Hemp fibers produced by different extraction techniques that were cultivated in the Portuguese districts of Bragança and Mirandela under various nitrogen fertilization and planting density conditions are examined and mechanically described in this paper. The objective of this study is to evaluate the influence of sowing density, nitrogen fertilization, sample location, and extraction method in order to improve the mechanical properties of hemp fibers. To achieve this, we determined the modulus of elasticity, the tensile strength, and the density. The mean value for the modulus of elasticity was 92.44 ± 7.44 GPa, the mean of tensile strength was 564.98 ± 167.03 MPa, and the mean of the density was 1.64 ± 0.24 g cm⁻³. We performed a statistical analysis of all parameters using ANOVA and found that the retting method had the greatest influence among all parameters. The associated effects of nitrogen fertilization and sowing density revealed an important influence on tensile strength and specific tensile strength, respectively.

Fused Filament Fabrication (FFF) has become a widely adopted additive manufacturing technology due to its cost-effectiveness, material versatility, and accessibility. However, optimizing process parameters, predicting material behavior, and ensuring structural reliability remain major challenges. This review analyzes state-of-the-art computational methods used in FFF, which are categorized into four main areas: melt flow dynamics, cooling and solidification, thermal–mechanical behavior, and material property characterization. Notably, the integration of Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) has led to improved predictions of key phenomena, such as filament deformation, residual stresses, and temperature gradients. The growing use of fiber-reinforced filaments has further enhanced mechanical performance; however, this also introduces added complexity due to filler orientation effects and interlayer adhesion issues. A critical limitation across existing studies is the lack of standardized experimental validation methods, which hinders model comparability and reproducibility. This review highlights the need for unified testing protocols, more accurate multi-physics simulations, and the integration of AI-based process monitoring to bridge the gap between numerical predictions and real-world performance. Addressing these gaps will be essential to advancing FFF as a precise and scalable manufacturing platform.

Although polylactic acid (PLA) is one of the most used materials in additive manufacturing, its mechanical properties are quite limiting for its practical application, therefore, to improve these properties it is frequent to add fibers and, in this way, create a more resistant composite material. In this paper, the authors developed PLA composites reinforced with flax fibers to evaluate the improvement of tensile and flexural strength. The experimental design of experiments was based on the L18 Taguchi array where the control factors were the extruder temperature (three levels), number of strands (three levels), infill percentage of the specimens (three levels), and whether the flax fiber had surface chemical treatment. The tensile and flexural specimens were made on a 3D printing machine and was a mold was developed to fix and align the fiber strands during the printing process. The tensile and flexural experimental tests were performed in agreement with ASTM D638.14 and ISO 14125 standards, respectively. Analyzing the results, it was verified that the surface chemical treatment (NaOH) of the fiber did not show any influence in the mechanical properties of the composites; in contrast, the infill density demonstrated a huge influence for the improvement of mechanical strength. The maximum values of tensile and bending stress were 50 MPa and 73 MPa, respectively. The natural fiber reinforcement can improve the mechanical properties of the PLA composites.

One of the most important agricultural products in the northeast of Portugal is sweet chestnuts. In this region only, several tons of this product are produced every year. Sweet chestnuts are enclosed in a hedgehog covered with a large number of short and fine spines and containing more than one nut. This hedgehog, with long spines, does not have any commercial value, and they are considered an agricultural waste that is left in the fields to be incorporated into the soil. In this work, the authors propose to study the influence of some factors in the short-fiber non-woven chestnut hedgehog spine-reinforced polyester composite performance. For this purpose, the tensile properties of chestnut spines were determined by implementing a design of experiments (DOE) approach developed by Taguchi. Thus, the factors that will be controlled are the fiber dispersion which is defined by fiber/matrix mass ratio or weight fraction, the interfacial strength that could be improved using an alkali–silane treatment and, finally, the moisture level in the fibers. For each factor, three levels were used, an L₉ orthogonal array of experiments was implemented to evaluate tensile and flexural properties. These properties were determined based on results of 54 tests, and the maximum mean values for tensile and flexural strength were 23.5 MPa and 33.6 MPa, respectively. The most influential factor for maximizing the tensile strength was the volume fraction of fiber with a contribution of 30%, and most influential factor for maximizing the flexural strength was the moisture level with a contribution of 65%. The determined mean value of tensile strength of the spines was 148 MPa.

Natural fibre composites have some mechanical and environmental advantages when compared with synthetic ones, and the environmental advantages can be improved if the base materials are autochthonous. In this work, two factors concerning the characteristics of natural fibre composites are analysed: the influence of fibre type and fibre surface alkali–silane treatment. For this purpose, an orthogonal array of experiments was defined, where sodium hydroxide (NaOH) at varying concentrations was used on flax and hemp fibres. The matrix of the composite was always polyester resin, and six plates were manufactured with different combinations of alkali–silane treatment and fibre types. To evaluate the composites’ mechanical characteristics, 18 tensile tests were performed and the average tensile strength for each combination was calculated. The combination that gave the highest value of tensile strength (113 MPa) was the flax composite associated with the alkali–silane fibre surface treatment with an NaOh concentration of 5%. The most influential factor for maximizing the tensile strength was the alkali–silane fibre surface treatment, with a contribution of 53.0%.