-bend samples. The predictions revealed that the inclusion of the narrow weld region of the laser-welded blanks within the hardness-mapped TWB models resulted in an improvement of the predictions in terms of the load–displacement response and fracture locations compared to the “without weld” scenario. Therefore, the correlation of variations in the flow and fracture response of the hot-stamped TWBs of Ductibor® 500-AS and Usibor® 1500-AS with the trend of changes in hardness across the weld can be a more suitable strategy for modelling the mechanical behavior of such TWBs in crash-safety simulations.

Performance Evaluation of Planar Anisotropy Yield Criteria for Aluminum Sheet Forming Analysis

The present work aims making a step by identifying a set of standardized accuracy measures based on laboratory experiments. Uniaxial tension, plane strain tension, hemispherical dome, and square cup tests were conducted for a 6000 series aluminum sheet used as automotive panels. Principal strain distribution, punch height at fracture, draw-in, and wrinkling were measured and compared with numerical simulation predictions. The performance of the five yield criteria, Barlat89, BBC2005, Hill48, Vegter2017, and YSD6 (Yoshida 6th order polynomial function) was measured and evaluated using the conventional accuracy index and the proposed accuracy indexes that represent manufacturing defects encountered in practice. It is found that state of the art material models such as BBC2005, Vegter2017 and YSD6 are fully able of representing standardized material test results, which is also well reflected in the predictive accuracy of simulated hemispherical dome and square cup tests. Because earlier models such as Hill48 and Barlat89 lack in capturing a high number of experimentally determined data points, they should be used with caution even Barlat89 shows good performance in the hemispherical dome tests.

Forming limit evaluation of adhesive bonded sandwich steel sheets and numerical prediction

AbstractIn the present work, the effect of hardener to resin ratio of epoxy adhesive on the formability of adhesive bonded steel sheets has been studied. Forming limit curve has been evaluated by hemispherical dome tests at predefined strain‐paths. They are predicted by finite element simulations using strain mapping method. Cohesive zone model has been used to model the interface between skin and adhesive core. An improvement in the forming limit of sandwich steel sheets has been observed when the hardener to resin ratio is changed from 0.6 : 1 to 1 : 1 due to the improved plasticity of adhesive core layer at 1 : 1 ratio as compared to others. The forming limit of sandwich sheet made at 1 : 1 ratio is equivalent to base steel sheet of same thickness and grade. This shows the potential use of sandwich sheet in place of base steel sheet. The forming limit curve predictions agree well with experimental data for base sheet, while reasonable agreement is observed in case of sandwich sheet. Numerical prediction of interface delamination show insignificant influence of hardener to resin ratio on the onset of delamination and significant effect of strain‐paths.

Material characterization of high strength sound-deadening sheets and its application on a square cup drawing simulation

In this study, the mechanical properties of a high strength sound-deadening sheet (SDS) in the stretch deformation mode was characterized, and it successfully apply to the formability prediction of the square cup drawing tests. The high strength SDS was fabricated with dual phase (DP) 590 steel sheets as outer skins and polymeric adhesive layer as a core by a roll bonding process. Uniaxial and biaxial mechanical characteristics were evaluated by the uniaxial tension tests and the in-plane biaxial tension tests, respectively, while the forming limit was investigated by the punched dome tests. Then, based on the experimental data, the mechanical properties of SDS was characterized, viz., hardening curve, yield function and forming limit curve. Square cup drawing was carried out using the SDS, and its deformation and failure behaviour were successfully predicted via the finite element simulation performed by inputting characterized mechanical properties.

Characterization of the Mechanical Properties of a High-Strength Laminated Vibration Damping Steel Sheet and Their Application to Formability Prediction

In this study, the mechanical properties of a high-strength laminated vibration damping steel sheet (LVDSS) with stretch-dominated deformation behavior were characterized and successfully applied to formability prediction. The high-strength LVDSS was fabricated with dual-phase 590 steel sheets as outer skins and polymeric adhesive layer as a core via roll bonding. The uniaxial and biaxial mechanical behaviors were evaluated by performing uniaxial tension and in-plane biaxial tension tests, respectively, while the forming limit was investigated by conducting punched dome tests. Then, based on the experimental data, the mechanical properties of the LVDSS were characterized, viz., the hardening curve, yield function, and forming limit curve. As an example of LVDSS application, a square cup drawing test was conducted and a dashboard prototype was fabricated using an LVDSS, and the deformation and failure behaviors were successfully predicted via a finite element simulation performed by inputting the characterized mechanical properties.

Studies on texture and formability of Zircaloy-4 produced by pilgering route

Zircaloy-4 is a widely used material in nuclear industries due to its excellent corrosion resistance at elevated temperatures, low neutron absorption cross section and balanced strength and ductility. In the present study, the formability of Zircaloy-4 sheets produced through pilgering route has been investigated. Nakajima dome tests have been conducted on sheet samples taken along rolling, transverse and diagonal directions with respect to the rolling direction of the sheet to construct the forming limit diagrams (FLDs). The strain signature of deformed samples has also been studied to determine the peak limiting strains. Further, limiting dome height values are calculated and all the results are validated with FE-Simulations. Finally, XRD studies have been performed to understand the relationship between the texture and formability. The FLD and texture studies reveal that the Zircaloy-4 possesses higher drawability in comparison to that of the stretchability. The orientation of the specimen has negligible influence on FLD in tension–tension region and marginal influence in tension–compression region.

Forming of Adhesive-Bonded Sandwich Sheets with a Rubber Pad

The effect of a rubber pad, its quality, and diameter, on the forming limit of adhesive-bonded sandwich steel sheets is studied using hemispherical dome tests. The forming limit curve (FLC) is predicted with finite element simulations and the strain mapping method. The FLCs of sandwich sheets are compared with that of base, i.e., 0.4- and 1.2-mm-thick sheets. The forming limit of sandwich sheets when formed using rubber pads improve significantly, and is equivalent to that of the base sheet of equivalent thickness formed without a rubber pad. The rubber pad leads to a uniform strain distribution due to a cushioning effect provided by the rubber pad which also delays the onset of interface delamination. The polyurethane rubber performs better compared with the silicone rubber in improving the FLC. The effect of the rubber pad diameter is insignificant. The FLCs and forming height predicted by numerical simulations are reasonably accurate, when compared with the experimental results.

Failure prediction of AZ31B Mg sheet at room temperature considering material anisotropy and differential work hardening

Failure predictions of AZ31B Mg sheet at room temperature were conducted considering material anisotropy as well as differential work hardening for the range from uniaxial to balanced biaxial loading paths. The maximum shear stress criterion was used as a failure criterion, and both the associated and non-associated flow rules were employed along with the quadratic anisotropic functions to derive the analytic expressions for failure. Different levels of material anisotropy were considered in formulation, and procedures of determining the material coefficients considering the differential work hardening behavior were provided. Failure predictions using the analytical expressions were compared with the experimental results obtained by hemispherical dome tests. The results show that in the failure prediction of AZ31B Mg sheet, both the material anisotropy and the differential work hardening need to be addressed properly to make an accurate failure prediction.

Determination of forming limits of high strength sound-deadening laminated sheet

High strength sound-deadening laminated sheet consists of high strength steel sheets and a viscoelastic polymeric adhesive with high vibrational damping characteristics. In this study, to evaluate formability of sound-deadening laminated sheet, forming limit curves (FLC) were determined. The high strength sound-deadening laminate sheet was fabricated with dual phase (DP) 590 steel sheets as outer skins and polymeric adhesive as a core by roll bonding process. The punched dome tests (Nakajima tests) were performed with varying specimen width. On the specimen prior to deformation, a stochastic pattern was applied to the surface using a white and black spray. Until the fracture occurs, the strain distribution was measured in the stereo image using digital image correlation (DIC) system. FLC of the laminated was successfully determined by capturing limit strains near fracture as well as calculating them according to the ISO standards. FLC of the laminated sheet is slightly higher than that of DP590 sheet but is not significantly different.

Experimental and Numerical Investigation of Forming Limit Differences in Biaxial and Dome Test

For centuries, metals and materials have been characterized using a traditional method called a uniaxial tension test. The data acquired from this test found to be adequate for operations of simple forming where one axis stretching is dominant. Currently, due to the demand of lightweight component production, multiple individual parts eliminated by stamping a single complex shape, which also further reduces many secondary operations. This change is driving by the new fuel-efficiency requirement by corporate average fuel economy of 55.8 miles per gallon by 2025.1 Due to complex part geometry, this forming method induces multiaxial stress states, which are difficult to predict using conventional tools. Thus, to analyze these multiaxial stress states limiting dome height tests and bulge tests were recommended in many research publications. However, these tests limit the possibilities of applying multiaxial loading and rather a sample geometry changes are required to imply multiaxial stresses. Even this capability is not an option in bulge test due to leakage issue. Thus, a test machine called a biaxial test was devised that would provide the capability to test the specimen in multiaxial stress states by varying the independent load or displacement on two independent axis. In this paper, two processes, limiting dome tests and biaxial tests were experimented, modeled, and compared. For the biaxial tests, a cruciform test specimen was utilized, and conventional forming limit specimens were used for the dome tests. Variation of sample geometry in limiting dome test and variation of loading in biaxial test were utilized to imply multiaxial stress states in order to capture the limit strain from uniaxial to equibiaxial strain mode. In addition, the strain path, forming, and formability investigated and the differences between the tests provided. From the results, it was noted that higher limit strains were acquired in dome tests than in biaxial tests due to contact pressure from the rigid punch. The literature shows that the contact pressure (which occurs when the rigid tool contacts the deformed body), increases the deformation and thus increases the limit strains to failure. This contact pressure parameter is unavailable in biaxial test, and thus, a pure material behavior can be obtained. However, limit strains from biaxial test cannot be considered for a process where rigid tool is processing the metal, and thus, calibration is necessary.

Experimental fracture characterisation of an anisotropic magnesium alloy sheet in proportional and non-proportional loading conditions

A comprehensive experimental investigation was performed to characterize the fracture behaviour of a rare-earth magnesium alloy sheet, ZEK100-O, under both proportional and non-proportional loading conditions. This material possesses severe plastic anisotropy and tension-compression asymmetry that evolve with plastic deformation and is an excellent candidate to experimentally evaluate phenomenological fracture modelling strategies. Different types of specimen geometries were fabricated in different orientations with respect to the rolling direction of the sheet to reveal the anisotropic fracture response of the alloy. Moreover, three different types of plane-strain tension tests, namely, v-bend, butterfly, and Nakazima dome tests were conducted and compared in terms of their applicability for fracture characterization of sheet materials. To visualize directional dependency of the fracture response of the magnesium alloy, experimental fracture loci for different orientations were constructed. Furthermore, non-proportional tests were performed in which abrupt changes in stress state were imposed to study the role of the loading history on fracture behaviour of the alloy. The non-proportional tests entailed pre-straining the material in uniaxial and equi-biaxial tension up to a prescribed plastic work level, followed by extreme strain path changes to plane-strain tension and shear states. Non-proportional deformations with such severe strain path variations have not been reported in the literature for materials with complex anisotropic behaviour such as ZEK100-O. The results of which have enabled the direct experimental evaluation of phenomenological damage models without performing an inverse calibration from finite element simulations. Based on the results of the non-proportional tests, it was shown that simple damage indicators were unable to describe the influence of severe changes in the strain path on fracture.

Effect of anisotropy and differential work hardening on the failure prediction of AZ31B magnesium sheet at room temperature

In order to predict the failure of AZ31B Mg sheet at room temperature under uniaxial and biaxial loading paths, analytical expressions based on the maximum shear stress criterion in conjunction with the quadratic anisotropic yield function and plastic potential were developed. By utilizing the formulations based on the associated flow rule (AFR) and the non-associated flow rule (NAFR), different levels of material anisotropy were considered with the developed equations. Material coefficients of anisotropy were obtained from tensile and bulge tests taking into account the differential work hardening of AZ31B Mg sheet. Failure predictions were performed and compared with the experimental failure strains measured by the hemispherical dome tests at room temperature. In the failure prediction of AZ31B Mg sheet, it was found that the material anisotropy and the differential work hardening exhibited by the AZ31B Mg sheet were primary factors which have a major influence on the results of failure prediction, and the formulations based on the NAFR with the material coefficients obtained by taking into account the differential work hardening showed good accuracy by yielding failure predictions close to the experimentally measured failure strains.

Characterization of OT4-1 Alloy by Multi-Dome Forming Test.

In this study, the rheological characteristics of a titanium alloy have been obtained by multi-dome bulging test. Free bulging process is an experimental technique that can be used to characterize material in conditions of biaxial tension during superplastic, as well as conventional, hot forming. The constitutive constants are calculated on a base of the information about the bulge geometry, applied pressure, and forming time. A multi-dome forming test allows one to reduce the number of the experiments required for the characterization, since every multi-dome test produces several domes of different size. In this study, a specific die for multi-dome test was used. The die has six holes with different radiuses of 20, 25, 30, 35, 40, and 45 mm. During a test, the specimen is clamped between blank holder and die holder, heated to a specific temperature, and formed by applying constant gas pressure. The experiments were conducted at different temperatures for OT4-1 titanium alloy. The constitutive constants were obtained by processing the experimental data using two different techniques and compared with tensile test results. In order to estimate the influence of friction on the experimental results and to verify obtained material characteristics, finite element (FE) simulation was performed. Finally, the results of FE simulation were compared with the experimental data. The results of the simulation show the advantage of material characterization based on multi dome tests and its interpretation by inverse analysis. The deviations produced by the effect of friction are more significant when the direct approach is applied instead of inverse analysis with a semi analytical model of the bulging process.

Forming Limit Differences in Hemispherical Dome and Biaxial Test During Equibiaxial Tension on Cruciform

Traditionally, the mechanical properties of materials have been characterized using the uniaxial tension test. This test is considered adequate for simple forming operations where single axis loading is dominant. Previous studies, however, have noted that the data acquired from this type of testing are not enough and additional details in other axes under simultaneous deformation conditions are important. To analyze the biaxial strain, some studies have suggested using the limiting dome height test and bulge test. However, these tests limit the extent of using multi-axial loading and the resulting stress pattern due to contact surfaces. Therefore, researchers devised the biaxial machine which is designed specifically to provide biaxial stress components using multiple and varying loading conditions. The idea of this work is to evaluate the relationship between the dome test data and the biaxial test data. For this comparison, cruciform specimens with a diamond shaped thinner gage in the center were deformed with biaxial stretching on the biaxial testing machine. In addition, the cruciform specimens were biaxially stretched with a hemispherical punch in a conventional die-punch setting. Furthermore, in each case, the process was simulated using a three-dimensional (3D) model generated on abaqus. These models were then compared with the experimental results. The forces on each arm, strain path, forming, and formability were analyzed. The differences between the processes were detailed. It was found that biaxial tests eliminated the pressurization effect which could be found in hemispherical dome tests.

Hot stamping of AA6082 tailor welded blanks: experiment and FE simulation

An advanced forming technology, solution Heat treatment, Forming and in-die Quenching (HFQ ® ), has been employed to form AA6082 tailor welded blanks (TWBs). In comparison with conventional stamping of TWBs, the mechanical properties and formability of AA6082 laser TWBs could be improved under the HFQ ® forming condition. The TWB was divided into three physical zones, i.e. base metal, heat affected zone (HAZ) and weld zone, based on the hardness distribution. It was found that the degraded hardness of the weldment can be restored after HFQ ® forming. TWBs of AA6082 with different thickness ratios of 2 (2–1 mm), 1.3 (2–1.5 mm) and 1 (1.5–1.5 mm) were used to study the TWB thickness ratio effects on the forming behaviour. Hemispherical punch dome tests on the TWBs with varying thickness ratios demonstrated different formabilities, and indicated increased displacement of the weld line with increasing thickness ratio. Finite element (FE) modelling was adopted to analyse the weld line movement and strain distributions during HFQ ® forming.

Processing of thermoset prepregs for high-volume applications and their numerical analysis using superimposed finite elements

Prepreg compression moulding (PCM) potentially fulfils the requirements for high-volume applications of structural thermoset composites. Preforming of thermoset prepregs is a first step in PCM followed by consolidation and curing. High volume manufacturing necessitates automation of the preforming stage in order to achieve the required production rate and repeatability. The present work investigates the processing of thermoset prepregs based on in-plane shear characterisation tests along with preforming experiments using classical hemispherical dome tests. The second part of this work proposes a simplified numerical scheme for prepreg preforming analysis using superimposed finite elements. The elements share common nodes. One of the sets of elements is assigned with the bi-directional material data of fibrous reinforcement. The second set of elements is allocated to resin behaviour. The scheme has the advantages of defining controlled fibre-to-resin ratio and predicting the stress field within individual constituents in the subsequent structural analysis.Although the deformation behaviour of structural prepregs is largely driven by fibrous reinforcement, however, it has been observed here that the resin contributes to alter the shear rigidity of the material which eventually affects the shaping behaviour of the deformed prepreg. It has also been shown that the proposed numerical approach is capable of predicting the deformation behaviour of the prepreg.

The flow behavior and forming limit modeling of an Al-Cu-Mg aluminum alloy sheet considering the effects of pre-strain and solution heat treatment

Al-Cu-Mg aluminum alloy sheet is widely used for aircraft skin. Multi-stage forming combined with Solution Heat Treatment (SHT) is necessary for some geometrically complex productions in aircraft skin components. However, the effects of pre-strain and natural ageing time on the formability of Al-Cu-Mg aluminum alloy sheet are not fully understood. In this study, the deformation responses of an Al-Cu-Mg aluminum alloy sheet, over a range of pre-strain levels (0% to 6%) and natural ageing time levels (0 h to 2 h) were investigated. A modified Hollomon constitutive model coupling with pre-strain and natural ageing time was proposed to characterize the deformation responses of the Al-Cu-Mg aluminum alloy sheet. The hemispherical dome tests were performed at natural ageing time levels ranging from 0.25 h to 1.5 h. Marciniak and Kuczynski (M-K) theory was used to obtain the forming limit prediction model which takes pre-strain and natural ageing time into account. The theoretical Forming Limit Curves (FLCs) were compared with the experimental results.

HFQ forming of AA6082 tailor welded blanks

An advanced forming technology, solution Heat treatment, Forming and in-die Quenching (HFQ ® ), has been employed to form AA6082 tailor welded blanks (TWBs). In comparison with conventional stamping of TWBs, the mechanical properties and formability of AA6082 laser TWBs could be improved under the HFQ forming condition. The TWB was divided into three physical zones, i.e. base metal, heat affected zone (HAZ) and weld zone, based on the hardness distribution. It was found that the degraded hardness of the weldment can be restored after HFQ forming. TWBs of AA6082 with different thickness ratios of 2 (2-1mm), 1.3 (2-1.5mm) and 1 (1.5-1.5mm) were used to study the TWB thickness ratio effects on the forming behaviour. Hemispherical punch dome tests on the TWBs with varying thickness ratios demonstrated different formabilities, and indicated increased displacement of the weld line with increasing thickness ratio. Finite element (FE) modelling was adopted to analyse the weld line movement and strain distributions during HFQ forming. HFQ ® is a registered trademark of Impression Technologies Ltd.