FAILURE BEHAVIORS OF repaired COMPOSITE STRUCTURES

SummaryScarf bonding technique is increasingly used as a method of repairing thick composite structures, especially when flatness of repaired surfaces and high recovery rates of stiffness and strength are required. The partially damaged structure is first cut in form of circular or rectangular cutouts. A patch is then added to restore its shape and strength. Several types of bonding joints, such as over-lap, stepped-lap and scarf-lap, can be used to join the damaged part with the patch for repair. It is clear that understanding the failure behaviors and prediction methods of adhesively bonded joints is necessary in order to investigate those of the repaired structures. A schematic configuration of repaired laminates used in this study is shown in Fig.1.The aim of this research was to investigate the failure behaviors and to predict failure load of those scarf-repaired laminates. The specimens were manufactured and tested under in-plane tensile loading. Several repair parameters such as defect sizes, over-ply lap length and scarf angle were considered. Failure load and modes were recorded and investigated. A finite element analysis using damage zone method was conducted for the failure load prediction.

Fig. 1. Schematic configuration of repaired laminate used for tensile testingThe damage zone method bases on the assumption that failure will occur after an appropriate number, area or volume of elements failed in the finite element model. A random specimen was chosen as the reference and its experimental failure load was applied to the finite element model to evaluate the critical damage zone. This critical value was then used to predict the failure load of other repaired laminates.Two main failure modes were experimentally observed: cohesive failure of the adhesive layer and laminate failure in the parent laminates and the patch. The failure load of the specimens was reduced with the increase of scarf angle. Finite element models observed locally failed zone inside the specimen at experimental failure load. The predicted failure loads were compared to experimental results and a good agreement was obtained in virtually all cases of parameters.