Key Engineering Materials Vols. 504-506

Abstract: The numerical modeling of microstructure evolution in the forming processes is used different schemes and different tools. One of the methods is cellular automata (CA), and recently an interest in cellular automata for modeling of forming processes is increased constantly. Cellular automata are considered as one of the most optimal tool for microstructure modeling because of the computational effort and the obtained results. The paper presents the use of frontal cellular automata (FCA) for modeling microstructure evolution during the rolling process. The use of frontal cellular automata in combination with other methods is related to the possibility of obtaining more accurate and reliable results and has undeniable advantages. Results of the modeling of the process by finite element method (FEM) are input data for further simulation by FCA. Some examples of microstructure as results of FCA simulation are presented in the paper.

Abstract: In this paper, a new hybrid production technique is presented. It exploits the advantage of high temperatures and high forces in the ring rolling process. This manufacturing technique is not only suitable to increase the ring’s diameter but also to apply and compact powder metallurgical multi-functional coatings onto solid substrate rings with the same process. In order to design this new process parameterized 2D and 3D FE models are created in ABAQUS/EXPLICIT on the basis of a viscoplastic material model formulation. The control capability of the conventional control mechanisms are based on the assumption of volume consistency. However, this assumption is not well applicable for a ring furnished by multi-functional surfaces with non-isochoric plastic deformation behavior. Therefore, this paper deals with the implementation of a new control mechanism. Finally the paper is concluded with the integration of heat treatment of the rolled ring into the subsequent cooling process.

Abstract: This paper describes the possibilities for identifying temperature-dependent mechanical and thermo-physical properties of oxide scale components which are a necessary data input for a numerical simulation. Scale develops on steel surfaces in oxidising atmospheres above 570 °C and influences the surface quality as well as the roughness of semi-finished products. The main components of oxide scale are wustite, magnetite, and hematite. Intermediate layers can be formed depending on the chemical composition of the basic material, which can be assigned at the rolled samples with the help of the optical microscopy. Hence, the mechanical properties and the metal forming relevant properties of the several oxide layers are very different. Owing to these differences the deformation behaviour of the inhomogeneous oxide scale also varies during hot rolling. With this new method is it possible to determine the pure properties of the several oxide scale layers independently from each other. Thereby, the influences of the raw material as well as the alloys are excluded. The pure powder allows to characterising the physical properties by applying compression tests with similar stress condition then during hot rolling. Additionally, the 3 piont-bending tests are conducted in order to describe the fracture toughness of the oxides. The results will be implemented in a new model, which consist of single layers with specific material properties. Based on the new knowledge the forming technology can be managed and controlled to minimise the scale cracks on the surface and thus the resulting roughness as well.

Abstract: To produce preforms for complex long flat parts with an unsteady mass distribution along the longitudinal axis rolling processes, like cross wedge rolling, can be used. Tools for cross wedge rolling processes can be constructed as roller or flat, both with wedges. In the collaborative research project SFB 489 “Process chain for the production of precision forged high performance parts” the subproject “Innovative machine and tool technology for precision forging” deals with the development of a flashless forging process for a two cylinder crankshaft with pin and flange. This process is developed by IPH – Institut für Integrierte Produktion Hannover. The first preform of the developed forging sequence is produced by a cross wedge rolling process on the basis of flat with wedges. To consider the mass distribution of the two cylinder crankshaft in the preform for a rolling process four mass concentrations for the crank arms and mass concentrations for pin and flange are needed.

Abstract: In this paper, a discrete approach for the simulation of the preforming of dry woven reinforcement is proposed. A “unit cell” is built using elastic isotropic shells and axial connectors instead of bars and beams used in previous studies. Shell elements are used to take into account the in-plane shear stiffness and to manage contact phenomenon with the punch and die. Connectors reinforce the structure in the yarn directions and naturally capture the specific behavior of the fabric. To identify the material parameters, uniaxial tensile tests and bias tests have been employed. A numerical algorithm, coupling Matlab and Abaqus/Explicit, is used to determine the shear parameters by an inverse method. The model has been implemented in Abaqus to simulate hemispherical stamping. Experimental results are compared to numerical simulations, good agreement between both results is shown.

Abstract: Abstract Two experimental devices are used for the analysis of the deformation mechanisms of biaxial non-crimp fabric composite reinforcements during preforming. The bias extension test, commonly use for the shear behaviour characterisation of woven fabrics, allows to highlight the sliding between the two plies of the reinforcement. This sliding is localized in areas of high gradient of shearing. This questions the use of bias extension test in determining the shear stiffness of the studied reinforcement. Then a hemispherical stamping experiment, representative of a preforming process, allows to quantify this sliding. The slippage is defined as the distance, projected onto the middle surface, of two points initially opposed on both sides of the reinforcement. For both experiments, the characteristic behavior of the non-crimp fabric reinforcement is highlighted by comparison with a woven textile reinforcement. This woven fabric presents only a very little sliding between warp and weft yarns during preforming. This aspect of the deformation kinematics of the non-crimp fabric reinforcement must be considered when simulating the preforming.

Abstract: A hybrid finite element discrete mesoscopic approach is proposed to model the forming of composite parts using a unidirectional glass prepreg non-crimp fabric (NCF). The tensile behavior of the fabric is represented using 1-D beam elements, and the shearing behavior is captured using 2-D shell elements. The material is characterized using tensile and shear frame tests. These properties are then incorporated into an ABAQUS/Explicit finite element model via user-defined material subroutines. The shear frame characterization test is simulated using a finite element model of the fabric, and the finite element results are compared to experimental data as a validation of the methodology. The thermostamping of a double-dome geometry, which has been used in an international benchmarking program, is modeled as a demonstration of the capabilities of the proposed methodology.

Abstract: A flax fibre plain weave fabric has been used to form a complex tetrahedron shape. The global shape has been obtained. Even if no apparent defects are visible when observing the yarns, the strain of the tightest yarns of the preform has been measured and compared to the biaxial strains of the fabric determined independently of the process. The results show that the strains in the yarns close to the triple point of the shape (top of the tetrahedron) are higher than the strain at failure. This could lead to local lack of fibre density and to possible zones of weakness for the composite part. It is therefore necessary to increase the tensile performances of the yarns constituting the fabric.

Abstract: Design and production guidelines for UD reinforced thermoplastic composites are highly desirable. Therefore, forming experiments and simulations with a realistic complex shaped product were conducted. Thermoforming experiments with quasi-isotropic UD carbon/PEEK and 8HS woven glass/PPS composites showed a clear difference in formability. Many wrinkles develop near doubly curved areas for the considered UD composites, whereas significant in-plane shear is observed for the woven composites. Forming prediction tools can be utilised to optimise the product design with respect to formability. A forming prediction methodology is shown, which encompasses finite element modelling in combination with material models that describe major deformation mechanisms. Characterisation methods were developed to describe inter-ply friction and in-plane shear. Forming simulations are able to indicate the critical areas for the UD composites, as is concluded from the comparison of wrinkling and in-plane shear distributions within the formed specimens. Forming experiments and predictions match qualitatively well and this tool can successfully be utilised in the product design phases.

Abstract: The Continuous Fibre Reinforcements and Thermoplastic resin (CFRTP) are widely employed in the prepreg processes. Currently, the most used thermoplastic resins in aeronautics are PPS (polyphenylene sulfide) and PEEK (Polyetheretherketone). They present many advantages on their mechanical properties. However, these mechanical properties depend strongly upon the thermoforming conditions, especially the intraply shearing. In order to improve and complete the understanding about the in-plane shear behavior of thermoplastic composite materials in their forming processes, the thermo-mechanical analysis of PPS/carbon and PEEK/carbon commingled fabrics at different forming temperatures are performed by using the bias-extension tests. The experimental data leads to significant difference on the in-plane shear behavior under different temperature, as well as the wrinkles can be noted in certain thermoforming conditions. Therefore, in order to predict the feasible forming conditions and optimize the important forming parameters of the thermoplastic composites, the in-plan shear behaviors in function of temperature will be integrated into our numerical model to carry out the numerical simulations of thermoforming processes.