Review of fatigue of bulk structural adhesives and thick adhesive joints

Abstract

Fatigue of structural adhesives has been investigated through joints and a little number of works investigate bulk adhesive behaviour itself. Aerospace and automotive engineering focuses more on joint configuration studies, which are correlated with practical applications. Previous works showed that for thin adhesive joints, material properties measured by bulk adhesive testing and joint testing are similar despite the triaxial stress states developing in the adhesive bondlines. However, with the introduction of structural adhesives in construction industry, thicker bondlines have emerged where the bulk adhesive material dominates the joint behaviour. This review work summarises works on the fatigue of bulk structural adhesives used mainly in the construction industry investigating structural adhesives fatigue behaviour either through experiment on joints or on bulk adhesive specimens. The work focuses on thick adhesive bondlines in joints, and discusses the controversy that is over whether adhesive properties from joints or from bulk material should be used.

Testing mechanical performance of adhesively bonded composite joints in engineering applications: an overview

“…As pointed out by some authors, FRP composites (hybrid or not) are recommended as suitable material candidates for light, sustainable and resilient structures, in the (among others) aerospace, the civil, auto-motive, wind energy, naval and offshore industry. [1,32,142,324] Today, limited factors for the acceptance of composite bonded joints in these industries are mainly the lack of confidence and the lack of existence of appropriate international standards for covering the need of each industrial application. [1,395] Nevertheless, tailored experimental protocols and experience that is accumulated in the last three-four decades, come to fulfil this gap, providing, among others, information regarding the adhesive process preparation, appropriate type of adherends, the surface quality, treatment, the best curing process, the necessary protection of the adhesive bond, and the needed quality monitoring measures.…”

“…[1,32,142,324] Today, limited factors for the acceptance of composite bonded joints in these industries are mainly the lack of confidence and the lack of existence of appropriate international standards for covering the need of each industrial application. [1,395] Nevertheless, tailored experimental protocols and experience that is accumulated in the last three-four decades, come to fulfil this gap, providing, among others, information regarding the adhesive process preparation, appropriate type of adherends, the surface quality, treatment, the best curing process, the necessary protection of the adhesive bond, and the needed quality monitoring measures. Finally, this brief overview highlighted multitude of standards and approaches which often concern very similar loading conditions or materials used across different industries.…”

“…Probably the first "inventor" of a "glue" that is acknowledged in several documents is Daedalus, who, according to the legend of the Greek mythology, engineered wings fashioned from feathers joined with wax to escape from Crete together with his son Icarus . [1] The Greek legend dates more than 3000 years back in time, however, it was not before the 17-18 th century when Galileo suggested that there should be in nature a "gluey or viscous substance to bind materials together" and Newton conjectured that "There are agents in nature able to make the particles of bodies stick together by very strong attractions". [2] The first mention for a glue in patent literature comes from a British patent of 1754, although a massive production started much later, at the beginning of the 20 th century .…”

Can confined mechanical metamaterials replace adhesives?

“…In fast-growing industrial sectors for infrastructure engineering, such as transportation and energy, drives for efficiency and functionality inevitably rely on a deep understanding of confined materials and adhesive joints [1,2]i.e, joining or sealing two similar or dissimilar materials is paramount to progress. Examples are found in glass fibre composite (GFRP) parts of a wind turbine rotor blades [3], trending glass bridges connected to metallic or wooden frames [4], and timber construction with in-rod mounting [5].…”

“…It is now recognised, that the thicker and more compliant adhesive layers can offer a set of advantages that should no longer be ignored, e.g., enhanced fracture toughness and damage tolerance, or better damping and insulation properties [8,9]. Moreover, in many applications, thick layers cannot be avoided and they help accommodating manufacturing tolerances [1]. However, this comes at a price of volume-to-weight penalty, reduced stiffness and severe stress gradients close to the corners and edges created along the adhesive material surface [10,11,12].…”