Accuracy analysis of plane-strain bulge test for determining mechanical properties of thin films

The effect of a variety of geometrics, initial conditions and material properties on the deformation behavior of thin films in the plane-strain bulge test was systematically scrutinized by performing the finite element analysis, and then the accuracy of the plane-strain bulge test in determining the mechanical properties of thin films in terms of our finite element results was analyzed. The results indicate that although the determination of the plane-strain modulus in the light of the plane-strain bulge equation is fairly accurate, the calculation of the residual stress is not satisfied as expected, especially for low residual stress. Finally, an approach is proposed for analyzing bulge test data, which will improve the accuracy and reliability of this bulge test technique.

Theoretical prediction and experimental investigation on formability of tailor-welded blanks

Based on the elastoplastic mechanical properties of the weld and heat affected zone metals obtained by a nanoindentation test, a theoretical calculation model was established for the forming limit diagram（FLD） of tailor-welded blanks（TWBs） on the basis of plastic constitutive relations and Hosford yield criteria. Hemispherical punch bulging tests were performed to verify the FLD theoretical calculation results. The results demonstrated that not only the FLD theoretical calculation of base materials but also that of TWBs had a good agreement with their experiments. Besides, poorer formability of TWBs caused its FLD significantly lower than that of base materials. The theoretical calculation model offers a reliable approach to obtain the specific FLD of TWBs.

Formability determination of AZ31B tube for IHPF process at elevated temperature

Ring hoop tension test and tube bulging test were carried out at elevated temperatures up to 480 ℃to evaluate the formability of AZ31B extruded tube for internal high pressure forming （IHPF） process. The total elongation along hoop direction and the maximum expansion ratio （MER） of the tube were obtained. The fracture surface after bursting was also analyzed. The results show that the total elongation along hoop direction and the MER value have a similar changing tendency as the testing temperature increases, which is quite different from the total elongation along axial direction. Both the total elongation along hoop direction and the MER value increase to a peak value at about 160 ℃. After that, they begin to decrease quickly until a certain rebounding temperature is reached. From the rebounding temperature, they begin to increase rapidly again. Burnt structure appears on the fracture surface when tested at temperatures higher than 420 ℃. Therefore, the forming temperature of the tested tube should be lower than 420 ℃, even though bigger formability can be achieved at higher temperature.

Warm hydroforming of magnesium alloy tube with large expansion ratio

Process of warm tube hydroforming was experimentally investigated for forming an AZ31B magnesium alloy tubular part with a large expansion ratio. Effects of temperature on the mechanical properties and formability were studied by uniaxial tensile test and hydraulic bulge test. Total elongation increases with temperature up to 250℃, but uniform elongation and maximum expansion ratio get the highest value at 175℃. Different axial feeding amounts were applied in experiments to determine the reasonable loading path. A preform with useful wrinkles was then realized and the tubular part with an expansion ratio of 50% was formed. Finally, mechanical condition to produce useful wrinkles is deduced and the result illustrates that useful wrinkles are easier to be obtained for tube with higher strain hardening coefficient value and tubular part with smaller expansion ratio.

Comparison of experimental methods to evaluate seam welds quality in extruded profiles

The wedge and bulge expansion tests were compared in the assessment of the seam welds strength in a tubular profile extruded at two ram speeds.In the wedge test,the expansion was determined by moving a conical punch into the tube until the specimen fracture.In the bulge test,a hydrostatic tensile stress state was applied by expanding the specimen with an internal rubber plug.The two methodologies were compared in terms of load and hoop strain at fracture and by detecting the fracture morphology and location.Then,the effect of a number of design parameters was investigated in order to evaluate the robustness of the standard testing conditions.For both tests,ductile fractures appeared in the seam welds location,but the bulge test was more robust and conservative with respect to the wedge test,showing less scattered data.Thus,the performances of a second die for the tube profile,designed to optimize the seam welds quality,have been successfully assessed by the bulge test and results compared to those achieved by a novel numerical quality index,coming to a final good matching.

Characterization of mechanical properties for tubular materials based on hydraulic bulge test under axial feeding force

A T-shape tube hydraulic bulge test under axial feeding force is carried out to characterize the mechanical properties of EN AW 5049-O and 6060-O aluminium alloys.The punch displacement,T-branch height and axial compressive force are recorded online during the experiment.An intelligent inverse identification framework combining the finite element method and numerical optimization algorithm is developed to determine material parameters by fitting simulated results to the experimental data iteratively.The identified constitutive parameters using the inverse modelling technique are compared with those determined by the theoretical analysis and uniaxial tensile test.The comparison shows that the predicted bulge height and punch force based on the material parameters obtained by the three methods are different and the inverse strategy produces the smallest gap between numerical and experimental values.It is possible to conclude that the hydraulic bulge test can be applied to characterize the stress-strain curve of tubular materials at the large strain scope,and the automatic inverse framework is a more accurate post-processing procedure to identify material constitutive parameters compared with the classical analytical model.

An inverse strategy to determine constitutive parameters of tubular materials for hydroforming processes

This paper is to determine the flow stress curve of 5049-O aluminium alloy by a tube hydraulic bulging test with fixed end-conditions. During this test, several tubular specimens are bulged under different internal pressures before their bursting, and the corresponding bulging height and wall thickness at the pole are measured. An inverse strategy is developed to determine the constitutive parameters of tubular materials based on experimental data, which combines the finite element method with gradient-based optimization techniques. In this scheme, the objective function is formulated with the sum of least squares of the error between numerical and experimental data, and finite difference approximation is used to calculate the gradient. The tubular material behavior is assumed to meet the von Mises yield criterion and Hollomon exponential hardening law. Then, constitutive parameters identification is performed by minimization of the objective function. In order to validate the performance of this framework, identified parameters are compared with those obtained by two types of theoretical models, and tensile tests are performed on specimens cut from the same tubes. The comparison shows that this inverse framework is robust and can achieve a more accurate parameter identification by eliminating mechanical and geometrical assumptions in classical theoretical analysis.

Large deflection of clamped circular plate and accuracy of its approximate analytical solutions

A different set of governing equations on the large deflection of plates are derived by the principle of virtual work(PVW), which also leads to a different set of boundary conditions. Boundary conditions play an important role in determining the computation accuracy of the large deflection of plates. Our boundary conditions are shown to be more appropriate by analyzing their difference with the previous ones. The accuracy of approximate analytical solutions is important to the bulge/blister tests and the application of various sensors with the plate structure. Different approximate analytical solutions are presented and their accuracies are evaluated by comparing them with the numerical results. The error sources are also analyzed. A new approximate analytical solution is proposed and shown to have a better approximation. The approximate analytical solution offers a much simpler and more direct framework to study the plate-membrane transition behavior of deflection as compared with the previous approaches of complex numerical integration.

Influence of Heat Treatment on Hydroformability of TRIP Seamless Steel Tube

A low-carbon TRIP seamless steel tube, which is expected to be used in the hydroforming process, was successfully fabricated using piercing, cold-drawing and two-stage heat treatment process. The two-stage heat treatment is one crucial step because it significantly affects the microstructure and mechanical properties of TRIP seam less steel tube. In order to obtain the TRIP seamless steel tube with high hydroformability, several different heat treatment processes were conducted. The effects of heat treatment conditions （intercritical annealing （IA） and isothermal bainite treatment （IBT）） on the TRIP seamless steel tube hydroformability which was determined by free hydraulic bulge test were analyzed. Two different internal pressure boosting velocities of 0.2 and 0.5 MPa/s of free hydraulic bulge tests were adopted to determine the effective stress vs. effective strain curve of TRIP seamless steel tube. The results showed that for the predetermined IA condition, the maximum bulge height increased, but the maximum burst internal pressure decreased, with the increase of IBT holding time from 4 to 6 rain. For the predetermined IBT condition, the maximum bulge height decreased, but the maximum burst internal pressure increased, with the increase of IA holding time from 5 to 10 rain. By analyzing the free hydraulic bulge test results, it was found that the maximum bulge heights of TRIP seamless steel tubes with the internal pressure boosting velocity of 0.5 MPa/s were higher than those when the internal pressure boosting velocity was 0.2 MPa/s. This means that an appropriate deformation rate should be chosen to obtain the optimal hydroformability of TRIP seamless steel tube. In addition, the effective stress vs. effective strain curves of TRIP seamless steel tubes were ohtained with free hydraulic bulge test.