PSI - Issue 13

Mirko Maksimović et al. / Procedia Structural Integrity 13 (2018) 1888 – 1894 Author name / Structural Integrity Procedia 00 (2018) 000–000

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3.2 Residual Life Estimation Here residual life of cracked structural element, Fig. 4, is considered: numerical and experimental. To determine computation of residual life for this structural element here is used equation (1) along mixed mode crack growth trajectory. For that purpose analytic formula for equivalent stress intensity factor K eq is necessary. For determination analytic formula of K eq here are used discrete values of the stress intensity factors along crack trajectory. Discrete values of SIF`s determined by FEM are given in Table 1:

Table 1: Discrete values of SIF`s along mixed mode crack growth trajectory a [mm] 6 4 2.5 2 1.43 K I [daN/mm 3/2 37.6 90.8 123.5 162.5 177 K I I [daN/mm 3/2 21 9.5 2.2 -1 2.5 θ 32 5.9 0.7 3.2 1.4

Using discrete values of SIF`s from Table 1 and relation for equivalent SIF in the next form

1/ 4

K eq = (6) we can obtain analytic formulae for the stress intensity factor along the crack growth trajectory, in accordance to Fig. 5, in the next form: K eq = -2E+07 a 3 + 820616 a 2 - 5217.1 a + 20.311 (7) in which a is the crack length along crack trajectory. To determine residual life of structural component under mixed modes crack growth here Paris`s low (1) and SED method (2) are used. Paris`s constants for considered steel (C 4732) are: C=0.00000000058, n=2.57. Specimens are tested under cyclic load of constant amplitude in which σ max =250 MPa and σ max =25 MPa. The crack length versus number of loading cycles is shown in Fig. 6. 4 I II K K      4 8

Fig. 6 Comparisons computation models with experimental crack growth trajectory

Experimental determined number of cycle before failure is N exp =32200 cycles as shown in Figure 6. Residual life estimation under mixed modes crack growth is computed from point a 0 to point a 4 in accordance Figure 5b. From Figure 6 is evident better agreement SED method than conventional Paris crack growth method with experiments.