The Effects of Pre-Bond Moisture on the Fracture Behaviour of Adhesively-Bonded Composite Joints

Abstract

The results of an investigation into the effects of pre-bond moisture absorbed by fibrecomposite substrates prior to bonding with various structural epoxy adhesives are presented.Substrates were bonded in the as-received condition (where substrates had been exposed to atmospheric moisture for periods of greater than three months) and were also bonded in the fully-dried condition (after drying under vacuum at 105 ºC for 28 days). Additionally, substrates were conditioned by water submersion for various durations prior to bonding. Double cantilever beam tests were performed on the resulting joints to determine the adhesive fracture energy, G IC . The effect of pre-bond moisture on the glass transition temperature of the adhesive was also determined. One adhesive was shown to exhibit an extreme sensitivity to pre-bond moisture. A severe reduction in fracture energy accompanied a change in the fracture morphology and T g . Other adhesives were shown to be relatively insensitive to the levels of pre-bond moisture introduced.

Mode I fracture toughness of co-cured and secondary bonded composite joints

“…Comparing the results to the propagation values of fracture toughness in Figure 2d, it appears that T g is proportional to G IC , which would seem to concur with the findings of Blackman et al [14]. However, in that work, the locus of failure remained cohesive, regardless of the level of pre-bond moisture, while in the present study, the locus of failure for the CC180 joint system was interfacial.…”

“…The release of the additional water vapour explains the lower T g of the CC180 joint system. It should be noted that the differences in T g reported here are likely the result of a combination of both the cure temperature and prebond moisture stored in the prepreg, laminate or adhesive before curing [14]. While the experimental data from the thermal characterisation methods suggest that the adhesive has been plasticised due to diffusion of the water from the prepreg into the adhesive, it would be advantageous to model the diffusion process to determine if it is possible during the time span of a cure cycle (4 hrs).…”

“…It should also be noted that there may have been residual moisture left in the composite laminate or that the laminate reabsorbed moisture from the atmosphere in the few hours during preparation of the secondary bonded joints that could also have affected the glass transition temperature and fracture toughness [14].…”

The fracture behaviour of structural adhesives under high rates of testing

“…Because moisture absorbed by composite substrates prior to bonding (i.e. pre-bond moisture) can be very damaging to adhesive joints [8,9], all composite substrates were thoroughly dried before the adhesive joint manufacture process. This was achieved by placing the substrates in a vacuum oven at 60˚C until mass measurements indicated a 'fully dried' condition had been achieved.…”

The use of acoustic emission and composite peel tests to detect weak adhesion in composite structures

“…[2] Research shows that the fracture surface of DCB test coupons show adhesive failure when defects such as pre-bond contamination, pre-bond moisture and uncured adhesive are present at the bond line. [3,4,5] As a consequence, fracture toughness values under opening mode (G IC ) significantly decrease, which is a direct result from the crack growing at the interface instead of inside the adhesive layer (cohesive failure-good adhesion quality). [6,7] The same studies also show that neither ultrasonic C-scan nor X-ray tomography are able to detect those defects prior to testing.…”