crack-stop holes are often drilled undersized and left unreinforced. While undersized holes do improve fatigue life of a cracked structural member, it has been shown that various levels of cold expansion can increase fatigue life of an unreinforced crack-stop hole by an order of magnitude (6–14). The increase in fatigue life provided by cold expansion is a result of the three principal residual stresses induced by cold expansion: tangential, radial, and transverse. Among them, compressive tangential stress (also referred to as hoop or circum- ferential stress) is the major contributor to significant gains in fatigue life (9). Several techniques have been developed to cold-expand holes in metal structures, each having the common feature of inducing a layer of residual compressive stress around the outside of the hole. These compressive residual stresses are the direct result of forced, inelastic deformation of material around the circumference of a crack-stop hole. As a crack-stop hole is forced to expand through a mechanical process, yielding first initiates along the edges of the hole where stresses are highest. As further expansion is mechanically induced, the zone of plasticity spreads outward from the hole. Material beyond this plastically deformed region deforms elastically under applied stress. After the mechanically applied pressure or displacement is removed from the system, residual compressive stresses around the hole are created from the elastic rebounding, or “springback,” of the unyielded material surrounding the permanently deformed plastic zone (15). Figure 1 shows the level of residual tan- gential compressive stress that can be expected to develop around a mechanically expanded hole. A different technique, examined by Reemsnyder, involved the installation of high-strength bolts in crack-stop holes used to enhance fatigue performance (16 ). While the main focus of the study by Reemsnyder was the potential fatigue life improvement of previously cracked holes in riveted bridge connections, the study mentioned that high-strength bolts were installed in drilled crack-stop holes located a predetermined distance from the riveted connections (16). Cracks did not reinitiate from the crack-stop holes with the installed high-strength bolts; however, because fatigue life improvement of crack-stop holes was not the main focus of the study, no quantified fatigue life improvement was provided. In separate studies performed by Huhn and Valtinat (17 ) and Brown et al. (18), the influence of fully tensioned high-strength bolts on the fatigue life of bolt holes in slip critical connections was examined. According to both studies, tensioned high-strength bolts significantly increased fatigue life of the bolt hole plate. According to the authors, “this was due to the high pressure under the washers of the bolts. This high pressure gives a certain protection of the area around the hole, so that the stress distribution in the net section Development of a Technique to Improve Fatigue Lives of Crack-Stop Holes in Steel Bridges Josh S. Crain, Gary G. Simmons, Caroline R. Bennett, Ron Barrett-Gonzalez, Adolfo B. Matamoros, and Stanley T. Rolfe 69 A common technique to prevent the propagation of fatigue cracks in bridge girders is to drill crack-stop holes at crack tips. Stress concen- trations at the crack tips are reduced and fatigue life of the bridge is extended. The size of the crack-stop hole needed to prevent further crack growth is determined by using 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; physical limitations force crack-stop holes to be undersized in the field. To improve effectiveness of undersized holes to that of full-sized holes, a method is needed to strengthen undersized crack-stop holes. This study investigated the potential of a technique to improve the fatigue life of undersized, crack-stop holes. It uses piezo- electric actuators operated at ultrasonic frequencies to convert electrical signals into mechanical work. The technique produced residual compres- sive stresses of the same order of magnitude as those produced by static cold expansion. A suite of finite element models was created to quantify and characterize the residual stresses surrounding the cold-expanded, undersized, crack-stop holes. Results were compared with analyses in the literature. As a result of the relatively long propagation life between initiation of a fatigue crack and eventual failure, measures can be taken to retro- fit and preserve existing cracked bridge members if fatigue cracks are detected early. Several existing methods can retard or stop the propagation of fatigue cracks. These methods include repair welding or grinding of shallow cracks, metal reinforcements, adhesive carbon fiber reinforced polymer patching, altering connection details, and drilling stop holes at crack tips (1–5). The methods are attractive considering that the alternatives are to replace the cracked structural member or reduce external loads coupled with careful monitoring. The technique of drilling a hole at a crack tip is a well-known procedure used in everyday practice to enhance fatigue life of steel structures (2). The primary challenges associated with correctly applying this technique are that the theoretical size of the crack-stop hole is often too large for practical implementation in the field or the location is blocked by other members. To overcome these issues, J. S. Crain, G. G. Simmons, A. B. Matamoros, and S. T. Rolfe, 2150 Learned Hall; C. R. Bennett, 2155 Learned Hall; and R. Barrett-Gonzalez, 2124 Learned Hall, University of Kansas, 1530 West 15th Street, Lawrence, KS 66045. Cor- responding author: C. R. Bennett, crb@ku.edu. Transportation Research Record: Journal of the Transportation Research Board, No. 2200, Transportation Research Board of the National Academies, Washington, D.C., 2010, pp. 69–77. DOI: 10.3141/2200-09