AN INVESTIGATION ON THE GROWTH OF FATIGUE CRACK BETWEEN SEQUENTIALLY COLD EXPANDED ADJACENT CIRCULAR HOLES IN Al 7075 - T651 ALLOY

Critical structural holes located in close proximities are sequentially cold expanded one after the other in series to enhance their fatigue strengths by inducing beneficial residual stresses around hole regions. In some instances, where, several holes are closely located, the cold expansion-induced beneficial residual stress fields in the regions between the holes are considerably different in comparison to the case of cold expansion of a single hole that is free from a proximity hole. Therefore, an attempt is made in the present work to investigate the crack growth behavior in the residual stress field induced by the sequential cold expansion of closely spaced adjacent holes in typical aircraft-grade Al 7075-T651 alloy. In the present work, initially, Finite Element (FE) simulation on the sequential cold expansion of two adjacent holes in thin Al 7075-T651plate is carried out for 4% expansion level and resulting compressive residual stress fields around hole regions are predicted. Further, an experimental investigation on sequential cold expansion process is carried out using indigenously developed tooling set-up and Fatigue Crack Growth (FCG) behavior between cold expanded holes is measured through testing. The FCG measurement results indicate that crack propagation rate is higher between the cold expanded holes in comparison to the case of noncold expanded holes.