By proposing an adaptation of the methodology usually used in metal forming, this paper aims to provide a general procedure for determining the forming limits, by necking and fracture, of polymeric sheet. The experimental work was performed by means of Nakajima specimens with different geometries to allow to obtain strains in the tensile, plane, biaxial and equibiaxial states for Polycarbonate sheet with 1 mm of thickness. The application of the time-dependent and flat-valley approaches used in metals has been revealed appropriate to characterize the onset of necking and obtain the forming limits of polycarbonate, despite the stable necking propagation typical of polymeric sheets. An analysis of the evolution of the strain paths along a section perpendicular to the crack allowed for a deeper understanding of the steady necking propagation behaviour and the adoption of the methodology of metals to polymers. The determination of the fracture strains was enhanced with the consideration of the principal strains of the DIC system in the last stage, just before fracture, due to the significant elastic recovery typical of polymeric sheets. As a result of this analysis, accurate formability limits by necking and fracture are obtained for polycarbonate sheet, together with the principal strain space, providing a general framework for analysing incremental sheet forming processes where the knowledge of the fracture limits is relevant.

This work presents a comprehensive investigation of an experimental study conducted on ultra-high molecular weight polyethylene (UHMWPE) sheets using single point incremental forming (SPIF). The analysis is performed within a previously established research framework to evaluate formability and failure characteristics, including necking and fracture, in both conventional Nakajima tests and incremental sheet forming specimens. The experimental design of the SPIF tests incorporates process parameters such as spindle speed and step down to assess their impact on the formability of the material and the corresponding failure modes. The results indicate that a higher step down value has a positive effect on formability in the SPIF context. The study has identified the tool trajectory in SPIF as the primary influencing factor in the twisting failure mode. Implementing a bidirectional tool trajectory effectively reduced instances of twisting. Additionally, this work explores a medical case study that examines the manufacturing of a polyethylene liner device for a total hip replacement. This investigation critically analyses the manufacturing of plastic liner using SPIF, focusing on its formability and the elastic recovery exhibited by the material.

In this study, the surface of aerosil OX50 was treated with γ-methacryloxypropyltrimethoxysilane in order to evaluate its effect on light-cured dental composites. Four composites were prepared with Bis-GMA/tetraethylene glycol dimethacrylate/γ-methacryloxypropyltrimethoxysilane, Bis-GMA/tetraethylene glycol dimethacrylate, and a 45% treated or untreated aerosil OX50, using the camphorquinone/N,N-dimethylaminoethyl methacrylate pair as initiator. Evidence of filler silanization was provided by FTIR as a low-intensity absorption at 1707 cm⁻¹ (carbonyl functional group) while thermogravimetric analysis showed a mass loss of approximately 2% associated with the decomposition of γ-methacryloxypropyltrimethoxysilane. The experimental composites studied meet the requirements of the ISO 4049:2019 standard for depth of cure, water sorption, and solubility. The composites are shown to be thermally stable and presented a degree of conversion higher than 70%, being higher than that reported for many commercial composites. Based on the observed properties, the best formulations were those in which the silane is incorporated into the matrix and the filler was previously treated.

This work investigates the feasibility of applying single point incremental forming (SPIF) to polycaprolactone (PCL) sheets manufactured via fused filament fabrication (FFF). An experimental campaign was conducted for evaluating the response of FFF-printed sheets under SPIF plastic deformation, considering the influence of the spindle speed and step down. Formability and failure, forming forces, temperature, surface roughness, as well as shape accuracy were analysed on the testing pyramidal geometries. Strain states were evaluated with respect to forming limits assessed in prior research. In addition, a qualitative comparison with compression-moulded PCL sheets was carried out to determine the influence of the manufacturing route. Results showed that all FFF-printed sheets were formed without fracture within the tested conditions, exhibiting stable plastic behaviour under incremental deformation. Process parameters affected forming forces, temperature, and surface quality, while fair geometric accuracy and thickness distributions were achieved. The comparison with compression-moulded sheets suggested a similar forming response under the analysed conditions. Based on these results, the feasibility of the hybrid FFF + SPIF approach was explored. As a proof of concept, the hybrid process was successfully applied to the manufacturing of a customized cranial geometry, showing its potential to achieve the required anatomical accuracy and geometric fit for patient-adapted shapes for biomedical applications.

Additive manufacturing (AM) allows the production of thin metallic sheet blanks with customised geometry and thickness, which can later be shaped by conventional or advanced forming processes, such as incremental forming. These techniques enable hybrid manufacturing processes in which printing and forming are combined, allowing applications such components with a tailored distribution of thickness or porosity, as might be the case of customised biomedical implants. However, the formability and fracture behaviour of those printed sheet materials are still largely unknown. This work presents an experimental characterisation of fracture-controlled forming limits in selective laser melting (SLM) Ti6Al4V sheets under proportional loading. Uniaxial, plane strain and equi-biaxial tests were carried out on as-built and heat-treated material, using test geometries specifically selected to minimise material consumption and combining strain measurements with finite element simulations. The as-built condition shows very limited ductility and fractures shortly after yielding, whereas heat treatment leads to an increase in fracture strain with only a moderate reduction in strength. In all the tests, failure is controlled by fracture rather than by macroscopic necking, so the forming limits are described in terms of a fracture forming limit (FFL). Fracture results are represented in stress triaxiality versus equivalent strain space, leading to the assessment of a fracture locus that is physically consistent with void-growth-controlled failure, which is particularly relevant for SLM materials with intrinsic porosity. Classical uncoupled fracture criteria are evaluated and calibrated using a minimum number of experimental points, highlighting the strengths and limitations of triaxiality-based approaches for SLM sheets and providing a practical and cost-effective basis for modelling hybrid and incremental forming processes.