Institute for Composite Materials

ABSTRACTThe aim of this work was to develop and characterize sequential trial‐curable off‐stoichiometric thermosets based on two curing agents with different reactivities in terms of temperature. Trial‐curing refers to the occurrence of three curing reactions: epoxy‐amine polycondensation reaction triggered by polyetheramine D‐230 (reactive curing agent) at lower temperature, followed by adduct formation and anionic homopolymerization, both triggered by 1‐ethyl‐3‐methylimidazolium dicyanamide (latent curing agent) at higher temperature. Intermediate materials were studied after the first curing step. A wide range of properties such as reactivity, glass transition, network density, and thermal stability could be tuned by adjusting the relative contribution of the involved reactions. The glass transition of final samples, determined from the peak temperature of tan delta curve by Dynamic Mechanical Analysis, varied from 108.8°C to 166.4°C. This value increased by decreasing relative contribution of the epoxy‐amine condensation reaction. The curing process was additionally monitored by Fourier‐transformed infrared spectroscopy to identify curing reactions.

<abstract> <p>Ultra-thin flexible glass with thicknesses of 100 µm or below is a substrate in the fields of optics, electronics, and semiconductors. Its brittleness is challenging in production processes like physical vapor deposition processes, especially in roll-to-roll production. In many cases, multiple geometric deformations take place and each step, like coating or cutting, influences the glass strength. By now, the relation between the strength of ultra-thin glass under quasi-static conditions and its strength under cyclic load has not been studied. Moreover, the effect of coatings has not been investigated. Both aspects are crucial to design reliable production processes. Therefore, the strength of ultra-thin glass under cyclic load was studied for uncoated and coated substrates. Two coating types were investigated: a single indium tin oxide film and a seven-layer antireflective layer stack. The coatings significantly influence the strength of the underlying glass in both test modes. The barrier properties, thin film stress, and the morphology/crystalline structure are identified as the major characteristics influencing the strength.</p> </abstract>

To achieve short cycle times, CF/PEEK tape preforms are laid up quickly in ATL, thus tape preforms are only partially consolidated. Different gas volume flows are set to vary the energy input during tape laying at 1 m/s. The tape preforms are characterized by means of thickness measurements, microsection analysis, peel-test, compression thermal analysis and flexural test to assess the degree of consolidation. Subsequently to ATL, tape preforms are processed in stamp forming at different pressure settings in a rapid-closing downstroke press. The resulting laminates are analyzed to assess the influence of tape preform consolidation and molding pressure. The results reveal a dependency between gas volume flow during automated tape laying and consolidation of the correspondent tape preforms. Laminate quality of specimen after stamp forming is independent to the tape preform consolidation according to the results of this investigation.

Metal fiber hybrids (MFH) exhibit outstanding mechanical properties. They combine the advantages of ductile metallic materials with the well-known advantages of classical glass or carbon fibers in polymer matrices. Previous research has shown that these hybrid material concepts can improve structural integrity and energy absorption while maintaining their excellent weight-specific mechanical properties as well as allowing a wider range of multifunctional applications. In today’s component design process, simulation is a powerful tool for engineers to exploit the full mechanical potential of the material used. However, describing the material behavior including its multifunctional usability in numerically aided design processes of components is currently one of the major challenges for MFH. Against this background, this work focuses on the development and evaluation of a description method for MFH in the finite element analysis (FEA). A steel and carbon fiber reinforced epoxy resin (SCFRP) with hybridization at the laminate level is chosen as the reference material. To describe the behavior of unidirectional steel fiber reinforced plastic (SFRP) layers, a material model combining an orthotropic damage model and a 1D-plasticity model is proposed and implemented as a user-defined subroutine for LS-Dyna. In addition, SCFRP laminates are manufactured, tested under tensile loading, and used to parameterize the material models and to validate the description method for SCFRP. In this study, it is shown that the description method in combination with the newly developed material model is able to describe the complex failure mechanism of SCFRP. In particular, with respect to the material behavior up to the failure of the carbon fibers, a very good mapping accuracy can be achieved. Strain localization effects occur in both numerically predicted and experimentally measured post-failure behavior. Therefore, it could be concluded that the accuracy of the numerical predictions strongly depends on the geometric resolution of the discretization.

In contrast to thermosets, vitrimers have dynamic covalent bonds providing the potential to allow being thermoformed after stimulation by heat. However, it is unclear if the thermoforming of carbon fiber reinforced vitrimers (vitrimer-CFRPC) correlates to that of thermoplastic-CFRPC (TP-CFRPC). For this reason, their viscosity was investigated by means of a torsion clamp rheometer and tempered 3-point bending thermoforming. The results are compared with a well thermoformable TP-CFRPC. The investigated vitrimer-CFRPC base on epoxy formulations with disulfide crosslinks. The results show that the high viscosities of the vitrimers make defect-free thermoforming challenging. In the micrographs of the tempered 3-point forming of the vitrimer-CFRPCs wrinkling and no ply sliding occurred. A vitrimer-CFRPC was thermoformed to an omega profile, indicating that opposite forming radii lead to additional interlaminar shear forces promoting ply sliding. In order to approximate the thermoforming behavior of vitrimer-CFRPC, their viscosity at process temperature must be reduced.

Composite processing and subsequent characterization of microfibrillar composites (MFC) were the focus of this work. Compression molding of wound MFC filaments was used to fabricate MFC composites. The MFC composites were composed of polypropylene (PP) as matrix materials and polyethylene terephthalate (PET) as reinforcement fibers. The PP/PET blends were mixed with PET contents ranging from 22 wt% to 45 wt%. The effect of processing parameters, pressure, temperature, and holding time on the mechanical properties of the MFCs was investigated. Tensile tests were conducted to optimize the processing parameter and weight ratio of PET. Tensile strength and modulus increased with the increase in PET content. PP/45 wt% PET MFC composites properties reached the value of PP/30 wt% GF. Falling weight tests were conducted on MFC composites. The MFC composites showed the ability to absorb the impact energy compared to neat PP and PP/30 wt% GF.

Continuous compression moulding (CCM) is an efficient process for manufacturing endless fibre-reinforced thermoplastic composites, so called organic sheets. The semi-finished products are fully impregnated and consolidated and can be thermoformed into complex 3D-geometries. Applications benefit from excellent weight-specific features as well as functional integration. Nevertheless, limited production speed and lower than acceptable manufacturing quality are still a challenge, especially with the use of high shrinkage polymers. Hence, porosities and defects due to pressure drops inside the laminate during impregnation and solidification can cause degradation in material properties. With the integration of an active adaptive pressing tool and an inline pressure measurement system, the process can be optimised towards guided impregnation and improved pressure distribution. A calculation method based on the B-factor method by Mayer has been adapted for the CCM process in order to enhance the tool design. Both, production speed as well as organic sheet quality can be improved with the optimised processing system presented in the following work.