Here we investigate the adhesive properties of Selective Laser Melted (SLM) titanium surfaces in metal-composite cobonded joints without any prior surface treatment, to explore the inherent surface roughness of SLM parts to potentially create strong adhesive bonds. Double Cantilever Beam (DCB) tests were carried out to assess the joints mode I fracture toughness involving untreated SLM titanium and woven Carbon Fibre Reinforced Polymer (CFRP) adherends, co-bonded with an epoxy film adhesive. The same type of DCB joints, but now with sandblasted SLM titanium adherends, were tested to compare the adhesion strength of the untreated SLM surface versus the sandblasted surface. The findings reveal that joints with untreated SLM titanium adherends exhibit similar toughness to those with sandblasted SLM titanium adherends, indicating that the surface morphology of as-printed SLM titanium is suitable for manufacturing robust adhesive joints. The elimination of the surface treatment in the manufacturing of adhesive joints with SLM adherends could increase the interest of many industrial fields towards the use of adhesive bonding over other joining techniques.@en

This work aims to improve the damage tolerance of secondary adhesively bonded joints under quasistatic mode I loading conditions by architecting the carbon fibre-reinforced polymer substrates’ stacking sequences [1]. Double Cantilever Beam tests show that architecting the stacking sequence of the laminates composite substrates in combination with the adhesive layer’s fracture toughness affects the crack onset and triggers different crack paths throughout the joints’ thickness. In specimens bonded with a low-toughness bi-component adhesive, the tailored design, including a co-cured toughening layer, could increase the effective fracture toughness of the composite bonded joints up to 200%. From this study, it was possible to recognise the complexity and benefits of moving from the traditional cohesive failure to outbreaking multiple crack path propagation. @en

Using fiber-reinforced composite patches for repairing damaged structures made of metal or/and concrete is an interesting and widely available solution on the market using synthetic materials. These repairing patches are bonded on the structures’ surfaces to increase their strength against internal stresses, as well as protect them from external physico-chemical attacks, thereby limiting crack propagation. Natural fibers offer a potential alternative to replacing glass or carbon fibers commonly used for bonded repair patches. Similarly, bio-based polymers represent an important sustainable alternative for partially or entirely replacing the petroleum-based polymers. In this study, an epoxy matrix reinforced with flax fiber is proposed as the material for the patches, and bonded to a steel plate using four different types of adhesive materials, including a castor-oil derived polyurethane resin. Floating roller peel tests were performed to assess the adhesion and viability of these new patches. The resulting peeling loads and fracture surface analysis are presented. Polyurethane demonstrates promising performance for epoxy-to-steel joints, but major improvements of the bio-based polyurethane application process and curing conditions may be necessary for its successful industrial implementation.@en

Understanding the relationship between the sensors’ outputs and the damage evolution within the joints is becoming increasingly crucial to improving structural health monitoring systems and collecting data to improve the joint’s design. Therefore, a study of the acoustic emission method associated with visual fracture evaluation was proposed to give insights into the toughening of composite bonded joints and better understand the relationship between the acoustic emission features and the damage mechanism involved. Thus, two different layups were proposed for the substrates: [0]8 and [0/902/0]S. In addition, a toughened epoxy adhesive with an embedded carrier (AF163-2k) was used to bond the substrates. Five specimens of each stacking sequence were tested under quasi-static mode I loading conditions. A travelling microscope and a regular digital camera were used on the lateral sides of the specimens to track the crack propagation paths. One piezoelectric sensor linked to the AMSY-6 Vallen system was used to assess the acoustic emission features produced within the joints during the tests. Unsupervised machine learning algorithms based on artificial neural networks and the Morlet continuous wavelet transformation were used to pattern recognition of the acoustic emission data. Self-organising maps, together with k-means algorithms, were used for data clustering. Following that, the acoustic emission features of each cluster were associated with the insights obtained from the crack propagation images. Finally, it was observed that the different layups triggered simultaneous toughening mechanisms. The combination of the acoustic emission and the visual evaluation was crucial for a deeper understanding of the underlying phenomena.@en

Aiming to increase damage tolerance of adhesively bonded joints, this work explores the influence of CFRP layup of the adherends on the crack onset and crack propagation of composite bonded joints under mode I loading. Quasi-static Double Cantilever Beam tests were performed using four different CFRP layups bonded with two adhesives. Parallel to the experimental program, finite element analyses were performed to aid in understanding and identifying the various damage mechanisms in each specimen type. The results show that the CFRP layup and adhesive fracture toughness significantly influence the joint fracture phenomena at crack onset and further crack propagation. An enhancement of the joint's mode I fracture toughness values at crack onset was observed in the specimens where a crack competition between the propagation within the bondline and the composite's layers was triggered. During crack propagation, the fracture toughness of the joint increases at crack deflections between the different plies of the CFRP layup until reaching the 0° ply, where sudden delamination occurs. It has been shown that CFRP layup tailoring is a promising toughening method that, when carefully designed, has the potential to increase the maximum effective fracture toughness up to 100% when compared to pure cohesive failure.

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The use of adhesively bonded joints has increased considerably due to their lightweight, relevant strength-weight ratio and possibility to join multi-materials. Nevertheless, there are still some challenges in the application of this kind of joints in primary structures, such as guaranteeing their reliability during the components’ useful life. Structural health monitoring methods are suggested to ensure in-service safety and reliability of adhesive joints. The acoustic emission appears promising because it can detect the elastic waves produced within the material when it is under damage or straining. This research focuses on mode I fatigue damage monitoring metallic double cantilever beam adhesively bonded joints using the acoustic emission method. Digital image correlation and visual evaluation were applied during fatigue interruptions to track the crack-tip position within the adhesive and correlate them with the acoustic emission outcomes. The acoustic emission method is susceptible and different kinds of waves (background, friction and damage) can be easily assessed during the tests, producing an immense amount of data. So, unsupervised artificial neural networks for patterning recognition were proposed. Self-organising maps and k-means algorithms were used for data clustering and then classified regarding their sources. Finally, the acoustic emission results, digital image correlation and visual evaluations were compared.@en

Delamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation). @en

The feasibility of acoustic emission structural health monitoring to detect, localise and monitor crack propagation during quasi-static mode I loading of adhesively bonded joints was studied. Unsupervised artificial intelligence pattern recognition methods (Self-Organised maps and K-means) were used to classify acoustic emission raw data as either background noise or relevant information. After that, three different time-of-arrival picking algorithms were considered and implemented to determine the acoustic emission source's location, and their accuracy was discussed. Localised acoustic emission events were divided into well-defined groups with different energy levels and compared to Digital Image Correlation and visual evaluation results. It was possible to conclude that the highest energetic group allows the assessment of the onset of plasticisation ahead of the crack-tip within the studied adhesive, bringing novel standpoints to the use of acoustic emission as a structural health monitoring method for adhesively bonded joints.

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The feasibility of applying Optical Backscatter Reflectometry as a backface strain monitoring method for bonded joints subjected to mode I fatigue crack propagation was studied. Visual evaluation and Digital Image Correlation methods were applied to track the crack-tip position and the onset of plasticisation in the adhesive, respectively. The methods were compared to understand the relationship between the changes in the optical fibre measurements and the crack propagation. The absolute maximum strain values gave important information about the crack propagation stage (stable or high velocity) within the adhesive and fast-rate crack propagation, which resulted in a sudden increase in strain values. Finally, the strain peak position was found to be a valuable indicator of crack tip position.

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Natural fiber reinforced polymer composites (NFRCs) have demonstrated great potential for many different applications in various industries due to their advantages compared to synthetic fiber-reinforced composites, such as low environmental impact and low cost. However, one of the drawbacks is that the NFRCs present relatively low mechanical properties and the absorption of humidity due to the hydrophilic characteristic of the natural fibre. One method to increase their performance is hybridization. Therefore, understanding the properties and potential of using multiple reinforcement’s materials to develop hybrid composites is of great interest. This paper provides an overview of the recent advances in hybrid natural fiber reinforced polymer composites. First, the main factors that affect the performance of hybrid fiber-reinforced composites were briefly discussed. The effect of hybridization on the mechanical and thermal properties of hybrid composites reinforced with several types of natural fibers (i.e., sisal, jute, curauá, ramie, banana, etc.) or natural fibers combined with synthetic fibers is pre-sented. Finally, the water absorption behaviour of hybrid fiber-reinforced composites is also discussed. It was con-cluded that the main challenges that need to be addressed in order to increase the use of natural-natural or natural-synthetic hybrid composites in industry are the poor adhesion between natural fibers and matrix, thermal stability and moisture absorption of natural fibers. Some of these challenges were addressed by recent development in fibers treatment and modification, and product innovation (hybridization).

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The relationship between the response of backface strain distributed sensing by Optical Backscatter Reflectometry and the damage in the adhesive was studied using double cantilever beam specimens. Digital Image Correlation and visual inspection provided information about the crack-tip position and the extension of the cohesive process zone. The comparison between the features of the backface strain pattern and the position and size of the cohesive zone led to the conclusion that, for the studied adhesive, backface distributed sensing allows for the onset of plastic deformations ahead of the crack-tip to be identified, opening new perspectives for fatigue crack monitoring and in-service measurements.

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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.

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In this work, a comparative analysis of the mechanical and thermal properties of polyester and epoxy single and hybrid natural fibre-reinforced composites was performed. Pure jute, jute + curauá and jute + sisal composites with two distinct thermoset polymer resins (an epoxy and a polyester) were produced. Tensile, flexural and impact tests were carried out, in accordance to ASTM standards, to investigate and compare the mechanical properties of the composites as a function of matrix and hybridization. In addition, a thermogravimetric analysis (TGA) was used to complete the comparative analysis of the thermal properties. Finally, a scanning electron microscopy (SEM) was used to examine the fracture surface of the tested specimens. It was found that the hybridization process improved the mechanical properties of the non-hybrid jute fibre based composites for both matrices used. The resin used as matrix plays an important role on the mechanical properties of the composites. The epoxy matrix based composites presented higher tensile strength, while the polyester based composites presented higher tensile and flexural stiffness as well as higher impact energy, when compared to the epoxy-based composite. TGA analysis showed that the thermal stability of epoxy-based composites was higher compared to the polyester-based composites.@en

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 ei and σu,i (i = I, II) of a triangular traction separation law. In the third, only σu,i, was taken as variable, while ei 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.@en

The main objective of this work is to study the effect of chemical treatment on the thermal properties of hybrid natural fiber-reinforced composites (NFRCs). Different chemical treatments [i.e., alkalized and mixed (alkalized+ silanized)] were used to improve the adhesion between the natural fibers (jute, ramie, sisal, and curauá) and the polymer matrix. A differential scanning calorimetry, thermogravimetry, and a dynamic mechanical analysis were performed to study the thermal properties of hybrid NFRC. It was found that the chemical treatments increased the thermal stability of the composites. Scanning electron microscopy images showed that the chemical treatments altered the morphology of the natural fibers. A rougher surface was observed in case of alkali treated fiber, whereas a thin coating layer was formed on the fiber surface during the mixed treatment. However, for some fibers (i.e., sisal and rami), the chemical treatment has a positive impact on the composite properties, whereas for the jute and curauá composites, the best behavior was found for untreated fibers.

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The main objective of this work was to investigate the effect of hybridization and the effect of chemical treatments on the mechanical properties of a novel intralaminar natural fibre hybrid composite. Jute, sisal and curauá fibres were selected as natural fibre reinforcements for the epoxy matrix based composites which were produced by hand lay-up technique. The total volumetric fraction used in the fabrication of the hybrid composites was 30% of fibres and 70% of epoxy resin. Tensile, flexural and impact tests were carried out according to ASTM standards to characterize the novel hybrid composites. It was found that the mechanical properties evaluated are significantly improved by addition of the natural fibres to pure jute based composites. The fibre treatments had different effects on the mechanical properties of the hybrid composites. For sisal reinforced hybrid composites, the alkaline treatment had a positive impact on the composite properties (tensile, flexural and impact), while for the jute + curauá hybrid composites the alkali treatment had a negative effect on the tensile and impact properties. The mixed (alkalization + silanization) treatment had a positive effect on the jute + curauá flexural properties, while decreased the flexural properties for the jute + sisal composite. A scanning electron microscopy (SEM) was used to examine the fracture surface of the tested specimens.

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Fibre optic (FO) sensors are becoming increasingly popular for different applications in structural monitoring. Among their excellent properties, a strong interest for this type of sensors are represented by the possibility of embedding FOs inside composite components. In this case, one of the factors that significantly influence the efficiency of the whole Structural Health Monitoring (SHM) system is the interfacial adhesion between FO sensors and the host material. The main objective of this work is to investigate the interfacial adhesion between embedded fibre optic sensors and epoxy matrix to find the best type of optical fibre to be used in epoxy matrices to produce smart composites. Four types of optical fibres with different diameters and coatings (i.e. polyimide, polyacrylate and ormoceramic) were used. Pull-out tests were carried out and different methods were used to obtain the composite/optical fibre interfacial properties. Finally, an optical microscopy and Scanning Electron Microscopy (SEM) analysis were performed to characterize the fibre/matrix interfaces. It was found that the optical fibre that presented the highest energy required for interface rupture and, consequently, less invasiveness to the host material was the ormoceramic fibre with the smallest diameter.

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This work has a purpose to evaluate the electrode wear behaviour and the distribution of hardness in a joint welded by resistance spot welding process. The lifetime of electrodes was evaluated by examination of the electrode caps after more than 2000 points with acceptable welds. A microstructural evaluation by optical (OM) and scanning electron microscopy (SEM) was carried out. The microstructure of welded joint is primarily composed of ferrite grains. The microhardness test results indicated a significant increase in hardness in the weld nugget, probably due to the formation of martensite in the fusion zone. The hardness results for the four welding parameters selected at the end of the useful lifetime of the electrode were lower in all regions. The methodology used for parameter selection has highlighted an optimal combination of 200kgf electrodes force and 9 cy welding time (cycles), where more weld points were obtained, thereby increasing the lifetime of the electrode. The cover electrode tends to form brass (Zn + Cu) on the indentation obtained after spot welding, subsequently reducing electrode life. The results indicate the importance of combining welding parameters to reduce the electrode wear.@en