Scribe Marks at Fuselage Joints: Determination of Residual Stress and Effects of Fatigue Loading Using Nanoindentation and Snychrotron X-Ray Diffraction

Abstract

Empirical understanding of fatigue crack growth from small defects is of tremendous importance and of significant concern for structural integrity of aerospace structures. In fatigue, a crack initiates from a stress concentration location and causes premature failure. In principle fatigue life for scribes and scratches is function of the stress concentration around the root which depends upon the depth and root radius of the scribe, the associated microstructure, the residual stress field, work hardening from plastic deformation during scribing and relaxation or redistribution of these residual stresses in fatigue. The scope of the present work is on determination of the residual stress field around scribe marks of different geometries and the effect of fatigue loading on the residual stress field. The determination of a local residual stress field in a small area of 100 μm x 100 μm around shallow scribes (<150 μm deep) is a very challenging engineering problem. Additionally, the large grain size of A1 2024-T351 (-20 μm), anisotropy between grains and texture makes further difficulties. The residual stress field associated with scribes of different geometries produced by different tools has been measured using synchrotron X-ray diffraction. It was found that some tools produce a severe tensile stress field and work hardening around the root of scribe as compared to the other tools which also produce a tensile residual stress field but without work hardening. Additionally, a method of extraction of residual stress has been developed using the nanoindentation load-displacement data. The method has been applied to extract residual stresses while taking into account the plastic deformation around scribe roots. The residual stress field of twenty nine scribes of different geometries produced from different tools has been determined from the nanoindentation method. Depending upon the tool and root radius, residual stresses were different in magnitude and were found in the range of +100MPa to +200MPa. The effect of fatigue loading on the relaxation of pre-existing residual stresses was studied. It was found that stress field around scribes is not affected by fatigue loading. It was concluded that fatigue life of these scribes should be examined with consideration of residual stresses. Any crack initiation and propagation model without consideration of residual stress fields may predict more conservative lives in fatigue