The effect of manufacturing processes on the fatigue lifetime of aeronautical bolts

Many aeronautical fastners are exposed to cyclic stresses during service. T herefore, such parts are usually designed for limited fatigue lifetime. Var ious combinations of process type and sequence may be employed to produce t hreads, each resulting in different fatigue properties. Specifications of a ircraft bolts often require production of threads by heat treatment followe d by rolling, in order to improve the fatigue properties. Unfortunately, th ese specifications are not always followed to the letter. Therefore, for ei ther quality assurance or failure analysis purposes, it is important to be able to determine unambigiously the process by which threads were produced. The objectives of this work were to study the effect of varied thread manu facturing process type and sequence on the mechanical properties of AISI 43 40 stud bolts, and to develop a laboratory procedure for distinguishing bet ween them, Threads were produced on heat-treated and non-heat-treated stud bolts either by machining or cold-rolling. The non-heat-treated bolts were subsequently heat-treated. All bolts were then subjected to mechanical test ing (static tensile, dynamic fatigue, hardness and microhardness tests), me tallographic and fractographic examinations. While the fatigue properties w ere significantly affected by the manufacturing process used, no effects on the tensile strength of the bolt were observed. Metallographic inspection and microhardness testing, but not fractographic inspection, were found to be effective for distinguishing between different manufacturing procedures. (C) 2001 Elsevier Science Ltd. All rights reserved.