Solid State Phenomena Vol. 389

Abstract: Fiber-reinforced thermoplastics (FRTP) offer high strength-to-weight ratios as well as weldability and recyclability, making them attractive for lightweight applications. Conventional thermoforming of continuous FRTP, however, requires part-specific molds, limiting economic viability for prototypes, individual parts, and small series. This study investigates a robotic hot double-sided incremental forming (DSIF) process developed for dieless, flexible forming of continuous FRTP sheets together with metal dummy sheets. Five different generic demonstrator parts with varying wall angles, degrees of symmetry, forming depths, and sizes were formed to assess process capability. Results demonstrate that typical defects such as fabric wrinkling and deconsolidation can be successfully avoided, and that the geometric accuracy achievable is comparable to that of metal DSIF. Challenges exist in forming larger parts due to the failure of the employed metal dummy sheets.

Abstract: Incremental Sheet Forming (ISF) enables the flexible creation of shell-shaped structures. Unlike conventional forming, ISF does not require a bespoke forming tool, greatly reducing upfront costs and lead times, especially for small lot sizes. Several parameter classifications for the ISF of metals and polymers have been proposed in the past. Such classifications increase awareness of possible levers for process optimization, guide experimental analysis, and enable a holistic understanding. Lately, fiber-reinforced polymers (FRP) are of increasing interest in ISF. In previous studies, ISF systems for various kinds of FRP have been developed, and several parameters and target variables have been investigated. However, there is currently no classification that addresses the specific parameters and target variables relevant to this material class. Therefore, the goal of this work is to develop such a classification to create a comprehensive foundation for future FRP ISF investigations. This effort is undertaken by building upon existing classifications and reviews independent of the material class and synthesizing these with a systematic literature review of FRP ISF investigations. The resulting classifications cover a broad range of parameters and target variables and reveal a structure that guides a systematic understanding and ensures future expandability.

Abstract: The rebound-effect that frequently occurs during electromagnetic sheet metal forming is one of the main causes of deviations in the shape and dimensional accuracy of flat surfaces. The selection of the die material and its corresponding energy absorption capabilities has a critical impact on this effect. This article analyses materials with different physical properties in terms of their energy absorption behavior under dynamic impact load. A variety of model tests are being conducted to examine a wide range of impact velocities and energies. The experimental setups comprise two variations of a drop tower test, which can be used to determine the percentage of impact energy absorbed at high and low momentum. To achieve higher impact velocities, a third experiment involving an electromagnetically accelerated impact body was conducted for the material that demonstrated the best result in the preceding tests.

Abstract: The human skull can become fractured or injured through impact and often requires repair through a craniectomy and subsequent cranioplasty, surgery performed to repair defects or damage to the cranium. Challenges related to material choice, which must be biocompatible, and customization for each patient’s anatomy remain. One possible solution is fabrication of patient-specific cranial implants, out of biocompatible polymers, using single point incremental forming (SPIF). In this paper, polyetheretherketone (PEEK) and ultra-high molecular weight polyethylene (UHMWPE) are formed using SPIF at room temperature to manufacture a cranial implant. The SPIF process is used to produce formed parts from which test specimens were extracted to evaluate the tensile performance and thermal properties. Formed cranial implants were impacted using a drop weight to evaluate their suitability under relevant conditions. The geometric conformance of the SPIF process was studied to compare the material behavior for the specified polymers after forming. The results validate that SPIF can be conducted at room temperature with PEEK and UHMWPE biocompatible polymers to enable custom implant manufacturing. However, PEEK exhibited superior performance in terms of tensile strength, geometric conformance, energy absorption, and melting temperature, and is recommended over UHMWPE for future implant applications.

Abstract: This study presents an integrated finite element-based framework for analyzing the final protrusion length of enameled copper hairpins during stator manufacturing. Protrusion uniformity is essential for reliable laser welding, yet it is often degraded by layer-dependent deformation during expanding and twisting. To clarify the mechanisms governing protrusion variation, a bilayer material model for the copper–enamel system was developed and validated using tensile tests, indentation-based inverse characterization, and three-point bending experiments. Mechanically consistent boundary conditions for expanding and twisting were reconstructed from manufacturing observations and incorporated into FE simulations. The results indicate that twisting governs protrusion length through axial material redistribution, whereas expanding mainly serves as a feasibility-enabling step that establishes stable tool engagement. Based on these insights, a physics-guided regression formulation was introduced to relate key twisting kinematics to protrusion length. The proposed framework provides a mechanistic basis for understanding protrusion variability and supports further development toward rapid prediction and variability control.

Abstract: Thermoplastic-based sheets (TBSs) are increasingly adopted in the automotive and aerospace sectors due to their potential for producing lightweight and durable structures. However, conventional manufacturing techniques, such as compression molding, offer limited process flexibility, as they rely on costly, dedicated molds. Single Point Incremental Forming (SPIF) represents a promising die-less alternative. Nevertheless, its application to thermoplastics requires strict control of the process conditions to avoid material failure. This study focuses on the validation of a novel experimental apparatus for pressure-assisted hot SPIF. The developed equipment enables precise, real-time control and regulation of both the working temperature and the hydrostatic support pressure, which are critical parameters for enhancing polymer formability. A key aspect of the experimental procedure is the use of an aluminum sacrificial sheet placed between the forming tool and the polymeric blank. This intermediate layer fulfills a dual role by ensuring a hermetic hydraulic seal to prevent fluid leakage and by promoting uniform pressure distribution during the forming process. The experimental results demonstrate the effectiveness of the proposed setup, achieving successful deformation of TBSs with high geometric accuracy. Overall, this research confirms the feasibility and robustness of the designed equipment for processing unconventional materials, offering a flexible and efficient alternative to traditional rigid tooling technologies.

Abstract: Hollow embossing rolling (HER) is a promising continuous forming technology for producing thin metallic bipolar half plates (BPHP) with micro-channel structures used in fuel cells and electrolyzers. This study presents a simulation-based analysis of process-specific disturbances influencing the forming accuracy and quality of HER-formed BPHP. Using a validated LS-DYNA shell-based model, the effects of six disturbance variables, roller misalignments (axial, tangential, angular), roller gap variation, initial sheet thickness deviation, and changes in friction coefficient, were systematically investigated. Results show that even small roller misalignments of ±10 µm or manufacturing related roller gap deviations of +5 µm lead to significant changes in rolling force, strip draw-in, and sheet thinning behavior. Variations in friction coefficient notably affect draw-in and wrinkling tendencies. Overall, the study highlights the high sensitivity of the HER process to geometric and frictional disturbances and provides quantitative tolerance limits crucial for precision manufacturing and robust continuous BPHP forming.

Abstract: Incremental sheet forming is a viable method for manufacturing highly customized components from non-conventional materials. Among these, niobium is a metal of growing interest due to its potential in various technological applications. In this experimental study, the incremental forming of high-purity annealed niobium sheets was investigated, with particular attention given to the surface finish of the formed parts. To this end, the surface morphology of the components, specifically fixed wall conical frusta, and the forming forces were analyzed. The results indicate that, despite the material’s notable formability, the incrementally formed niobium surfaces exhibit poor quality. This is attributed to the unique properties of niobium, suggesting that the development of surface treatment strategies is advisable to improve this aspect.

Abstract: This study investigates the structural response of blank-holders (BHs) equipped with spatially distributed magnetorheological (MR) actuators for adaptive deep drawing. While MR actuators provide fast, independent, and high-resolution force modulation, their effectiveness depends critically on the BH’s ability to transmit spatially differentiated loads without excessive diffusion or unrealistic stress localization. The relationships between BH stiffness, actuator spacing, and pressure localization at the sheet interface remain only partially understood, limiting the implementation of distributed blank-holding strategies. To address this gap, a comprehensive finite element (FE) framework is developed, combining a full closed-cup deep-drawing model with a complementary simplified configuration that isolates local deformation mechanisms under single-actuator loading. Parametric analyses examine the influence of BH thickness, local actuator force, and actuator spacing on stress distribution, localization radius, and overlap between adjacent load paths. Results show that BH thickness is the dominant factor governing spatial resolution: thinner BHs enable sharp pressure localization, whereas thicker ones diffuse local loads and suppress stress peaks. The spacing between actuators must therefore be selected as a function of BH stiffness to avoid stress-free regions while preserving distinct pressure footprints. For the reference industrial configuration (60 mm BH thickness), an actuator spacing of approximately 150 mm achieves the optimal compromise between localization capability and continuous sheet support. The proposed framework establishes quantitative design criteria for BH geometries compatible with MR-based adaptive forming and supports the development of next-generation blank-holding systems offering enhanced process stability, reduced scrap, and improved material-flow control.