Key Engineering Materials Vols. 611-612

Abstract: In forming processes of thermoplastic composites, the combined forming behaviour of matrix material and fibre reinforcement determines the resulting geometry and structure. These specific characteristics of the components and their interaction vary during the processing steps, especially for the matrix material with change in temperature. During the forming step, the molten thermoplastic polymer exhibits viscoelastic behaviour. Therefore, the fibres encounter resistance if a forming load is applied. The resulting fibre alignment is dependent on the forming temperature, the forming speed, and the time between the release of load and cooling. An investigation into the specific matrix characteristics during the forming step is presented. In the experiments a representing fibre is drawn through a molten polymer specimen under variation of speed and temperature and the resistance force is measured. The experimental findings are compared to numerical results obtained with a computational fluid dynamics (CFD) package using a finite volume approach and its ability for the prediction of fibre movement in molten matrix during forming processes is evaluated. In addition, a better understanding of the impact of forming speed and temperature during forming processes due to the characteristics of the molten matrix is obtained.

Abstract: SiC composites have been developed by various processing methods, i. e. reaction bonding, CVD/CVI and liquid phase impregnation. This class of composites is handicapped by the high cost of investment and processing, interface coatings and raw materials and therefore only attractive for applications in astronautics and military aviation. Polymer Matrix Composites (PMC) are widely used in lightweight applications. The manufacturing technologies are fully developed and raw materials are cheap. The major drawback and limiting factor of these reinforced polymers is the limited service temperatures. Novel composite materials, bridging the gap between PMC and CMC, are manufactured by the use of polysiloxanes as SiOC matrix precursor. Such competitive free formable composites are capable for service temperatures up to 800 °C even in oxidative atmosphere. In order to make the material attractive for series applications, cost effective manufacturing technologies like Resin Transfer Moulding (RTM), filament winding or warm pressing techniques are employed. Beside the improved thermal and chemical resistivity in comparison to reinforced polymers and light metals, a major benefit of SiOC composites is investigated in the field of friction materials. A promising alternative to carbon and synthetic ceramic fibers are mineral based Basalt fibers. These lightweight fibers show high thermochemical stability up to 700 °C, are relatively cheap and became recently available in industrial scale.

Abstract: In this paper a new variable forming tool concept and associated numerical methods for calculating optimal actuator layout and estimating CFRP part quality are presented. The concept of the tool features a modular design and active control of the forming process to achieve the desired geometry. Initially the laminate is placed on the flat top layer of the forming tool. There it is fixed and compacted using vacuum bagging. After compacting, it is heated up to increase the performance of the forming process using water based tempering of the forming tools top layer. The heated laminate is then formed, pulling the tools top layer into the desired geometry using the actuators. Finally, the formed laminate is cooled and transferred into a mold for curing. The position of the forming tools actuators on the base plates is variable. Numerical optimization in combination with finite element (FE) technologies is utilized, to approximate the tool surface within given error margins, with as few actuators as possible. In addition, results of a numerical method for part quality estimation are shown. The influence of the forming process on mechanical properties due to fiber waviness is taking into account using a self-developed method that includes manufacturing characteristics in FE modeling of the part. The method is based on mathematical descriptions of fiber waviness, which are implemented into a FE model. Therefore a structure discretization assuming perfect fiber orientations is realized and the expected fiber waviness induced by the forming process is applied element-wise.

Abstract: This paper presents a novel test set-up to characterize the bending behavior of UD laminates in thermoforming conditions. An analysis of the set-up and the applied boundary conditions is presented. A validation of the test procedure using purely elastic specimens, shows its potential to accurately measure bending properties, to be used for numerical forming simulations. Preliminary bending tests were performed on UD carbon PEEK specimens, showing a visco-elastic response. The actual bending phenomenon is confirmed to be intimately related to the intra-ply shear deformation mechanism.

Abstract: Defects and shape distortions can arise during thermoforming of thermoplastic composites. Design guidelines for thermoplastic composites can help to prevent such defects. The material selection and geometry definition are part of the design guidelines. A test to check the formability of a material is developed as an aid for the material selection. Strips of varying widths are thermoformed inducing double curvature, in order to find the width at which the material starts to wrinkle. The thermoformed strips of quasi-isotropic UD carbon/PEEK and quasi-isotropic 8HS glass/PPS show a clear difference in formability. The UD carbon and the 8HS glass strips develop wrinkles through-the-thickness of the laminate at a width of 40 mm and 90 mm respectively. The test shows good potential to be used as part of the design guidelines.

Abstract: Friction and wear of lubricated machine elements can be reduced by the introduction of lubrication pockets produced by surface texturing. Different manufacturing methods can be taken into consideration whereas a forming process offers the possibility for mass production. Hot micro coining is a forming process, which allows manufacturing of surface textures with different shapes and dimensions into a flat and deformable material, e.g. seals made of steel. In this work hemispherical and ellipsoid pockets with a maximum depth of 100 µm have been embossed into stainless steel (AISI 304). To ensure that the used process parameters will not lead to tool damage, Finite Element simulations were performed and experimentally verified. First tribological experiments were conducted on a ball-on-disk tribometer in order to study the tribological properties of hemispherical structures with pockets depths of 50 and 100 µm. Different sliding velocities were applied to study the coefficient of friction and wear volume for different lubrication conditions. A comparison between coined and not‑coined specimens demonstrates that the micro coined surface textures lead to a significant reduction in the wear volume at boundary and mixed lubrication conditions.

Abstract: Roll forming process is an interesting process for the production of complex profiles because of its high production rate, low investment and efficient use of material. Furthermore, and due to their high yield strength, this technology is suitable for the forming of Advanced High Strength Steels which are being increasingly introduced in the automobile sector.

Abstract: Roll forming is an important economic forming process for manufacturing of profiles. For an optimal design of the process, it is important to determine the loads occurring during the forming process. Furthermore, the information of the load behavior enables an evaluation of the formability of the planned profiles with the chosen roll forming machine. An experimental determination of loads in roll forming processes requires a complex measurement setup in combination with a high amount of measurement devices. Hence, the analysis of roll loads by means of finite element simulation is of special interest. The use of roll forming simulations for the determination of geometrical outputs is state of the art. However, due to simplifications, a realistic and reliable output of roll loads in roll forming is impossible. Therefore, the compliance behavior under load and the frictional behavior have to be incorporated in the simulation model. The friction behavior in roll forming processes is presented to be very insignificant in literature. The value of the friction coefficients vary in a broad range. Due to lack of knowledge in the compliance behavior of the used stands, simulation models with rigid rolls are still state of the art. This paper will show the reproduction of realistic roll loads, e.g. torques and forces, in a roll forming simulation. Therefore, the friction coefficients of each roll-sheet metal contact will be gained experimentally and implemented in the numerical model. Furthermore, a characteristic compliance of the roll forming stands will be analyzed and also considered in the simulation. Finally, the influence of changing parameters, e.g. raise of the friction coefficients, on the roll loads will be investigated. To verify the simulation the numerical results will be compared to data gained by experiments.

Abstract: Cemented carbides present some characteristics that ensure high performances for cutting and wear-resistant tools. The aim of the work is to evaluate the influence of Chromium Carbide and Nickel on the properties (wear and friction) of a cemented carbide constituted by a hard phase of tungsten carbide and a binder phase of cobalt. Different tests were carried out by varying the percentage of Cr₃C₂ and, for one only case, also Nickel was added. The tests were carried out by a pin-on-disk apparatus and diamond abrasive sheet. The experimental campaign provided tests for different values of load and relative velocity. Friction coefficient was directly evaluated by the apparatus and data on the wear were obtained by measuring the loss of weight of the samples (parallelepipeds with hemispherical head). The tests allowed to determine the percentage of Cr₃C₂ that ensure an improvement of the aforementioned properties and to highlight the irrelevance of Nickel.

Abstract: The present work is motivated by the will to improve Finite Element (FE) Modelling of cutting tool wear. As a first step, the characterisation of wear mechanisms and identification of a wear model appear to be fundamental. The key idea of this work consists in using a dedicated tribometer, able to simulate relevant tribological conditions encountered in cutting (pressure, velocity). The tribometer can be used to estimate the evolution of wear versus time for various tribological conditions (pressure, velocity, temperature). Based on this design of experiments, it becomes possible to identify analytically a wear model. As a preliminary study this paper will be focused on the impact of sliding speed at the contact interface between 304L stainless steel and tungsten carbide (WC) coated with titanium nitride (TiN) pin. This experiment enables to observe a modification of wear phenomena between sliding speeds of 60 m/min and 180 m/min. Finally, the impact on macroscopic parameters has been observed.