Rosemere de Araujo Alves Lima

Multi‐material fused filament fabrication (FFF) allows hybrid joints to be manufactured in a single uninterrupted process. However, the load‐transfer behavior and failure mechanisms of co‐printed material transitions remain insufficiently understood. This study investigates PLA/PLA‐CF hybrid single‐lap joints manufactured by one‐step FFF with overlap lengths of 8, 12.5, and 25 mm, maintaining constant width and thickness. The results of the quasi‐static tests showed mean maximum apparent lap‐shear stresses of 11.6, 7.9, and 5.3 MPa, respectively. For the 12.5 and 25 mm overlaps, failure occurred in the PLA‐CF adherend. Therefore, the measured apparent stresses represent only lower bounds for the corresponding joint‐region failure limits. In contrast, the 8 mm specimens exhibited cohesive failure within the printed PLA‐CF architecture of the overlap, rather than separation at the PLA/PLA‐CF interface. An adapted Volkersen‐type model was used to examine load transfer based on the observed joint‐region failure and the lower‐bound constraints from the longer overlaps. Three‐dimensional finite element analyses were also performed to evaluate the stress distribution before failure. A preliminary environmental‐economic assessment showed that the preferred configuration depends on the selected mechanical objective, whether maximum specimen load or apparent lap‐shear stress. The manufacturing impacts were also influenced by the adopted specimen geometry.

This study investigates the fracture behavior of co-bonded joints between Carbon Fiber Reinforced Polymers and 316L Stainless Steel manufactured by Laser Powder Bed Fusion, with the aim of improving the reliability of multi-material joints and reduce assembly time. To promote adhesive interlocking between the two materials and reduce dependence on both surface preparation and metal type, the metal adherends were designed with interface lattice patterns. Mode I fracture toughness is evaluated through Double Cantilever Beam tests, supported by post-mortem analysis, which provides valuable insights into the effects of the pattern. Among the investigated topologies, the TPMS Gyroid unit cell exhibited the highest amount of cohesive failure thanks to its morphology and hence, it achieved a fracture toughness approximately 40 % higher than the Simple Cubic (SC) unit cell. A homogenized 3D Finite Element Model of DCB tests, which includes 3D elasto‐plastic behavior and Cohesive Zone Model, successfully reproduced the experimental load-displacement curve while maintaining low computational cost. Overall, this study demonstrates that surface lattice patterns in bi-material joints effectively anchor and retain the adhesive within the interface cavities, with TPMS Gyroid significantly enhancing Mode I fracture toughness compared to the SC unit cell, while elasto-plastic homogenization of the adhesive infilled unit cells enables a computationally efficient and accurate FE modeling of bi-material joints.

Adhesively bonded joints are extensively utilised in multi-material lightweight structures, including those with high load-bearing demands, such as wind turbine blades, and in the automotive and naval sectors. However, for certification in critical safety applications, adhesively bonded joints cannot be relied on alone and require backup measures, such as rivets and bolts, to mitigate the risk of sudden failure. A typical example is the secondary bonded parts in aircraft. To increase the reliability of adhesively bonded joints, it is crucial to have monitoring methods that accurately identify their damage evolution and different fracture modes, even under fatigue loading. This work aims to show how acoustic emission can act as a real-time monitoring technique for identifying various fracture modes, such as adhesive and cohesive failure, during mode I fatigue loading. To this end, an acoustic emission system was used to monitor adhesively bonded double-cantilever beam specimens under continuous mode I fatigue loading. The acquired datasets were analysed using a workflow that combines unsupervised artificial neural networks for clustering, followed by waveform inspection, and statistical analyses including distribution fitting, bootstrapping, and confidence intervals. After post-processing, it was possible to distinguish the fracture modes of bonded joints under fatigue failure, cohesive or adhesive, by observing distinct patterns and behaviours in the acoustic emission features. At the specimen level, cohesive failure resulted in more hits and higher cumulative energy than adhesive failure.

This work addresses the effect of the test temperature on the cohesive model parameters for the 3M Scotch-Weld^™ 7260 B/A epoxy adhesive. It extends a previous experimental work done at room temperature and further develops a previously proposed parameter identification method based on optimization. Double-Cantilever Beam (DCB) and End-Notched Flexure (ENF) tests were conducted at four different temperatures: 20°C, 40°C, 55°C and 70°C. Moreover, Single Lap Joints (SLJ), and bulk specimens’ tensile tests were carried out. Finite element analyses of DCB and ENF tests were performed and optimization algorithms were used to calculate constitutive cohesive model parameters. Three approaches were followed. First, cohesive parameters were derived from bulk tests. Then, optimization with two variables for each mode was conducted, to identify the parameters e_i and σ_u,i (i = I, II) of a triangular traction separation law. In the third, only σ_u,i, was taken as variable, while e_i was derived from the elastic modulus of the bulk adhesive. Finally, the cohesive models were applied to simulate the response of the SLJ, and numerical results were compared with experiments. The two free variables optimization method allowed to obtain the most accurate predictions of the maximum load and SLJ displacement at failure.

The objective of this work was to analyze the influence of the alkalization and alkalization + silanization (mixed) treatments on the interfacial adhesion and mechanical properties of intralaminar hybrid composites (jute + sisal, jute + curauá, jute + ramie, and jute + glass fiber). Single‐fiber tensile tests, fragmentation, and short beam tests were performed. Finally, a SEM analysis was used to study the influence of surface chemical treatments on the morphology of the natural fibers used. SEM images showed that the chemical treatments altered the morphology of the natural fibers. The chemical treatments improved the tensile strength of the fibers studied. It was found that the alkaline treatment is the best suited for jute, ramie, and curauá fibers, while the sisal fiber presented the best results after the mixed treatment. The superficial chemical treatments also affect the interfacial/interlaminar adhesion of the hybrid composites studied here. The best superficial treatments for each type of hybrid composite were: alkaline treatment for jute + ramie and jute + sisal, while for jute + curauá the best results were obtained for the untreated specimens.