13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- fracture phenomena on fatigue life. Figure 12 : SEM micrograph showing a zoom in the centre of the fish-eye crack displaying the fractured nitride particle and the rough micro-propagation with crystallographic character I. Large nitride broken during loading (or during forging) The nitride cracking is dependent on the particle size and applied stress [9, 11]. As a consequence the distribution of the nitrides through the material plays a role on the fatigue life of a structure. The crack initiation always occurred on the largest nitrides for the present study when comparing according to surface characterization of the nitrides distribution realised (Table 3). II. Vacuum near threshold micropropagation with crystallographic character The question is if a cracked nitride leads to crack propagation from the first load cycle onwards. If the value of stress intensity factor generated by the cracked nitride is superior to the threshold value this will be the case. Based on the size of the particles found in the crack initiation site, the initial stress intensity factor range, ΔKini, can calculated for a penny shaped crack in a cylinder using the following formula [22]: inclusion a ini inclusion area K π σ 0,5 , = Δ (2) Where, σa is the stress amplitude. The initial stress intensity factor calculated from a nitride size is between 4MPa√m and 6MPa√m for different specimens displaying internal crack initiation. According to Lawless and King’s studies [23, 24], the stress intensity threshold in air for 718 DA, at 538°C and 550°C respectively, is about 8MPa√m. At 427°C in air [19] the threshold for a fine microstructure varies between 5MPa√m and 9MPa√m depending of the heat treatment and the stress ratio. This threshold decreases when the grain size decreases [19, 23]. These values have been found for crack propagation in air; the initiation of the internal fish eye crack occurs, however, in vacuum. Few studies about propagation threshold were performed in vacuum. In [25] the threshold seems to be closed from the one in air when extrapolating the given curve. Material 2 of this study has a small mean grain size (about 7µm) which can reduce the propagation threshold. In addition the residual stress around the particle may be in tension and lead to an effective stress intensity factor which is higher than the one calculated here which would facilitate the propagation. Furthermore most the threshold values given in the literature have been obtained for long cracks. However, for the fish eye crack initiation, the initial crack is the cracked nitride which is a short crack. It may be assumed that due to the short crack behavior crack progression may occur below threshold values obtained for long cracks [26]. The last parameter which can influence the initial stress intensity factor is the shape of the particle. The corners of the cubic nitrides may generate a stress intensity factor that is superior to the one obtained by the equation (2) for a penny shaped crack The propagation speed and consequently the fatigue life of this step is certainly low because of the
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