43 Tests on the Ductility of Stainless Steel and   Titanium Alloy Aerospace Fasteners  

Jarrod T. Whittaker * and D aniel P. Hess* 

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 compara ble size and tensile strength. A test procedure is  developed and tensile tests ar e 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 displacem ent  curve s and used to compute the ductility index for each test fastener . The ductility index quantifies the  relative ductility between the different fastener materials . The aver age 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 A28 6 test  fasteners fracture across three or four threads which corresponds to about a 45 degree angle. Both t he  relative ductility index values and fracture surface characteristics indicate much less ductility in the titanium  alloy fasteners . These result s 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.  

Introduction and B ackground  

Ductility is ge nerally a desirable characteristic for threaded fasteners since 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 failure.    Titanium alloy fasteners are currently avail able for aerospace applications . They provide lower weight and  better performance in extreme temperature than traditional aerospace fastener materials such as A286 stainles s 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 uncertainty 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 relative ductility . Tensile tests are performed on both titanium alloy and traditional aerospace fasteners  of comparable  size and tensile strength. Load versus displacement data are obtained. Elastic deformation and plastic  deformation at rupture are extracted from the data and used to com pute a ductility index [4- 6]. In addition, the  fracture surfaces of the different fasteners are examined and documented.    While it is found that titanium alloy fasteners are significantly less ductile than A286 stainless steel fasteners, these results are not intended to disqualify the use of titanium alloy fasteners . Rather, the test procedure  and/or resulting data from this work can assist in proper joint design with titanium alloy fasteners  for safe use.                                               * University of South Florida, Tampa, FL 

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016