Tube forming technologies based on internal forming pressures, such as hydroforming or hot tube gas forming, are state of the art to manufacture complex closed profile geometries. However, materials with excellent specific strengths and chemical properties, such as titanium alloys, are often challenging to shape due to their limited formability. In this study, the titanium alloy Ti-3Al-2.5V was processed by superplastic hot tube gas forming to manufacture a helically shaped flex tube. The forming process was investigated in terms of process simulation, forming tool technology and process window for the manufacturing of good parts. Within a simulation study, a strain rate optimized forming pressure–time curve was defined. With the newly developed tool design, forming temperatures up to 900 °C and internal forming pressures up to 7 MPa were tested. A process window to manufacture good parts without necking or wrinkling has been successfully identified. The experiment data showed good agreement with the numerical simulations. The detailed study of the process contributes to an in-depth understanding of the superplastic forming of Ti-3Al-2.5V during hot tube gas forming. Furthermore, the study shows the high potential of superplastic hot tube gas forming of titanium alloys for the manufacturing of helical flex tubes and bellows.

https://www.mdpi.com/2075-4701/10/9/1150/pdf

Process Development for a Superplastic Hot Tube Gas Forming Process of Titanium (Ti-3Al-2.5V) Hollow Profiles

This paper studies the quality of modal damping ratios estimates based on ambient and free vibration tests using both numerical simulations and data collected on large Civil Engineering structures where both tests were performed. The simulated data allowed to study the influence of factors like non-proportional damping or proximity of natural frequencies on the quality of the estimates and also to illustrate the influence of the identification algorithms parameters on the accuracy of the results. The analysis of data collected on a cable-stayed bridge, on the suspended roof of a stadium and on a footbridge permitted to compare the results of both testing approaches in real applications where some factors that cannot be realistically included in the numerical simulations can play an important role. The processing of ambient vibration responses is performed with two output-only identification approaches: frequency domain decomposition and stochastic subspace identification methods. The numerically simulated and the measured free decays were analyzed with a simple method based on filters and fitting of exponential decays and also with the use of subspace models.

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Damping Estimation Using Free Decays and Ambient Vibration Tests

Deformation behaviors of four-layered U-shaped metallic bellows in hydroforming

Metal bellows are widely used in piping systems, aerospace, automobile and other industries due to their favourable properties including absorption of expansion, light weight and flexibility. In this paper, the fittability was presented to evaluate the convolution shape precision of the metal bellows. By establishing a finite element model of bellows hydroforming process during the bulging and forming stages, the influence of internal pressure, axial feeding and feeding loading path on the wall thickness variation and fittability of one-convolution bellows was investigated. On that basis, the hydroforming process of multi-convolution bellows was studied, and an experiment was carried out. The results showed that with the increase in internal pressure, the wall thickness of the bellows thinned overall, and the fittability of the root zone of the bellows gradually deteriorated. Some region near the crown zone did not fit the dies well when the internal pressure is small. To improve the fittability of the bellows, the actual axial feeding should be a bit less than the theoretical axial feeding. It is of importance in developing the hydroforming technique and improving the hydroforming quality of bellows.

Influence of process parameters on thinning ratio and fittability of bellows hydroforming

Bending characteristics of the reinforced S-shaped bellows under internal pressure

Mathematical description and mechanical characteristics of reinforced S-shaped bellows

Reinforced s-shaped bellows is a kind of typical reinforced metal bellows that can withstand high pressure. Hydroforming process and its simulation technology of reinforced s-shaped bellows were investigated in this paper. The hydroforming technology of reinforced s-shaped bellows is a severe plastic deformation process. Most area of the bellows is in the state of severe plastic yielding with a maximum equivalent plastic strain of 30.6%. At the same time, the wall thickness shows a linear thinning trend from wave trough to peak. The control of hydraulic pressure is a key factor of hydroforming process. The best hydroforming hydraulic pressure is 12 MPa, which is in good agreement with the designed shape. At last, the key factors affecting plastic strain and wall thickness thinning of the hydroforming process were revealed. Plastic strain and wall thickness thinning of the large-pitch bellows in the hydroforming process are obviously higher than those of the small-pitch ones. The waveform parameters directly affect the hydroforming process. However, the influences of initial wall thickness and number of layers of the bellows were not obvious. The maximum thinning rate of the bellows basically maintains around 20%.

Study on the hydroforming technology of reinforced s-shaped bellows

Experimental study on scheme optimization of composite damping structure was carried out and the effect of environmental temperature on damping characteristic was investigated in the present study. Damping coefficient of three layers composite damping structure is nearly ten times of that of light plate. The scheme of composite damping structure has a significant effect on damping characteristic. Effect of environmental temperature on damping characteristic was analyzed under temperature range from −10 °C to 80 °C. It shows that both of frequency and damping coefficient of composite damping structure are influenced by environmental temperature. There is a minimum frequency value which occurs at nearly 30 °C in the temperature range. Damping coefficient decreases with the increase of environmental temperature. The maximum damping coefficient is more than twice of the minimum one. Environmental temperature needs to be well considered in the practical application of composite damping structure.

Experimental Study on Scheme Optimization of Composite Damping Structure

Purpose




Computational efficiency is always the major concern in aircraft design. The purpose of this research is to investigate an efficient jig-shape optimization design method. A new jig-shape optimization method is presented in the current study and its application on the high aspect ratio wing is discussed.




Design/methodology/approach




First, the effects of bending and torsion on aerodynamic distribution were discussed. The effect of bending deformation was equivalent to the change of attack angle through a new equivalent method. The equivalent attack angle showed a linear dependence on the quadratic function of bending. Then, a new jig-shape optimization method taking integrated structural deformation into account was proposed. The method was realized by four substeps: object decomposition, optimization design, inversion and evaluation.




Findings




After the new jig-shape optimization design, both aerodynamic distribution and structural configuration have satisfactory results. Meanwhile, the method takes both bending and torsion deformation into account.




Practical implications




The new jig-shape optimization method can be well used for the high aspect ratio wing.




Originality/value




The new method is an innovation based on the traditional single parameter design method. It is suitable for engineering application.

Shihui Huo

Papers

Mathematical description and mechanical characteristics of reinforced S-shaped bellows

Study on the hydroforming technology of reinforced s-shaped bellows

Experimental Study on Scheme Optimization of Composite Damping Structure

A new jig-shape optimization method for the high aspect ratio wing