Solid State Phenomena Vol. 387

Abstract: While the use of composite materials increases the specific stiffness of structural parts, their manufacture using automated fiber placement processes such as Tailored Fiber Placement (TFP) allows for the addition of functionalization. An example of such a part is the hydrofoil, which can gain hydrodynamic performance if its shape adapts to the different loads encountered in the three modes of navigation. One method that can meet these requirements is passive functionalization. In this context, the development of digital design support tools is essential. Among them, topology optimization is a well-established method. This work focuses on the development of a strategy for optimizing the topology or the fiber density distribution of the part and the orientation of the fibers for composite materials with an objective function of path generating type allowing passive functionalization. A method for generating fiber trajectories for the TFP process is also presented. The topology optimization results of a cantilever type test case and a shell plate are shown and discussed.

Abstract: Direct compounding of long fiber thermoplastic (LFT-D) materials in compression molding are two complex processes in series linked by the plastificate. Continuous compounding and sequential compression create a time-dependent property progression along the extrusion direction of the plastificate. Under variation of secondary parameters, extruder die temperature, and die height of the LFT-D line, samples of plastificates, flow fronts and plates are manufactured and characterized. The plastificate density progression along the extrusion direction is primarily influenced by the temperature of the die. Lofting of the plastificate is higher at high temperatures while the density difference along the extrusion direction is lower. This density difference is known to influence fiber orientations and mechanical properties. The flow front of the material filling the mold is skewed because of the density difference. We show that the skewness is mainly influenced by the die height and is lower at high die heights. The fiber content distribution in the plate is discussed and found to be influenced by the length of the plastificate which is in turn determined by the secondary parameters. These secondary parameters of the LFT-D line can play a role in process optimization once the primary parameters are selected. This work provides clues and observations of principles for such optimizations.

Abstract: Some composite preforms lose dimensional stability after mandrel removal during tube winding, leading to pronounced longitudinal expansion and radial contraction for specific layup configurations. This work presents an experimental method for tracking the reorganization of individual wound tapes during mandrel release and uses these measurements to validate a finite element (FE) model designed to capture the underlying deformation mechanisms. The experiments reveal a nonlinear dependence of ribbon elongation on the initial winding angle. Complementary FE parametric studies investigate the effects of tape geometry and mechanical properties, demonstrating that increased mechanical anisotropy amplifies the elongation response. The study also introduces a layer-equivalent stiffness formulation and concludes with FE simulations of multi-tape preforms. Together, these results support the development of improved winding strategies aimed at reducing defects, lowering production costs, and enhancing the structural performance of tubular composite components.

Abstract: In virtual process chains for discontinuous fiber-reinforced polymers, clustering of fiber orientation tensors reduces the number of macroscopic material cards required for downstream structural and warpage simulations. However, it remains unclear whether including the additional information provided by the fourth-order fiber orientation tensor improves clustering quality. This study investigates the influence of second-order vs. fourth-order informed clustering on clustering outcomes and the resulting orientation-averaged mechanical properties. Using parameterizations based on harmonic decomposition, rotation-invariant clustering is performed in both the second-order and fourth-order parameter spaces. Results from injection molding simulation data indicate that the level of tensorial information has limited effect when the fourth-order tensor is computed via a closure approximation, as the deviatoric parameters are nonlinearly dependent on the second-order parameters. In contrast, the choice of clustering algorithm -- KMeans vs. Birch -- has a more pronounced influence on cluster shapes and allocations. Furthermore, we demonstrate that clustering affects orientation-averaged stiffness properties, with deviations most pronounced near cluster boundaries and rarely occurring tensor shapes.

Abstract: Wrinkling during diaphragm forming of engineering fabrics compromises structural integrity and surface quality. This study investigates two strategies—diaphragm pre-tensioning and fabric inter-ply lubrication—to mitigate wrinkle formation. A pre-tensioning (PT) blank holder was designed and evaluated through deep-draw experiments supported by finite element analysis (FEA). Results show that pre-tensioning (50% strain) significantly reduces wrinkle severity compared to a conventional blank holder. The PT blank-holder creates initial equi-biaxial strain consistent with analytical and FEA predictions. Resin inter-ply lubrication also decreased wrinkling and reduced forming force by approximately 70%, primarily by lowering inter-ply friction. However, pre-tensioning proved more effective overall. These findings demonstrate the potential of diaphragm pre-tensioning for improving forming quality and provide a foundation for advanced multi-step thermoforming processes.

Abstract: Fibre tension is an important process parameter during filament winding. It strongly affects the void content and fibre volume fraction, which in turn determine the mechanical performance of the part. Fibre tension also contributes to the residual stresses that develop in the filament-wound material. This study focuses on how the fibre tension shapes the stress distribution in filament-wound composites. To this end, a numerical predictive model was developed. Experimental validation was conducted using a force sensor, and good agreement was observed between the model predictions and the experimental measurements. These findings provide deeper insight into the role of fibre tension in filament winding and offer practical guidance for optimising the process to enhance the performance of composite pressure vessels.

Abstract: This work develops and validates thermo-chemical models for pultrusion of glass fiber–reinforced polyurethane composites on an industrial PulFlex production line. A reduced one-dimensional model combining a calibrated Kamal–Sourour (KS) cure law with an Arrhenius type chemo-rheological viscosity formulation is cross-validated against a three-dimensional ANSYS Composite Cure Simulation using identical material inputs along a three-zone, 0.9144 m heated die. Embedded thermocouples provide in-die temperature histories at 50.8 cm·min⁻¹ for calibration, while additional differential scanning calorimetry (DSC) measurements supply degree of cure (DoC) profiles for independent validation. At the industrial operating speed of 50.8 cm·min⁻¹, the mathematical and ANSYS models both reproduce the measured temperature peak location and exit temperature within a few degrees Celsius and predict a die-exit DoC of approximately 0.95, confirming near-complete curing. Using these calibrated fields as inputs to an analytical pulling-resistance formulation, both models predict comparable pulling force magnitudes and plateau behavior, demonstrating that the simplified 1D framework can capture not only thermo-chemical evolution but also process resistance trends over a range of pulling speeds. The validated 1D model therefore enables efficient exploration of speed–temperature–force tradeoffs for process window design, while the 3D ANSYS model provides a higher-fidelity reference for local gradients and future thermo–chemo–mechanical extensions.

Abstract: Novel multifunctional resins and composites present multiple manufacturing challenges that need to be overcome to allow for wider adoption. These challenges include high viscosity, short cure windows, low permeability preforms, and non-standard cure kinetics. Standard liquid composite molding methods (such as resin infusion under flexible tooling [RIFT], resin transfer molding [RTM], or compaction-RTM) are poorly equipped to manufacture these new materials. A new system is presented that combines the abilities of RIFT/RTM/c-RTM while introducing controlled deformation of the preform during infusion to remove flow through the preform. This manufacturing method allows the preform to lie uncompacted, while still under vacuum, during infusion which allows resin to flow unrestricted between plies. Then once infusion has occurred, compaction proceeds to produce the final composite geometry. This method has been successfully implemented to manufacture structural power devices with a biphasic resin system and metal coated carbon aerogel preform, as well as a vitrimer composite with a high-viscosity/short-cure resin thought high weight carbon fiber preform. The novel and flexible manufacturing parameters of this new system present a low-cost route towards optimizing the manufacture of challenging and novel resin systems, allowing for a faster understanding and implementation of these materials.

Abstract: Incomplete impregnation is a remaining challenge in the production of thermoplastic unidirectional (UD) tapes, particularly for the melt impregnation. This study investigates an approach analogous to multi-stage die impregnation by re-calendaring partially impregnated UD thermoplastic tapes, where different closing forces under controlled processing conditions are used to enhance impregnation quality. In this work, polypropylene–carbon fiber (PP-CF) tapes with a width of 20 mm and a thickness of 400 µm were produced by using an Xplore UD Tape Line with a closed pultrusion die. As a model sample, these tapes were deliberately produced under conditions expected to result in partial impregnation by setting the die pressure and pultrusion rate accordingly. To improve impregnation, the tapes were subsequently subjected to calendaring with different nip force passes in a different line, thereby mimicking a staged or multi-die consolidation approach. Results show that calendaring induces pronounced geometric reconfiguration accompanied by improved impregnation quality. Tape thickness was reduced by up to approximately 65%, while tape width increased by up to approximately 70%, indicating effective lateral spreading under compressive and shear stresses. Optical microscopy of polished cross-sections revealed a reduction of dry fibre regions and improved resin continuity within inter-filament gaps at intermediate calendaring nip force. Density-based fiber volume fraction of the tape measurements showed only a slight increase in the range of 2–3%, suggesting that consolidation was governed primarily by fibre rearrangement and spreading rather than significant resin squeeze-out. The findings provide practical insights into how the calendaring unit of a thermoplastic tape manufacturing line can be adapted for multi-stage consolidation, offering improved impregnation quality in thicker thermoplastic tapes that are more prone to impregnation defects. This approach may also serve as a bridge solution when the pressure build-up is limited.