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.

Short fiber reinforced polymers are widely used in the construction of electronic housings, where they are often exposed to harsh environmental conditions. The main purpose of this work is the in-depth study and characterization of the water uptake behavior of PBT-GF30 (polybutylene terephthalate with 30% of short glass fiber)as well as its consequent effect on the mechanical properties of the material. Further analysis was conducted to determine at which temperature range PBT-GF30 starts experiencing chemical changes. The influence of testing procedures and conditions on the evaluation of these effects was analyzed, also drawing comparisons with previous studies. The water absorption behavior was studied through gravimetric tests at 35, 70, and 130 ∘C. Fiber-free PBT was also studied at 35 ∘C for comparison purposes. The effect of water and temperature on the mechanical properties was analyzed through bulk tensile tests. The material was tested for the three temperatures in the as-supplied state (without drying or aging). Afterwards, PBT-GF30 was tested at room temperature following water immersion at the three temperatures. Chemical changes in the material were also analyzed through Fourier-transform infrared spectroscopy (FTIR). It was concluded that the water diffusion behavior is Fickian and that PBT absorbs more water than PBT-GF30 but at a slightly higher rate. However, temperature was found to have a more significant influence on the rate of water diffusion of PBT-GF30 than fiber content did. Temperature has a significant influence on the mechanical properties of the material. Humidity contributes to a slight drop in stiffness and strength, not showing a clear dependence on water uptake. This decrease in mechanical properties occurs due to the relaxation of the polymeric chain promoted by water ingress. Between 80 and 85 ∘C, after water immersion, the FTIR profile of the material changes, which suggests chemical changes in the PBT. The water absorption was simulated through heat transfer analogy with good results. From the developed numerical simulation, the minimum plate size to maintain the water ingress unidirectional was 30 mm, which was validated experimentally.

Adhesives are widely used in the automotive industry as they can be used to manufacture lightweight multimaterial structures with improved strength-to-weight ratio, contributing to lower energy consumption and emissions. In this industry, it is also crucial to ensure passengers' safety and, thus, mechanical behavior of the complete automotive structure should be tested for impact conditions, including the adhesive joints within it. This work presents the development of a finite element, which models the mechanical behavior of adhesives and takes into account the strain rate dependent property variation, in mode I. The trends for the property variation were set based on an experimental study of two adhesives under three different loading conditions: quasi-static, intermediate speed, and impact. The strength was determined using bulk tensile tests and toughness using double cantilever beam, respectively. Both adhesives studied have shown an increase in ultimate stress and critical energy release rate with increasing strain rate. The property variation was then implemented on a finite element, which revealed a good agreement with the experimental results.

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The increasingly larger use of adhesive joints in the automotive industry demands a full comprehension of the adhesive behaviour when subjected to dynamic loadings. So far, some authors studied the effect of the strain rate regarding the adhesives performance, usually for the range of approximately 0–105/s; nevertheless, few studies are clear regarding the method used to calculate strain rate, especially when fracture mechanics analysis is the focus of study. Those who present approaches to assess the values of strain rate usually consider a constant value for each test. In this paper, a numerical approach is proposed to assess the strain rate in modes I and II in double cantilever beam and end notch flexure tests, respectively. The results of this study demonstrate that the strain rate in the adhesive bondline along the crack propagation in the double cantilever beam and end notch flexure tests is not constant when loaded at a constant cross-head speed. This finding also helps to justify why experimental R-curves of double cantilever beam tests, when loaded at speeds above quasi-static conditions, do not have a perfect plateau contrasting with those, usually presented by numerical simulations, that do not take into account the effect of the strain rate.