Micro-slotting Residual Stress Measurement Technique for Understanding Fatigue Performance of Open-Hole Ti-6Al-4V Sample
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JMEPEG https://doi.org/10.1007/s11665-019-04312-8
Micro-slotting Residual Stress Measurement Technique for Understanding Fatigue Performance of Open-Hole Ti-6Al-4V Samples Elizabeth Burns, Joseph Newkirk, James Castle, and Jennifer Creamer (Submitted October 29, 2018; in revised form March 22, 2019) Micro-slotting, a microscale relaxation residual stress measurement technique, has been shown in recent years to be a reliable method for measuring local residual stresses in metallic materials. This study demonstrates the unique application of the micro-slotting residual stress measurement technique for measuring near-edge tangential residual stresses around cold-expanded holes in Ti-6Al-4V plates. Knowledge of the near-edge elastic strains induced by the hole processing, in combination with plastic strain information obtained using electron backscatter diffraction, allowed for interpretation of fatigue life differences and crack growth behavior between the as-drilled and cold-expanded conditions. The similar crack initiation lives of the as-drilled and cold-expanded open-hole coupons were attributed to the similar elastic and plastic strains present at the hole edges. The subsequent crack growth resistance observed for the cold-expanded holes was a result of the large compressive residual stress region induced by cold expansion. Keywords
residual stress, micro-slotting, fastener holes, cold expansion, drilling, fatigue, Ti-6Al-4V
1. Introduction Reliable measurement of near-edge residual stresses around fastener holes plays a significant role in successful fatigue life prediction of metallic airframe structures. Fastener holes act as sites of stress concentration and are prone to fatigue cracking in aging aircraft (Ref 1). Furthermore, the complexity of the drilling process results in numerous parameters that can affect fatigue performance (Ref 2). The process of cold expansion (Ref 3) is a widely used method for extending the fatigue life of fastener holes. The improvement in fatigue life is a result of the compressive tangential residual stress region around the hole that is introduced by the cold expansion process (Ref 4). The use of this process in the design stage, however, is limited by the ability to accurately measure this resultant residual stress distribution at the hole edge. Established residual stress measurement techniques are limited with regards to measuring residual stress around holes. X-ray diffraction (XRD) and neutron diffraction are nondestructive methods that can measure residual stress in two dimensions (Ref 5), yet both techniques are unable to resolve
Elizabeth Burns, Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO; and Boeing Research and Technology, St. Louis, MO; Joseph Newkirk, Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO; and James Castle and Jennifer Creamer, Boeing Research and Technology, St. Louis, MO. Contact e-mails: [email protected], [email protected]
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