Review on the effect of moisture and contamination on the interfacial properties of adhesive joints

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. </jats:p

A strain rate dependent cohesive zone element for mode I modeling of the fracture behavior of adhesives

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. </jats:p