PSI - Issue 42

D. Weiß et al. / Procedia Structural Integrity 42 (2022) 879–885 Author name / Structural Integrity Procedia 00 (2019) 000–000

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4. Experimental investigations and results Fatigue crack growth rate analysis is performed using an electro-mechanical tensile testing machine E LECTROPULS E10000 by I NSTRON . The crack growth rate is determined under Mode-I-loading for the same environmental conditions for both the extended Mini-CT-specimen as well as the CC-specimen. The experimental test setup consists of a tested specimen in which the crack grows and a reference specimen with the same initial crack length. Using the calibration function mentioned above, a PC with evaluation software as well as a measuring device called DCM-2 by M ATELECT LDT, the crack length a can be determined during the experiment, Richard and Sander (2012). The crack growth rate da/dN of the CC-specimen at a cyclic stress intensity factor value of Δ K = 20 MPam 0.5 should be compared with the value of the crack growth rate curve of the extended Mini-CT-specimen. Therefore, a compensating curve according to Forman and Mettu (1992) is used for the description of the entire crack growth rate curve by means of the following formula to obtain a compensation value for the crack growth rate:     =    1 1 − −        1 −  ,     1 −  ,     The parameters for the equation are illustrated in Table 1. The given parameters C FM , n , p and q in the Forman-Mettu equation are material-dependent values obtained from experiments, and in the included crack opening function  the crack closure phenomenon is described. Table 1. Parameters of Forman-Mettu equation. Parameter Value C FM 2.6 E-12 n 2.5 p 0.3 q 0.87 In Fig. 5, the mathematical description of the entire curve according to Forman-Mettu is emphasized by a yellow line. In a circle it can be seen that the crack growth rate of the extended Mini-CT specimen at Δ K = 20 MPam 0.5 is about 2.02 E−05 mm/cycle. This value is served as a comparative value for the tests with the CC-specimen to identify a possible influence of the outer forming area of the clinched joint on the fatigue crack growth rate.

Fig. 5. Crack growth rate curve of the base material and a compensating curve according to Forman-Mettu (yellow).