The stress intensity factor for a cracked stiffened sheet repaired with an adhesively bonded composite patch

The problem of a cracked, stiffened metallic sheet adhesively bonded by a composite patch is analyzed. The composite patch is assumed to be either an infinite orthotropic sheet or an infinite orthotropic strip normal to the crack. Due to the high stress concentration around the crack and on the interface, an elliptical disbond is assumed to exist around the crack. The crack is asymmetric with respect to the stiffener's locations as well as to the patch's center. The effect of thermal stresses in curing process is also considered. The fracture problem is solved by the displacement compatibility method, using the complex variable approach and the Fourier integral transform method.The problem is dealt with in two steps. First, starting with an uncracked, patched stiffened sheet, the stress at the prospective location of the crack is determined in a closed-form solution. The second step is to introduce a crack into the stiffened patched sheet. The multivalue of the analytical formulation is treated in detail to ensure proper implement in the computer. The results show that the effect of the stiffeners on the stress intensity factor is not significant for a crack fully covered by a patch.For the repairs by Boron/Epoxy patches, the difference in KI between the infinite sheet patch and the infinite strip model is only minor (less than 5 percent) in the absence of the curing thermal stresses and it becomes more pronounced when these stresses are taken into consideration. The stress intensity factor for a crack repaired by an infinite composite strip also can be estimated with a good or reasonable accuracy via a simplified analysis in which the patch is considered as an infinite strip in the first step and is treated as an infinite sheet in the second step of the solution procedure mentioned above.The latter simplified analysis is based on the approach originally proposed by Rose for a relatively simple repair configuration. For most cases, that approach seems to work well for the repair of a stiffened sheet by an infinite composite strip with the effects of thermal stresses and a disbond included. It should be emphasized that the present methodology can apply to the problem of a crack in a metallic stiffened sheet growing beyond the patch's boundary and also to the repairs by an infinite adhesively bonded composite strip parallel to the crack.