Issue 48

M. Laredj et alii, Frattura ed Integrità Strutturale, 48 (2019) 193-207; DOI: 10.3221/IGF-ESIS.48.21 200 constant hole diameter d = 5.8 mm. The authors concluded that the increase of thickness leads to an increase of residual compression stresses at the edge of the hole. In the following the influence of the (Ce) on the CRSEH is investigated. This study was carried out for an expansion degree ranging from 2 to 6  . Figure (6-b) illustrates the variation of the CRSEH as function of the (Ce). We found that, the stresses decrease with the increase of the expansion degree (absolute value). Moreover, the effect of yield strength according to the variation (CRSEH) is analysed and illustrated in figure (6-c). This figure shows that an increase in the yield strength reduces the CRSEH after expansion. The process is valid between [240- 340 MPa]. However, in the range [340-430 MPa] the (CRSEH after the expansion increases slightly depending on the value of yield strength. It should be noted that no experimental or numerical study had examined the effect of the degree of expansion and the yield strength on CRSEH. Regarding the second factor (MCRS), which represents an important parameter for the residual stress profiles, we studied the variation this factor with respect to the (t), (Ce) and (Ys). The analysis results are illustrated respectively in figures (7-a), (7-b) and (7-c). We find that an increase of the thickness and the degree of expansion lead to reducing the maximum compressive residual stresses (in absolute value) which is less marked in the case of the degree of expansion. Hence, the increase of these parameters leads to the increase of the lifetime of the structure. These results are in agreement with the findings research of Z. SEMARI and J. VOGWELL [29-2] concluding that the compression stresses are inversely proportional to the degree of expansion. Unlike the other effects, we found that the increase of the yield strength causes an increase of the MCRS (in absolute value) which confirms the conclusions of others authors [30 .1], performing several tests and simulations on Aluminium alloys and some steels . Figure 7 : The variation of the maximum compressive residual stress after expansion In the following section, we have studied the effects of the (t), (Ce) and (Ys) on the depth variation of the maximum compressive residual stress. The examination of the figure (8- a), permit us to notice that the thickness has a minor influence on the compressive residual stress. In fact the increase of thickness by 133 % leads to an increase of 7.05% in the (DMCRS). The Figure (8-b) shows clearly that the degree of expansion has a higher influence on DMCRS. According to the figure (8- c), we notice that the increase of the yield strength leads to a decrease of DMCRS. In this part, we are interested in studying the evolution of other important parameter on residual stress profiles which is the compressive residual stresses zone. The results in the Figure (9-a) indicate that an increase of the thickness leads to an increase of CRSZ. Consequently, the use of thicker structure conducts to an important improvement of lifetime. Therefore we can also indicates that there exist a critical thickness (t = 4.93 mm) beyond which the CRSZ is almost independent of this geometrical parameter. The results obtained are consistent with the work of V. NIGRELLI, A.T. ÖZDEMIR and [28-10]. Regarding the degree of expansion, we notice that it had an important effect on the compressive residual stresses zone. These results are in agreement with the results of A.AMROUCHE and M. SU [32, 31] who found that the degree of expansion has a significant influence on CRSZ. Also, the authors had shown, based on experimental measurements and finite element numerical calculations study that the growth of CRSZ stops at Ce = 5%. It can be interpreted as the convergence difficulty of 3D calculation. On the other hand, beyond the degree of expansion value 4 the influence of the