Issue 46

A. Baltach et alii, Frattura ed Integrità Strutturale, 46 (2018) 252-265; DOI: 10.3221/IGF-ESIS.46.23 257 Figure 7 : Comparison of the actual results of the residual circumferential stresses along the distance from the hole edge for: a) Entrance face, b) Exit face and c) Mid-thickness plane. Considering the problematic outlined in this paper, the side that interests us here is the entry of the mandrel. Because, it is established, unanimously that it represents the side where the residual stresses are the lowest, compared to all the hole thickness. Indeed, the results in Fig. 7a (Entrance face) indicates that the circumferential residual stresses at the hole edge are significantly lower than those in Fig. 7b (Exit face) and more lower than those of Fig. 7c (mid-thickness plane). Also, the circumferential stress distribution is not uniform throughout the plate thickness having a maximum compressive value around the mid-plane position and a minimum value at the pin entrance of the hole edge. This last finding agrees well with the literature, thus, the fatigue crack usually initiates from the mandrel entry side. This leads to the weakness of the structure by this side. This is confirmed by Figs. 8 and 9, where, examples of the residual stresses distribution in the vicinity of the hole edge are given. The iso-values clearly indicate that the residual stresses are no uniformly distributed around the hole and in its vicinity. This is more pronounced in the case of the expansion by the ball where tensile stresses are present in the vicinity of the hole at the entrance face. Figure 8 : Distribution of the circumferential residual stresses around the expanded hole for the case of the ball mandrel a) b) c)