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Machine learning is emerging as a powerful tool for the discovery of novel high-performance functional materials. However, experimental datasets in the polymer-science field are typically limited and they are expensive to build. Their size (< 100 samples) limits the development of chemical intuition from experimentalists, as it constrains the use of machine-learning algorithms for extracting relevant information. We tackle this issue to predict and optimize adhesive materials by combining laboratory experimental design, an active learning pipeline and Bayesian optimization. We start from an initial dataset of 32 adhesive samples that were prepared from various molecular-weight bisphenol A-based epoxy resins and polyetheramine curing agents, mixing ratios and curing temperatures, and our data-driven method allows us to propose an optimal preparation of an adhesive material with a very high adhesive joint strength measured at 35.8 ± 1.1 MPa after three active learning cycles (five proposed preparations per cycle). A Gradient boosting machine learning model was used for the successive prediction of the adhesive joint strength in the active learning pipeline, and the model achieved a respectable accuracy with a coefficient of determination, root mean square error and mean absolute error of 0.85, 4.0 MPa and 3.0 MPa, respectively. This study demonstrates the important impact of active learning to accelerate the design and development of tailored highly functional materials from very small datasets.

Adhesive joints provide an effective and lightweight solution for the assembly of structures and offer both mechanical and operational advantages over conventional mechanical fastening systems. In this study, a new single-layer adhesive joint is investigated in which a thin, pretensioned textile reinforcement is inserted into the adhesive layer. In the first part, a simplified analytical model is proposed to describe the distribution of axial stresses in the adhesives and the reinforcement as well as the shear stresses in the adhesive layer. In the second part, the effects of geometric, mechanical and loading variables are investigated in a parametric analysis, focussing on the role of the initial pre-compression on the tensile response of the joint. The third part of this study compares the theoretical results with experimental data obtained with static tests on specimens made of unreinforced GFRP and epoxy resin. The results show small deviations (3–8%) between model and test. Finally, a simplified method for estimating the load-bearing capacity of brittle joints, both conventional and reinforced, is proposed. It is shown how the introduction of reinforcement and prestressing can modulate the stiffness and improve the stability of the joint without significantly affecting the load-bearing capacity.

This paper presents the results of an experimental study of adhesive joint strength with consideration of the inaccuracy of the hardener dosage, in the context of evaluating the degradation of joints when used either at ambient or elevated temperatures. The butt joint strength characteristics were assessed for two types of adhesives—rigid and flexible—and two curing scenarios—with and without heat curing. An excess hardener was shown to be significantly more unfavourable than its deficiency, which can ultimately be considered as a recommendation for forming epoxy adhesive joint assemblies. In order to fully understand the relationship between the analysed mechanical properties of the material and the influence of component ratio excesses and heating, a process of fitting basic mathematical models to the obtained experimental data was also performed.

Most structures are comprised of a number of individual parts or components which have to be connected to form a system with integral load transmission path. The structural adhesive bonding represents one of the most enabling technologies to fabricate most complex structural configurations involving advanced materials (e.g. composites) for load-bearing applications. Quite recently there has been a lot of activity in harnessing nanotechnology (use of nanomaterials) in ameliorating the existing or devising better performing structural adhesives. The 10 chapters by subject matter experts look at the following issues: Surface preparation for structural adhesive joints (SAJ) Use of nanoparticles in enhancing performance of SAJ Optimization of SAJ Durability aspects of SAJ Debonding of SAJ Fracture mechanics of SAJ Failure analysis of SAJ Damage behavior in functionally graded SAJ Impact, shock and vibration characteristics of composites for SAJ Delamination arrest methods in SAJ

The use of adhesive joints in naval applications requires a thorough understanding of their fatigue performance. This paper reports on the fatigue experiments performed on double cantilever beam specimens with thick adhesive bondline manufactured under shipyard conditions. The specimens have an initial crack at the steel–adhesive interface and are tested in unaged, salt-spray-aged and immersion-aged conditions to determine the interface mode I fatigue properties. The strain energy release rate is calculated using the Kanninen–Penado model, and the fatigue crack growth curve is determined using a power law model. The crack growth rate slope for salt-spray-aged specimens is 16.5% lower than for unaged specimens, while that for immersion-aged specimens is 66.1% lower and is shown to be significantly different. The fracture surfaces are analyzed to identify the failure mechanisms and the influence of the aging process on the interface properties. Since the specimens are manufactured under shipyard conditions, the presence of voids and discontinuities in the adhesive bondline is observed and as a result leads to scatter. Hence, Bayesian linear regression is performed in addition to the ordinary least squares regression to account for the scatter and provide a distribution of plausible values for the power law coefficients. The results highlight the impact of aging on the fatigue property, underscoring the importance of considering environmental effects in the qualification of such joints for marine applications.

In this study, the influence of sandblasting process parameters as a surface preparation method on the strength of single-lap adhesive joints of EN AW 2024 T3 aerospace aluminium alloy sheets was determined. Eleven sets of sandblasting parameters were used, which were determined according to a determined experimental plan. The variable factors in the sandblasting process were pressure, nozzle distance, and workpiece displacement speed. The sand jet incidence angle was constant. Garnet 80 E+ was the abrasive material that was used. The joints were made using an epoxy adhesive composition of Epidian 5 epoxy resin and a PAC curing agent. The influence of the surface preparation method on the surface roughness and contact angle to determine the surface free energy was evaluated. The shear strength of the adhesive joints was also determined, which finally allowed the evaluation of the applied surface treatment variants. The obtained results were subjected to statistical analysis, which indicated that the highest shear strength of the adhesive joints was obtained for samples whose surfaces were treated by sandblasting at parameter configurations in which the pressure was 5–6 × 105 Pa; the distance between the nozzle and the sandblasted surface should not be greater than 97 mm, and the speed at which the workpiece moves in relation to the nozzle should not be greater than 75 mm/min.

This paper presents the results of a study of adhesive joints, focused on the heterogeneity of the properties of the adhesive material in the adhesive joint. The main objective of the study was to determine potential differences in the material properties of adhesive joints made with selected structural adhesives. Due to the impact of the joined material on the adhesive during the curing of the joint as well as the impact of phenomena occurring during the curing of the adhesive, the properties of the adhesive joint may vary along the thickness of the joint. Determining the differences in material properties over the thickness of the adhesive bond is important for more accurate prediction of adhesive bond strength in FEM simulations. In order to observe changes in the material properties of bonds, nanoindentation tests have been carried out on eight adhesive joint bonds made with common structural adhesives used to join sheets of aluminium alloy or corrosion-resistant steel. Basing on the achieved test results, load/unload curves were developed for imprints at characteristic spots of the joints. Distinct differences in the achieved average force value were observed for imprints located in the wall-adjacent zone and in the centre of the adhesive joint; this can be interpreted as areas of the joint with different material structures of higher or lower density of imperfections or porosities. Differences in the load/unload curves for ‘rigid’ and ‘flexible’ adhesives were analysed. The summary includes a conclusion that an adhesive joint is characterised by heterogeneous properties along its thickness.

The thermal effect on the shear load and fracture of MP55420 adhesively bonded joints of plates of 590/600 dual phase steel and AA6061-T6 aluminum alloy was investigated by means of single lap shear (SLS) testing, visual and SEM fractographic analysis applying radical temperatures (−20, 25, 60 and 100 °C). Results indicated that as the SLS load decreases the displacement increases in function of temperature increments for the steel-aluminum (St-Al), aluminum-aluminum (Al-Al) and steel-steel (St-St) unions. Three different fracture mechanism were detected in the fracture surface of each condition. The ST-St joint exhibited the better results at higher temperature than zero degrees with the largest values of SLS load and displacement. In this case, the failure mechanism consisted of cohesive and adhesive on both sides of the fracture joints containing morphology of micro-voids with different sizes depending on temperature. The St-Al joint exhibited mechanical degradation with the lowest load (0.45 kN) and middle displacement at 100 °C resulting in unacceptable adhesive fracture with the most of resin on one adherend, generating superficial shallow micro-voids with different size as a function of temperature. At −20 °C condition, the Al-Al joint reached the maximum load (11.55 kN), large displacement and cohesive failure assuring adequate mechanical behavior associated with the adhesive contraction, while inappropriate adhesive-cohesive failure was generated in the St-St joint leading to a reduction in the load and the lowest displacement (0.29 mm) linked to the degradation of the adhesive resistance.

Over recent decades, the need to comply with environmental standards has become a concern in many industrial sectors. As a result, manufacturers have increased their use of eco-friendly, recycled, recyclable, and, overall, more sustainable materials and industrial techniques. One technique highly dependent on petroleum-based products, and at the edge of a paradigm change, is adhesive bonding. Adhesive bonding is often used to join composite materials and depends upon an adhesive to achieve the connection. However, the matrices of the composite materials and the adhesives used, as well as, in some cases, the composite fibres, are manufactured from petrochemical products. Efforts to use natural composites and adhesives are therefore ongoing. One composite that has proven to be promising is wood due to its high strength and stiffness (particularly when it is densified), formability, and durability. However, wood must be very carefully characterised since its properties can be variable, depending on the slope of the grains, irregularities (such as knots, shakes, or splits), and on the location and climate of each individual tree. Therefore, in addition to neat wood, wood composites may also be a promising option to increase sustainability, with more predictable properties. To bond wood or wooden composite substrates, bio-adhesives can be considered. These adhesives are now formulated with increasingly enhanced mechanical properties and are becoming promising alternatives at the structural application level. In this paper, wooden adhesive joints are surveyed considering bio-adhesives and wood-based substrates, taking into consideration the recent approaches to improve these base materials, accurately characterise them, and implement them in adhesive joints.