Moisture degradation of open-faced single lap joints

Abstract

To obtain experimental data in short time on the degradation of adhesives exposed to moisture, a valuable technique is represented using the open-face configuration. With this technique, a layer of adhesive is first applied on one adherend and exposed to the humid environment; then, the second adherend is bonded and the joint can now undergo mechanical testing. Apart from the acceleration of moisture uptake which is obtained due to the larger area exposed, a further advantage is the uniformity of degradation. A further acceleration can be obtained by adding a hygroscopic contaminant at the adhesive/adherend interface, which speeds up moisture uptake and accentuates the interfacial nature of the failure. The main aim of this work was to evaluate the decay of the mechanical strength in the absence or presence of a contaminating agent. The specimens studied were single lap joints, tested under static shear loading. Two sets of specimens were considered; in the first set, the adhesive was applied in standard way and in the second set, the adhesive/adherend interface was contaminated with droplets of CaCl 2 aqueous solution. Both sets were subjected to humid and warm environment (100% relative humidity, 50°C). After the desired exposure times in the range 1-5 weeks, groups of specimens were dried and bonding of the second adherend was carried out. Then, mechanical testing was performed; the fractured surfaces were examined by scanning electron microscopy. The results show that before degradation the failure type is cohesive, but it changes to interfacial failure as the degradation proceeds. Uncontaminated specimens exhibit gradual degradation during the exposure time; contaminated specimens achieve almost half of the degradation in less than one week; after that, the process continues at lower speed and at the end of the observed period both methods show similar values of failure loads. Additional tests were carried out to assess the moisture absorption in the adhesive layer and relate it to the exposure time.

Review on the effect of moisture and contamination on the interfacial properties of adhesive joints

“…Acknowledging the interfacial nature of the failure of adhesive joints subjected to humidity, this technique can be adopted to speed up the water uptake process and obtain a more uniform degradation of the adhesive layer. Wylde and Spelt 91 and Loh et al 92 used this technique to study the interfacial fracture energy of aged adhesives and Goglio et al 90 used it to analyze SLJs immersed in water. The process of joint manufacturing consists of applying the first layer of adhesive onto each substrate, the primary adhesive bonding, Figure 11a, which is followed by drying this layer in an oven to remove any moisture present, Figure 11(b).…”

A review on the temperature and moisture degradation of adhesive joints

“…Another advantage is that the moisture uptake is uniform in the entire adhesive layer. Goglio et al 67 found that the strength of unaged open-faced single-lap joints was reduced by about 10% when compared to the strength of closed specimens. 2.…”

“…Goglio et al. 67 found that the strength of unaged open-faced single-lap joints was reduced by about 10% when compared to the strength of closed specimens. Specimens with reduced thickness: Some authors have used specimens with smaller width in order to allow for a faster saturation. Viana et al.…”

Deformation and damage evolution of a full-scale adhesive joint between a steel bracket and a sandwich panel for naval application

“…2,[11][12][13] The results demonstrate that the most severe damage occurs at the adhesive/adherend interface. 14 Different environmental exposure can cause a change in the failure mode of the joint. In order to understand the complete behaviour and failure mode of multi-material adhesive joints, one can implement non-destructive testing (NDT) techniques for in-situ evaluation of the damage.…”