Abstract
The main objective of this research is to investigate the effect of moisture on the degradation of adhesively bonded aluminium joints under both static and fatigue loading. This has been achieved through a combination of experimentation and progressive damage finite element modelling (using a cohesive zone approach). Moisture uptake behaviour in the adhesive was studied to obtain the coefficient of moisture diffusion and of moisture expansion. The coefficient of thermal expansion was also measured to provide the data for the finite element modelling, which included residual stresses due to cooling from cure temperature and swelling of the adhesive layer. The moisture dependent properties of the adhesive were obtained from bulk adhesive tensile tests. The joints investigated included monolithic single lap joints loaded in tension and laminated doublers loaded in bending and tension. Various widths were used to get the full and partial saturation of the adhesive layer. Under both static and fatigue loading, the degradation increased with increasing moisture content and tended to level out when the moisture content approached the saturation level. Most of the failures were cohesive in the adhesive layer, showing that the degradation was due to adhesive, rather than interfacial degradation. Calibration of the cohesive properties was achieved by combining backface strain and load data measured in the monolithic single lap joint. These were then utilised to predict the residual strength of the doublers in bending. In fatigue, the calibrated cohesive zone properties were integrated with a strain-based fatigue damage model to simulate the fatigue response of the monolithic single lap joint and doubler loaded in bending both in the unaged and aged condition. The backface strain technique has been successfully used to monitor the fatigue damage evolution in the joints considered, to calibrate the parameters in the strain based fatigue model, and also to study the effect of adhesive fillet on the fatigue damage evolution in the doubler loaded in bending. Doublers loaded in tension exhibited an entirely different failure mechanism including rupture of both the adhesive and aluminium layer. The effect of moisture on the degradation of this joint was not significant. The butts between aluminium sheets that inevitably exist in laminates were shown to affect the strength of the joint. For these joints, the progressive damage modelling used a cohesive zone approach for the adhesive layer and the butt, and continuum damage for the aluminium layer. In fatigue, the cohesive zone and the continuum damage were integrated with the strain-based fatigue damage model to predict the response. The predicted response under both static and fatigue loading was found to be in good agreement with the experimental data. Finally, experimental and numerical studies have been undertaken on hybrid fibre-metal (aluminium-Glare) laminate (FML) doubler joints to investigate their response under static and fatigue tension loading. The specimens had fibres either parallel to the loading direction (spanwise) or perpendicular to the loading direction (chordwise). Again, the effect of the butt position was investigated. The spanwise specimen was found to have the highest strength followed by chordwise specimens without butts and finally chordwise specimens with butts. The most critical position for a butt was found to be adjacent to the doubler end. Without butts, the static strength for spanwise and chordwise specimens was controlled by the failure in the Glare layer whilst the fatigue failure was precipitated by failure in the aluminium sheet. Where butts are present, they significantly influence the joint response. A progressive damage numerical analysis was undertaken and was found to be in good agreement with the experiment data in terms of both the strength and the failure mechanisms.