Abstract
The objective of this research was to find patterns of fatigue initiation in adhesively bonded structures. Fatigue initiation plays a very important role in the useful life of any structure, but it is a difficult phenomenon to quantify. Three types of aluminium-FM73M single lap joints (SLJ) were tested in tensile mode at different loads. The damage was recorded using the backface strain technique. Six strain gauges (SGs) were installed to record damage. Several types of tests were performed: tests to failure, tests to limited damage and sectioning, and tests to limited damage and a residual strength test. The tests to failure were performed to obtain backface strain patterns. The specimens tested to limited damage were sectioned, polished and inspected under a microscope to study how the damage affected the adhesive. The tests to limited damage, followed by a static test to failure, were carried out to find the effect of damage on the static strength. The load-life data obtained matched previous data well. Experimental tests found that the damage appeared in the fillet as a microcrack formation, merging at the end of the test into a major crack. This pattern depended on geometry and load. The residual strength tests in specimens with limited damage showed that the joints kept a significant proportion of original static strength, even if the joint had been damaged significantly. Numerical simulations were performed in ABAQUS to match and predict fatigue life and backface strain patterns at different loads. Fortran was used to develop damage models based on user-defined field subroutines. Two elastic damage models were developed (one and two phase), which reduced the elastic modulus as damage increased. A more complete elasto-plastic damage model was also developed. In this model the elastic modulus and yield stress were reduced. This gave good predictions of both fatigue life and backface strain patterns. This model can be used to determine fatigue lives in other bonded structures and represents an important step forward in this area.