Analytical and numerical analysis of a “springback-forming” process dedicated to stiffened panels

Highlights

• We developed a forming process dedicated to stiffened panels.

• We obtained a uniform single curved panels using the proposed process.

• We developed an analytical and a numerical model of the process.

• We analyzed the capabilities and the limitations of the process.

• We carried out experimental tests to demonstrate the feasibility of the process.

Abstract

The aim of this paper is to present and to analyze the capabilities of a process named “springback-forming”, dedicated to stiffened panels such as airplane׳s fuselage panels. The principle of this forming process is to apply a tension on the stiffener, before the assembly stage with the sheet in a flat configuration using fasteners, adhesives, or a welding process, etc., the bending of the structure is then achieved by springback energy of the stiffener when its tension is released. Using an analytical and finite element models, we studied the capabilities of this process in terms of curvature limits in the case of a single-curved stiffened panel. The results of both models are in good agreement. Through a parametric study, numerical simulations show that when the structure is relatively slender the curvature radius obtained is uniform. Moreover, the value of this radius is independent of the structure׳s length and is mainly limited by the stiffener׳s height. The carried out experimental tests, using laser beam welding as a joining process, demonstrated the feasibility of the process. From the proposed modeling, it is possible to evaluate the range of achievable curvature radius and its uniformity for different values of both geometrical and mechanical parameters of the structure.

Introduction

The transportation sector, including aeronautics, automobiles, railway and naval, is based in a large proportion on forming metallic materials. In these sectors, there is a constant need of reducing costs such as product development cost (in prototyping or in industrialization stage); tools cost by making them, for example, more reusable; and manufacturing costs by having less parts and reducing the assembly time. This constant need led to a global approach aiming to have the most suitable manufacturing processes for each type of parts, and a robust simulation tools to analyze the performance of these processes. In this context, the airplane manufacturers are interested in the development of innovative forming processes dedicated to stiffened panels such as fuselage panels. These structures are constructed primarily from thin sheets, called also web or skin, and stiffening elements such as beams [7].

An assessment of existing manufacturing technology for metallic fuselage structure was carried out by Pettit et al. [10]. We distinguish, in this assessment, two categories of manufacturing strategy of these stiffened panels: in the first category, sheets and stiffeners are formed separately and then assembled, mostly by riveting and in the second category, sheets and stiffeners are first assembled and then formed together to the correct shape.

In the first category, the manufacturing of each element of the structure is based on conventional processes. The most used process for sheets is roll forming to make singly curved panel, as reported by Megson [7]. This process is usually replaced by stretch forming for doubly curved or more complex panels. The stiffeners are extruded or machined and then assembled with the sheet (using bolts, rivets, or a welding process). In this category, the precision of the final shape is largely dependent on the precision of each component. Furthermore, automatizing of such assembly operations is costly in terms of machines and tools, mainly because of the curvature of the stiffened panels. In contrast, the assembly in flat configuration requires less sophisticated machines and therefore is more cost effective and easier to control.

In the second category, press bend-forming is an effective and often used process, as reported in NASA-CR-124075 [9]. On the one hand, the main advantage of this process is the use of a universal die for all panels; on the other hand, the key problem is the design of the forming path of the punch used in the bending process. This issue is often solved by using finite element models instead of an experimental approach. Because of the time consuming simulations, Yan et al. [14] developed an equivalent model to improve the efficiency of the finite element model and optimize the bend forming path. Moreover, the cost of this process increases because of the considerable realignment work needed to achieve the imposed tolerances [8]. A more versatile process, in the same category, with lower machine and manufacturing costs, is shot peen-forming. This process is a major process for manufacturing wing skins [13] and is also used successfully to form fuselage panels [8]. Its versatility comes from its adaptability to all panel sizes, reduced machines costs since neither the die nor the punch is needed, and its good production rate. However, with this process only small curvature is achievable and special precautions are necessary to avoid producing doubly curved panel. Li [5] studied experimentally the use of pre-bending of the panel while it is formed, using peen-forming, as a way to form single-curved stiffened panel. He showed that the increase of the pre-bending loads induces the decrease of the curvature radius in the pre-bending direction and the increase of the curvature radius in the perpendicular direction. Similar to other processes, to determine the process parameters, the trial-and-error approach is more and more replaced with efficient numerical models. Wang and Platts [13] presented a numerical procedure to obtain the initial blank shape from the final formed surface. Gariepy [3] developed a finite element model of the process capable of predicting accurately the final shape and the effect of different parameters on the process.

A variation of press bend-forming is warm forming. In this process, the bending capability of the panel is extended by increasing the working temperature, during forming, for an adequate amount of time. Generally, the working temperature is around 200–300 °C for aluminum alloys [12]. The warm temperature increases the material ductility and lowers its yield strength. As a result, smaller curvature radii are achieved compared with cold forming processes. However, because of the warming equipment necessary additional cost is added.

A more favored process, in the aerospace industry, is creep age-forming [6]. In this process a heat treatment (artificial aging of aluminum alloys like the 2000 series) takes place, in an autoclave, simultaneously with the forming process. The latter is a bend-forming process using vacuum bagging technique. Holman [4] showed, by experience, that the smallest residual stresses are obtained using creep age-forming, compared with roll forming, press bend forming, and shot peen-forming. Brewer [1] tested successfully its feasibility in the case of wing skins and fuselage panels. However, with this process a large springback occurs. An exploratory experimental work was led by Airbus Saint-Nazaire to form a single-curved stiffened panel using this process. The springback varies from 65% to 90%. To help predict the curvature achievable and control the springback, robust numerical models are more and more used. In addition, these models serve to study the feasibility of applying creep age-forming to stiffened panels. Lin et al. [6] developed a numerical model to estimate the springback of a non-stiffened sheet. The results are between 65% and 80% of the tool׳s radius. Takafumi et al. [11] studied experimentally and numerically the forming of doubly curved stiffened wing skins (using creep age forming). The springback obtained is between 50% and 70% (of the tool׳s radius) and the difference between the two approaches is less than 7%. Davoodi [2] studied numerically the forming of a single-curved stiffened panel. The springback obtained is between 65% and 90%.

In this paper, we investigate the feasibility of a process that we named “springback-forming”. The proposed process belongs to the second category; does not need a die or a punch as in shot peen-forming; and the springback is absent, contrary to the processes mentioned above. To study and analyze the performance of this process, we illustrate its principle in the case of the forming of a single-curved panel with one stiffener. We also developed an analytical and a numerical model to determine the capabilities and the limitations of the process in terms of the achievable curvature radius. We find a good agreement between the two models. We conducted experimental tests, using laser beam welding as a joining process, which demonstrated the feasibility of the process and found a qualitative agreement with numerical model. Hence, using the analytical tool we can evaluate quickly the effect of various parameters on the process.