Proceedings of the Institution of Mechanical Engineers,Part C: Journal of Mechanical Engineering Science

Melamine-formaldehyde is the most common resin used in the production of high-pressure laminates and low-pressure laminates (melamine impregnated paper), due to its properties of durability, good fire, and heat resistance and good moisture resistance. High-pressure laminate is a common choice for horizontal and vertical applications in the construction and furniture industries, thanks to its durability and versatility. The manufacture of such laminates commonly starts with the impregnation of decorative and kraft papers with thermosetting resins and after a drying process, the impregnated paper sheets are bonded together by high pressure and temperature pressing. The aim of this work was to evaluate the influence of the melamine-formaldehyde resins condensation degree after synthesis on high-pressure laminates performance. For that purpose, melamine-formaldehyde resins with different condensation degree were synthetized and characterized. Fourier-transform infrared–attenuated total reflection spectroscopy and dynamic mechanical analysis were used to study the chemical and mechanical cure of melamine-formaldehyde resins. Melamine-formaldehyde condensation degree has shown to have influence on resin pot-life, on characteristic reaction temperatures, on the degree of cross-linking achieved, and finally on the surface quality of high-pressure laminates.

Over the years, the use of structural adhesive bonding has significantly grown in numerous technological sectors, including the aeronautical, aerospace, medical and automotive industries. The growing need to design lighter and better performing structures has pushed designers to improve their construction techniques, and consequently adhesive joints have appeared as an optimal joining solution, providing the necessary high strength and stiffness, low cost and excellent capabilities to join multi-material structures. In many of these applications, perhaps most importantly in the automotive industry, it is fundamental to ensure that when the joint is loaded to destruction, such as in a vehicle collision, failure is always cohesive and adhesive failure is avoided. This work proposes a novel technique to ensure that the failure mode is not adhesive, forcing a failure mode that does not propagate through or near the interface. To accomplish that, an epoxy adhesive typically used in the automotive industry was studied and reinforced with microparticles of cork. This study was validated experimentally with joint configurations typical of industrial applications, such as single lap joints, supported by numerical simulations performed to better understand the failure mechanism. The influence of the amount and size of these particles on the fracture type was evaluated. Overall, both the experimental and numerical results showed that by increasing both the size and the amount of the particles in the adhesive, the failure mode tends to be more cohesive (in the middle of the bondline) with a small reduction in joint strength, demonstrating that this can be a viable technique if cohesive failures in the adhesive layers are necessary.

The development of lighter structures and materials has been one of the main research concerns of the transportation industry during the last decade, driven by the necessity to decrease fuel consumption and emissions. Therefore, the use of several different new lightweight materials, such as special metal alloys, reinforced polymers and new advanced composite materials has been explored, leading to optimized structures which combine these novel materials. To manufacture these multi-material structures, adhesive bonding is one of best joining techniques available, as fasteners add weight to the structure and require holes to be drilled and welding is not easily applicable to reinforced plastics, composites and some high strength metal alloys. However, adhesive bonding also presents some limitations that need to be considered, such as the appearance of singularities and the resultant stress concentration at the edges of the bond line, which result in a reduction of the joint strength. In order to mitigate this effect, several techniques have been proposed, being the use of functionally graded adherends one of them. Functionally graded adherends consist in an adherend where the mechanical properties gradually change throughout the material, usually in the thickness or length direction. The present work introduces the concept of a layered functionally graded adherend, varying the flexibility of each layer through the thickness direction. Different ratios of stiffness variation, combined with different adhesive properties, were numerically evaluated for single lap joints, comparing the stress distribution of the adhesive layer and the resultant joint strength, using cohesive zone modelling. Moreover, an optimization process of typical graded material properties, where different distribution laws that consider material weight and strength are considered, is presented.

Additive manufacturing has gained increasing attention in recent years in numerous industrial sectors due to its inherent characteristics, e.g. tool-free production and unprecedented freedom of design. However, in some applications such as heat exchangers the design has to follow certain restrictions, e.g. to allow for the removal of unfused powder, which can be enclosed in cavities. Moreover, in case multi-material parts are considered, the use of different powders during processing is often uneconomical since powder recycling is highly challenging. Therefore, the production of complex structures being characterized by limited accessibility and components made of different materials often require a subsequent joining process. Based on an analysis of state-of-the-art joining technologies employed for additively manufactured metal components, research gaps related to adhesive bonding are deduced. In light of the prevailing gaps, the influence of selective laser melting process parameters like laser power and build direction on the surface topography and, thus, on the bondability of the substrates are investigated. The mechanical tests reveal a high bond strength for the vertically oriented samples and the samples manufactured with a laser power of 400 W. Furthermore, a laser post-treatment of the SLM samples lead to an improvement of lap shear strength. Finally, results reporting on the ageing behaviour of these joints and an outlook on further research activities are given.

Adhesives play an important role in many key industrial sectors, such as the automotive industry, enabling the construction of lightweight, multi-material structures, combining polymers, composites and metals. However, adhesives are usually polymeric materials, which can be affected by environmental and working conditions, such as moisture and contamination. Although moisture and contamination degrade the adhesive, the failure of a bonded joint is often ultimately interfacial. Therefore, a literature review on the influence of those factors on the interfacial properties of adhesive joints is performed to understand the phenomena that take part in the degradation on adhesive joints when subjected to humid and contaminated environments, which can oftentimes be the case in factory conditions, especially for parts from third-party suppliers. The mechanisms and effects of moisture aging and contamination are presented, as well as experimental testing methods and practical case studies. It is concluded that both moisture and other contaminants may lead to a reduction in joint strength and catastrophic adhesive failure. Moisture absorption can occur through the adhesive, but in an adhesive joint, it can additionally occur through the substrate, the interface between the adhesive and the substrate or in the cracks and pores of the adhesive. After water ingresses into the adhesive, it decreases its mechanical properties and plasticizes it. However, in an adhesive joint, the water diffusion occurs much faster than in bulk adhesive due to the complementary diffusion paths, which typically leads to adhesive failure at the adhesive/substrate interface. Additionally, in an adhesive joint, water may induce the hydrolysis of the substrates or have other chemical interactions with them. Contaminants can diffuse through the joint or remain at the adhesive/ substrate interface. When they diffuse through the joint, they have consequences similar to those of water sorption. However, when they remain at the interface, they can produce locally debonded areas, which may lead to joint failure.

Understanding the fatigue response of adhesive joints under cyclic shear stress is important to avoid fatigue failure of the bonded structures. Recently, a cohesive zone modelling (CZM) technique was developed where the cohesive properties of the elements were degraded by a proposed empirical relation. However, based on this degradation approach, the fatigue life of the joints should be known before running the analysis. The aim of the current study is to improve the numerical method in which the fatigue life of the joints will be automatically estimated during the analysis. To achieve this, the concepts of fracture mechanics are considered by using the Paris’ law combined with the degradation model. A user material subroutine is developed to conduct the numerical calculations based on the concepts of CZM. End notched flexure (ENF) tests were carried out to evaluate the numerical data. Based on the results, it was found that the modified approach, in which the total fatigue life is obtained automatically, can be employed for fatigue life assessment of adhesive joints subjected to cyclic shear stresses.

This work studied the capability of structural adhesives to bond an aluminium rail used to assemble the seats inside the train. Scaled specimens of these joints were mechanically tested under a range of temperatures (−40 °C up to 80 °C) before and after ageing in distilled water in order to simulate real life conditions. A three-dimension numerical simulation was carried out to understand the magnitude of stresses present in the adherends and in the adhesive layer. A new cohesive element was developed using the finite element (FE) to predict the behaviour of an adhesive joint after environmental degradation. Experimental tests were carried out to assess the joint strength and the failure mechanism, experimentally. The model gave accurate results and was able to successfully predict cohesive failure of every joint that failed cohesively in the adhesive layer.

The increasing interest for the application of adhesive joints in naval superstructures motivates researchers to gain an in-depth understanding of the mechanical behaviour and failure mechanisms of these joints. This work reports on an experimental study of the deformation behaviour and damage evolution of a full-scale multi-material joint using different instrumentation techniques. Adhesively bonded joints of steel to sandwich panel components have been subjected to quasi-static tensile tests during which the global deformation of the joint and local strain distributions were monitored using digital image correlation (DIC). During one particular tensile test, fibre optic Bragg sensors (FBG) were also applied to the specimen’s surface at different locations in order to quantify the evolution of local strains. Additionally, acoustic emission (AE) sensors were installed in order to monitor damage initiation and evolution with increasing levels of imposed deformation. This test showcased adhesive failure at the interface of the steel adherend and the adhesive, while cohesive failure was observed within the adhesive and skin failure at the interface between adhesive and the composite skin of the sandwich panel. The post-mortem observed failures modes were compared to the acoustic events that originated during the test due to damage initiation and propagation within the joint. The evolution of the different sensor signals, i.e. the damage expressed as cumulative AE energy and local strains measured with Bragg sensors and DIC, are mutually compared and acceptable correlation is found.

The key to sustainable development in the footwear industry through the principles of circular economy lies in taking care of the design, as well as the introduction of innovative and more resource efficient materials and processes to reduce or avoid the use of water, energy, hazardous chemicals and to minimise emissions and waste. In fact, the environmental footprint is already being considered as another requirement of the footwear through eco-design. In this sense, previous studies carried out by INESCOP regarding its environmental impact in terms of carbon footprint showed that 15% of it corresponds to the assembly processes, mainly by adhesive joints, due to their content on organic solvents, hazardous chemicals and polymers from fossil origin.

Therefore, this paper focuses on recent developments carried out by INESCOP on more sustainable adhesives and adhesion processes for the upper-to-sole assembly in the footwear manufacturing process, through different approaches. Firstly, bio-based reactive polyurethane hot melt adhesives have been synthesised using polyols from different renewable sources. Secondly, the use of the low-pressure plasma surface treatment to improve the adhesion of polymeric materials used as soling materials was assessed in order to reduce volatile organic compounds emissions, as well as the use of hazardous chemicals for total automation of the bonding process.