PSI - Issue 19

S. Periane et al. / Procedia Structural Integrity 19 (2019) 415–422 S.Periane et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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As around 8 samples were used in this method the cyclic load value for the first sample is given upto two third of the ultimate tensile strength of the sample. If the sample is broken for that particular value the next test is continued for lesser value, if not broken it will be continued with the value higher than the previous value. The figure 8 shows the values which are like a staircase, by doing subsequent tests the values converge at the end. The mean line which passes nearer through the maximum points is taken as the endurance limit for that particular condition. For example in dry condition the endurance limit is denoted as 1040MPa and in emulsion condition as 1030MPa below which the material will never break under 10 5 cycles. The fractographic images showed in figure 9 and 10 reveals the crack propagation mechanism along with the presence of defects. On close examination of the surface some defects present have been the crack initiation site due to maximum tensile stress acting on it. Apart from defects there are many other factors which affects the fatigue life of the sample which quite evident from the figure 9 (b). It clearly shows an inclusion of the base metal present inside a deep crater which disperses the single crack in many fragmented cracks around its boundary. These fragmented cracks again converge after passing the crater to a certain micrometer distance. In dry machined sample the fatigue crack initiated and propagated exactly at the middle of the sample, whereas for the emulsion machined sample it initiated at certain distance (4mm) away from the center of the sample. An overview of both the fractured surface of the sample shown in figure 9 (a) and 10 (a) has two distinctive zones 1 and 2 of failure which is similar in both the type of machined samples. Figure 9 (a) and 10 (a) shows the crack initiation and propagation to a certain distance in zone 1. Whereas in zone 2 the crack propagation literally stops and leads to sudden failure were the large high energy dimples are present. In the tests about 85% of the cycle time to failure is required to fracture the first zone and remaining 15% of the cycle time to failure for the second zone. 4.4.2. Fractographic analysis of fatigue fractured samples machined under dry and emulsion conditions

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(b)

2

(a)

(c)

Fig. 9. Fractured surface of the sample machined under dry condition

The fatigue fractured surface of the sample machined under emulsion condition figure 10 (a) has many deep craters along with many high energy dimples and shear bands were formed along the crack propagation direction.