Effect of humidity on the mechanical properties of adhesively bonded aluminium joints

Abstract

The presented work focuses on the effects of water degradation on the long-term behaviour of adhesive joints. The objective of this study is to measure the evolution of various mechanical properties such as tensile stress and fracture toughness as a function of humidity for two distinct adhesives, using bulk adhesive and double cantilever beam specimens in unaged and aged conditions in order to understand the influence of humidity on the adhesive properties. A mathematical equation that allows the prediction of each property degradation as a function of water is proposed and validated, which takes into account various parameters such as the diffusion coefficient, resulting in a general equation for mechanical property degradation prediction of potentially any adhesive. It was also found that the distinct adhesive properties such as strength, stiffness and fracture toughness all decreased due to water degradation with the exception of the strain that increased, concluding that water reduces the joint strength and lifespan of the studied adhesives, although in different ways.

A cohesive zone element for mode I modelling of adhesives degraded by humidity and fatigue

“…The presence of humidity in an adhesive translates into a degradation of its mechanical properties [22,23,24], and the way such degradation affects specific adhesives must be understood, specifically for properties that are necessary for the definition of the CZM law (such as Young's modulus, tensile strength and fracture toughness). The traction-separation law parameters are thus adapted in the beginning of the simulation as represented in Figure 4.…”

“…Experimental work is therefore needed to characterize how the mechanical properties of the adhesive itself decrease with different humidity levels, and relationships should be found on how each property decreases with an increase in humidity. Previous work has been accomplished with such objective, and the degradation of two distinct epoxy adhesives has been studied [23], which has resulted in equations that allow the CZM code to determine the mechanical properties at any level of humidity content using the following equation:…”

“…where y is the tensile property to be degraded (Young's modulus, tensile stress or strain at failure), subscripts 0 and f are, respectively, the initial and final values for that property, x is the level of relative humidity present in the joint (such that 0 ≤ x ≤ 1), and D and m ∞ are parameters for the Fick's law of each adhesive. Equation 7 is plotted in Figure 4 (right) for the case of tensile stress (other properties follow a similar evolution), and more details on this formulation may be found in the literature [23]. Thus, by simply providing the base mechanical properties, Fick's law parameters and relative humidity content, the cohesive element may estimate the value of each property for the specific humidity concentration and adapt the traction-separation law automatically.…”

Water Diffusion in Double Cantilever Beam Adhesive Joints

“…Moisture degradation of adhesives include reduction of their mechanical properties, inducing plasticization (Costa et al 2016a, Costa et al 2016b, Costa et al 2016c, Sugiman, Crocombe and Aschroft 2013, Wylde and Spelt 1998. The deleterious effects are greater in adhesive joints as the degradation of the adhesive-adherend interface may cause interfacial failure.…”

Determination of the fracture envelope of an adhesive joint as a function of moisture

“…Epoxy resins are widely used as matrix for structural adhesives due to their good mechanical properties [5]. Nevertheless, these properties are strongly affected by water absorption, which causes plasticization, decrease of the glass transition temperature and reduction of their mechanical strength and also the integrity of the adherend [9][10][11][12][13][14]. Because of this, it is important to determine the influence of water on the fracture toughness.…”