T-bolt bearing strengths in composite blade applications

T-bolts have been extensively used to attach composite blades to rotor hubs in both tidal and wind energy applications. Fitting of t-bolts to the composite component is achieved through the intersection of an in-plane hole with a through plane hole, altering the geometry of the bearing interface. It is important to understand whether this geometry affects the laminates ultimate bearing strength. In this study, monotonic tests were made to measure the ultimate bearing strengths of pin loaded double shear and t-bolt loaded connections for E-glass / Epoxy laminates with a high percentage of reinforcement orientated to the load axis. The internal failure mechanisms at ultimate load are observed by Micro CT. Both fibre matrix shearout and delamination are observed prior to ultimate failure. This study demonstrates that the ultimate bearing strength of t-bolt loaded connection can be 20% lower than an equivalent double shear connection, and that for highly orientated laminates with specific reinforcement architecture the first visible damage occurs at a similar level to the generic Germanischer Lloyd design allowable for load introduction zones. Bearing pressure distribution was determined through radial strain measurement of a 1/4 strain gauge array applied below the bearing interface and was found to resemble a Gencoz distribution.