Key Engineering Materials Vols. 651-653

Abstract: Numerical and analytical models dealing with different physics involved in pultrusion are combined in the optic of an integrated analysis of the process. The impregnation stage is simulated by means of a CFD multiphase model, evaluating the pressure and velocity field in the liquid resin. Composite temperature and degree of cure are inferred using 3D thermo-chemical models. Finally, contact conditions, stresses and strains are derived applying computational simulation and analytical models, in order to predict the final pulling force. Different product sizes are considered, simulating suitable processing condition.

Abstract: A numerical method to predict failure performance was developed for resistance spot welded AHSS (advanced high strength steel) sheets in this work and applied for a TRIP (transformation induced steel sheets) 980 sheet. For the numerical analysis, utilizing the numerical inverse calibration method, hardening data and fracture criteria were characterized for the base and the weld nugget based on the standard simple tension test and a newly designed miniature tension test, respectively. The characterized properties of the base and the weld nugget were then applied to analyze failure performance in the coupon tests of the lap-shear and U-shape tension tests carried out for welded sheets. The analysis was performed under the quasi-static condition in this early effort and showed reasonably good agreement with experiments both in failure modes and strength.

Abstract: Ti-6Al-4V is an alloy that is increasingly used in aeronautics due to its high mechanical properties coupled with the lightness. An effective technology used to manufacture titanium components with a reduced buy to fly ratio is the laser beam welding. Previous studies showed that the key factor that rules the mechanical properties and the fatigue life of the joint is its morphology. The aim of this paper was to investigate the influence of the main process parameters, such as welding speed and laser power, on the geometrical features of the joint, in terms of undercut, underfill, reinforcement and so on. 3.0 mm thick Ti-6Al-4V rolled sheets were welded in butt joint configuration by using a Nd-YAG laser source. The joint performances were studied in terms of weld morphology, microstructure and Vickers microhardness. Then, defects such as underfill and reinforcement, controlling the whole weld morphology, were observed, and the relationship between the occurrence, the entity of these defects and the process parameters was studied.

Abstract: The aim of the present work is to investigate the influence of the Vacuum Assisted Resin Transfer Molding process steps on the impregnation quality of the laminates as well as on mechanical and tribological properties of the processed material. Composite laminates were realized using epoxy resin reinforced with carbon (CF) or glass continuous (GF) fibers. Two different textile architectures, namely non-crimp fabrics (UD) and woven-mat (0/90), were used and various processing conditions were employed. Optical observations revealed an unexpected trend relatively to the intra and inter bundle voids concentration with respect to the impregnation velocity, especially using UD-CF and UD-GF reinforcements and low impregnation rate. Tensile and three points bending tests highlighted the strong impact of fiber material and architecture on mechanical properties, whereas the presence of voids played a slightly influence on the fiber dominated characteristics analyzed. Tribological outcomes evidenced a reduction of the friction coefficient when the resin is reinforced by carbon or glass fibers as well as when the sliding direction of the counterbody is oriented parallel to the fiber direction.

Abstract: Metallic foams are known for their interesting physical and mechanical properties such as high stiffness, very low specific weight, high compression strength, unusual acoustic and thermal properties and good energy absorption characteristics. These materials are currently manufactured by means of several different processes. The limit of these conventional technologies is the impossibility to produce foams with complex geometry.This paper deals with the study of an innovative method to produce complex shaped precursors for aluminum foams through cold gas dynamic spray deposition process (CGDS). Aluminum alloy (Al-Si) fine powders were previously mixed with the blowing agent (titanium-hydride, TiH2). The mixing process was carried out by means of sound-assisted fluidized bed apparatus that allows to obtain an homogeneous mixture of the two elements. The mixed particles were then sprayed by means of the CGDS process on a stainless steel sheet that simulates the real component to be reinforced with the foam. Subsequently the obtained precursor was heated up in a furnace The produced metal foam was characterized using optical and electron microscopy in order to study the cell morphology and distribution.The obtained results showed the effectiveness of this method to produce aluminum foams with complex shape.

Abstract: The main objective of the present work is to assess the influence of several parameters relevant for Finite Element Analysis (FEA) in modelling Friction Stir Welding (FSW) processes on AA2024-T3 plates. Several tests were performed including variations on the type of shell elements, number of integration points across thickness direction and mesh refinement levels, aiming for good accuracy and low computational cost. On the one hand, several setups of the mechanical boundary conditions, modelling the clamping systems, were also tested, leading to the conclusion that the results, in terms of longitudinal residual stresses, are significantly affected by this factor. On the other hand, variations on the heat input distribution showed a reduced effect, or almost null, on the final results.

Abstract: The production of prostheses is still not completely optimized, especially for those districts where both functional and aesthetic requirements have to be combined with the urgency of intervention. The prostheses manufactured by machining using CAD/CAM techniques represent the conventional way to obtain a “custom-made” part. However, the above-mentioned solutions are penalized by the too long manufacturing time. This limit can be overcome by using an innovative metal-forming process, i.e. the Incremental Sheet Forming (ISF), which also allows to obtain complex patient-specific geometries even if characterized by a lower precision compared to the conventional process. In this paper, alternative approaches to manufacture a skull prosthesis (i.e. conventional milling and ISF) are compared from technological and economical points of view.

Abstract: The use of FEM (finite element method) to assistant in ramp-up processes of car body construction lines is increasing, thanks to developments in recent years [1-3]. Car body manufacturing begins with sheet metal forming, while in subsequent steps the inner structures of the vehicle are assembled and connected to the outer skin by hemming. With reference to the current state of the art, there is no methodology which can reliably predict the dimensional accuracy of body parts through metal forming [4].Additionally, several methods to predict the distortion of joining and the dimensional effect of clamping during the assembly process were presented and validated [4-11]. Dimensional effects of the clamping process are basically the result of a deliberate alignment, other than the given values of construction to compensate dimensional inaccuracy of single parts from the body shop. These deliberate alignments are generally effected through a translation of clamps and pins in the clamping device. Until now, most of the methods of clamping and joining simulation presented have been verified using academic samples.In this report, the quality of forecasting in real problems during a ramp-up process will be verified and expanded. As part of a national project, co-funded by Sächsische Aufbaubank (SAB), the potential of FEM to assist in the ramp-up process were reviewed in a cooperative effort between Porsche Leipzig GmbH and Fraunhofer Institute for Machine Tools and Forming Technology (IWU). Furthermore, it will be shown that developed methods are able to represent the influence of deliberate positioning of clamps in complex samples. For the first time the quality of forecasting through the translation of locating pins is numerically and experimentally qualified.

Abstract: In this study we construct and analyze a two-well Hamiltonian on a 2D atomic lattice.The two wells of the Hamiltonian are prescribed by two rank-one connected martensitic twins,respectively. By constraining the deformed con gurations to special 1D atomic chains withposition-dependent elongation vectors for the vertical direction, we show that the structure ofground states under appropriate boundary conditions is close to the macroscopically expectedtwinned con gurations with additional boundary layers localized near the twinning interfaces.In addition, we proceed to a continuum limit, show asymptotic piecewise rigidity of minimizingsequences and rigorously derive the corresponding limiting form of the surface energy.

Abstract: Metal forming processes involve continuous strain path changes inducing plastic anisotropywhich could result in the failure of the material. It has been often observed that the formation andevolution of meso-scale dislocation microstructures under monotonic and non-proportional loading have substantial effect on the induced anisotropy. It is therefore quite crucial to study the microstructureevolution to understand the underlying physics of the macroscopic transient plastic behavior. In thiscontext the deformation patterning induced by the non-convex plastic energies is investigated in amulti-slip crystal plasticity framework. An incremental variational approach is followed, which resultsin a rate-independent model exhibiting a number of similarities to the rate-dependent formulationproposed in [Yalcinkaya, Brekelmans, Geers, Int. J. of Solids and Structures, 49, 2625-2636, 2012].However there is a pronounced difference in the dissipative character of the models. The influenceof the plastic potential on the evolution of dislocation microstructures is studied through a Landau-Devonshire double-well plastic potential. Numerical simulations are performed and the results arediscussed with respect to the observed microstructure evolution in metals.