PSI - Issue 20

N.A. Makhutov et al. / Procedia Structural Integrity 20 (2019) 63–74 N.A. Makhutov and V.V. Zatsarinnyy / Structural Integrity Procedia 00 (2019) 000–000

70

8

When calculating the ψ k ψ coefficient was set as 1 in formula (6); it contributes to the safety factor. In a similar manner the basic strength and plasticity characteristics were calculated for the temperature of -85 o С and probability of Р=0.01. Numerical values amounted to σ т t =566 MPa, σ в t =712 MPa, σ в t =MPa, Sk t = 1267 MPa and ψ k t = 55.7%, correspondingly. This required to fully repeat the specified calculation procedure having introduced the new initial data σ т , σ в , ψ k and S k at the temperature of 20 o С with the probability of Р= 0.01 and new coefficients ß т и ß в into the equations (2 – 6). Table 4. Comparison of average experimental and calculated characteristics of the mechanical properties of steel 15H2NMFA depending on temperature. Type of characteristic σ в, MPa σ т, MPa S k , MPa ψ k, % Experimental 20 0 С 350 0 С 20 0 С 350 0 С 20 0 С 350 0 С 20 0 С 350 0 С 690 595 558 433 1648 1131 78,3 66,6 Calculated - 632 540 481 1446 1370 - 83,3 Error Δ, % - +5,8 -3,2 +9,9 -12 +17 - +20 Note . The “Error” row represents the difference between calculation and experimental calculation values ascribed to calculation values. Average values (at probability Р=50%) calculated for characteristics σ т , σ в , ψ k and S k for specified temperatures are given in the Table 5. t cross-sectional narrowing for temperature –85 o С the n Knowing the average values for these basic characteristics the theoretical distributions can be built. For this, the following procedure was proposed. First, a normal distribution function is built for each of the characteristics at room temperature with known experimental variation coefficient, for example, v σв exp = 0.0266. Then, based on the σ в0,5 calculated average value for temperature of -85 o C a second approximate distribution is built with the same variation coefficient. It is apparent that the true calculated distribution at the temperature of -85 o C must have a different variation coefficient, which additionally factors in the effect of temperature. If it is assumed, that during the static rupture of a specimen at room temperature the uncertainty of temperature measurement appears due to the fact that (in the local zone) the temperature may increase by approximately +3…5 o С, the variation coefficient will amount to υ t =5/293≈0.02. When this in the case, it is suggested that the summarized calculated variation coefficient, which includes the initial experimental variation coefficient and variation coefficient conditioned by υ t change can be expressed as Stepnov and Zinin (2016) did: ����� � �� � � ���� � � � � (8) After that, the redefined theoretical normal distribution of strength characteristic σ в is built with new variation coefficient for temperature of -85 o C. Additionally, an alternative formula was proposed for the ψ k variation coefficient of cross-sectional narrowing: Table 5. Calculated values (average) of the mechanical properties of steel 15X2NMFA. t 0 ,C T, 0 К Т= t+273  т t , MPa  в t , MPa S k t , MPa ψ k t , % +20 +80 - 85 293 353 188 540 520 610 689,7 1446 1422 1521 78,3 80,0 72,3 671 756