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

64 2

1. Introduction Probabilistic strength estimates for technosphere objects are predicated on systematic investigations on dispersion of the parameters characterizing main mechanical properties (MMP) of structural steels over a wide temperature range. It is contentedly labor-intensive and intricate technical problem, in the first place due to necessity to test considerable series of specimens with the purpose to substantiate experimentally proper theoretical distribution curves and also due to the need to ascertain low and upper boundaries of dispersion which are determined usually on a level of the fracture probabilities from 1 to 5 percent and below. Development of Siberia and Extreme North areas where infrastructures, unique and serial industrial objects, machines and equipment operate in a tough natural environment at negative climatic temperatures has set a challenge to give scientific substantiation of expected lifetime for strength, survivability and reliability of their load carrying components. So, for example, according to the data revealed by Makhutov (2015) it was established that main key factors for failure occurrence (up to 70…80% of failures) of ore mining, building equipment and highroad engineering, power installations and cross-country pipelines that operate under the conditions of negative climatic temperatures are negative operation temperature, seismic, wind, snow and ice loads. One of the most important divisions of these researches is to provide low temperature strength with determination of probabilistic characteristics of strength and yield ability under the conditions of material homogeneous stress state at negative temperatures for further engineering designs of machine and structure components. In the absence of data obtained with the aid of specimen tests at negative temperatures it is possible to use the primary data of short-time static tests at room temperature for MMP determination as reference on a first approximation. These data are usually acquired at the series of specimens at the rate from 20…100 to 1000 and more; which provides an opportunity to determine the probabilities Р from 0.1 to 0.001 for such main MMP characteristics as ultimate strength σ в , yield stress σ т , critical rapture strength S k and ultimate plasticity as measured by ψ k . From these characteristics for defined Р values other calculation parameters used in strength design (i.e. fatigue endurance limit σ -1 , strain-hardening factor m, characteristics of response to change of temperatures β Т , β в , and a number of other parameters pointed by Makhutov (2008, 2015) can be determined. In this paper the task is allotted to quantitatively evaluate the probabilistic basic and derivate MMP characteristics for two representative structural steels – low-alloyed steel with high strength and yield ability 15H2NMFA and ordinary non-alloyed steel 3 in a range of positive and negative temperatures with following use of these characteristics for probabilistic estimates of safety factors based on proposed approaches.

Nomenclature σ B

ultimate strength , MPa

σ T

yield stress, MPa

σ 0,2

technical yield stress at residual elongation 0,2%, MPa

S k ψ k

critical rapture strength, MPa

relative narrowing of cross section area, %

σ -1

fatigue limit, MPa

δ P

relative elongation of break, %

probability

m β T β B

strainhardening factor

temperature influence parameter temperature influence parameter

ν

coefficient of variation for different parameters critical temperature at brittle fracture, °C

tk