High-strength threaded fasteners are critical components in the assembly of virtually every form of transportation and industrial machinery. Industry-accepted standards for the measurement and performance of these critical parts have existed for decades, but recent failures of threaded fasteners in service and qualification testing are bringing new focus to the critical area between the head and shank of the fastener. This juncture has been highly analyzed in large-diameter (0.5 inch or more), high-strength, externally wrenched bolts (hex head, 12 point, etc.) used in automotive and aerospace applications, but less attention has been paid to the many bolts and screws below this threshold.
Recent qualification failures of M5 diameter fasteners made to a European standard have brought new focus on this issue. The aerospace industry has always been concerned with the weight of the airframe structure, and the thin materials used in airframe construction necessitate the use of 100-degree countersunk flush-head fasteners. These thin-profile heads—as well as other low-profile designs intended to reduce weight in aerospace, automotive and industrial applications—present unique challenges for the design of an effective torque transfer mechanism (internal or external) while still assuring head-to-shank integrity.