Residual Strength of Glued-in Bolts After 9 Years In Situ Loading

In 1993 one of the wooden blades of the Nibe-B windmill was struck by lightning and subsequently demounted after 9 years of use. The mishap offered a unique opportunity to investigate the residual strength of the 28 glued-in bolts used to form the blade to rotor hub connection.The test method developed is presented along with the load history of the bolts, the recorded residual strength, the observed fracture modes and the obtained force displacement curves.

A total of 11 bolts were pulled out between undamaged bolts. The mean strength of these bolts was found to be 362 kN with a standard deviation of 37 kN. Comparing this mean residual strength with previously recorded test data reveals a 5% strength reduction. The force displacement curves shows no plastic capacity in the connections.

At fracture, a displacement between 0.4 mm and 1.0 mm was observed. In the majority of failures the bolts were pulled out like a cork from a bottle. This localised failure mode explains the surprisingly high mean residual strength of 304 kN of bolts pulled out between failed bolts.