## 5 Conclusions  
According to the stepped-lap scarf joint repair for spar cap damage of a blade in service, two investigations were carried out in this study. The basic configuration test confirmed the sensitivity parameters of the spar cap stepped-lap scarf joint repair and the weak zone where damage was most likely to occur after the repair. The health monitoring test used carbon nanotube buckypaper sensors to monitor the failure of the stepped-lap joint surface. Under static and fatigue loads, the sensors could successfully capture the damage evolution and crack propagation signals. The experimental results provided an important parameter basis for evaluating the spar cap repair scheme and presented a promising method of health monitoring for spar cap after repair. 1. Three groups of test specimens with 64 different configuration parameters of stepped-lap scarf joint repair unidirectional tape were designed to carry out tensile tests. The results made clear the influence of the stepped-lap mode, stepped-lap joint surface number, and reinforcement layer on the failure load. The reinforcement layer could effectively improve the load-carrying capacity of the repair zone and it was confirmed that using N reinforcement layers to reinforce N stepped-lap joint surfaces could increase the failure load by 65-95% of the original value. This provided an important parameter basis of the repair scheme design for a spar cap with different damage degrees. 2. Under tension loading, the initial failure mode of the spar cap repaired using the stepped-lap scarf joint method was the delamination of the stepped-lap joint surface. The ultimate failure appeared as the shear failure of the adhesive layer on the stepped-lap joint surface and the coupling of the fiber fracture at the adhesively bonded joint. The stepped-lap joint surface was identified as the weak part of the spar cap repair and the healthy operation status of the surface should be monitored. 3. The quasi-static tensile test and fatigue test of the specimens that were repaired using the reinforcement layer of the double stepped-lap scarf joint method were carried out via embedding  
Figure 19: Responses of tension–compression fatigue: (**a**) resistance response of sensor L2-2 and (**b**) resistance response of sensor M2-2. the carbon nanotube buckypaper sensors on the stepped-lap joint surface. The sensors successfully monitored the damage and crack propagation signals. 4. The quasi-static tensile test for health monitoring confirmed the failure processes: the stiffness turning point, damage evolution, crack propagation, and fracture. The results provided an important basis for monitoring the failure on the stepped-lap joint surface with carbon nanotube buckypaper sensors and verified that the shear failure of the adhesive layer on the stepped-lap joint surface was the main failure mode of the repaired structure. 5. The fatigue test of the health monitoring successfully captured the progressive failure: the initial damage, damage accumulation, initial crack, and crack propagation to structural failure. The resistance change rate of all sensors increased with the increase of the fatigue cycle. The carbon nanotube buckypaper sensors could accurately detect the progressive damage of the specimen repaired using a stepped-lap scarf joint method. The results provided an important basis for fault judgment when carbon nanotube buckypaper sensors were applied to the health monitoring of a spar cap after repair. In this paper, the actual size of the spar cap with a large aspect ratio was considered in the basic configuration test, which provided important support for the numerical simulation analysis of the test model that followed. Although the health monitoring test is in the laboratory stage, this monitoring method can be applied to the spar cap repair of a blade in service in the field in the future, and a basis set of judgments for running a fault diagnosis on a blade can be formed as well.