COMPOSITE PATCH REPAIRS OF METAL STRUCTURES - ADHESIVE NONLINEARITY, THERMAL CYCLING, AND DEBONDING

Comparison with the experimental data has been carried out to determin e the ability of the finite element alternating method in predicting t he fatigue response of a cracked metallic panel with a partially debon ded composite patch, A total of 15 different specimens are considered in this comparison. Some of the parameters that are varied in these sp ecimens include the disbond location, the disbond area, the initial cr ack length, the maximum stress loading, and the stress ratio, For all of these specimens, it has been found that the numerical results corre late very well with the experimental data when the adhesive nonlineari ty is properly accounted for in the analysis, In addition to this comp arison with the experimental data, numerical studies have been carried out to examine the effect of the thermal cycling on the fatigue respo nse of a bonded repair, It was found that, due to the strong differenc e in the thermal expansion coefficient of the boron/epoxy patch and th e aluminum panel, the fatigue life of a specimen, which undergoes cycl es of high stress at low-temperature and low stress at high-temperatur e 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 spe cimen that undergoes mechanical fatigue loading at a constant temperat ure. In addition to this study, numerical analysis has been carried ou t to study the interaction between two nearby composite patches, The s tudy found very little interaction between the two patches when these two patches lie next to each other, with both the cracks under the pat ches being collinear and both being normal to the direction of the loa d, 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 bein g on the top of the other, both the cracks being normal to the directi on of the load, the fatigue life of this specimen can increase substan tially when these two patches are very close to each other; however, t his may lead to failure of the metal in between the patches, bat this is not considered in the present study.