Development of the Testing Machine for Biaxial Loading of Sheet Metals

2017 5th International Conference on Mechanics and Mechatronics (ICMM 2017) ISBN: 978-1-60595-541-4

Yan-Zhi GUAN1,2,a, Wei-Qiang XUE1,2, and Hai-Bo WANG1,2

1School of Mechanical and Material Engineering, North China University of Technology, Beijing 100144, China 2Beijing Engineering Research Center of Variable Cross-Section Roll Forming, Beijing 100144, China

Abstract. In sheet metal forming process, most of the materials are under complex stress state. So the mechanical behaviour can’t be applied to the actual projects only by the uniaxial tensile test. In order to get more accurately mechanical properties of materials especially for the anisotropic materials such as high strength steel and composite materials, it is necessary to do the research of biaxial tensile and press test machine. In this study, the biaxial loading machine was developed. The machine can be used for biaxial tensile test under different loading path and uniaxial tensile test. With the developed software, the experimental data can be processed through the program, and the biaxial tensile curve and yield criterion can be obtained. From the perspective of the development of modern industrial equipment and the experimental methods, the research and development of the equipment in this study play an important role in the field of the biaxial tensile test in domestic.

1 Introduction universal testing machine imported from Japan, has been made by YueYang University in China. But it only At present, the material tensile tests of domestic are provides the load ratio of 1:1 and 1:2[4]. Beihang mainly focused on the uniaxial tensile which has been University has done lots of independent research about completed by the universal testing machine. It mainly biaxial tensile and press test machine, and they developed applied to aviation, material, metallurgical, aerospace and the machine based on the hydraulic control. It can other fields. The plate is affected by complex stress in the achieve synchronous control of the two chucks in coaxial, process of production and application, namely in the bi- and is capable of the biaxial tensile test under different directional stress state. So the mechanical behaviour can’t loading path [5-8]. The biaxial tensile and press test be applied to the actual projects only by the uniaxial machine we research and develop is different from the tensile test. In order to get more practical and accurate one researched by Beihang University. The machine we material performance parameters and deduct and verify invented makes use of Siemens SIMOTION control the yield criterion, it is necessary to develop the biaxial system [9-13], and it can complete the four axis synergy tensile and press test equipment [1, 2]. in two directions with functions of loading, displacement and strain control. The machine is capable of the biaxial tensile test under different loading path. The machine has 2 The Development of Biaxial Tensile the characteristics of relatively high precision, low noise, Test Machine wider application and easy to operate. In the study, the biaxial tensile testing machine of In early 60’s, the concept of biaxial tensile testing hardware development including the assembly design of machine based on the cruciform specimen was proposed. mechanical transmission parts, the hardware selection It has many factors to be considered in developing biaxial and the construction of the control system. The loading equipment, such as the design of the cruciform construction of the control system takes advantage of the specimen, the coordinated motion control, precision of multi-axis cooperative movement of Siemens biaxial tensile testing machine, the determination of the SIMOTION. And the electrical design and the hardware strain and the stress in the central regions of the construction of the contactor control part were developed. cruciform specimen, etc. In recent years, the biaxial Figure 1 shows the structures of the machine. loading testing machine has been developed successfully due to the constant optimization of the motion control system and the high level of production. Professor Kawabata [3] developed several biaxial tensile test machines with the cruciform specimen, the load ratio of which is adjustable. In the 1980s, the mechanical biaxial tensile test machine developing from DCS-25T, a aCorresponding author: [email protected] 34

Figure 1. An essential framework of the Machine. Figure 4. The fixture. 2.1 The Design of Mechanical Parts

The design of mechanical parts in the biaxial tensile and 2.2 The Hardware and Control System of the press test machine refers to the biaxial tensile test Biaxial Tensile and Press Testing Machine machine researched by Professor Kuwabara and Wu et al. The biaxial tensile and press testing machine adopts a system (SIMOTION system) to complete the four axis linkage function. SIMOTION provides three kinds of platform: SIMOTION C based on the controller platform, SIMOTION D based on the drive platform and SIMOTION P based on a platform. The machine has Figure 2. The machine mechanical structure diagram. SIMOTION D425 system for multiple spindles, the model for 6AU1425-0AA00-0AA0. The internal of D425 Figure 2 shows the load direction of one of the axe of integrates an S120 control unit CU320. And the the mechanical structure of the biaxial tensile and press SIMOTION system adopts a modular system that makes test machine. Among them, 1 for the servo motor, 2 for the selection more flexible and makes the system more speed reducer, 3 for coupling, 4 for bearing group, 5 for compact, figure 5. the ball screw, 6 for nut (the sliding table), 7 is the tension sensor, 8 for fixture, 9 for the rack. Two-way Control IPC/UI Limit button switch thrust ball bearings was assembled by large two-way LAN axial force, models for 51417; next to the thrust ball was I/O SIMOTION D425 Motion controller I/O the double row deep groove ball bearing which was used DRIVE-CLiQ for large radial force and two-way axial force, models for DC 6317. The sliding table reciprocating motion runs on the TM31 TM31 SMC30 Double BUS BLM A/D A/D Sensor Motor Power rail fixed on the rack. Four servo motors in two loading module module module module module direction drive the coordinated movement of four axes, as seen in figure 3. Signal Pressure Grating Servo Reactor amplifier sensor ruler motor Filter Extensometer

Bidirectional Tensile Test Machine Figure 5. The control system structure of the SIMOTION D425.

3 Experimental In engineering practical applications, most of the materials are under complex stress state in the process Figure 3. The biaxial tensile and press test machine. like in-plane bidirectional stress [14]. The same sheet metal is in different stress-strain relationship with The fixture of the traditional universal testing different bidirectional stress. Pearce [15], Wood Thorpe machine is mechanical vertical clamping. This fixture is [16], et al. got the stress-strain curve of the different manual horizontal mechanical clamping as reference performance of sheet metals with the method of hydraulic standards, figure 4. The screws on both sides should bulging. The conclusion is that the loading path has a tighten first to ensure enough clamping force in the great impact on stress-strain curve [17]. M. Wan, X.D. stretching process. Then the middle screw should be. Wu, et al. [18] had studied the biaxial deformation Finally, a Z glyph can be got in specimen ends. behaviour of many sheet metals such as BH220, SPEN, et al., which has a very important practical significance.

The cruciform test specimen was designed in thinned For the unconspicuous yield phenomenon material, central region and slit arm, figure 6. The total length of it the yield strength is the stress of 0.2% plastic strain is 350mm with the width of 60mm; the length and width corresponding. The plastic strain ratio (represented by r) of the thinned central region are all 57mm; the seam is could be used to evaluate deep draw ability of sheet 0.2mm with a length of 60mm. Each arm has the same metal. The larger the r is, the better the deformability in size with equally spaced seven seams. The thickness is thickness is. 1.5mm, and the central region is 0.5mm on both sides. The plastic strain ratio: Laser cut its shape and seam, and the central region takes shape by CNC. The rounded-corner size between arms  r  b (3.3) should be precise. The rolling and directions should be  marked. s

In the equation, r is the plastic strain ratio; εb is the strain in width direction; εs is the strain in the thickness direction. b   ln 0 (3.4) b b s s  ln (3.5) s0 b bl r  (ln0 ) / (ln ) (3.6) Figure 6. The picture of real cruciform specimen. b b00 l Considering there is a large amount of experimental Where b is the initial width of specimen centre; b is data in the uniaxial tensile and biaxial tensile test to deal 0 the width after tensile; S is the initial thickness of with, an independent data processing program need to run 0 specimen centre; S is the thickness after tensile. L is the in Visual Studio.Net 2003 with the related theory of length after tensile in pulling force direction. The change metal plastic forming. With this program using Access of r can be obtained with every calculated site. The strain Database for data input and output uniaxial and biaxial of 15% was used because of the uncertainty of tensile stress-strain curves can be obtained. In this way, measurement of 10% is larger than before, and the 20% the efficiency can be greatly improved when dealing with value is smaller than before. the data of a plastic strain and a yield point.

3.2 Biaxial Tensile Test Data 3.1. Uniaxial Tensile Test Data X: Y= 4:1; 4:2; 4:3; 4:4; 3:4; 2:4 and 1:4. It set seven In order to deal with uniaxial tensile stress-strain curve, groups of different load proportional for this experiment. the centre cross-sectional area of the specimen After all the preparations finished, the proportion value (represented by P) should be measured. The n denotes the and the initial thickness of the centre area (m ) should be initial deviation of the extensometer. The processing 0 input to the operation interface. Where X is the rolling algorithm of uniaxial tensile test data as follows: direction; Y is the perpendicular to the rolling direction. Calculate the nominal stress and nominal strain of The n and q devotes the initial deviations of uniaxial tensile test. extensometer in x and y direction. In the biaxial tensile S n F test, the centre of the specimen was continuously pulled iiii (3.1) nom， nom by the bidirectional tensile which made the centre cross- I0  n P sectional area changed. An algorithm based on the theory of volume invariance in plastic deformation was Where Si is the displacement value of the i site in the developed to reflect the changing cross-sectional area. centre feedback by extensometer; I0 is the scale distance of extensometer; Fi is the load of the i site at both end of i i the specimen; σ nom is the nominal stress; ε nom is the 3.2.1 The True Stress of X Axis nominal strain. The true stress and strain should be calculated to get a ()XX12ii 1i  (3.7) more realistic data and describe the changes of centre 2(L02 q +ii ) m cross sectional area in large deformation process.

i i i i i 2i H()/() S 2 i  q L 0  S 2 i (3.8) true  nom(1   nom )，  true  ln(1   nom ) (3.2) 2 i i mH0 (3.9) Where σ true is the true stress; ε true is the true strain. mi  (LSLS0 1i   1 i )( 0  2 i   2 i )

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