PSI - Issue 14

Sachin Bandgar et al. / Procedia Structural Integrity 14 (2019) 330–336 Sachin V Bandgar/ Structural Integrity Procedia 00 (2018) 000–000

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There are three basic factors necessary to cause fatigue: a maximum tensile stress of sufficiently high value, a large enough variation or fluctuation in the applied stress and a sufficiently large number of cycles of the applied stress. The process of fatigue consists of three stages:  Initial fatigue damage (stress concentrations) leading to crack nucleation and crack initiation.  Progressive cyclic growth of a crack (Crack propagation) until the uncracked cross section of a part

becomes too weak to sustain the loads imposed.  Final, sudden fracture of remaining cross section.

2. Experimental Procedure ASTM E647 standard is followed to perform FCGR test. The testing was done on make: MTS 100KN machine. The Compact Tension (CT) specimen is used for FCGR tests as shown in Fig 2. Precracking and actual FCGR test of the CT sample was done at 10 Hz. The maximum load for all specimens kept constant i.e. 12 KN with different load ratio 'R'.

Fig 2. CT sample drawing

3. Results and discussion 3.1. FCGR at load ratio of 0.1

FCGR test of steel A was conducted at constant load corresponding to ∆K value of 27 MPa*m 0.5 with a load ratio of 0.1. The data obtained from COD gauge and load cell were processed with available post processing software to generate da/dN vs ∆K. The plotted data of da/dN vs ∆K is shown in Fig 3. The data obtained was smoothened by sixth order polynomial and then fitted by power law to obtain 'm' and 'C' values. The 'm' and 'C' values are calculated from the graph is mentioned in the table 6. Similar procedure was followed for obtaining FCGR of steel B with a load ratio of 0.1. From the table it is evident that there is marginal change in the value of 'm' for steel A and steel B at load ratio of 0.1, whereas the value of intercept 'C' increased considerably for steel B. The FCGR at R=0.1 is higher in steel A than steel B for almost same value of crack length at different ∆K, which is shown in table 4 and 5. Similarly for R=0.5, FCGR for steel A is higher than steel B at different ∆K. Table 4. Steel A at R=0.1 ∆ K 30 35 40 45 50 da/dN(mm/cycle) a(mm) 2.04 x 10 -4 16.99 3.06x 10 -4 19.978 4.35 x 10 -4 22.43 5.96 x 10 -4 24.47 7.38 x 10 -4 26.12