Issue 47

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 47 (2019) 303-320; DOI: 10.3221/IGF-ESIS.47.23 311 Fracture mechanics parameters versus crack length The whole set of the interference fringe patterns is the source of initial experimental data, which represent in-plane displacement components measured by electronic speckle-pattern interferometry in the vicinity of the crack tip. Availability of high-quality interference fringe patterns, which are free from rigid-body motion, serves as a reliable indicator of the real stress state near the crack tip. Some of these interferograms are presented in Figs. 1–4. Absolute values of in-plane displacement components u and v are determined in two specific points located at the crack borders immediately. The first point, denoted as n–1, lies in the beginning of each crack length increment. The second point n–0.5 is located in the middle of the crack length increment. Averaged crack mouth opening displacement (CMOD 1 n v    , n = 1, 2, 3) together with crack opening displacement related to half of the crack length (COD 0.5 n v    ) are obtained for symmetrically centred cracks in all specimens of both types. These data are essential for the transition from the initial experimental data to the required SIF ( n I K  , n = 1, 2, 3) and T-stress ( n T  , n = 1, 2) values by using the relationships of the modified version of the crack compliance method developed in [36]. Dependencies of CMOD, SIF and T-stress values from total crack length in specimens of both groups, constructed for different loading cycles, are shown in Figs. 5, 6 and 7, respectively. a b Figure 5 : CMOD values as a function of total crack length for specimens of T5-H1 (a) and T5-H2 (b) group. T-stress dependencies along the crack length shown in Fig. 7 are presented as straight lines. This is because of three crack length increments ( 1 a  , 2 a  and 3 a  ) enable 1 T  and 2 T  can be only derived from experimental data [36]. Relative arrangement of these straight lines is of importance for further analysis. Graphical information in Figs. 5a and 6a reveals indicators, which can be used for fatigue life assessment. Indeed, CMOD values 0 v   related to the first crack length are equal to zero for N = 5000 and N = 6000 cycles (Fig. 5a). The second crack length demonstrates negative CMOD values 1 v   for N = 5000 and N = 6000 cycles. Positive SIF value 2 I K  for N = 5000 cycles is less than positive SIF values at the other stages of fatigue loading (Fig. 6a). It is safe to assume that previous measurement point N = 4000 cycles corresponds to reaching 63% of lifetime. This fact will be confirmed later in the next section. Negative SIF value for N = 6000 cycles is related to 95% of lifetime. Note that no surface cracks were observed after applying N = 4000, 5000 and 6000 cycles. Fig. 5b demonstrates that CMOD values for cracks of 2 a  length, denoted as 1 v   , obtained for N = 3000, 6000, 9000 and 12000 cycles are practically the same. But, graphical information in Fig. 6b shows that SIF value 2 I K  for N = 9000 cycles is more than SIF values for the second crack lengths 2 a  at the other stages of fatigue loading. This indicator corresponds to reaching 57% of lifetime.