Understanding the Differences in Hemispherical Dome and Biaxial Test During Equi-Biaxial Tension on Cruciform

Author(s):  
Chetan P. Nikhare ◽  
Emmett Vorisek ◽  
John Nolan ◽  
John T. Roth

One metal manufacturing process which uses thousands of processes to trim, stretch, draw, bend etc. under a big umbrella is sheet metal forming. Using heavy equipment, the sheet metal parts are deformed into complex geometries. The complexity in these parts produces multi-axial stress and strain, a state for which it is critical to analyze using conventional tools. 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 is 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 bi-axially stretched with a hemispherical punch in a conventional die-punch setting. Furthermore, in each case, the process was simulated using a 3D model generated on ABAQUS. These models were then compared with the experimental results. The forces on each arm, strain path, forming and formability was 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.

Author(s):  
Chetan P. Nikhare ◽  
Emmett Vorisek ◽  
John R. Nolan ◽  
John T. Roth

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.


Author(s):  
Chetan P. Nikhare

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.


Author(s):  
Eren Billur ◽  
Muammer Koc¸

Hydraulic bulge testing is a material characterization method used as an alternative to tensile testing with the premise of accurately representing the material behavior to higher strain levels (∼70% as appeared to ∼30% in tensile test) in a biaxial stress mode. However, there are some major assumptions (such as continuous hemispherical bulge shape, thinnest point at apex) in hydraulic bulge analyses that lead to uncertainties in the resulting flow stress curves. In this paper, the effect of these assumptions on the accuracy and reliability of flow stress curves is investigated. The goal of this study is to determine the most accurate method for analyzing the data obtained from the bulge testing when continuous and in-line thickness measurement techniques are not available. Specifically, in this study the stress-strain relationships of two different materials (SS201 and Al5754) are obtained based on hydraulic bulge test data using various analysis methods for bulge radius and thickness predictions (e.g., Hill’s, Chakrabarty’s, Panknin’s theories, etc.). The flow stress curves are calculated using pressure and dome height measurements and compared to the actual 3-D strain measurement from a stereo optical and non-contact measurement system ARAMIS. In addition, the flow stress curves obtained from stepwise experiments are compared with the ones from above methods. Our findings indicate that Enikeev’s approach for thickness prediction and Panknin’s approach for bulge radius calculation result in the best agreement with both stepwise experiment results and 3D optical measurement results.


PRILOZI ◽  
2015 ◽  
Vol 36 (1) ◽  
pp. 225-230 ◽  
Author(s):  
Aneta Mijoska ◽  
Mirjana Popovska

Abstract Metal-ceramic and all-ceramic prosthetic restorations in the patient mouth are often damaged by esthetic and functional problems that reduce their success and longevity. Аim: To evaluate methods for testing mechanical characteristics of dental ceramics through analysis of different testing methods. Material and methods: Dental ceramic materials are tested with in vivo and in vitro methods for their most important mechanical characteristics: hardness, toughness, flexural strength and abrasion. In vitro testing methods are faster and more efficient, without subjective factors from the patient according to ISO standards. Testing is done with universal testing machines, like Zwick 1445, Universal Testing Machine (Zwick DmbH & Co.KG, Ulm, Germany), Instron 4302 (Instron Corporation, England), MTS Sintech ReNew 1123 or in oral chewing simulators. Results: According to the testing results, flexure strength is one of the most important characteristic of the dental ceramic to be tested, by the uniaxial and biaxial tests. Uniaxial tests three-point and four-point flexure are not most appropriate because the main stress on the lower side of the tested specimens is tension that causes beginning fractures at the places with superficial flow. Uniaxial results for flexural strength are lower than actual force, while with biaxial test defects and flows on the edges of tested specimens are not directly loaded. Conclusion: Biaxial flexural method has advantages over uniaxial because of real strength results, but also for simple shape and preparing of the testing specimens.


2014 ◽  
Vol 626 ◽  
pp. 495-501 ◽  
Author(s):  
Rong Shean Lee ◽  
Ta Wei Chien

In most situations, original Cockcroft criterion underestimates material formability in the first quadrant of FLD. So far, some modified Cockcroft criteria have been reported for different applications. This presentation will focus on the modified Cockcroft criterion which takes strain-path effect into consideration. This paper demonstrates the accuracy of this criterion through limiting dome height test, free bulge test, and the biaxial tensile test using cruciform specimen respectively. The results showed that the modified Cockcroft criterion with strain path effect has good agreement with experimental results.


1976 ◽  
Vol 11 (1) ◽  
pp. 1-6 ◽  
Author(s):  
D A Kelly

Two types of creep testing machine, one to apply bi-axial tension to cruciform specimens and the other, torsion to disc-shaped specimens, have been constructed in the course of an investigaton of the behaviour of copper subject to multi-axial stress systems. Problems of bending in biaxial tension and buckling in torsion were encountered subsequent to construction of the machines. In the former case the deficiency was accepted and eccentric loading used to control the bending within acceptable limits. In the latter case microscopic examination was found necessary in order to define creep failure.


2020 ◽  
Vol 856 ◽  
pp. 128-134
Author(s):  
Chalida Udomraksasakul ◽  
Thanasan Intarakumthornchai ◽  
Yingyot Aue-u-Lan

Hydraulic bulge test or biaxial test is a well-known mechanical test used to determine a flow stress of material because of the large level of effective strains and not interfered by the necking unlike in uniaxial tensile test. However, the flow stress obtained is influenced by the anisotropy effect. That flow stress needs to be corrected by the anisotropic values (r-values) obtained from the uniaxial tensile test which limited by the necking. Therefore, to obtain the accurate flow stress the r-values should be determined directly from the biaxial test. The elliptical tests with ratio of 2 (the ratio between major and minor axis) at different sheet orientations (0๐ and 90๐ from the rolling direction) and the equibiaxial test were proposed. In this research, the effect of the sheet orientations upon the flow stress (K and n values) under biaxial tests was investigated by experiment and equation of material grade SPCD with the thickness of 0.8mm. The results showed that the flow stress without correcting r-values gave more variations than those with correcting one with the r-values obtained from the uniaxial test. Therefore, the r-values used to correct the flow stress under biaxial test should be directly determined from the biaxial test.


2000 ◽  
Author(s):  
James C. Gerdeen

Abstract The literature on formability and LDH (Limited Dome Height) testing is reviewed. The LDH tooling uses a 4.0 inch (100 mm) diameter punch, and various widths of specimens are used to give various strain ratios. The strains are then plotted on forming limit curves (FLD’s). In this paper, the development of a new mini-LDH test for thin sheet metal is described. The test involves a new geometry of specimens and a fixture which can be mounted in any universal testing machine with a low enough force range. The size effect is studied by conducting LDH tests with smaller diameter punches: 2.0 inch (50.8 mm), 0.50 inch (12.7 mm), and 0.25 inch (6.35 mm) diameters. Experimental results are presented for 0.010 inch (0.254 mm) thick 1100-H19 aluminum, and 5182-H19 aluminum. An analysis is also presented for the fully biaxial case and the plane strain case in order to model the experimental results.


Author(s):  
Chetan P. Nikhare

From centuries the metals and materials has been characterized using a traditional method called uniaxial tension test. The data acquired from this test found adequate for operations of simple forming where one axis stretching is dominant. Currently due to the demand of lightweight component production, multiple individual parts are eliminated by stamping in a single complex shape which further reduces many secondary operation. This need is driven by the requirement of 54 miles per gallon by 2025. Due to the complex part geometry, the forming method induces multi-axial stress states, which found difficult to predict using conventional tools. Thus to analyze these multi-axial stress states limiting dome height test and bulge test were recommended in many research. However, these tests limit the possibilities of applying multi-axial loading and resulting stress patterns due to contact surfaces. Thus a test machine called biaxial test is devised which would provide the capability to test the specimen in multi-axial stress states with varying load. In this paper, two processes, limiting dome test and biaxial test were modeled and compared. For this, the cruciform test specimens were used in biaxial test and conventional forming limit specimens for dome test. Variation of loadings were provided multi-axially in both test to capture the limit strain from uniaxial to equi-biaxial strain mode. In addition, the strain path, forming and formability was investigated and difference between the tests were provided.


1985 ◽  
Vol 107 (4) ◽  
pp. 372-378 ◽  
Author(s):  
H. M. Shang ◽  
F. S. Chau ◽  
C. J. Tay ◽  
S. L. Toh

In this investigation, the clamped portion of copper test specimens is allowed to be drawn slightly into the die throat when hydroformed into axisymmetrical shells. Changing the blank holding load results in different draw-in characteristics. In general, the drawing-in action affects the shape of the shell and also lowers the severity of the deformation and surface area of the deformed shell. Although the nominal thickness uniformity of shells formed with draw-in permitted is improved, the actual thickness is less uniformly distributed. Experimental results also show that, at the beginning of the forming process, the infeeding material due to draw-in is unstretched; but as deformation progresses, stretching of the infeeding material becomes necessary for attaining higher polar heights. The findings of this investigation show that care should be taken in interpreting the test results when the bulge test is used as a formability test. The results also have relevance to many current studies on the “limiting dome height” test for sheet metal.


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