Key Engineering Materials Vols. 554-557

Abstract: Medium to high strength aluminum alloys, such as 2xxx, 6xxx, and 7xxx series, are actually considered of great interest in the transport industries. For aeronautical applications, the precipitation hardenable AA2024 (Al-Cu) alloy is gaining considerable attention, in particular for the realization of nose barrier beam or fuselage panels. In this context, remarkable research effort is currently focused on the application of the Friction Stir Welding (FSW) process, as a suitable alternative to fusion welding processes. The interest in aeronautical application of FSW process is also justified by the reduction of production costs and weight and by the increase of strength and damage tolerance with respect to riveted lap joints. The implementation of the technique in safety-critical components, however, requires a deeper understanding of static strength as well as of fatigue behavior of FSWed assemblies. In this sense some experimental results have already been presented in the inherent literature, relatively, for instance, to AA6082-T6 and AA6061-T6, AA6063-T6, AA2024-T351, AA2024-T8 alloys processed by FSW. Despite the unavoidable relevance of experimental testing, a numerical approach able to predict the mechanical behavior of FSWed assemblies is very desirable, in order to achieve time and cost compression and to implement computational optimization procedures. This paper deals with a numerical investigation on the influence of FSW process parameters, namely the rotating speed and the welding speed, on fatigue crack growth in AA2024-T3 butt joints. The computational approach is based on a combined Finite Element Method (FEM) and Dual Boundary Element Method (DBEM) procedure, in order to take advantage of the main capabilities of the two methods. In particular, linear elastic FE simulations have been performed to evaluate the process induced residual stresses, by means of a recently developed technique named contour method. The computed residual stress field has then been superimposed to the stress field produced by an applied fatigue traction load in a Dual Boundary Element Method (DBEM) environment, where the simulation of a crack, initiated and propagating along the previously mentioned cutting line, can be performed in an automatic way. A two-parameters crack growth law is used for the crack propagation rate assessment. The DBEM code BEASY and the FEM code ANSYS have been sequentially coupled in the aforementioned numerical approach by using a BEASY interface module and in house developed routines. Computational results have been compared with experimental data, showing a satisfactory agreement. The influence of process parameters on the residual stresses distribution has also been highlighted.

Abstract: This paper investigates the integrated modeling of a pultruded NACA0018 blade profile which is a part of the FP7 EU project DeepWind. The pultrusion process simulation is combined with the preliminary subsequent in-service load scenario. In particular, the process induced residual stresses and distortions are predicted by using a new approach combining a 3D Eulerian thermo-chemical analysis, in which the temperature and the cure degree distributions are obtained, and a 2D quasi-static plane strain mechanical analysis. The post-die region where convective cooling prevails is also included in the process model. The bending into shape of the pultruded blade profile is simulated with and without taking the residual stresses into account. The internal stress distribution in the profile is evaluated after the bending analysis and it is found that the process induced residual stresses have the potential to promote or to demote the internal stresses in the structural analysis.

Abstract: One of the common problems in forging processes is the lack of key process parameters control, as well as their identification. Certain controlled parameters exist, such as temperature or stroke length, which are usually identified and controlled through a systematic approach. Their selection depends particularly on the part to produce or on customer’s constraints, rather than a rational approach. In this paper, a methodology is proposed to select the key process parameters. There are some methodologies which already exist, such as the DMAIC, which are used to determine the control parameters and their influences on the desired specifications. However, this approach has certain drawbacks. For example, the key parameters are selected by experts, which makes each case study time consuming. The aim is to develop a generic methodology to improve the manufacturing process in the forging industry. The methodology is represented as a decision support system that connects product specifications (geometry, absence of defects…) or other forging specifications (tool wear, involved energy...) to the process parameters. The latter will be able to define the key parameters, their values and their appropriate way of control. These links will be setup using the empirical rules and physical laws.

Abstract: The aim and scope of this paper is centered to analyze the influence of the geometry of V-shaped dies on the closure of internal centerline porosity defects in ingots during multistep open-die forging. The investigation is performed with small scale physical models made from lead using V-shaped dies with 90º and 120º and a reference pair of flat parallel platens. Holes drilled through the center of these preforms are produced to mimic centerline porosity in full scale cast ingots and intermediate rotation of the preforms replicate a multi-stage forging sequence under laboratory testing conditions. The presentation is supported by finite element modelling using an in-house developed computer program and the overall investigation shows that better results in closure of centerline defects are obtained with a V-shaped die with 120º die angle.

Abstract: Shear tests of rectangular sample are widely used by the scientific community for characterizing the material behavior due to large strains obtained. However, for some hard metals, such as the dual-phase steel DP 980, premature rupture occurs in the vicinity of the grips. Due to this fact, the shape of the shear specimen is optimized in this work with the aim of maximizing the deformation achieved in the central part of the specimen without the occurrence of rupture near the grips. As the rupture occurs at the corners of the shear specimen only the boundaries are subjected to shape optimization. A representation with cubic splines is adopted for the definition of the boundaries geometry. The material is defined by Hill’s 1948 yield criterion combined with an isotropic hardening law. Two macroscopic rupture criteria are considered and an objective function approach based on the maximization of the shear strain average value is defined. For this study, a direct search optimization method is used for minimizing the objective function. The optimized geometries obtained for the different rupture criteria and different set of design variables are compared. The use of a larger number of design variables allows to obtain optimized geometries with higher average shear strain. The best specimen geometry shape allows increasing the maximum deformation of DP 980 steel to 1.05 without occurrence of rupture. In addition, the final specimen geometries show a concave shape for the boundaries which means that this kind of shape is the best one to delay the rupture in shear specimens.

Abstract: Pultrusion is one of the most effective manufacturing processes for producing composites with constant cross-sectional profiles. This obviously makes it more attractive for both researchers and practitioners to investigate the optimum process parameters, i.e. pulling speed, power and dimensions of the heating platens, length and width of the heating die, design of the resin injection chamber, etc., to provide better understanding of the process, consequently to improve the efficiency of the process as well the product quality. Numerous simulation approaches have been presented until now. However, optimization studies had been limited with either experimental cases or determining only one objective to improve one aspect of the performance of the process. This objective is either augmented by other process related criteria or subjected to constraints which might have had the same importance of being treated as objectives. In essence, these approaches convert a true multi-objective optimization problem (MOP) into a single-objective optimization problem (SOP). This transformation obviously results in only one optimum solution and it does not support the efforts to get more out of an optimization study, such as relations between variables and objectives or constraints. In this study, an MOP considering thermo-chemical aspects of the pultrusion process (e.g. cure degree, temperatures), in which the pulling speed is maximized and the heating power is minimized simultaneously (without defining any preference between them), has been formulated. An evolutionary multi-objective optimization (EMO) algorithm, non-dominated sorting genetic algorithm (NSGA-II [Deb et al., 2002]), has been used to solve this MOP in an ideal way where the outcome is the set of multiple solutions (i.e. Pareto-optimal solutions) and each solution is theoretically an optimal solution corresponding to a particular trade-off among objectives. Following the solution process, in other words obtaining the Pareto-optimal front, a further postprocessing study has been performed to unveil some common principles existing between the variables, the objectives and the constraints either along the whole front or in some portion of it. These relationships will reveal a design philosophy not only for the improvement of the process efficiency, but also a methodology to design a pultrusion die for different operating conditions.

Abstract: ALEXANDER IVANOVICH OLEINIKOV Aircraft Engineering Faculty, Komsomolsk-on-Amur State Technical University Lenina prospect 27, 681013 Komsomolsk-on-Amur, Russian Federation a.i.oleinikov@mail.ru Keywords: forming, creep, age, transversely isotropic, kind of the stress state effect, wing panel, inverse problem, reverse engineering, computer-aided process design system. Abstract. Problems of inelastic straining of three-dimensional bodies with large displacements and turns are considered. In addition to the sought fields, surface forces and boundary displacements, original size and shape have also to be determined from specified residual displacements in these problems. Currently, forming of light metals poses tremendous challenges due to their low ductility at room temperature and their unusual deformation characteristics at hot-cold work: strong asymmetry between tensile and compressive behavior, and a very pronounced anisotropy. We proposed the constitutive models of steady-state creep of initially transverse isotropy structural materials the kind of the stress state has influence [1]. The forming process considered includes two stages: active stage of elastoviscoplastic straining of the blank in the die tooling and passive stage of unloading of the blank withdrawn from the die tooling. The final stress-strain state at the active stage is the initial state for the passive stage. Unloading is considered as purely elastic straining, with no increments of inelastic strains. The active stage, in turn, also includes two steps. At the first step, the frontal faces of the “cold” blank are pressed to the working surfaces of the die tooling, which results in elastoplastic straining of the blank. The second step includes the processes of stress relaxation and creep strain in the blank fixed in this die tooling during a given time at an elevated ageing temperature. Computer modeling of these forming processes involves the use of the finite element method for consecutive solutions of three-dimensional quasi-static problems of elastoplastic straining, relaxation, and unloading, and also determining boundary conditions from given residual displacements [2] . The paper gives basics of the developed computer-aided system of design, modeling, and electronic simulation targeting the processes of manufacture of wing integral panels. System application data resulting from computation of 3D-involute of a CAD-based panel model, determination of working surfaces of die tooling, three-dimensional analysis of stresses, and simulation of panel shaping under diverse thermo-mechanical and speed conditions are demonstrated. Modeling of forming of wing panels of the SSJ-100 aircraft are considered [2,3]. The modeling results can be used to calculate the die tooling, determine the panel processibility, and control panel rejection in the course of forming [3]. References [1] A.I. Oleinikov, Models for the steady-state creep of transversely isotropic materials with different tension and compression characteristics, J. Ind. Appl. Math. 5 (2011) 406-409. [2] B.D. Annin, A.I. Oleinikov and K.S. Bormotin, Modeling of forming of wing panels of the SSJ-100 aircraft, J. Appl. Mech. Physics 51 (2010) 579-589. [3] A.I. Oleinikov, A.I. Pekarsh, Integrated Design of Integral Panel Manufacture Processes. Dalnauka, Vladivostok, 2010.

Abstract: System integrity of a flanged connection requires that no leakages occur. Metallic flanges and their joining is of great importance when it comes to avoiding leakages from hydrocarbon lines. The American standard ASTM A182 demands that flanges must be forged to shape, thereby excluding other manufacturing methods. Mechanical properties of duplex stainless steel bars have been examined by doing tensile and charpy tests. A finite element model of a typical ASME-flange assembly was made and was used to calculate stress levels in the flange. The measured mechanical properties of the bar, showed that it is suitable for flange use.

Abstract: Nowadays, the characterization of material is becoming increasingly important due to ma\-nu\-fac\-tu\-ring of new materials and development of computational analysis software intending to reproduce the real behaviour which depends on the quality of the models implemented and their material parameters. However, a large number of technological mechanical tests are carried out to characterize the mechanical properties of materials and similar materials may also have properties and parameters similar. Therefore, many researchers are often confronted with the dilemma of what should be the best set of numerical solution for all different results. Currently, such choice is made based on the empirical experience of each researcher, not representing a severe and objective criterion. Hence, via optimization it is possible to find and classify the most unique and distinguishable solution for pa\-ra\-me\-ters identification. The aim of this work is to propose a methodology that numerically designs the loading path of multiaxial testing machine to characterize metallic thin sheet behavior. This loading path has to be the most informative, exhibiting normal and shear strains as distinctly as possible. Thus, applying Finite Element Analysis (FEA) and Singular Value Decomposition (SVD), the loading path can be evaluated in terms of distinguishability and uniqueness. Consequently, the loading path that leads to the most distinguish and unique set of material parameters can be found using a standard optimization method and the approach proposed. This methodology has been validated to characterize the elastic moduli for an anisotropic material and extrapolated for an hyperelastic material.

Abstract: Superplastic forming is a near net shape process used to produce various items with complex geometry. However in many cases, only some portions of the workpiece undergo superplastic deformation. In these cases, instead of choosing expensive starting sheet material with superplastic properties, a low-cost conventional material can be chosen and a grain refinement process can be performed in the selected regions to enhance superplastic properties locally [1]. This process is known as “selective superplastic forming” [R.S. Mishra, M.W. Mahoney, US Patent 6,712,916, 2002]. In some previous works the use of Friction Stir Processing (FSP) was used to obtain locally a microstructure with ultrafine grains in the AZ31 magnesium alloys [2, 3]. In this study a modeling approach was adopted thanks to a commercial FE code and different simulations were conducted in order to correlate the experimental and numerical results for the model optimization [4, 5]. Free bulge forming tests of friction stir processed AZ31 sheets, in conjunction with numerical simulations, were used to evaluate the proposed optimization approach, with the aim to reduce the time and costs in the design of components with complex geometry.