ORAL/POSTER REFERENCE: ICF100982OR GROWTH OF FATIGUE CRACKS FROM COLD EXPANDED HOLES M. J. Pavier, A. A. Garcia-Granada, V. D. Lacarac and D. J. Smith Department of Mechanical Engineering, University of Bristol, Queen's Building, University Walk, Bristol, BS8 1TR, UK ABSTRACT The cold expansion of fastener holes in aircraft components is a standard technique to improve fatigue life. However, there is uncertainty of the method for quantifying the improvement. In addition, there is concern that the beneficial residual stresses arising from cold expansion may relax due to creep, particularly in aircraft subjected to higher temperatures. This paper begins with experimental measurements and finite element predictions of cold expansion residual stresses and their redistribution after creep. The results of fatigue crack growth experiments are then presented, demonstrating the benefits of the cold expansion process, even when creep relaxation occurs. Finally, a comparison of the fatigue crack growth rate is given with a prediction using base line data combined with the finite element calculation of residual stress. KEYWORDS Fatigue, residual stress, cold expansion, finite element analysis INTRODUCTION Cold expansion of holes in aluminium aircraft structures is commonly used to improve fatigue life [1]. When airframes are subjected to higher temperatures there is concern that the beneficial residual stresses resulting from cold expansion may relax due to creep. Predictions and measurements of residual stresses caused by the cold expansion process have been made [2, 3] and more recently the effects of creep relaxation have been investigated [4]. In this paper, experimental measurements and finite element predictions are presented of residual stresses in cold expanded holes, before and after creep relaxation. During creep relaxation, additional tensile load was superimposed to simulate the conditions in the aircraft. Experimental measurements of residual stress used a new method based on Sachs’ boring to measure the average tangential residual stress. Axisymmetric and three dimensional finite element analyses were used to predict the stress distribution through the thickness of the component. The results of fatigue crack growth experiments for cracks growing from cold expanded holes are also presented. Comparisons of experimental crack growth with predicted growth using base line data and finite element residual stress predictions are made.
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