Fatigue Enhancement of Undersized, Drilled Crack-Stop Holes

Abstract

A common technique used to prevent the propagation of cracks in bridge girders is drilling crack-stop holes at the crack tips. By doing so, stress concentrations at the tip of the cracks are reduced and fatigue life of the bridge is extended. The size of the crack-stop hole is determined by utilizing known material properties and relationships developed through experimentation. However, these equations often result in a crack-stop hole diameter larger than can be practically drilled; therefore, physical limitations force crack-stop holes to be undersized in the field. To raise the effectiveness of the undersized holes to that of full-sized holes, a method is needed to strengthen undersized crack-stop holes. A similar problem was reported in the aerospace industry with fastener holes, which are potential sites for cracks to initiate and propagate. Static mechanical coldworking generated a great deal of interest in the 1970s and was among several processes that were investigated for improving fatigue life of fastener holes. Extensive literature exists showing that static coldworking of fastener holes can increase fatigue-life-to-failure by a factor from three to ten, depending on stress range. The purpose of this study is to develop a technique to improve the fatigue lives of undersized, crack-stop holes. The technique under development uses piezoelectric transducers operated at ultrasonic frequencies to improve upon recognized coldworking techniques. The piezoelectric transducers duplicate the residual compressive stresses produced by static cold expansion and hopefully change grain size. These residual compressive stresses act as a barrier to crack initiation by reducing the stress concentration at the hole. In addition, this new technique is expected to change the crystalline structure of the steel in the immediate vicinity surrounding the under-sized hole. It is thought that the excitation from the piezoelectric transducers will refine the grain size and create a more uniform grain size thereby improving fatigue performance. A tool is being developed that utilizes the piezoelectric transducers ability to convert electrical signals into mechanical work. Initially, the tool being developed is a small-scale laboratory device; once the technique is proven, it will be scaled up for use on full-scale bridge members under laboratory conditions. Lastly, a tool using the same technology will be developed for field application. The research includes a set of finite element models created to aid in tool design and to quantify and characterize the residual stresses surrounding the cold expanded crack-stop holes. Results were compared and agreed quite favorably to analyses found in past literature.