PSI - Issue 16

Ihor Dzioba et al. / Procedia Structural Integrity 16 (2019) 97–104 Ihor Dzioba, Sebastian Lipiec/ Structural Integrity Procedia 00 (2019) 000 – 000

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off the highest values and the normal direction in relation to the plane of the crack, these stresses are of decisive importance in the implementation of brittle fracture. In the known RKR criterion it is stated that cracking will be realized if the stress level exceeds some critical value over a length greater than the critical one (Ritchie et al. (1974)). Unfortunately, this criterion does not allow clear determination of the level of critical stress or the critical length. Because of this, many attempts have been made to modify it. Important assumptions which allowed more accurate determination of stress distribution in the area before the crack tip was formulated by Neimitz et al. (2007, 2010)). However, the questions of determining the critical level of stress and the length of the section are still open.

a

b

Fig. 7. The stress components (a) and plastic strain (b) distributions before the crack tip at T = – 120 0 C (where r is distance from crack tip)

On the basis of fractographic examination of the fracture surface for specimen tested at temperature of – 120 0 C, was found that fracture process has fully brittle character (Fig. 3a). So, in this case, the conditions required to perform the brittle fracture are met. Let’s consider the obtained stress and strain distributions shown in Fig. 7. Values of opening crack stress,  22 , just before the crack tip are higher than 1500 MPa and max. value reaches 1760 MPa at a distance of 0.15 mm from the crack tip (Fig. 7a). Level of effective plastic strain before the crack tip at a distance further 0.10 mm are low, lower than 0.02 (mm/mm). Only just before the crack tip, at a distance of less than 0.05 mm, the level of plastic strain increases to 0.025 – 0.15 (mm/mm). So, in the case of brittle fracture, there is a high level of normal stresses and slight strains in the area before the crack tip. Now let’s consider the case of mixed brittle-ductile fracture of the specimen tested at – 80 0 C (Fig. 3b). In this case more complex processes take place during fracture of specimen. Therefore, the evolution of the stress and strain fields before crack tip during loading the specimen was analyzed in several point according to marks (P1...P5) shown in Fig. 6b. The stress  22 and effective plastic strain distributions for respective load moments are presented in Fig. 8a and Fig. 8b. The level of stresses  22 growths with specimen loading. During specimen loading the maximum point of the stress  22 is increase reaches value of 1670 MPa and this point moves away from the tip of the crack to a distance of 0.5 mm. Whereas in the area directly before the crack, the level of stresses  22 decreases significantly, even to the value of 750 MPa. At the same time with the offset of the point of max stress  22 value, in the zone immediately before the crack tip the level of plastic strain increases and reaches value of 1.6. On the other hand, at the max stress  22 point the strain level is about 0.05, which is similar to the one in the specimens that broke completely brittle.