No abstract

Fracture tests were conducted on precracked panels reinforced with various crack stoppers. Motion pictures and continuous graphical records of load and local strains were taken during the tests. The purpose of these tests was to study variables affecting residual strength of reinforced panels. Results show that, for 2024-T3 aluminum skin panels reinforced with riveted or bonded flat straps made of various alloys, the residual strength increases with the product of reinforcement area and reinforcement strength. Reinforcement stiffness, which is of primary importance for other classes of panel configurations, was found not to be a significant variable for the panel configurations tested. A crack opening displacement model is proposed to illustrate the influence of the reinforcements, the skin fracture toughness, and the slow stable tear characteristics on the arrest of a stably propagating crack. The model helps to elucidate the interactions between skin variables and reinforcement variables.

The mixed-mode fracture behavior of shear panels is analyzed in this study to provide suitable fracture criteria for the prediction of the residual strength of crack panels under a combined tension and shear loading condition. Finite element models which incorporate a special crack tip element are utilized in the analysis to compute the values of KI and KII and the detailed stress distribution in the crack tip regions. For comparison purposes, analyses are made using the initial crack configurations of the shear panels and a number of selected configurations after a small increment of slow stable crack growth. Failure analysis is made using these results together with test results obtained in a previous experimental study by Liu. It is found that a simple failure criterion of KI/(KI)cr+KII/(KII)cr = 1 is applicable to materials that are not extremely ductile. However, the maximum tensile stress theory should be used to complement this criterion for the purpose of predicting the crack growth direction. The mixed-mode criteria of the strain energy density function and the “angular” stress intensity factor are also examined. In addition, it is shown that a predominantly Mode I condition is produced by oblique crack growth even after a relatively small crack growth increment. Hence, a pure Mode I fracture criterion can be used to predict the slow stable crack growth and subsequent failure of damage structures subjected to combined loads if a practical method can be found to estimate KI during the crack growth period. It should be noted that the present study is restricted to a monotonically increasing loading condition.