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Bi-axial stress state hot bulging behavior and plane-stress visco-plastic material modelling of TA32 sheets

Peng Heli, Luo Zhiqiang, Qu Shuguang, Shi Wenzhan, Fu Kunning, Xiao Wenchao, Zheng Kailun

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 3

art. no e167, 2023

Bi-axial state is the dominant stress state experienced by the sheet metal during various forming processes, which requires a thorough understanding and modelling for process designs. In this paper, effects of equal bi-axial stress-state on the hot deformation behavior of titanium alloys are thoroughly investigated using hot bulging tests, and is further compared to the uniaxial stress state. Firstly, a specific hot bulging test device enabling a uniform temperature field and constant control of strain rate was established, using which, systematic hot bulging tests at various temperatures (750–850 °C) and strain rates (0.001–0.1 s−1) of the near-alpha phase TA32 sheets were conducted to determine the hot equal bi-axial bulging behavior. Based on the testing data of force and geometry variations of bulged domes, the equivalent stress–strain curves were calculated. Secondly, a plane-stress visco-plastic plane-stress model of near-alpha TA32 sheets was developed for the first time, enabling both the uniaxial and biaxial flow behavior and forming limits to be precisely predicted. The prediction accuracies for uniaxial and biaxial cases are 93.5% and 89%, respectively. In the end, the uniform deformation resulting from the strain and strain rate hardening was determined, which contributes to the understanding of the stress-state effect on hardening preliminarily. The plane stress visco-plastic model provides an efficient and reliable material model for finite element (FE) simulations of hot forming titanium alloy sheets.

Comparison between direct and indirect measurement methods for bulge tests

Marinho E., Silva-Jr A., Batalha G.

Archives of Materials Science and Engineering

2013

Vol. 61, nr 2

77--86

Purpose: To reinforce the standardization of the bulge test measuring procedures by comparison of two different bulge forming measurement methods. Design/methodology/approach: Two different bulge forming measurement methods are used simultaneously in order to reinforce the standardization of the bulge test measuring procedures. An indirect method, Digital Image Correlation (DIC), is compared with a direct one, ultrasound pulse-echo method. Findings: The main conclusion is that the DIC system is a valid indirect measurement method to study bi-axial sheet metal forming. Research limitations/implications: The constant pressure bulge test method was used and it yielded positive results for comparing the direct and indirect method (considering thickness measurement of the bulge dome), as an important research implication is that the touch less measurement method could be applied to other sheet metal forming processes. Practical implications: Tension tests are used as a standard accepted procedure to determine material parameter values for characterizing the forming sheet behaviour. However, by using a tension test, only a limited strain range can be considered for determining the true stress – true strain curve. Based on this limitation, the bulge test is used to achieve a much larger strain range under bi-axial loading conditions. Originality/value: An indirect method, Digital Image Correlation (DIC), is compared with a direct one, ultrasound pulse-echo method, in situ, real time and on the same specimen.

Investigation of biaxial stress-strain relationship of steel sheet metal

Slota J., Spisak E., Stachowicz F.

International Journal of Applied Mechanics and Engineering

2004

Vol. 9, no 1

161-168

The stress-strain relationship of the deep drawing quality (DDQ) and intersitial free (IF) steel sheets was determined by uniaxial and equibiaxial (hydraulic bulging) tensile tests. The sheet thickness gradation in different points of the hemisphere formed in the bulge test was analysed, both experimentally and theoretically. The Hollomon equation was used to describe uniaxial and biaxial strain hardening curves, and the differential (strain dependent) strain hardening exponent nt was determined on the basis of the results of uniaxial and biaxial testing.