Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024 T3 alloy – An experimental study H.D. Gopalakrishna * , H.N. Narasimha Murthy, M. Krishna, M.S. Vinod, A.V. Suresh Department of Mechanical Engineering, RV College of Engineering, Bangalore 560 059, Karnataka, India article info Article history: Received 5 May 2009 Accepted 17 August 2009 Available online 20 August 2009 Keywords: Cold expansion Fatigue life Residual stresses Taper pin Ball mandrel abstract This paper presents the experimental results of fatigue life enhancement and the residual stresses around the cold expanded holes in Al 2024, a widely used aerospace alloy. Two techniques for cold expansion of holes, namely split-sleeve with taper pin technique and split-sleeve with ball technique were considered for comparison, as the former involves surface contact and the latter has line contact during expansion. The techniques were com- pared based on the fatigue life enhancement in the expanded holes, the induced and the residual stresses due to expansion. The holes were expanded by 2%, 3%, 4%, 5%, and 6% using INSTRON machine in both the techniques. While both the techniques resulted in improvement in fatigue life of the expanded holes, the taper pin technique yielded 200% higher fatigue life improvement than that obtained by ball technique. The induced residual stresses were measured by mounting strain gages of 0.2 mm gage length. These are drawn as a function of induced strain. In both the techniques residual stresses increased with increase in percentage of expansion until 5% and then decreased for 6% expansion. The increase in fatigue life at 5% expansion was 1.88 times and 5.3 times higher than that of the non-expanded holes for ball and tapered method, respectively. The corresponding improvement in taper method was greater than the non-expanded holes. While, it was observed that the residual stresses decreased with respect to the distance from the hole in both the techniques, the ball technique resulted in lower residual stresses than that of taper pin technique. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Fatigue performance of structures is greatly affected by the presence of stress raisers such as fastener holes, manufactur- ing errors, corrosion pits and maintenance damage which serve as nucleation sites for fatigue cracking when the crack length reaches a critical dimension. From economic point of view, a costly component cannot be retired from service simply on detecting a fatigue crack [1]. Also, demand for longer service life and the acceptance of pre-existing flaws have encouraged the aircraft industry to enhance the fatigue life by giving controlled yielding on the holes by introducing compressive resid- ual stress [2–5] with cold expansion process. Of the several existing techniques, the split-sleeve cold expansion method has been widely used in the aerospace industry [4–9]. When the mandrel is removed, the material which is elastically deformed springs back from the expanded state and influ- ences even the material in the elasto-plastic region to contract [10,11], which results in the compressive residual stress accu- mulation at the plastic zone (i.e., near the hole). Hence there are greater compressive residual stresses near the hole and reduces away from the hole. And later becomes constant to indicate the non-effect of expansion at that distance. The induced 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.08.002 * Corresponding author. Tel.: +91 98867 35465; fax: +91 80 2860 2148. E-mail address: gopalakrishna.hd@gmail.com (H.D. Gopalakrishna). Engineering Failure Analysis 17 (2010) 361–368 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal