TORQUE LIMIT FOR BOLTED JOINT FOR COMPOSITES PART A TTTC PROPERTIES OF LAMINATED COMPOSITES

The existing design code for torque limit of bolted joints for composites at Marshall Space FlightCenter is MSFC-STD-486B, which was originally developed in 1960s for metallic materials.The theoretical basis for this code was a simplified mechanics analysis, which takes into accountonly the bolt, nut and washers, but not the structural members to be connected. The assumptionwas that metallic materials would not fail due to the bearing stress at the contact area betweenwasher and the mechanical member. This is true for metallic materials; but for compositematerials the results could be completely different. Unlike most metallic materials, laminatedcomposite materials have superior mechanical properties (such as modulus and strength) in thein-plane direction, but not in the out-of-plane, or through-the-thickness (TTT) direction. Duringthe torquing, TTT properties (particularly compressive modulus and compressive strength) play adominant role in composite failure. Because of this concern, structural design engineers atMarshall are currently using a compromised empirical approach: using 50% of the torque valuefor composite members. Companies like Boeing is using a similar approach.An initial study was conducted last summer on this topic to develop theoretical model(s) thattakes into consideration of composite members. Two simplified models were developed basedon stress failure criterion and strain failure criterion, respective. However, these models couldnot be used to predict the torque limit because of the unavailability of material data, specifically,through-the-thickness compression (TTTC) modulus and strength. Therefore, the task for thissummer is to experimentally determine the TTTC properties. Due to the time limitation, only onematerial has been tested: IM7/8552 with [0