Measurements of the residual displacement fields surrounding cold expanded holes were made by using several optical techniques for experimental strain analysis. The residual strain fields were computed from the displace- ment field data for two levels of cold expansion. The experimentally determined results were compared with the- oretical predictions of the displacement and strain fields. The results show that the presently available theories do not accurately predict the residual strains. This may be due, in part, to the three-dimensional nature of the split-sleeve cold expansion process. Finally, the results of this investigation are compared with the results of another investigator. HE presence of fastener holes in a loaded structural ele- ment has a limiting effect on the fatigue life of the struc- ture due to the stress concentration at the hole boundary. In order to minimize the detrimental effect of the stress concen- tration, it is common practice to prestress fastener holes by in- stalling interference-fit fasteners or by cold working the holes prior to installation of the fastener, thereby introducing resid- ual compressive stresses at the boundary of the hole. The re- sidual stresses are a result of plastically deforming the material surrounding the hole. The cold working process used in this investigation was a split-sleeve cold expansion process marketed by Fatigue Tech- nology Inc. of Seattle, Washington. In this process, a hard- ened, stainless-steel split sleeve is placed over a tapered man- drel, and the mandrel/sleeve combination is inserted into a hole (see Fig. la). The mandrel is then drawn back through the hole while the sleeve is retained in the hole by the nosepiece of the mandrel puller as indicated in Fig. Ib. As the major diame- ter of the mandrel engages the sleeve and the hole, interference among the mandrel, the sleeve, and the hole causes plastic de- formation of the material adjacent to the hole. The split sleeve serves two purposes. The sleeve is prelubricated and prevents galling of the hole wall since there is no movement of the sleeve relative to the hole wall during the expansion process. The mandrel slides along the inside of the sleeve, which has a molybdenum-disulfide lubricant on its inner diameter. The sleeve is discarded after it has been used. The presence of the split in the sleeve allows for the sleeve to be pushed past the major diameter of the mandrel before the mandrel is inserted into the hole, providing for a one-sided operation. This is im- portant from a practical standpoint. Because of the split in the sleeve, there is a small discontinuity along the hole wall where the split in the sleeve is oriented. Fatigue tests of split-sleeve cold expanded holes indicate that there is no measurable effect of the discontinuity on the fatigue life.1'2 The hole is post- reamed after cold expansion to ensure that the hole is circular before a fastener is installed.
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Link et al. (1990) studied this question.