Ductility of Titanium Alloy and Stainless Steel Aerospace Fasteners
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TECHNICAL ARTICLE—PEER-REVIEWED
Ductility of Titanium Alloy and Stainless Steel Aerospace Fasteners J. T. Whittaker . D. P. Hess
Submitted: 28 July 2015 Ó ASM International 2015
Abstract This paper presents results from tests aimed to assess the relative ductility of titanium alloy Ti 6Al-4V and stainless steel A286 aerospace fasteners of comparable size and tensile strength. A test procedure is developed, and tensile tests are performed on test fasteners. All test fasteners fracture in the threaded region. Elastic and plastic deformation at rupture are extracted from the resulting load versus displacement curves and used to compute the ductility index for each test fastener. The ductility index quantifies the relative ductility between the different fastener materials. The average ductility index for the titanium alloy fasteners is about one-tenth the average value for the A286 fasteners. In addition, the fracture surfaces of the titanium alloy test fasteners fracture perpendicular to the axis of tensile loading, whereas the A286 test fasteners fracture across three or four threads, which corresponds to about a 45° angle. Both the relative ductility index values and fracture surface characteristics indicate much less ductility in the titanium alloy fasteners. These results are not intended to discourage the use of titanium alloy fasteners but rather to provide additional data for use in proper joint design when the benefits of lower weight or extreme temperature use are required. Keywords Fastener Bolt Ductility Titanium Stainless steel Aerospace Aviation
J. T. Whittaker D. P. Hess (&) Department of Mechanical Engineering, University of South Florida, 4202 E. Fowler Avenue, ENB 118, Tampa, FL 33620, USA e-mail: [email protected]
Introduction and Background Ductility is generally a desirable characteristic for threaded fasteners since elastic tensile stretch is essential for proper fastener function in a preloaded joint. As a result, traditional fastener materials are designed to provide sufficient ductility to help avoid abrupt and potentially catastrophic failures. Titanium alloy fasteners are currently available for aerospace applications. They provide lower weight and better performance in extreme temperatures over traditional aerospace fastener materials such as A286 stainless steel. However, engineers are reluctant to embrace this material due to uncertainty about its ductility and potential catastrophic failure that can result in brittle materials [1–3]. This concern is valid since fasteners by design are often used close to or at yield. Preload and external load uncertainties compound the problem. This concern was repeatedly raised during recent meetings of the NASA NESC Standard Development for Spaceflight Fastening Systems Team and is the premise for the work reported in this paper [4]. This paper reports on a series of tests performed to assess titanium alloy fastener failure characteristics compared to a more traditional aerospace fastener material with particular interest in quantifying r
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