The role of the interphase in the environmental failure of adhesive joints

doi:10.1016/S1359-6454(00)00240-8

Acta Materialia

Volume 48, Issues 18–19, 1 December 2000, Pages 4543-4553

https://doi.org/10.1016/S1359-6454(00)00240-8 Get rights and content

Abstract

The cyclic-fatigue behaviour of adhesive joints, which consisted of an aerospace-grade epoxy-adhesive bonding aluminium-alloy substrates, has been investigated. Fracture-mechanics tests were used to obtain the relationship between the rate of fatigue crack growth per cycle, da/dN, and the maximum strain-energy release rate, G_max, applied during the fatigue cycle. These cyclic-fatigue tests were conducted in both a “dry” environment of 23±1°C and 55% relative humidity and a “wet” environment of immersion in distilled water at 28±1°C. In particular, the effect of using various surface pretreatments for the aluminium-alloy substrates, prior to forming the adhesive joint, has been investigated. X-ray photoelectron spectroscopy and electron microscopy techniques have been used to identify the locus of joint failure and the mechanisms of environmental attack.

Introduction

There are many advantages that polymeric adhesives can offer compared to the more traditional methods of joining such as bolting, brazing, welding, mechanical fasteners, etc. These include [1], [2] the ability to join dissimilar materials to give light-weight, but strong and stiff, structures, such as honeycomb sandwich panels. Also, polymeric adhesives may be used to join thin-sheet material efficiently. Further, adhesive bonding frequently represents the most convenient and cost-effective joining technique and, indeed, the bonding operation can often be readily automated. For these reasons, adhesive bonding is widely used in the aerospace industry.

However, a major concern is that the mechanical performance of adhesive joints involving metallic or ceramic substrates may deteriorate upon being exposed to aqueous environments [1], [2], [3]. Further, these previous reviews have revealed that it is the interphase of the joint, i.e. the region adjacent to the interface between the substrate and the polymeric adhesive, which is susceptible to such attack and on which attention must be focused. Also, a common type of mechanical loading encountered by aerospace structures, especially adhesively-bonded components, is cyclic-fatigue loading. For most materials, the presence of this type of loading is found to lead to a much lower resistance to crack growth than under monotonic loading, and polymeric adhesives are no exception to this observation. Thus, as would be expected, the combination of an aqueous environment and cyclic-fatigue loading is a severe test for any adhesive system.

Hence, in the present study the cyclic-fatigue behaviour of adhesive joints has been investigated, and an assessment has been made of the environmental performance and durability of the joints under such loading conditions. A typical epoxy-film adhesive, as used by the aerospace industry, has been employed and, in particular, the effect of using various surface pretreatments for the aluminium-alloy substrates, prior to forming the adhesive joints, has been investigated. Fracture-mechanics tests were used to obtain the relationship between the rate of fatigue crack growth per cycle, da/dN, and the maximum strain-energy release rate, G_max, applied during the fatigue cycle. These cyclic-fatigue tests have been conducted in both a “dry” environment of 23±1°C and 55% relative humidity and a “wet” environment of immersion in distilled water at 28±1°C. X-ray photoelectron spectroscopy and electron microscopy techniques have been used to identify the locus of joint failure and the mechanisms of environmental attack.

Section snippets

Materials

An aerospace grade of aluminium alloy was employed (UK code: 2L93; international code: 2014A; this is a high yield stress alloy containing 4.65% w/w copper). A hot-curing toughened-epoxy adhesive, “EA9628 UNS” (unsupported), designed for structural bonds requiring high toughness, was used. The adhesive, manufactured by “Hysol Dexter Inc.”, was supplied in the form of a film and had a nominal thickness of 240 μm. The adhesive was cured by heating the joints for 60 min at 90°C, followed by 30 min

Values of the adhesive fracture energy, G_c

Fracture-mechanics tests were performed under monotonic loading at a constant rate of displacement of 1.0 mm/min. No “R-curve” behaviour was observed, i.e. the value of the adhesive fracture energy, G_c, was not seen to increase as the crack propagated through the joint. Indeed, a constant value of G_c was recorded as the crack steadily propagated through the TDCB joint. The values of G_c for the joints which had been subjected to the various surface pretreatments are shown in Table 1.

Several

The GBD-pretreated joints

Initial XPS studies on the surface of the aluminium alloy after the GBD pretreatment revealed a relatively high level of carbon, present

Conclusions

It is evident that the fracture-mechanics approach provides an excellent method for evaluating the effects of the different surface pretreatments on the durability of the adhesively-bonded joints. In particular, the combination of cyclic-fatigue loading and the presence of an aqueous environment leads to an assessment of the environmental resistance of the bonded joint within a matter of weeks, as opposed to the more typical accelerated ageing tests which involve exposing the joint, unstressed,

Acknowledgements

The authors would like to thank DERA (Farnborough) for financial support and Professor S. J. Shaw and Dr I. Ashcroft of DERA (Farnborough) for helpful discussions, and Dr P. Midgley and Mr P. Thomas (University of Cambridge) for their assistance in undertaking the EFTEM studies.

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The role of the interphase in the environmental failure of adhesive joints

Abstract

Introduction

Section snippets

Materials

Values of the adhesive fracture energy, Gc

The GBD-pretreated joints

Conclusions

Acknowledgements

Int. J. Adhesion Adhesives

Int. J. Adhesion Adhesives

Int. J. Adhesion Adhesives

Int. J. Adhesion Adhesives

Adhesion and Adhesives: Science and Technology

Proc. Inst. Mech. Engrs.

J. Adhesion

Surf. Interface Anal.

J. Adhesion

Values of the adhesive fracture energy, G_c