Name: A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions (Video) | JoVE Journal
Uploaded: 2017-04-04T12:30:00.000+00:00
Duration: 7 min 40 s
Description: Learn a novel biaxial testing method for determining forming limits of sheet metals under hot stamping conditions. This technique enables experimental measurement of complex forming conditions.

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5 minutes

This research was supported by the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 604240, project title "An industrial system enabling the use of a patented, lab-proven materials processing technology for Low Cost forming of Lightweight structures for transportation industries (LoCoLite)."

1. Preparation of Specimens
<ol>
	<li>Machine flat dog-bone and cruciform specimens from commercial material aluminum alloy 6082 (AA6082) using a laser cutter and a computer numerical control (CNC) milling machine (for formability tests at different strain paths including uniaxial, plane strain, and equi-biaxial straining states).</li>
	<li>Measure the thickness of each cruciform specimen and each dog-bone specimen with a vernier caliper three times in the central gauge region and calculate the average values. Ensure that the thickness of the gauge section in a cruciform specimen is 0.7 &#177; 0.05 mm and that the thickness of the uniaxial specimen is 1.5 &#177; 0.1 mm.</li>
	<li>Spray-paint the entire top surface of a cruciform specimen by using a flame-resistant, black spray paint (capable of withstanding temperatures up to 1,093 &#176;C). Wait until the paint dries and then spray flame-resistant, white paint dots from arm&#39;s length to create a stochastic spraying pattern to be recognized by the DIC system (see the example in Figure 1).</li>
	<li>Weld a pair of thermocouples to the center of the back surface (opposite to the painted surface) of the specimen. Connect the other end of the thermocouple to the feedback temperature control system of the uniaxial testing machine to monitor and control the temperature change history.</li>
</ol>
2. Assembly of the Biaxial Testing Apparatus
<ol>
	<li>Assemble all parts of the biaxial testing apparatus, including a base plate, a central shaft, input and output rotatable plates, carriages, a clamp, guide rails, and rigid connecting rods (the assembled apparatus is shown in Figure 2).
	<ol>
		<li>Using a connecting rod, couple the input rotatable plate directly to the movable jaw of a resistance heating uniaxial tensile test machine, which provides the uniaxial tensile force. Couple the input rotatable plate to the central drive shaft and couple this central drive shaft to the output rotatable plate.</li>
		<li>Ensure that the rotation of the input rotatable plate around the axis of rotation rotates the drive shaft, thereby rotating the output rotatable plate to which it is coupled around the axis of rotation.</li>
		<li>At one end, couple each of the rigid connecting rods to one of the connection points on the output rotatable plate. Couple the other end to one of the carriages. 
		NOTE: This will cause the carriages with specimen holders to slide back and forth along the guide rails with low friction, which can apply a biaxial force to the cruciform specimen.</li>
		<li>Using screw bolts, clamp each arm of the cruciform specimen to a carriage with a specimen holder and a top plate.</li>
	</ol>
	</li>
	<li>Set up grips in the chamber of the uniaxial tensile test machine, as shown in Figure 3 (a). Attach four welding cables to each pair of grips, which are made of stainless steel and copper, respectively, and thus connect the welding cables to the electrical power supply. 
	NOTE: The conductor area of the welding cables is 50 mm2 and the current rating is 345 A.
	<ol>
		<li>Put the grips and the clamp of the biaxial testing apparatus into the two jaws of the uniaxial tensile test machine and tighten them inside (Figure 3 (a)).</li>
	</ol>
	</li>
	<li>Set up the biaxial testing apparatus in the chamber of the uniaxial tensile test machine, as shown in Figure 3 (b).
	<ol>
		<li>Use two frames and screw bolts on the top and the bottom sides of the base plate to fix the apparatus in the chamber of the uniaxial tensile test machine.</li>
		<li>Put the specimen into the specimen holder on top of the biaxial testing apparatus.</li>
		<li>Connect each terminal of the welding cables to each clamping region of the specimen.</li>
	</ol>
	</li>
</ol>
3. Setup of the Heating and Quenching System
<ol>
	<li>Tightly connect each clamping region of the specimen to the stainless steel top plate, which serves as the electrode for resistance heating.</li>
	<li>Tighten the welding cables with crimp ring terminals to the top plate of each clamping region.</li>
	<li>Connect flared nozzles with hoses to the high flow quench system with regulated air supply at 8,000 kg/m2 pressure for cooling.</li>
	<li>Use four nozzles to blow air from the arms of the specimen to the central region of the specimen. 
	NOTE: The nozzles are not directed onto the gauge section for cooling to avoid blocking the central zone from the camera&#39;s view.</li>
</ol>
4. Setup of the DIC System
<ol>
	<li>Connect the high-speed camera of the DIC system with a micro lens to a computer. Adjust the frame rates of the camera to 25 fps, 50 fps, and 500 fps from the menu of frame rates (for the tests at the stretching strain rates of 0.01/s, 0.1/s and 1/s, respectively). Set the resolutions of all images to 1,280&#215;1,024 pixels. 
	NOTE: The frame rates depend on the number of data points to be collected; at least 200 data points can be collected using the above settings.</li>
	<li>Use an additional spotlight with a power of 300 W for tests at high strain rates. Point the spotlight directly at the chamber of the uniaxial tensile test machine.</li>
	<li>Adjust the camera lens so that it is parallel to the top surface of the specimen in the chamber and focus the camera on the gauge section.</li>
</ol>
5. Experimental Program
<ol>
	<li>Run the resistance heating uniaxial tensile test machine by clicking the triangular run button in the control software. 
	NOTE: Electricity runs through the AA6082 material and heats it to the solution heat treatment temperature of 535 &#176;C14 at a heating rate of 30 &#176;C/s. The material is soaked at 535 &#176;C for 1 min, which is sufficient for the full resolution of precipitates. Air blowing from the quench system is used to quench the material at a cooling rate of 100 &#176;C/s15 to one of 3 designated elevated temperatures in the range of 370-510 &#176;C.</li>
	<li>Stretch the specimen with the biaxial testing apparatus at a constant strain rate in the range of 0.01-1/s and record the deformation history by manually pressing the trigger button connected to the high-speed camera. 
	NOTE: The input displacement from the uniaxial testing machine to the biaxial testing apparatus was controlled by the built-in software of the uniaxial testing machine.</li>
	<li>Perform the tests at different strain paths consisting of uniaxial, plane strain, and biaxial straining states3 by adjusting the configuration of the biaxial testing apparatus.
	<ol>
		<li>Disconnect two opposed connecting rods for uniaxial tests. Clamp a dog-bone specimen on the biaxial testing apparatus and connect it to welding cables, as in steps 3.1-3.4. Repeat steps 5.1-5.2.</li>
		<li>Fix two opposed carriages to the base plate with screw bolts to restrict the deformation on the corresponding direction for testing under plane strain state. Clamp a cruciform specimen on the biaxial testing apparatus and connect it to welding cables, as in steps 3.1-3.4. Repeat steps 5.1-5.2.</li>
	</ol>
	</li>
	<li>Repeat steps 5.3.1-5.3.2 for each test condition three times, using new dog-bone and cruciform specimens.</li>
</ol>
6. Data Processing
<ol>
	<li>Import all images recorded by the high-speed camera into the post-processing software and follow standard steps for data analysis according to the software manual.</li>
	<li>Use the ISO standard3 to determine the forming limits by clicking the FLC Mode button in the software. 
	NOTE: This method has already been integrated into the image correlation processing software.</li>
	<li>Mark each result of the forming limits at various temperatures, strain rates, and strain paths in a diagram.</li>
	<li>Plot the forming limit curves at all test conditions to obtain an FLD of an alloy under hot stamping conditions.</li>
</ol>

<ol><li>Karbasian, H., Tekkaya, A. E. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S092401361000213X">A review on hot stamping.</a> J. of Mater. Process. Tech. 210 (15), 2103-2118 (2010).</li>
<li>Miller, W. S., et al. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S092150939900653X">Recent development in aluminium alloys for the automotive industry.</a> Mater. Sci. and Eng. 280 (1), 37-49 (2000).</li>
<li>Shao, Z., Li, N., Lin, J., Dean, T. A. <a target="_blank" href="https://link.springer.com/article/10.1007/s11340-016-0183-9">Development of a New Biaxial Testing System for Generating Forming Limit Diagrams for Sheet Metals Under Hot Stamping Conditions.</a> Exp. Mech. 56 (9), 1-12 (2016).</li>
<li>Ayres, R. A., Wenner, M. L. <a target="_blank" href="https://link.springer.com/article/10.1007/BF02686404">Strain and strain-rate hardening effects in punch stretching of 5182-0 aluminum at elevated temperatures.</a> Metall. Trans. A. 10 (1), 41-46 (1979).</li>
<li>Shao, Z., et al. <a target="_blank" href="http://journals.sagepub.com/doi/abs/10.1177/0954405416645776">Experimental investigation of forming limit curves and deformation features in warm forming of an aluminium alloy.</a> P. I. Mech. Eng. B-J. Eng. , (2016).</li>
<li>Marciniak, Z., Kuczynski, K. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/0020740367900665">Limit strains in the processes of stretch-forming sheet metal.</a> Int. J. Mech. Sci. 9 (9), 609-620 (1967).</li>
<li>Li, D., Ghosh, A. K., et al. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0924013603008550">Biaxial warm forming behavior of aluminum sheet alloys.</a> J. of Mater. Process. Tech. 145 (3), 281-293 (2004).</li>
<li>Palumbo, G., Sorgente, D., Tricarico, L. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0890695508001223">The design of a formability test in warm conditions for an AZ31 magnesium alloy avoiding friction and strain rate effects.</a> Int. J. Mach. Tool. Manu. 48 (14), 1535-1545 (2008).</li>
<li>Raghavan, K. S. <a target="_blank" href="https://link.springer.com/article/10.1007/BF02670679">A simple technique to generate in-plane forming limit curves and selected applications.</a> Metall. Mater. Trans. A. 26 (8), 2075-2084 (1995).</li>
<li>Ragab, A. R., Baudelet, B. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/0378380482900274">Forming limit curves: out-of-plane and in-plane stretching.</a> J. Mech. Work. Technol. 6 (4), 267-276 (1982).</li>
<li>Fan, X. -b, He, Z. -b, Zhou, W. -x, Yuan, S. -j <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0924013615301680">Formability and strengthening mechanism of solution treated Al-Mg-Si alloy sheet under hot stamping conditions.</a> J. of Mater. Process. Tech. 228, 179-185 (2016).</li>
<li>Zidane, I., Guines, D., Léotoing, L., Ragneau, E. <a target="_blank" href="http://iopscience.iop.org/article/10.1088/0957-0233/21/5/055701/meta">Development of an in-plane biaxial test for forming limit curve (FLC) characterization of metallic sheets.</a> Meas. Sci. Technol. 21 (5), 055701(2010).</li>
<li>Hannon, A., Tiernan, P. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0924013607009533">A review of planar biaxial tensile test systems for sheet metal.</a> J. of Mater. Process. Tech. 198 (1-3), 1-13 (2008).</li>
<li>Garrett, R., Lin, J., Dean, T. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0749641904001664">An investigation of the effects of solution heat treatment on mechanical properties for AA 6xxx alloys: experimentation and modelling.</a> Int. J. Plasticity. 21 (8), 1640-1657 (2005).</li>
<li>Milkereit, B., Wanderka, N., Schick, C., Kessler, O. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0921509312005540">Continuous cooling precipitation diagrams of Al-Mg-Si alloys.</a> Mater. Sci. Eng. A. 550, 87-96 (2012).</li>
<li>Crammond, G., Boyd, S. W., Dulieu-Barton, J. M. <a target="_blank" href="http://www.sciencedirect.com/science/article/pii/S0143816613001103">Speckle pattern quality assessment for digital image correlation.</a> Opt. Laser. Eng. 51 (12), 1368-1378 (2013).</li>
</ol>

The authors have nothing to disclose.

Conventional formability test methods used to determine forming limits are usually applicable only at room temperature. The presented technique can be used to evaluate the formability of metals for hot sheet stamping applications by introducing a novel biaxial testing apparatus to a resistance heating uniaxial testing machine. This cannot be performed using conventional methods for hot stamping applications. The setup of heating and cooling systems and the DIC system is critical to controlling the uniformity of temperatu...

The automotive industry is facing a huge global challenge of reducing fuel consumption and minimizing environmental pollution from vehicle emissions. Weight reduction is beneficial to improving the performance of automobiles and can directly reduce energy consumption1. Due to the low formability of sheet metals at room temperature, hot stamping and cold die quenching processes (referred to as hot stamping)2 are used to improve the formability of alloys and thus to obtain complex shaped components in automotive applications.
A forming limit diagram (FLD) is a useful tool to evaluate the formability of an alloy3. Out-of-plane tests, such as the Nakazima test4,5, and in-plane tests, such as the Marciniak test6,7,8, are conventional experimental methods to obtain the FLDs of sheet metals under various conditions9,10,11. A servo-hydraulic biaxial testing machine has also been used to investigate the formability of alloys at room temperature12,13.
However, none of the methods above are applicable to formability tests under hot stamping conditions, since a cooling process prior to forming is required along with control of the heating and cooling rates. The deformation temperature and strain rate are difficult to obtain accurately. Therefore, a novel formability testing system is proposed in this study to experimentally determine the forming limits of sheet metals under hot stamping conditions.

<table><tbody><tr><td>Aluminium Alloy&amp;nbsp;</td><td>Smiths Metal</td><td>6082</td><td>Specimens machining</td></tr><tr><td>Laser cutter</td><td>LVD Ltd</td><td>HELIUS 25/13</td><td>Laser cutting specimens</td></tr><tr><td>CNC machine</td><td>HAAS Automation</td><td>TM-2CE</td><td>Machine specimens by milling</td></tr><tr><td>Vernier caliper</td><td>Mitutoyo</td><td>575-481</td><td>Thickness measurement</td></tr><tr><td>Resistance heating uniaxial testing machine</td><td>Dynamic System Inc</td><td>Gleeble 3800</td><td>Thermo-mechanical materials simulator</td></tr><tr><td>High flow quench system</td><td>Dynamic System Inc</td><td>38510</td><td>For air cooling</td></tr><tr><td>Thermocouples</td><td>Dynamic System Inc</td><td>K type</td><td></td></tr><tr><td>Nozzles</td><td>Indexa</td><td></td><td>Nozzle flared 1/4 inch bore</td></tr><tr><td>Welding cables</td><td>LAPP Group</td><td>H01N2-D</td><td></td></tr><tr><td>High-speed camera</td><td>Photron</td><td>UX50</td><td>For DIC testing</td></tr><tr><td>Camera lens</td><td>Nikon</td><td>Micro 200mm</td><td></td></tr><tr><td>Lamp</td><td>Liliput</td><td>150ce</td><td>300 W</td></tr><tr><td>Laptop</td><td>HP</td><td>Campaq 2530p</td><td>For images recording</td></tr><tr><td>Biaxial testing apparatus</td><td>Manufactured independently</td><td></td><td>All parts were designed and machinced by authors for biaxial testing</td></tr><tr><td>Steel&amp;nbsp;</td><td>West Yorkshire Steel</td><td>H13</td><td>Mateials of the biaxial testing apparatus</td></tr><tr><td>Image correlation processing software</td><td>GOM</td><td>ARAMIS</td><td>Non-contact measuring system and data post-pocessing</td></tr></tbody></table>

a novel biaxial testing apparatus for the determination of forming limit under hot stamping conditions

A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions

The hot stamping and cold die quenching process is increasingly used to form complex shaped structural components of sheet metals. Conventional experimental approaches, such as out-of-plane and in-plane tests, are not applicable to the determination of forming limits when heating and rapid cooling processes are introduced prior to forming for tests conducted under hot stamping conditions. A novel in-plane biaxial testing system was designed and used for the determination of forming limits of sheet metals at various strain paths, temperatures, and strain rates after heating and cooling processes in a resistance heating uniaxial testing machine. The core part of the biaxial testing system is a biaxial apparatus, which transfers a uniaxial force provided by the uniaxial testing machine to a biaxial force. One type of cruciform specimen was designed and verified for the formability test of aluminum alloy 6082 using the proposed biaxial testing system. The digital image correlation (DIC) system with a high-speed camera was used for taking strain measurements of a specimen during a deformation. The aim of proposing this biaxial testing system is to enable the forming limits of an alloy to be determined at various temperatures and strain rates under hot stamping conditions.

Since FLDs are highly strain path-dependent, the linearity of the strain path for each test condition was verified by analyzing the DIC results; the strain paths are proportional throughout deformation for each test condition. The range of the minor-to-major strain ratio is approximately -0.37 (uniaxial condition) to 0.26 (near biaxial condition). By processing data for different AA6082 conditions, forming limit data for different strain paths were determined and hence, the FLDs for AA608...

This protocol proposes a novel biaxial testing system used on a resistance heating uniaxial tensile test machine in order to determine the forming limit diagram (FLD) of sheet metals under hot stamping conditions.

The hot stamping and cold die quenching process is increasingly used to form complex shaped structural components of sheet metals. Conventional ...

The overall goal of this experiment is to determine the forming limits of sheet metals under hot stamping conditions through the use of a novel biaxial texting system. The main advantages of this technique is that it enables forming limits of sheet metals to be determined experimentally and accomplish forming conditions. Generally conventional formability tests are not applicable to hot stamping for two reasons.First, the cooling process occurs prior to deformation and the second, controlling the forming parameters is very difficult. We were the first in the field to have the idea of using a novel biaxial testing system to measure forming limits and the complex forming conditions. For a formability test, set different strain paths including uniaxial plain strain and equibiaxial straining states.Machine flat and dog bone cruciform specimens from commercial material aluminum alloy 6082. Use a laser cutter and a computer controlled milling machine. From each specimen, take the average of three thickness measurements made with vernier calipers or micrometers at each specimen's central gauge region.The cruciform specimen should measure 0.7 millimeters plus and minus five microns and the uniaxial specimen should measure 1.5 millimeters plus and minus 10 microns. Now, spray paint the entire top surface of a cruciform specimen with one coat of flame resistant black spray paint. The paint must withstand temperatures in excess of 1, 000 degrees Celsius.After the paint dries, use white flame resistant spray paint to add dots for recognition by the DIC system. Spray from one arm's length away to create a stochastic pattern. Then weld a pair of thermal couples to the center of the unpainted back surface.First, assemble all parts of the biaxial testing apparatus including a base plate, a central shaft, input and output rotatable plates, carriages, a clamp, guide rails, and rigid connecting rods. Next, attach four welding cables to each pair of stainless steel and copper grips. Thus, connect the welding cables to the electrical power supply.Now secure the grips of the biaxial testing apparatus in the uniaxial tensile test machine, and for added security, attach the apparatus with bolts to the topside, and to the bottom side of the base plate. Now load the specimen onto the specimen holders of the test apparatus and connect the thermal couples to the feedback temperature control system of the testing machine to monitor and control the temperature. Next, set up the heating and quenching system.Connect each terminal of the welding cables to each clamping region of the specimen. Then, tighten each clamping region to the top plate which serves as the electrode for resistance heating. For cooling, connect hoses with flared nozzles to a high flow quench system.Set the system to produce an air pressure of 8, 000 kilograms per square meter. Now, position four nozzles to move air from the specimen's arms to its central region. Be sure to position the nozzles so they are not blocking the camera's view of the central zone.Now, attach micro lens to the high speed camera of the DIC system and set it up to view the central zone. Then connect the camera to a laptop. Adjust the frame rate to the test according to the stretching strain.Plan to collect at least 200 frames of data per test. Set the capture resolution to 1, 280 by 1, 024. Now focus the camera on the gauge section with the lens parallel to the top surface of the specimen.For high strain tests point an additional 300 watt spotlight directly into the test chamber. To start the resistance heating uniaxial tensile test machine, click the triangular run button in the control software. Then start the camera using a trigger button so it records the deformation history.The specimen is now heat treated and stretched at the selected constant temperature and strain rate. Perform the tests at different strain paths consisting of uniaxial, plain strain, and biaxial states by adjusting the configuration of the biaxial testing apparatus. For uniaxial tests, disconnect the two opposed connecting rods and clamp a dog bone specimen to the apparatus.Then connect the welding cables and repeat the other preparatory steps. For plain strain state tests, fix two opposed carriages to the base plate with screw bolts. This restricts the deformation on the corresponding direction.Then proceed to collect data. Formability tests using the novel plain biaxial tensile testing system were conducted at the designated deformation temperatures and strain rates after the heating and cooling processes. A forming limit curve identifies the boundary between uniform deformation and the onset of plastic instability or diffuse necking which leads to failure.As the strain rate increased, the forming limit of the alloy increased. A monotonic increase in the forming limit from 370 to 510 degrees Celsius indicates that high formability of the alloy can be obtained at a higher temperature under hot stamping conditions. The three forming limit curves are quite close to each other.Thus, the sensitivity of the temperature dependence is larger for tension tension biaxial strain paths than for tension compression strain paths. After watching this video you should have a good understanding of how to set up an experiment and conduct the formability test at various temperatures and strain rates by using the proposed system. Following this procedure, forming limits of alloy sheet metals like magnesium alloy or boron steel, can also be melded in the hot stamping conditions.

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Learn a novel biaxial testing method for determining forming limits of sheet metals under hot stamping conditions. This technique enables experimental measurement of complex forming conditions.

A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions

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