Detailed strain field analyses of fatigue cracks in friction stir welded joints

Abstract

This paper presents the results from Fatigue Crack Growth (FCG) tests on Friction Stir (FS) welded aluminium alloys. To obtain more understanding how the reduced yield strength and the residual stresses in the FS weld affect the FCG behaviour, welds were rotated under an angle of 0°, 45° and 90° with respect to the applied load. The goal of this study was to observe the change in FCG direction due to the presence of the FS weld. A new measurement technology, Digital Image Correlation (DIC), was used to measure the strain field in the vicinity of the crack tip. The tests were highly successful resulting in changes in FCG direction and rate, all recorded by DIC. One specimen showed a fatigue crack which turned 90° upward, parallel to the applied load and continued to grow in this direction. Moreover the effect of yield strength and residual stress was measured successfully. The main conclusion is that the relation between the yield strength and residual stress exist, but that the microstructure in the FS weld has also an influence. © Springer Science+Business Media B.V. 2009.