Repairs based on adhesively bonded fiber reinforced composite patch are more structurally efficient and much less damaging to parent structure than traditional repairs based on mechanically fastened metallic patch. Because of the high reinforcing efficiency of bonded patches, fatigue crack can be successfully repaired. Adhesively bonded composite patch repair technique has been successfully applied to military aircraft repair and expanded its application to commercial aircraft industry. In addition, this technique has been expanded its application to the repair of load bearing primary structure from secondary structure repair. Therefore, a through understanding of crack growth behavior of thick panel repaired with bonded composite patch is needed. We investigated the fatigue crack growth behavior of thin and thick panel repaired with bonded composite patch using the stress intensity factor range ( K) and fatigue crack growth rate (da/dN). The stress intensity factor of patched crack was determined from experimental result by comparing the crack growth behavior of specimens with and without repair. Also, by considering the three-dimensional stress state of patch crack, three dimensional finite element analyses were performed to obtain the stress intensity factor of crack repaired by bonded composite patch. Two types of crack front modeling, i.e. uniform crack front model and skew crack front model, were used. The stress intensity factor calculated using FEM was compared with the experimentally determined values. The fatigue lives were predicted using the various averaged stress intensity factors by FEM and compared with the experimental values.
When adhesively bonded composite patch repair are applied to primary structures, it is needed to demonstrate that its loss can be immediately detected. This approach is based on the "smart patch" concept in which the patch system monitors its own health. Based on the smart structure concept, optical fiber sensors have been increasingly applied to monitor the various engineering and civil structural components. These fiber optic smart structures allow engineers to add nerve systems to their designs, giving structures capabilities that would be very difficult to achieve by other means, including continuous assessment of damage processes. Several studies associated with crack monitoring using optical fiber sensors have been reported. In this study, we used recently developed Transmission-type Extrinsic Fabry-Perot Interferometric (TEFPI) optical fiber sensors for the monitoring of fatigue crack growth behavior of cracked thick aluminum plate repaired with bonded composite patch. The TEFPI optical fiber sensor has both the advantages of reflection-type EFPI optical fiber sensors and a simpler and more effective function to distinguish strain direction than do reflection-type EFPI optical fiber sensors. The objective of this study is to evaluate the potentiality of the application of TEFPI optical fiber sensors to the monitoring of the fatigue crack growth behavior of composite patch repaired structures. The sensing principle and the senor construction of TEFPI optical fiber sensor are presented. The experimental results from fatigue tests of center cracked tension (CCT) aluminum specimens repaired with bonded composite patch are presented and discussed. TEFPI optical fiber sensors are embedded and surface bonded to the composite patch at several locations. The experimental results show that it is possible to monitor the fatigue crack growth behavior of structures repaired with composite patch using TEFPI optical fiber sensors.
