PSI - Issue 23

Sergiy Kotrechko et al. / Procedia Structural Integrity 23 (2019) 413–418 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Based on these ideas a technique was proposed for the experimental determination of the magnitude of threshold stress of structural steels th  [Kotrechko (2013)]. According to this technique:

2 0 75 0 95 NF     . ) ( . , th

(6)

where the coefficient 0.75 corresponds to low strength steels, and the coefficient 0.95 – to high-strength ones; 2 NF  is the nominal fracture stress of notched tensile specimens at 2% average strain. For experimentally determining the value of 2 NF  it is enough to

1000 1100

test 5-6 notched tensile specimens at the boiling point of liquid nitrogen (-196 °C ) and to find the value of nominal fracture stress NF  at an average plastic strain in the notched section that is equal to 2%. 2. Results and discussion Experimental studies were carried out on reactor pressure vessel (RPV) steel and cast low-alloyed manganese steels. Determination of the threshold stress values th  for these steels was performed according to the technique described above. To ascertain the parameters of Weibull distribution, standard pre-cracked specimens CT-1T made of RPV steel and 1T SENB specimens made of cast low alloyed manganese steel were tested. The values of the shape parameter m and scale parameter u  were determined by a calibration procedure. For this purpose, the experimental values C K I of the investigated steels were used (Fig. 3 ( a and b )). The values m and u  were ascertained for the Weibull three- and two-parameter distribution, i.e., taking into account th  and at its zero value (see the table). In addition, the values c  and  were determined. In this case: 13 3 c 4 10 m     , 0 024 .  MPa -1 . Probability of the cleavage initiation for the finite element was taken to be zero, if the value of  , calculated from (2), became less or equal than 0.

S f

200 300 400 500 600 700 800 900

R MC

Stress, (MPa)

 Y

20 40 60 80



50 100 150 200 250 300 0

Reduction in area, (%)

Temperature, T (K)

Figure 2. Temperature dependence of the main mechanical characteristics of iron: MC R is the brittle strength; f S is the fracture stress, Y  is the yield stress,  is the reduction in area.

Table. Mechanical properties and values of the Weibull distribution parameters

Three-parameter Weibull distribution

Two-parameter Weibull distribution

Y  ( МP a)

ul  ( МP a)

u e (-)

th  ( МP a)

 (-)

Material

m

m

u  ( МP a)

u  ( МP a)

RPV steel Cast steel

610 319

714 481

0.07 0.26

0.75 0.74

1100

6835 3700

5.4 8.0

4895 3800

12.2

720 13.3 Here Y  is the yield stress; ul  is the ultimate strength; u e is the uniform elongation;  is the reduction in area; th  is the threshold stress; m and u  are the shape and scale parameters (values Y  , ul  , u e and  are given for room temperature). According to the data obtained, accounting for the threshold stress th  give rise to a significant (1.5 – 2 times)