Springback experimental evaluation and validation of aircraft industry sheet metal

Abstract

The tendency of the metal to regain its original shape will result in the spring back when it is being formed to make aircraft components like outer body panels and brackets. When spring back occurs, the components will not meet the requirements of the design and there will be a need for shims to fit the component into place. This will lead to increase in weight, fabrication cost and also the assembly line time. So the Finite-Element Analysis (FEA) is used to accurately predict this deflection so that manufacturing processes can be optimized to produce a perfect output with least deviations from the design. This thesis bridges a relation between the experimentally evaluated spring back and FEA calculated spring back. Also in order to validate the FEA Analysis regression analysis has been performed. Variations have been tabulated and graphed as it bridges the gap between experimentally evaluated spring back and analysis of the design. INTRODUCTION Aircraft construction involves a wide range of materials. Out of these sheet metal plays a major role. Sheet metal aircraft construction is the most prevalent aircraft construction material by all measures, used extensively from jetliners to light, single engine airplanes and kits over the past five decades. Furthermore, virtually all other aircraft types use sheet-metal construction to some degree whether an instrument panel on a composite aircraft, or a firewall on a wood or steel tube and fabric design. New and modern metal alloys and materials have allowed aviation technology to advance, and is the reason it continues to dominate over other aircraft building methods. Steel’s and Aluminum’s unique combination of properties makes it one of the most versatile engineering and building materials in existence:  Low weight / high strength relationship.  Corrosion resistance, especially with newer alloys and modern primers.  Low cost and widespread availability. The Bending process is the forming of sheet metal where angled or other shaped parts are produced. The process involves the uniform straining flat metal sheets around a linear axis, but it also may be used to bend tubes, drawn profiles bars, and wire. In bending, the plastic state is brought by a bending load. In fact, one of the most common processes for sheet metal forming are bending, which is used not only to form pieces such as L, U or V-profiles. Bending has the greatest number of applications in the automotive, aircraft and defense industries and for production of other sheet metal products. Typical examples of sheet-metal bends are illustrated in Fig 1. The basic characteristic of bending is tensile elongation on the outer surface and compression on the inner surface as shown Fig 1. Fig 1: Typical examples of sheet metal bend parts. The entire stress-strain curve is transverse, elastic stresses result in spring back and the residual stress pattern. Here, the bend radius Ri is measured on the inner surface of the bent piece. The bend angle φ is the angle of the bent piece and T is the material thickness. In bending process, since the outer fibers of the material are in tension and the inner fibers are in compression, theoretically the strain values on the outer and inner fibers are equal in magnitude and are given by the following equation: [Gupta*, 2.(9): September, 2015] ISSN 2349-6193 Impact Factor (PIF): 2.243 IJESMR International Journal OF Engineering Sciences & Management Research http: // www.ijesmr.com © International Journal of Engineering Sciences & Management Research [43] e0= e1 = 1 ( 2R T )+1 Experimental research indicate that this formula is more precise for calculating the deformation of the inner fibers of the material, e1, than for the deformation of the outer fibers, e0. The deformation in the outer fibers is notably greater, that is why neutral fibers move towards the inner side of the bent piece. As Ri/T ratio decreases, the bend radius becomes smaller; the tensile strain at the outer fibers increases and the Material eventually cracks. LITERATURE SURVEY Since sheet metal forming industry has become one of the major manufacturing centers for automobile and aerospace and defense industries, the popularity of sheet metal products is attributable to their light weight, great interchangeability, good surface finish, and low cost. There has been a growing interest during the past decade in using finite element method for Springback prediction following forming of arbitrary shapes. While it is apparently simple in concept, the prediction of Springback has proven challenging for a variety of reasons, including numerical sensitivity, physical sensitivity, and poorly characterized material behavior under reverse loading and unloading conditions. Springback of sheet metal parts after forming causes deviation from the designed target shape and produce downstream quality problems as well as assembly difficulties. Its economic impact in terms of delayed production, tooling revision costs, and rejection of unqualified parts is estimated to exceed $50million per year in the U.S. automotive industry alone. It is obvious that controlling Springback is a vital concern in manufacturing.Several studies has been done for past decades in order to develop Springback reduction and compensation methods. S. Nishino et al.[4] examined a new method of predicting a shape fixation property by combining free bending test data with the results of the computer simulations conducted using the finite element method (FEM). With the increased use Finite Element Simulation in tooling departments, the forming analyses of sheet metal components are used more frequently in the design feasibility studies of production tooling. These computer based tools allow the design engineer to investigate the process and material parameters controlling the material floe over the die surfaces. Several studies were done in past decade. M. Firat [17] studied U-Channel forming analysis to predict Springback. He established a kinematic hardening model based on additive back stress form in order to improve the predicted sheet metal deformation response .S.K.Panthi et al. [18] were also studied on a large deformation algorithm based on Total-Elastic-Incremental-Plastic Strain (TEIP) which was used for modeling atypical sheet metal bending process. The process involves large strain, rotation as well as Springback. N.Narasimham et al. [19] aimed to introduce a coupled explicit to-implicit finite element approach for predicting Springback deformations in sheet metal stamping that can be utilized for minimizing die prototype design time. In this study, they have utilized the explicit method initially to analyze the contact based forming operation of stamping process. Then an implicit solution has been performed to simulate the Springback developing in a blank after the forming pressure removed. They have coupled ANSYS/LS-DYNA explicit and ANSYS implicit codes to solve sheet metal forming processes that involves a high degree of Springback. One of the important studies of finite element analysis of Springback in bending was done by V. Esat [20]. In the mentioned work, V. Esat et al. developed a finite element simulation in order to simulate Springback by means of a Springback factor using commercially available finite element program. They reached a good agreement between the finite element simulation and empirical data. Their finite element model is based on 2-D shell elements and Chung-Hulbert dynamic implicit as time integration scheme. They used penalty method on analytically defined rigid bodies to handle contact algorithm. D.W.Park et al. [22] proposed a new shell element to improve accuracy and efficiency of Springback simulation by describing complicated bending deformation accurately. They applied the new element both implicit Finite Element Method and explicit Finite Element Method to conduct Springback simulation. Many studies had been carried out on different perspectives of Springback. Micari et al. [33] presented a Springback prediction technique in three dimensional stamping processes which is based on a combined approach in which an explicit finite element code has been employed to simulate the forming phase while a traditional implicit procedure has been used to analyze the Springback phase. Gauand Kinzel [34] performed an experimental study for determining the Bauschinger Effect on Springback predictions which seems very significant in wipe bending operations. Since Springback is a vital concern in manufacturing industry, beyond evaluating and simulating attempts of Springback, some researchers studied the parameters that effect Springback in sheet metal forming operations in order to control these disturbing parameters. [Gupta*, 2.(9): September, 2015] ISSN 2349-6193 Impact Factor (PIF): 2.243 IJESMR International Journal OF Engineering Sciences & Management Research http: // www.ijesmr.com © International Journal of Engineering Sciences & Management Research [44] FACTORS AFFECTING BENDING Bend radius Ri, is one of the most important parameter which affects bending operations of sheet metals. The bend radius in bending operations always pertains to the inside radius of bend. Minimum bend radius is dependent on the material thickness and the mechanical properties of the material. Minimum bend radii vary for various metals; generally, most annealed metals can be bent to a radius equal to the thickness, T and sometimes to T/2, for a given bend angle and bend length. Bend angle is another crucial factor in bending operations. As the bend angle becomes larger, especially with bend angles over 90°, many difficulties arise. In this case, the amount of bend radius become more critical and the material hardness becomes more detrimental to the success of the bending process. In bending process, some deformations occur in the bent-up region of the work piece depending on the dimensions of the work piece, bend angle, and bend radius. As th