Issue 48

K. Kimakh et alii, Frattura ed Integrità Strutturale, 48 (2019) 429-441; DOI: 10.3221/IGF-ESIS.48.41 439 previous study [13]. Equation 4 express the model used which relates the roughness to the cutting parameters. Therefore, from this model, we could define the appropriate turning machining parameters to obtain the surface state required by the functionality of parts. Finally, through this model and the present study which partially predicts the fatigue limit, we can control the choice of machining parameters to obtain a part with a longer lifetime. 1 1 1 2 2 2 0 1 1 1 1 1 1 1 1 1 2 2 1 1 k k k k k k k k i i ij i j ii i ijj i j iij i j i i j i i i j i i j i k k iijj i j i j i y b b x b x x b x b x x b x x b x x                                             (4) with k : number of factors, in our case k=3. x 1 : tool nose radius rε, x 2 : feed rate f x 3 : cutting speed Vc. The coefficients b i , b ii , biij, b ijj and b iijj are: b 0 = 14.9 b 1 = 8.717 b 2 = -275 b 3 = -0.2619 b 11 = -2.286 b 12 = 153.2 b 13 =-4206.10 -5 b 22 = 1006 b 23 = 3.617 b 33 = 1254.10 -6 b 112 = -80.09 b 113 =-8399.10 -6 b 122 =-872.9 b 133 =-4879.10 -7 b 223 = -10.42 b 233 =-1463.10 -5 b 1122 = 442.4 b 1133 =3806.10 -7 b 2233 = 4214.10 -5 C ONCLUSION n this study, the adverse effect of the surface state on fatigue behavior of AISI1045 carbon steel were highlighted in the case of turning process. The fatigue resistance increases with the decrease of the surface roughness. In fact, the roughness is the series of geometric irregularities generating local stress concentrations and the crack initiation depends on the size of these irregularities. Based on the S-N curves obtained from fatigue tests, it was noted that for a stress amplitude of 225 MPa, the fatigue lifetime went up from 2.10 5 cycles for a surface roughness of Ra = 3.12 µm to 2.75 10 5 cycles in the case of Ra = 2.26 µm, to 6.5 10 5 cycles for roughness of Ra = 1.25 µm and for a surface roughness of 0.5 µm the number of cycles to failure reaches 1.6 10 6 cycles. Therefore, the fatigue lifetime increase with the decrease of surface roughness. Actually, the fatigue resistance of mechanical parts could be improved by improving the surface roughness. To achieve this interesting result, the cutting parameters should be controlled. In our case, a mathematical model were proposed that express surface roughness function of the cutting parameters. Besides this results, the fatigue limit were predicted considering the surface roughness defect. The model adopted present an interesting results as compared with experimental data. R EFERENCES [1] Novovic, D. Dewes, R. C. Aspinwall, D. K. Voice, W. and Bowen, P. (2004). The e ff ect of machined topography and integrity on fatigue life, International Journal of Machine Tools & Manufacture, 44, pp. 125–134. DOI:10.1016/j.ijmachtools.2003.10.018. [2] Campbell, F. C. (2008). Elements of Metallurgy and Engineering Alloys, ASM International. pp. 243-263. [3] Ming, Z. Weiqiang, W, Pengfei, W. Yan, L and Jianfeng Li. (2016). The fatigue behavior and mechanism of FV520B-I with large surface roughness in a very high cycle regime. Engineering Failure Analysis, 66, pp. 432-444. DOI: 10.1016/j.engfailanal.2016.04.029. [4] Suraratchai, M. Limido, J. Mabru, C. and Chieragatti, R.. (2008). Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy, International Journal of Fatigue, 30, pp. 2119–2126. I