Issue 46

A. Baltach et alii, Frattura ed Integrità Strutturale, 46 (2018) 252-265; DOI: 10.3221/IGF-ESIS.46.23 258 In contrast, the iso-values of Figs. 9 show a significant improvement in the residual stresses distribution. Indeed, at first, one can note the vanishing of the tensile stresses (positive values) through the hole edge which become compressive stresses (negative values). More even, a decrease of the difference between the induced residual stresses in the entry side and in the exit side of the hole is obtained. The residual stresses in the middle planes remain the higher, compared to the two borders (entrance and exit). Figure 9 : Distribution of the circumferential residual stresses around the expanded hole for the case of the tapered pin mandrel Subsequently, we present the distribution of residual stresses for the cases studied in this work in order to analyze the effect of the shape of the mandrels used in the cold expansion simulation. Expansion with a ball After cold expansion with a ball, tensile stresses were obtained surrounding the entrance and the exit of the expanded hole. Fig. 10 compares the residual stress resulted through the plate thickness. This latter shows that, difference appears at the entrance face. In fact, positive values of the circumferential stresses surrounding the entrance location are present (Fig. 10a). Then, if present, far away the hole edge, the compressive circumferential stresses are insignificant (Fig. 10a). However, the compression zone is significantly reduced, where the residual stresses remain unchanged at the edge of the hole. Fig. 10d; compare the radial stresses obtained through the plate thickness. Note that lower the tensile radial stresses on the edge of the hole. Indeed, one can observe that the radial stresses are completely compressive (Fig 10b) with a peak compressive value up to -400 MPa in the inlet face. Figure 10 : Distribution of the residual stresses trough the thickness for the ball expansion technique: a) circumferential stress, b) radial stress. Expansion with a tapered pin In this section, all results are referred to the different taper used in this work as  =11.5 %,  =7.667 %,  =3.883 %,  =2.775 % and  =2.3 %. So, The results obtained using 11.5 % tapered pin model are presented in Figs. 11a and 11b for circumferential and radial residual stresses, respectively. Fig. 11 highlights the effect of the mandrel geometry on the radial and the circumferential stress components compared to the ball mandrel expansion technique. Fig. 14a reveals the presence of tensile stresses at a) b)