Yong Wu1,2*, Shiqiang Zhu3 and Gang Liu2,4
1 National Key Laboratory of Science and Technology on Helicopter Transmission, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
2 National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
3Capital Aerospace Machinery Company, Beijing,100076, China
4 Institute of High Pressure Fluid Forming, Harbin Institute of Technology, Harbin 150001, China
Keywords: Ti-alloy tube, Gas bulging test, GTN model, Microstructure.
Abstract:
The Gurson-Tvergaard-Needleman model (GTN model) was employed to predict the ductile fracture behaviour in the hot gas bulging of TA18 tube. The effects of gas pressure on strain rate and formability by the Ti-3Al-2.5V tube freely gas bulging tests by 7.5MPa, 10MPa, 12MPa, 14MPa and 14-10-6MPa at 800oC. The micro-crack distributions and fracture morphologies of the bulged tubes were observed by the scanning electron microscope. The fracture and micro-crack distribution were affected by the strain rate. The results show that the GTN fracture criterion can give a good prediction.
After the bulging test,the strains of the tubes were measured by ASAME 4.1 system. Fig.
1(a) shows the minor strain distribution of the bulged tube with constant pressure 12MPa. The minor strain was only about 0.2. The bulging limits strain state can be drawn by fitting the principle strain values measured to coordinate system. Fig. 1(b) shows the forming limit positions of tested Ti-tubes bulging with different gas pressures. The limit strain state points of the bulged tube with loading paths for 7.5MPa, 10MPa, 12MPa, 14MPa and 14-10-6MPa can be showed as (0.77, 0.27), (0.62, 0.19), (0.52, 0.19), (0.41, 0.19) and (0.57, 0.20), respectively.
The equivalent strains were 1.08, 0.84, 0.73, 0.61 and 0.79, respectively.
Fig. 2 shows the cavities distribution near to the fractures of the tested tubes with different gas pressure loading paths. The number and size of cavities increase with the increasing of gas pressure, as shown in Fig. 2(a-c). Fig. 2(d) shows the cavities distribution of the bulged tube by 14MPa. Because of the decreasing of the strain, the number and sizes of cavities also decrease.
However, there are many small cavities in Fig. 2(e). The sizes of the cavities are smaller than that in Fig. 2(b) and (c). And the number of the cavities is much bigger than that in Fig. 2(b) and (c). So, during the hot bulging of Ti-3Al-2.5V tube, the ductile failure is assumed to involve with degradation of structure due to nucleation and growth of defects, i.e. the cavities and micro-cracks, and their coalescence into macro-cracks.
The GTN damage model is a powerful model for the prediction of fracture in ductile metals, its performance is influenced by the accuracy of its parameter identification. And the GTN damage parameters can be identified using a hybrid experimental–numerical method based on the biaxial hydraulic bulging test [1, 2].
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Figure 1: The ASAME 4.1 results of bulged tubes, (a) the strain distribution of the bulged tube with gas pressure 12MPa, (b) the bulging limit.
Figure 2: Cavity distribution near the fracture, (a) 7.5MPa, (b) 10MPa, (c) 12MPa, (d) 14MPa, (e) 14-10-6MPa. Where, the equivalent strains were 1.08, 0.84, 0.73, 0.61 and 0.79,
respectively.
REFERENCES
[1] Kami A, Dariani B M, Comsa D S, Banabic D, Vanini A, Liewald M, 2016, Calibration of GTN damage model parameters using hydraulic bulge test, Rom. J. Techn. Sci. Appl.
Mechanics, 61(3), pp. 248-264.
[2] Cui X.L, Zhang W.W, Zhang Z.C, Chen Y.Z, Lin P, Chi C.Z, 2018, Prediction of Forming Limit of Dual-Phase 500 Steel Sheets Using the GTN Ductile Damage Model in an Innovative Hydraulic Bulging Test, JOM, 70(8), pp. 1542-1547.
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9th INTERNATIONAL CONFERENCE ON TUBE HYDROFORMING (TUBEHYDRO 2019) November 18-21, 2019, Kaohsiung, Taiwan WA2-1
One-sided Rubber Bulging Test to Measure Forming Limit Strains of Metal Tube
Kana Nakanahara1,* and Hidenori Yoshimura1
1Faculty of Engineering, Kagawa University,Hayashi-cho 1891-10, Takamatsu, 761-0396, Japan
Keywords: Forming limit diagram, Rubber, Bulging, Biaxial.
Abstract
In the recent tube forming process technique, tubes undergo more complicated and more severe deformation. For the process design, it is favorable to evaluate the formability of tubes precisely and easily. The proposed measurement test in this research, one-sided rubber bulging test shown in Fig. 1, satisfies it and the forming limit of various biaxial strain paths can be obtained easily.
Unlike the conventional hydraulic bulging test [1], since pressure medium is rubber, the complicated sealing mechanism is unnecessary and the measurement can be carried out by an inexpensive simple apparatus and a universal testing machine. While the condition near the uniaxial tension is achieved in free rubber-pressure bulging test [2], biaxial tension can be realized by bulging locally at one-sided opening. Moreover, to change the strain ratio of the circumferential direction to the axial one, we suggest to cut the specimen out at the opposite side of the local bulging area as shown in Fig. 2. Since the cutout shape and size effect on the amount of material pulled in the circumferential direction, various strain paths of the local bulged area can be obtained from the planar strain condition to the equibiaxial strain condition.
In this method, the complex tube-axial loading mechanism of a pipe is not required unlike the conventional biaxial hydro-pressure bulging test machine.
Often, there are some initial defects such as uneven distribution of thickness and hardness caused by the manufacturing processes. Actually, a welding steel tubes also have welded and heat affected zones. Although the fracture occurs at these zones for the free rubber-pressure test machines, forming limit of the other zone can be obtained because the welded and heat zones are covered with a closed die and the opening zone bulges locally in one-sided rubber pressure bulging test. Thus, we conduct preliminary and main tests as shown in Fig. 3 and 4. Thinner or low hardness locations is known by the preliminary bulging, and then the weak point is set to of the opening center. Strain path is calculated from the distance between
(a) Inside (b) outside
Figure 1: Illustration of one side rubber pressure bulging test.
Universal testing machine
Die
NI-1776-SmartCamera
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the marks which are drawn previously to the surface. The distance between the centroid of the marks is dynamically measured by image processing. The proposed method was applied to pure aluminum (1070TD), and Fig. 5 shows the strain paths acquired.
REFERENCES
[1] Sugawara, F., Kuwabara, T., 2013, Development of a Multiaxial Tube Expansion Testing Machine That Enables the Continuous Measurement of Large-Strain Biaxial Stress-Strain Curves of Sheet Metals, Journal of Japan Society for Technology of Plasticity, 54-624, pp. 57-63.
[2] Yoshimura, H., Tajima, M., Mihara, Y., 2011, Estimation of ductile fracture of tube material by ring tensile test, Proc. of 5th TUBEHYDRO2011, pp. 203-206.
(a) Initial (b) Deformed
Figure 2: Deformation behavior of round tubal specimen with cutout.
Figure 3: Preliminary test method. Figure 4: Main test method.
Figure 5: Strain paths with rectangular cutout of 𝜃 = 70°.
0 0.2 0.4 0.6
0 0.2 0.4 0.6
Axial direction strain εφ
Circumference direction strain εθ
No cutout θ70w10 θ70w30 θ70w60 Equal biaxial
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9th INTERNATIONAL CONFERENCE ON TUBE HYDROFORMING (TUBEHYDRO 2019) November 18-21, 2019, Kaohsiung, Taiwan WA2-2