María Consuelo Pérez

This study investigates the combined influence of re-entrant angle variation and nodal sphere incorporation on the mechanical behaviour of auxetic metamaterials. Three re-entrant configurations (15°, 30°, and 45°) were experimentally evaluated under compression, comparing architectures with and without spherical nodal reinforcement while maintaining remaining geometric parameters constant. Differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) confirmed an equivalent fully cured polymer state and high dimensional fidelity across all specimens, ensuring that the mechanical variations derive exclusively from structural and topological modifications. The results demonstrate that the re-entrant angle governs the transition from bending-dominated to stretch-dominated deformation. The 15° and 30° configurations exhibit stable plateau responses associated with progressive bending collapse, whereas the 45° configuration triggers a nodal reconfiguration that induces a sequential layer-by-layer collapse, yielding a 100% increase in compressive strength. The incorporation of nodal spheres significantly enhances initial stiffness and peak compressive strength across all geometries, most notably in the 45° re-entrant angle, which achieved a superior compressive strength and specific energy absorption (SEA). Furthermore, the node-reinforced 45° architecture exhibited an exceptional time-dependent viscoelastic dimensional recovery of over 99% post-testing, while the 30° configuration provided the most balanced, oscillation-free plateau response among the bending-dominated structures.

In this study, the relationship between processing conditions and final properties of injection-moulded polypropylene (PP) was investigated, focusing on the influence of melt temperature and injection pressure. An experimental design including three melt temperatures (200 °C, 220 °C, and 240 °C) and two injection pressures (5 MPa and 10 MPa) was implemented. The polymer’s performance was characterised, starting with dimensional measurements to assess geometric stability, followed by tensile testing to evaluate mechanical behaviour. Finally, the thermal properties and stability were determined using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA), complemented by dynamic rheological analysis to assess melt flow behaviour. Results show that melt temperature strongly governs mechanical performance, while injection pressure has a limited influence on ductility and only a minor effect on strength. Increasing melt temperature produced a drastic reduction in elongation at break, together with a systematic increase in shrinkage and dimensional dispersion. DSC revealed similar crystallinity levels for all conditions, indicating that mechanical and dimensional changes are not controlled by crystalline content. TGA confirmed the absence of significant mass loss during processing. Rheological analysis showed a progressive reduction in the high-molecular-weight fraction with increasing temperature, evidencing thermally induced chain scission. These results establish a processing-structure-property relationship, in which thermal history reduces molecular weight and entanglement density, leading to lower ductility and higher shrinkage, while injection pressure plays a secondary role.

The pitting corrosion resistance and the tribological behaviour of a ferritic stainless steel with high Mo content (AISI 436) and a commonly employed austenitic stainless steel (AISI 304) are compared. Special attention was paid to the role of Mo in improving corrosion resistance of ferritic stainless steels. Since the surface condition is an important parameter related to the onset of pitting corrosion in the presence of chlorides, three different surface finishes were tested for both steels. Two commercial finishing grades and laboratory polishing down to 1 µm were compared. Moreover, the influence of surface condition on the tribological properties for both steels was also evaluated. The study demonstrates that surface finishing plays a decisive role in both the electrochemical and mechanical response of stainless steels. A comprehensive microstructural and tribological analysis reveals not only how commercial finishing treatments modify passive film behaviour, but also how they affect friction stability and wear mechanisms. Special emphasis is placed on the synergistic effect between molybdenum content, passive film integrity and manufacturing processes. The obtained results provide valuable insight for industrial applications where durability against chloride exposure and abrasion is critical.

Auxetic metamaterials, characterised by a negative Poisson’s ratio, offer excellent energy absorption but often present limited compressive strength due to their strut-based architectures. Selective laser sintering (SLS) enables the precise fabrication of these structures, yet enhancing their mechanical performance remains challenging. This research investigates the influence of nodal spheres on re-entrant dodecahedral unit cells produced in PA12, varying node-to-strut diameter ratios (1:1, 2:1, and 3:1). Compression tests reveal significant increases in stiffness and compressive strength, reaching up to 88.70% for the 3:1 ratio. When normalised by relative density, the 2:1 configuration proves most effective, achieving a 35.33% increase in specific strength and a 19.58% improvement in specific energy absorption. The deformation behaviour indicates a mixed bending–stretching mechanism, with geometry exerting a stronger influence than the base material. Although larger nodal spheres enhance absolute strength, they also increase mass and relative density, which may limit their suitability for weight-sensitive applications. Overall, these findings highlight nodal reinforcement as a promising strategy to enhance the mechanical efficiency of auxetic metamaterials while maintaining their auxetic response. These improvements support applications in aerospace, automotive engineering, personal protection systems, lightweight structural panels, and energy-absorbing components.

The pitting corrosion resistance and the tribological behaviour of a ferritic stainless steel with high Mo content (AISI 436) and a commonly employed austenitic stainless steel (AISI 304) are compared. Special attention was paid to the role of Mo in improving corrosion resistance of ferritic stainless steels. Since the surface condition is an important parameter related to the onset of pitting corrosion in the presence of chlorides, three different surface finishes were tested for both steels. Two commercial finishing grades and laboratory polishing down to 1 µm were compared. Moreover, the influence of surface condition on the tribological properties for both steels was also evaluated. The study demonstrates that surface finishing plays a decisive role in both the electrochemical and mechanical response of stainless steels. A comprehensive microstructural and tribological analysis reveals not only how commercial finishing treatments modify passive film behaviour, but also how they affect friction stability and wear mechanisms. Special emphasis is placed on the synergistic effect between molybdenum content, passive film integrity and manufacturing processes. The obtained results provide valuable insight for industrial applications where durability against chloride exposure and abrasion is critical.

The objective of this research is to enhance the mechanical and corrosion resistance properties of a cast Ni-Al bronze (NAB). To achieve this, the effect of deep cryogenic treatment (DCT), a process that has shown promise in other alloys, is initially investigated. It is demonstrated that, in the case of NAB, DCT induces only minor microstructural changes, which do not lead to any significant improvement in its properties. Consequently, it is proposed that a combined treatment be employed, involving annealing either before or after DCT. The results indicate that annealing at 675 °C for 2 h following cryogenic treatment at −180 °C increases the yield strength by approximately 11%. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in simulated seawater further confirm that this combination results in the formation of oxide layers with enhanced protective capacity. These improvements are attributed to the significant refinement and homogenization of the microstructure, including the globularization of the kI, kII, and, particularly, kIII phases, and an increase in the precipitation of the kIV phase in a finer and more homogeneous form within the alpha phase.

The use of thermoplastic materials has had significant growth in recent years. However, with great mechanical requirements, thermoplastics have limitations to their use. To improve these restrictions, these materials are reinforced to obtain better properties. Polypropylene is one of the most versatile polymers and is used in almost all modern industries. Thus, the aim of this study is to create composite materials that offer performance for various industrial fields using carbon fiber fabric reinforcement, which is an inexpensive material widely used by the aerospace, automotive, and marine industries. The samples are produced by the over-injection molding of polypropylene. The investigation is focused on the impact of two critical control parameters in the injection molding process: temperature and pressure. Twelve experiments have therefore been considered, taking into account the combination of three factors: the presence or absence of carbon fiber fabric reinforcement, three levels of temperature (200 °C, 220 °C, and 240 °C), and two injection pressures (5000 kPa and 10,000 kPa). To evaluate the influence of these factors, three analyses were carried out: first, on the samples’ shrinkage using a portable metrology-grade 3D laser scanner; second, on the internal defects using computed tomography (CT); and third, on the mechanical properties with tensile tests. From the results obtained, it is observed that the mold shrinkage fell slightly when PP samples were reinforced with carbon fiber, with both materials (PP and carbon-fiber-reinforced PP) having linear behavior with temperature. It is also noticed that polypropylene behaves as a crystalline material when processed at higher temperatures and pressures. From tests on the mechanical properties, it is concluded that the mean yield strength of PP-CF for injection temperatures of 220 °C and 240 °C represents an increase of 43% compared to the non-reinforced material.

The influence of annealing treatment on the microstructure and the corrosion behaviour of a complex bronze Al-Ni alloy was analysed. The microstructural characterization after thermic treatment shows that the retained β-phase disappears and the K_II-phase undergoes a spheroidization process. Moreover, there is an augmentation of K-phase precipitates within the α-phase, particularly K_IV and K_V, which homogenizes the distribution of alloying elements between the phases. The corrosion behaviour was investigated by means of selective corrosion tests, potentiodynamic measurements, and electrochemical impedance spectroscopy. All of the experiments showed that thermic treatment greatly improves the corrosion resistance of the alloy. As such, annealing treatment influences a microstructure to be more dealloying resistant, which allows the development of a more protective air-formed passive film. On the other hand, the lower current registered after anodic peaks and Electrochemical Impedance Spectroscopy data seem be correlated with a more protective layer formed on annealed material. As such, this protective character is due to a greater incorporation of Al³⁺, Ni²⁺ and Cu²⁺ in the passive film, as revealed by X-ray Photoelectron Spectroscopy results.

This study intended to help practitioners selecting the appropriate motion according to their level of expertise for glide path and complete root canal preparation of curved canals in terms of instrument fracture and preparation time. A total of 160 curved root canals (angle >30°, radius <6 mm) were allocated randomly to 4 groups: A/B for expert operators and C/D for nonexpert operators. A/C were shaped with rotary and B/D with reciprocating instruments. Preparation time was registered and compared with Kruskal–Wallis non-parametric test and Dunn’s post hoc test. Instrument separation was also registered and compared with chi-square test. A fractographic analysis was performed with scanning electron microscopy. The use of reciprocation motions allowed nonexpert operators to avoid instrument breakage during glide path and to prepare root canals faster. Experts were equally effective in rotary/reciprocating root canal shaping and faster with rotary motions than nonexpert operators, who also fractured significantly more rotary instruments.