Fatigue crack growth in pin-loaded cold-worked holes

An experimental program has been carried out for the evaluation of the influence of the split sleeve expansion process on the fatigue crack growth in 2024-T351 aluminium alloy specimens. In previous ICAF Symposia, some information has been already given about the first part of the experimental activity (Helsinki, 2015) and about the development of a numerical analysis method (Nagoya, 2017). Both such preliminary papers referred to open hole specimens, tested under Constant Amplitude loading, while recently a pin loaded hole configuration has been evaluated. The non-inspectability of the configuration required some particular experimental effort, in order to collect test results in terms of crack growth as a function of number of cycles. To this end, a marker load technique was adopted: a block of high R ratio cycles (R=0.9) was inserted in the R=0.1 sequence, with the aim of obtaining information about the crack front evolution at different number of cycles. Due to the three-dimensionality of the residual stress field, which is of lower intensity at the face of mandrel entrance, a 1 mm radius quarter-circular notch was inserted by means of EDM on the mandrel entry side face. Moreover, it was necessary to include also mechanically milled notches, of similar dimensions. The results show a rather regular front evolution, and have provided important material for the development of accurate numerical methods, based on the evaluation of the residual stress field and on the subsequent modification of the stress intensity factor distribution along the corner crack front. The numerical analysis methodology is a specialization of the technique, already presented in the Nagoya Symposium, to the problem of single or double corner crack in a pin loaded hole. In particular, the strong three-dimensionality of the stress field poses a challenge to the block-by-block propagation analysis.