Crack growth in structural adhesive joints in aqueous environments

Abstract

The adhesive fracture energy, G c , of metallic joints, bonded with a rubber-toughened epoxy adhesive, has been measured using monotonically-loaded tests. Such tests have been conducted in various relative humidities and in water, at 21 o C. Two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease ('GBD') pretreatment or a grit-blast, degrease and silane primer ('GBS') pretreatment. The joints were formed using metallic substrates which were either (a) aluminium-alloy substrates, (b) steel substrates, or (c) 'dissimilar' substrates (i.e. one substrate being aluminium-alloy with the other one being steel).For both test environments, when G c was plotted against the crack velocity, three regions of fracture behaviour could be distinguished. At low rates of displacement the crack grew in a stable manner, visually along the interface, and relatively low crack velocities could be readily measured. This was termed 'Region I' and here the value of G c measured in the aqueous environment was relatively low compared to that measured in a relatively dry environment of 55% relative humidity. On the other hand, at relatively high rates of displacement the crack always grew in a stick-slip manner mainly cohesively in the adhesive layer at approximately 20 km/minute. This was termed 'Region III', and here the value of G c was relatively high and independent of the environmental test conditions employed. In this region the crack was considered to grow faster than the water molecules were able to reach the crack tip, which explains the independence of G c upon the test environment. In between 'Region I' and 'Region III', a transition region was observed which was designated as 'Region II'. The major effect of the 'GBS' pretreatment, compared to the 'GBD' pretreatment, was to increase the value of G c both in 'Regions I and III', although the presence of the silane primer had the far greater effect in 'Region I'.

Crack growth rate and crack path in adhesively bonded joints: Comparison of creep, fatigue and fracture

“…This is similar to data reported for fracture tests in humidity-controlled environments using steel or aluminum adherends at a range of loading speeds [26]. Interfacial failure was only observed at low crack growth rates when moisture could diffuse ahead of the crack tip [26]. Therefore, the present experiments show that, despite the dissimilar strain energy release rates and damage zone sizes in creep and fatigue testing, condensed moisture caused interfacial failure in creep crack growth just as it did in mixed-mode near-threshold fatigue because of the low crack speeds.…”

“…This was attributed to moisture diffusion ahead of the crack tip being faster than the crack growth rate near the threshold, thus hydrating the aluminum oxide and weakening the interface, causing interfacial failure [22]. This is similar to data reported for fracture tests in humidity-controlled environments using steel or aluminum adherends at a range of loading speeds [26]. Interfacial failure was only observed at low crack growth rates when moisture could diffuse ahead of the crack tip [26].…”

Experimental Study of Tensile Test in Resistance Spot Welding Process

“…Moreover, the development of epoxy adhesive has contributed to an increasingly important role of adhesive bonding in mechanical fastening due to its high static and fatigue strength, weight reduction, etc. However, the adhesive-bonded specimens are often easily damaged in certain environmental conditions such as high humidity as Kinloch et al (2007) and low temperature accordingly Miyoshi (1991). Therefore, the weld-bonded DP steel using epoxy adhesive which is a combination of RSW and adhesive bonding has been the subject of particular attention by Miyoshi (1995) and Darwish (2003).…”

The static and fatigue responses of aged metal laminate doublers joints under tension loading

“…[17] Water then decreases the mechanical properties of adhesive (tensile strength and Young's modulus) [4][5][6][7][8][9], may (depending on surface treatment) cause corrosion in the substrate at the interface [18][19][20], and hydration of strong metal oxides [21,22]. The consequences of the diffused water in the adhesive joint are reductions in the joint strength, both static [23][24][25][26][27][28] and fatigue [29][30][31][32].…”

“…[19,35] Furthermore, water can swell the adhesive inducing the residual stress in the joint. [3] Finite element studies have revealed that the residual stresses due to swelling were sufficient to cause damage of the adhesive at the end of overlap.…”