Dielectric and mechanical studies of the durability of adhesively bonded aluminium structures subjected to temperature cycling. Part 2: examination of the failure process and effects of drying

Abstract

Additional dielectric measurements and mechanical tests on adhesively bonded joints are reported for bonds having been aged by complete immersion in water at 50 and 65 C and by changing between these temperatures. Dielectric frequency domain measurements are discussed in terms of the effects on the adhesive and the oxide layer as a result of water ingress to the bond line. Initially, the moisture adsorption follows a pseudo-Fickian behaviour, and correlation with the mechanical properties would suggest that the initial loss in mechanical strength is consistent with plasticization of the adhesive layer. In all the cases studied, acceleration in the rate of increase in the permittivity was observed consistent with the moisture converting the interfacial oxide layer to hydroxide. X-ray diffraction measurements indicate that the hydroxide formed was predominantly of the boehmite [g-AlO(OH)] form, but also with some of the diaspore [a-AlO(OH)] form. The change of the oxide to hydroxide changes the bond line characteristics and is clearly identifiable in the time domain reflectometry (TDR) measurements. Further analysis of the high-frequency dielectric data indicate that the changes in the adherend surface resistivity that occur in the latter stages of ageing may be indicative of the conversion of oxide to hydroxide. Comparison of the TDR measurements with ultrasonic and optical images indicated that the dramatic loss of shear and cleavage strength is coincident with the observation of defects having been created in the interfacial layer. The ability for a joint to recover on drying was investigated, and the data indicate that, depending on the distribution of the moisture in the bond, recovery of mechanical properties is or is not possible. In the initial stages, when moisture is in the adhesive, recovery is in principle possible. However, once moisture has changed the interfacial layer, recovery becomes more difficult.

Effect of humidity on the mechanical properties of adhesively bonded aluminium joints

“…21 It is expected that the fracture toughness decreases with an increase in the water content, a trend often seen in the literature. 22,23 For these tests the specimens shown in Figure 2(d) were used, and Figure 6 shows the effect of humidity on the fracture toughness of the studied adhesives. Due to the amount of time needed to fully saturate each DCB specimen the relative humidity was studied up to 60%, which already presents a very clear trend in degradation of the fracture toughness.…”

Dielectric and mechanical studies of the durability of adhesively bonded CFRP structures subjected to aging in various solvents

“…Mode I failure was also studied by means of a cleavage/ peel test. The test was designed around ASTM D 3433-93 [17] and ASTM D 3807-93 [18] and has been discussed previously [1,2]. The load was applied using the Zwick 1445 tensile testing machine, with a constant cross-head separation rate of 3 mm/min.…”

“…High-frequency non-destructive dielectric measurements were conducted using a Hewlett Packard HP8753A network analyser connected via a HP-IB interface to a computer, scanning through the frequency range of 300 kHz to 3 GHz. The details of the measurements and the calculations have been discussed previously [1,2] as with the details for the above shear tests and Mode I failure tests.…”

“…Certain of these fluids are known to have dramatic effects on the strength of joints and others are fairly benign. In previous papers, the non-destructive dielectric monitoring of hygrothermal aging in aluminium adhesively bonded structures has been reported [1,2]. The aluminium is a good electrical conductor and the bond forms a waveguide that can be used to characterize the dielectric properties of the joint.…”

“…In this paper, a study of the aging characteristics of CFRP/epoxy/CFRP bonded structures in water and a range of other solvents systems is reported. The application of the dielectric method to the study of aging in aluminium adhesive bonds has been investigated previously [11][12][13][14][15] and it has been shown [1,2] that a correlation can be made between the change in the mechanical properties of the bond and the variation in its dielectric properties. This study attempts to explore whether a similar correlation might exist in the case of CFRP bonded structures.…”