Issue 48

K. Kimakh et alii, Frattura ed Integrità Strutturale, 48 (2019) 429-441; DOI: 10.3221/IGF-ESIS.48.41 430 Zhang and al [3] studied the fatigue behavior of FV520B-I steel for different surface roughness (Ra = 0.6 μm, Ra = 0.2 μm and Ra = 0.05 μm). For smooth specimens crack initiate from subsurface for the majority of specimens except for very high stresses, the cracks initiate from the surface. However, for rough specimens the stress concentrations are very important which promote the crack propagation. In fact, the fatigue lifetime decrease considerably. Surmarchai and al [4] analyzed the influence of surface condition on the fatigue lifetime of 7010 aluminum alloy tested in 4-point flexion. They explain the strong dependence between the surface condition and the fatigue propriety. Indeed, roughness streaks generate stress concentrations. Based on this phenomenon, a model of fatigue lifetime prediction has been proposed. For the residual stresses, they have the most predominant influence when the surface state varies between 2.5 μm and 5 μm. Guolian Liu et al [5] discussed the impact of milling cutting conditions on surface integrity and fatigue strength. The parameters studied are the cutting speed, the feed rate and the depth of cut. Three groups of test specimen were machined. For each group a single parameter were varied over five levels. They confirmed that the cutting conditions affect significantly the surface integrity, therefore the fatigue strength is affected too. It decreases with the increase of cutting speed. For a feed rate less than 0.1 mm/rev, the lifetime increases with the increase of feed rate. For a feed rate between 0.1 and 0.2 mm/rev the fatigue life increases but beyond 0.2 mm/rev, it decreases with the fast increasing of residual stresses. Indeed, the fatigue lifetime of the machined parts depends on the interaction between the different physical parameters involved. In 2005 Marbu and al [6] investigate the influence of surface state on fatigue behavior of aluminum alloy (7010-T74511). They developed a mechanical model to establish the physical origin of this influence. Alang [7] studied the effect of surface state on fatigue strength. The roughness was varied over three levels (1.77 μm, 2.88 μm and 5.48 μm) and the specimens were tested for rotating bending. He notes that for lifetimes less than 10 5 cycles, the surface roughness considered didn’t present a significant effect, but beyond 10 5 cycles, the specimens with a smooth specimens have a longer lifetime than the rough test specimens. Youngsik Choi and Khadija Kimakh and al [8, 9] analyzed the impact of feed rate on fatigue strength of the machined surfaces of steel. He concluded that the manufacturing process directly affects the microstructure, surface state and residual stresses. The modifications generated on surface or subsurface influence the fatigue behavior of the steel studied. Several studies didn’t establish correlations between cutting parameters and fatigue lifetime of mechanical parts. Few researchers are interested to this problem. Unfortunately, in their studies, the number of parameters has often been limited given the complexity of the problem [10, 11]. The aim of our study is to analyze the fatigue behavior of AISI 1045 carbon steel and to predict the fatigue limit for different surface roughness. But the most interesting is to propose a model that allows to have a mastered surface state from a chosen cutting parameters to generate mechanical parts with a better fatigue resistance. For this purpose, four batches of AISI 1045 carbon steel fatigue test specimens were machined with different cutting parameters generating different surface roughnesses. They were tested in uniaxial fatigue with a stress ratio of R = 0.1. The results obtained were presented as an S-N curves and the fracture surface were observed using a scanning electron microscope SEM. From these results the effect of the roughness on the fatigue lifetime has been demonstrated and Based on roughness profiles and an appropriate models the fatigue limit has been predicted. E XPERIMENTAL PROCEDURES Material he chemical composition was identified for different samples by spectrometric analysis. Tab. 1 summarizes the chemical composition of the AISI 1045 steel considered for our study. Table 1 : Chemical composition of the AISI 1045 steel. In order to determine the mechanical proprieties of the AISI 1045, tensile tests were carried out according to the ASTM E8 / E8M-13a [12]. Also hardness measurements were performed. Tab. 2 groups the mechanical properties obtained. T Steel Chemical composition (%) AISI 1045 C Mn S P Si Cr Ni Cu 0.42 0.72 0.02 0.04 0.19 0.09 0.11 0.17