Ulrich Sachs

Evaluation of master equations for the droplet size distribution in condensing flow

The kinetic equation (KE) and its first- and second-order approximations, the general dynamic equation (GDE), and the Fokker–Planck equation (FPE), respectively, have been evaluated based on (a) their equilibrium distributions, (b) a nucleation pulse experiment, and (c) an expanding nozzle flow. Large differences are observed between the equilibrium distributions of the FPE and KE, whereas the GDE does not have an equilibrium distribution at all. For the nucleation pulse experiment, good agreement is found between the KE, FPE, and GDE due to quasisteady nucleation. For the condensing nozzle flow, the difference between the GDE and the KE distributions is large, although the relevant flow variables show fair agreement. A sensitivity study of the KE solution with respect to uncertainties in (a) the surface tension model, (b) the sticking probability, and (c) the equilibrium distribution revealed that both the sticking probability and the equilibrium distribution have a significant influence on the predicted condensation onset. Furthermore, it is found that the proposed Wolk and Strey-corrected Courtney equilibrium distribution yields the best agreement with the reported measurements

A Friction-Test Benchmark with Twintex PP

This paper presents an update on a friction benchmark, that was proposed during the 13th ESAFORM conference. The goal is to compare different friction test set-ups [1–4] by determining the coefficient of friction (CoF) for Twintex® PP. The benchmark instructions are based on the ASTM standard D1894 [5] but also account for different friction velocities, pressures and temperatures. At the time of writing five research groups contributed to the benchmark, each with a custom designed test set-up, differing in size, mechanism, force control and temperature regulation. All tests will be conducted with woven glass reinforced polypropylene, from the same Twintex® batch. Conclusions will be drawn about the comparability of different testing methods by recognizing and analyzing systematically deviating results.

Forming of Thermoplastic Composites

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.

Friction in Forming of UD Composites

Inter‐ply and tool/ply friction play a dominant role in hot stamp forming of UD fiber‐reinforced thermoplastic laminates. This research treats friction measurements of a PEEK‐AS4 composite system. To this end, an in‐house developed friction tester is utilized to pull a laminate through two heat controlled clamping platens. The friction coefficient is determined by relating the clamp force to the pull force. The geometry of the gap between the clamping platens is monitored with micrometer accuracy. A first approach to describe the relation between the geometry and frictional behavior is undertaken by applying a standard thin‐film theory for hydrodynamic lubrication. Experimental measurements showed that the thin‐film theory does not entirely cover the underlying physics. Thus a second model is utilized, which employs a Leonov‐model to describe the shear deformation of the matrix material, while its viscosity is described with a multi‐mode Maxwell model. The combination of both models shows the potential to...

Bending Characterization of UD Composites

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.

Formability of Fiber-reinforced thermoplastics in hot press forming process based on friction properties

High performance composites are used in commercial applications in a steadily growing degree. This increase of advanced materials is accomponied with the development of fully automated fabrication processes. It aims to drive down the time and costs of the production while ensuring a high quality of the product. This can achieved by considering the process of hot press forming with continuous fiber reinforced thermoplastics. The development of the process is, however, accompanied with a few difficulties, which require more research. For example, composite materials with different architectures, lay-ups, and constituents, show large differences in formability. This research examines the effect of friction on the formability of thermoplastic composites. Both experiments and simulations were conducted. Demonstrator products have been press-formed from laminates with different materials and architectures (UD-carbon PEEK, UD-carbon-PEI, 8hs-glass PPS, 5hs-carbon PEEK and UD-glass PPS), to investigate their effects on formability. Creating a doubly curved shape from a flat laminate requires at least three deformation mechanisms, namely in-plane shear, bending and inter-ply slippage This paper focuses on the sliding mechanism and the corresponding friction. In order to quantify the amount of sliding in the press-formed product, a dot pattern has been applied to both surfaces of the laminate. The slip between the outer plies can be analyzed by means of photogrammetry. Besides, the friction coefficient of each material is measured in a special designed friction test set-up. It can be seen that the composite formability is directly linked to its friction properties. FE simulations of the press-form process will be performed based on the measured material properties, to demonstrate the influence of the materials friction coefficient.