Crack Growth of Structural Adhesive Joints in Humid Environments

Abstract

The adhesive fracture energy, G c , of aluminium-alloy and steel joints, bonded with a rubbertoughened epoxy adhesive, has been measured using monotonically-loaded tests. Such tests have been conducted at different levels of relative humidity and two surface pretreatments have been employed for the substrates prior to bonding: a simple grit-blast and degrease ('GBD') pretreatment or a silane primer ('GBS') pretreatment. 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 the adhesive fracture energy was relatively low and decreased steadily as the relative humidity was increased. On the other hand, at relatively high rates of displacement the crack 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 relative humidity. 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 greater effect in 'Region I'. INTRODUCTIONThe use of structural adhesives offers many advantages when compared with other more traditional joining methods such as welding, riveting and mechanical fasteners [1]. However, as with any technology, there are some potential disadvantages associated with the use of adhesives.In particular, adhesive joints may suffer environmental attack when exposed to relatively hot and humid environments. The development of a sound accelerated-ageing test method to assess the susceptibility of an adhesive 'system' (i.e. the adhesive/primer (if any)/substrate and substrate surface pretreatment in combination) to such attack would represent a significant advance, especially if such tests could be completed in a relatively short time-scale [2].The aim of a sound and meaningful accelerated ageing test must be to accelerate the mechanisms of attack seen during the service-life of the bonded joint, and not to induce misleading and irrelevant mechanisms. For example, it is well known that to accelerate the rate of environmental attack it is not possible simply to raise the temperature of the environment, since unrealistically high test temperatures will indeed change the mechanisms of attack. One method of accelerating the rate of attack which has recently been explored in detail by Kinloch and coworkers (e.g. [3]) has been the use of cyclic-fatigue tests, using a fracture-mechanics approach, conducte...

Crack growth in structural adhesive joints in aqueous environments

“…These values lead to the conclusion that the epoxy/oxide interfaces will indeed be susceptible to attack and degradation upon exposure to water molecules. These ideas have been supported by the use [18] of surface-analytical methods which confirmed that fracture indeed occurred mainly at the interface between the adhesive and the metallic-oxide layer on the substrate. However, it should be borne in mind that the thermodynamic approach describes the behaviour of the joints at equilibrium (i.e.…”

“…the RH, are critical factors. Finally, it is noteworthy that these observations imply that, in 'Region I', the water molecules reach the vicinity of the crack tip at a sufficiently fast rate to enable the environmental attack mechanism to occur readily, since the G c versus crack velocity dependence is controlled by the viscoelastic nature of the epoxy adhesive, not by the RH employed [18].…”

“…Firstly, it must be emphasised that the interfacial fracture seen in 'Region I' in the 55% RH environment is not intrinsically the result of environmental attack, since joints tested at 0%RH were observed to exhibit 'Region I' behaviour and hence to fail visually along the interface between the adhesive and the substrate [18]. Secondly, however, there is a clear effect of the relative humidity of the test environment in 'Region I': the higher the relative humidity, the lower the adhesive fracture energy, as may be seen in Figure 9.…”

“…(This observation may be explained [18] from work by Maugis [19] who has considered the transition from stable to stick-slip crack growth that occurs in brittle materials as the crack velocity is increased. )…”

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

“…[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.…”