Materials Science Forum Vol. 1187

Abstract: This study analysed the mechanical performances and the environmental sustainability of hybrid hemp/carbon fibre reinforced polymer composites produced adopting different stacking sequences. In this context, three carbon layers were replaced with hemp ones and were positioned either at the mid-plane of the laminate in a symmetric configuration (S sample) and near to the external side of the composite material in an asymmetric configuration (A-HC sample). Additional full carbon sample (CFRP) and hemp sample (HFRP) were manufactured and used as reference materials. The mechanical behaviour of these materials was investigated through flexural, interlaminar shear and low-velocity impact (LVI) tests, and a cradle-to-grave Life Cycle Assessment (LCA) analysis was performed to quantify their environmental impacts in terms of Global Warming Potential (GWP). The experimental results revealed that hemp/carbon hybridisation in composite systems makes it possible to achieve a trade-off between mechanical performances and sustainability. Some of the investigated hybrid configurations exhibited mechanical properties comparable to conventional CFRPs thanks to strength, stiffness and enhanced energy absorption capability which depend on the stacking strategy and the presence of natural fibres that contribute to the damage mitigation. From an environmental perspective, thanks to numerous advantages in the use of hemp fibres, hybrid solutions significantly reduce the global warming potential compared to CFRPs, confirming that hemp/carbon hybridisation represents a promising strategy to balance structural performance and environmental.

Abstract: This study investigates the applicability of deep learning models for automated quality classification of cold spray coatings, focusing on three deposition categories: good, degraded, and poor deposition. Three state-of-the-art convolutional architectures, ResNet-50, EfficientNet-B0, and ConvNeXt-Tiny, were evaluated across two training phases designed to assess the impact of dataset balancing, data augmentation, and higher input resolution. In the first phase, models were trained on an imbalanced dataset using only class weighting; EfficientNet-B0 achieved the best performance (ACC 80%, F1 77%), while ResNet-50 showed notable instability (ACC 60%, F1 56%). In the refined second phase, oversampling, advanced augmentation, 380×380 resolution, and early stopping led to substantial performance gains for all models. ConvNeXt-Tiny achieved the most robust and balanced results (ACC 93.3%, F1 90.3%), outperforming EfficientNet-B0 and ResNet-50 particularly in sensitivity and specificity for minority classes. Grad-CAM analysis provided qualitative insights into the decision-making process: poor samples elicited strong, spatially extended activations corresponding to defective regions, degraded samples produced more localized responses aligned with mid-scale irregularities, and good samples yielded diffuse, low-intensity activation patterns associated with surface uniformity. These interpretable attention maps validated the physical relevance of the learned features and confirmed the suitability of ConvNeXt-Tiny for reliable and explainable cold spray quality assessment.

Abstract: Direct molding is a technology where thermoset powders are agglomerated by compression molding without adding any additional substance or linking agent. It has been applied to powders from composite recycling, such as fiberglass. Agglomeration depends on residual reactivity of powders, the intrinsic re-activation of the particle surfaces because of the broken chemical links, and an incipient degradation mechanism during molding. In the case of continuous fiber laminates, the mechanical properties of the virgin item cannot be recovered as the recycled composite is made by particles. Nevertheless, high values may be reached, potentially interesting for such applications, depending on some precautions during the molding phase. Powders from grinding of fiberglass have been recovered from industry. They have been compression molded, and samples have been extracted from different parts of the molded plate to evaluate the distribution of the mechanical properties by bending tests. Results show that a bending strength up to 27 MPa can be achieved, without using any virgin material or additional substance, and a bending modulus over 3.5 GPa. However, pressure distribution during molding is not uniform and mechanical properties strongly vary from the periphery of the plate to the inner zones.

Abstract: Nano-coating fragmentation (NCF) is patented technology which allows producing thermoplastic matrix nanocomposites without the step of nanoparticle preparation. Thermoplastic pellets are PVD (physical vapor deposition) coated by the metal of the desired nano-reinforce. In the following processing step, by extrusion or injection molding, nano-coatings are fragmented into nanoplatelets because of the action of the screw. In this study, nano-silver (Ag) filled nanocomposites with polypropylene (PP) matrix have manufactured by this innovative technique and tested in open environment for their anti-fouling behavior. PP pellets have been PVD coated into a large chamber with the aid of a rotating drum. Coated pellets were physically mixed with virgin in the percentage of 0, 5, 10, 20, and 100%. Consequently, the expected Ag percentage ranged from 0.036% wt to 0.103% wt. Square nanocomposite samples (80x80 mm² and 3 mm thick) were injection molded in a fully electric press. One sample for each nano-Ag content was selected to be exposed in open environment. A smart buoy, especially designed for water cleaning and monitoring, has been used for experimentation. Results show that Ag-NPs provide a significant contribution to reduce the growth of vegetation on the molded plastic surfaces. However, at very low contents, the negative effect of the Ag NPs on the surface morphology of the molded samples nullifies this contribution.

Abstract: Shape memory polymers (SMPs) can recover from a programmed temporary shape to their original configuration when exposed to an external stimulus, most commonly heat, making them attractive materials for soft actuation in functional and biomedical devices. Among them, thermoplastic polyurethanes (TPUs) display reliable shape-memory behavior under various conditions. The emergence of biomedical-grade TPUs and their compatibility with additive manufacturing provides new opportunities for fabricating customized components with tunable actuation capabilities. In this study, biomedical TPU filaments were processed via fused deposition modeling (FDM) to produce block-shaped specimens of controlled size and weight. The samples were mechanically deformed into a C-shaped geometry at room temperature, fixed in fridge at the temperature of –20 °C, and subsequently tested under constrained recovery at room conditions, using a universal testing machine. The recovery load has been measured for a time of 30 min. The results show that TPU-based SMPs can develop substantial recovery forces during shape restoration. The shape memory behavior has been modeled by using a logistic function, which has been able to identify a characteristic time, the same for all the samples despite their printing conditions and architecture. These findings highlight the potential of FDM-processed biomedical TPUs for compact soft-actuation systems requiring high force output.

Abstract: Current developments in the transition to renewable energy and the electrification of mobility are leading to higher demands with regard to resource efficiency in the production of electric motors. One current trend is the segmentation of the stator pack, which enables higher material utilization and novel assembly processes. In this paper, different connection geometries for individually stacked stator segments are examined. Experiments were conducted by laser beam cutting and stacking of samples with different segmentations and connection geometries. Afterwards the geometric dimension and tolerances are compared with an unsegmented stator. Furthermore, the impact of the additional connection on the mechanical behavior under load are investigated using Finite Element Analysis. The required force to join segments across different connection geometries ranges from 462 N to 1875 N, while the cylindricity of segmented stator cores spans from 33 µm to 59 µm, compared to 40 µm for the unsegmented sample. Simulation results show that the elastic strain on the connections is largely influenced by the size of air gaps between segments, as well as the geometry and number of segments.

Abstract: Pulsed Electrochemical Machining (PECM) is an established process that is characterized by the machinability of metallic workpieces regardless of their mechanical properties. Applications of PECM, such as the manufacturing of punches made from hardened tool steel, often utilize the lateral working gap for the final shaping of the workpiece. A major challenge in designing an economically viable removal process is the prediction of the lateral gap for certain targeted feed rates. This case study presents a design strategy for the design of PECM applications utilizing the lateral gap. Based on a characterization of the material removal characteristics of the hardened tool steel S390, preliminary experiments were conducted to characterize the relation between (1) the feed rate, the current density in the frontal gap and the voltage and (2) the lateral gap. Further, multiple parameter sets were derived for the machining with a targeted lateral gap. The validity of these parameter sets is verified experimentally. Based on these results, a cathode for the manufacturing of a demonstration punch was designed and manufactured. These demonstration punches were machined and the resulting dimensions evaluated. Lastly, fine adjustments regarding the process parameters were applied to achieve the targeted geometric accuracy.

Abstract: Beading has been used in metal construction for decades to reinforce and stabilize thin sheets of metal. In aircraft, washing machine and car manufacturing, this allows for cost-effective, lightweight and material-saving designs to be realized. These indentations are embossed into thin metal sheets to increase their rigidity and stability, thereby preventing fluttering or deformation. The bending stiffness is significantly increased by reshaping the material. The increased stability allows thinner sheets to be used, which reduces the overall weight of the structures and components. Beading is often used on larger surfaces to prevent fluttering or vibrations and to ensure greater dimensional stability. The combination of two old production processes, beading and steam bending for wood is examined in this paper. The use of beads to reinforce thin wooden panels saves material, resources and weight, thereby making production more sustainable. The investigations carried out examined the possibilities of introducing beads into thin panels made from different types of wood. The temperature, water content, water vapour content, soaking time and pressing pressure were varied. In a first step, a test specimen was produced that serves as a mould for the surround. This shape was pressed into the thin wooden panels when varying the processing parameters shown above. In a next step, the indentation depths achieved were measured. The deflection of the thin wooden panels was then measured under different loads and compared with the calculated results.

Abstract: While bending processes for producing tube bends with a constant radius have been extensively investigated in recent years, only a limited number of studies have addressed form-bound bending processes for generating variable radii. In particular, a systematic investigation of the influence of bending die geometry with a variable radius profile on tool reaction forces, geometrical and non-geometrical bent part properties is still lacking. In this study, compression bending using bending dies with a continuously varying radius is investigated by means of finite element (FE) simulations. The geometry of the bending dies is parameterized using an Archimedean spiral curve, allowing the bending radius to be described as a function of the bending angle. The introduced radial gradient, defined as the derivative of the radius with respect to the bending angle, dR/dα, serves as the central design parameter of the bending die and is systematically varied from constant radius with 0 mm/° to 1 mm/°. The influence of the radial gradient dR/dα of the bending die geometry on tool reaction forces as well as on the geometrical and non-geometrical properties of the bent part is investigated by means of a numerical parametric study for a selected bending task. The results show that for small to moderate values of dR/dα, all investigated metrics exhibit a pronounced linear dependence on the radial gradient. This behavior is further confirmed by the evaluation of the maximum values of the process and geometric parameters as a function of dR/dα, yielding high coefficients of determination (R²). For larger values of dR/dα, however, the sensitivity of both process-related and geometric characteristics decreases.