Composite patch repairs of metal structures: Adhesive nonlinearity, thermal cycling, and debonding

Comparison with the experimental data has been carried out to determine the ability of the finite element alternating method in predicting the fatigue response of a cracked metallic panel with a partially debonded composite patch, A total of 15 different specimens are considered in this comparison. Some of the parameters that are varied in these specimens include the disbond location, the disbond area, the initial crack length, the maximum stress loading, and the stress ratio, For all of these specimens, it has been found that the numerical results correlate very well with the experimental data when the adhesive nonlinearity is properly accounted for in the analysis, In addition to this comparison with the experimental data, numerical studies have been carried out to examine the effect of the thermal cycling on the fatigue response of a bonded repair, It was found that, due to the strong difference in the thermal expansion coefficient of the boron/epoxy patch and the aluminum panel, the fatigue life of a specimen, which undergoes cycles of high stress at low-temperature and low stress at high-temperature loading, is dramatically reduced, It was also found that the fatigue life of a specimen that undergoes a thermal-mechanical-fatigue cycle is more sensitive to disbonds in the adhesive layer than a similar specimen that undergoes mechanical fatigue loading at a constant temperature. In addition to this study, numerical analysis has been carried out to study the interaction between two nearby composite patches, The study found very little interaction between the two patches when these two patches lie next to each other, with both the cracks under the patches being collinear and both being normal to the direction of the load, On the other hand, when the two patches lie vertical to each other, i.e., when the two cracks under the patches are parallel and one being on the top of the other, both the cracks being normal to the direction of the load, the fatigue life of this specimen can increase substantially when these two patches are very close to each other; however, this may lead to failure of the metal in between the patches, bat this is not considered in the present study.