Spectrum fatigue of composite bolted joints

The new Swedish fighter JAS39 Gripen has a large number of primary structures made of composites. On those structures a large number of bolted joints are used which during the aircraftʹs service life will be subjected to spectrum fatigue loading. Consequently it is important to study the spectrum fatigue life of bolted joints. Specimens with a double-lap configuration and six bolts have been fatigue loaded at the load ratios R=−0.2 and R=−5. Specimens were also fatigue loaded with a vertical fin spectrum which had different amounts of elimination of load cycles. A linear damage rule, Minerʹs rule, was used to predict the spectrum fatigue life. The experimental results show that the shortest fatigue life occurs for specimens loaded at R=−1 followed by specimens loaded at R=−0.2. The longest fatigue life occurred for specimens loaded at R=−5. It was found that 50% elimination of load cycles in the spectrum can be used without affecting the fatigue life. The Minerʹs rule predictions appeared to overestimate the spectrum fatigue life. From bolt failure it was found that the first bolt row transfers the largest amount of load in the specimens.