PSI - Issue 59

V. Sidyachenko et al. / Procedia Structural Integrity 59 (2024) 265–270 V. Sidyachenko and V. Pokrovskii / Structural Integrity Procedia 00 (2019) 000 – 000

266 2

1. Introduction A large number of experimental data concerning the influence of warm prestressing (WPS) on the fracture toughness of heat-resistant steels is limited to studying the fracture strength by mode I (opening) mechanism, such as presented by Chell et al. (1981), Pokrovsky et al. (1994), Smith et al. (2004), Yasnii et al. (2010) and others. At the same time, in real structures crack plane orientation with respect to applied loads is random. That is why it is important to consider that WPS and subsequent in-service loading result in deformation of the specimen with a crack not under mode I, but under any mode of loading, including mixed modes of loading: I+II+III. In general, available data for reactor pressure vessel steels demonstrate that WPS (mode I loading) causes an increase in fracture toughness in the lower shelf of the temperature dependence. However, it is noted here that an increase in loading of the in-plane shear component (mode II) facilitates the transition from brittle to ductile behavior even at low temperatures (- 120ºC) , as demonstrated by Swanki et al. (1998). In these cases, WPS (mode I and II loading) does not influence the fracture toughness under in-plane shear conditions. When WPS is of mode II type, the mode I fracture toughness decreases by 60% compared to the initial value. When the specimen is reloaded in mode II, mode I and mode II pre-loading have negligible effects on the mode II fracture load, as shown by Ayatollahi et al. (2007). The use of a modified compact specimen with a slant crack permits a satisfactory description of experimental data on mode I+III static fracture toughness, including WPS, and by Pokrovskii et al. (2017) has been established that the longitudinal shear component reduces “the positive e f fect” of W PS. The aim of the study is to analyze the effect of WPS with mixed mode I+II+III on the fracture toughness at the corresponding mixed mode fracture and to study the effect of WPS (modes I and II of loading) on the fracture toughness of reactor steels in modes II and I, respectively. 2. Methodology 2.1. Experimental procedure The experimental investigations were performed on heat-resistant reactor pressure vessel steels 15Kh2NMFA(II), 15Kh2MFA(II) and 10G2NMFA. The mechanical material properties under study at a temperature of WPS (+325 ºC), at a temperature of reloading after WPS ( -12 0ºC) and at room temperature +20 ° C were obtained using the standard procedure on cylindrical specimens and are presented in Table 1.

Table 1. Materials properties Steel

Temperature ° С

Yield strength σ YS , MPa

Ultimate strength σ UTS , MPa

Elongation δ, %

Reduction in area Ψ, %

n strain hardening

325

929

1063 1157 1260

15.2 16.6 14.2 14.1 18.8 19.2 21.2 22.0 24.7

54.3 67.2 54.0 68.9 67.1 66.2 66.4 68.5 65.7

0.041

15Kh2MFA(II)

20

1100 1160

0.0384 0.0323

-120

325

590 636 970 416 430 645

668 735

0.129

15Kh2NMFA(II)

20

0.0309 0.0280

-120

1024

325

540 550 776

0.092 0.088 0.057

10G2NMFA

20

-120

To study the influence of WPS under mixed modes I+II loading, the specimen should meet the following criteria: implementation of the entire range of modes I+II loading, compactness and possibility of performing tests at high and low temperatures, ease of manufacture and possibility to initiate a crack under mode I loading. A prismatic specimen used for asymmetric four- or three-point bending meets these requirements.