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

269

5

The analysis of fracture in corresponding specimens showed that for both steels WPS does not influence the angle of crack propagation under mode II loading. Moreover, for steel 15Kh2NМFА(II), which offset yield stress is lower than for steel 15Kh 2МFА (II), plastic growth of the crack of approximately 6.5-7 mm similar to that one without WPS was observed at the temperature of -150 0 С and under mode I loading after corresponding WPS under the same mode. The analysis of the experimental data for steel 15Kh2МFА(II), which illustrates the influence of WPS under mode II conditions on the mode-I static fracture strength characteristics, showed that after WPS the value for K f is only 24% and 64% of the K Ic value at temperatures of -150 ° С and +20 ° С, respec tively. At the same time, for more plastic steel 15Kh2NМFА(II) WPS under mode I does not influence the values K f and K f = K IC . The difference between the characteristics of fracture toughness for the steels studied after WPS under mode II loading can be caused by features of deformation at the crack tip occurring under WPS conditions. At the cross section one side of the crack blunts, and maximum hydrostatic stress localizes in this region, whereas an equivalent plastic strain localizes on the sharpened side. The intensity of the processes can be different depending on the mechanical properties of the material. If residual asymmetrical blunting of one side of the crack dominates due to WPS, then the other side is characterized by the formation of the region of tensile residual stresses, which causes redistribution of stresses under repeated mode I loading and a decrease in the fracture toughness. If, on the other hand, strain processes dominate in the crack plane under mode II preliminary thermomechanical loading conditions, then it exerts a small influence on the mode II fracture toughness. 4. Numerical simulation of WPS Let us simulate the most unfavorable situation for decreasing the value K f under mode I at the temperature of -150 ° С after WPS under mode II for steel 15Kh2МFА(II). An attainment of the critical value σ c for the maximum tensile stress at a characteristic distance r c around the crack tip was taken as brittle fracture criterion at the temperature of -150 ° С (Ritchie et al. (1973)). The simulation was performed according to the following algorithm. At the first stage, the distribution of normal stresses at the crack tip under loading of the specimen in three-point bending to a level of K Ic = 40 МPа m 1/2 at the temperature of -150 0 С was determined . The characteristic distance ( r c =170 µm) was determined as a distance from the crack tip to the point, where normal stresses reaching the value for opening-mode cleavage stress in the crack plane. At the second stage, the effect of WPS was simulated: mode II loading (four-point bending) at a temperature of +180 ° С and, then, total unloading. At the third stage, mode I loading at the temperature of -150 ° С was conducted up to obtaining different levels of SIF and distribution of the maximum tensile stresses at a distance r c around the crack tip considering previous stress history was plotted, i.e. WPS under mode II (Fig. 4).

250

500 1000 1500 2000

1

200

1

 c

1/2 )

2

150

6

4 3

5

50 SIF K f (MPa m 100

 (MPa)

4

3

0

2

-500

-200 -100 0 100 200 300 400 0

360

180

288

72

0

 , 0

t, 0 С

Fig. 4. Distribution of stresses depending on the angle α in the layer at a distance r c around the crack tip, t = -150 0 C, mode I (a): 1 – without WPS, K Ic = 40 МPа m 1/2 ; 2-6 after WPS – К=2 2 МPа m 1/2 , 6 – К = 40 МPа m 1/2 .

Fig. 3. Temperature dependence of the fracture toughness under various schemes of deformation: 1, 2 – mode I; 3, 4 – mode II; , – WPS under mode II, mode II fracture; , - WPS under mode II, mode I fracture; – WPS under mode I, mode II fracture; 1, 4, , , – 15Kh2NМFА(II); 2, 3, , – 15Kh2МFА(II).