PSI - Issue 30

S.P. Yakovleva et al. / Procedia Structural Integrity 30 (2020) 193–200 Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2020) 000–000

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1. Introduction The loss of load-carrying capability and destruction of technical objects are usually conditioned by local damages of material structure, the accumulation and merger of which give rise to micro- and macro-cracking as it described by Panin (1998), Botvina (2008), Sakai (2011), Romanov (2014), Murakami (2002). Therefore, to ensure the reliability and safety of technical objects, it is relevant to study the development of structural damages taking into account the loading conditions (including natural and climatic) and their influence on the properties of metals and alloys as it shown by Makhutov (2013), Matvienko (2014), Sangid (2013), Romanov (2006). For the issues of safety of machinery and technical systems in the North and in the Arctic, which have recently become particularly important, as noted, for example, by Makhutov et al. (2013), one of the main external factors is the factor of low air temperatures, which increases the probability brittle fractures. As is known machine parts experience mostly repeated and alternate loads, which lead to the occurrence and development of fatigue damage. With reference to climatic conditions of the North and the Arctic, in order to avoid the transition fatigue cracks to spontaneous propagation by a brittle fracture mechanism, one of the main requirements for the metal is an ability to remain cold resistant during the entire period of operation. The operating of trucks carrying out a large volume of transportation in the northern regions shows that the structural elements that account for the largest number of failures include suspension elements as shown by Ishkov et al. (2004) and Kim (2002). Accordingly, suspension elements made of spring steels largely determine the level of reliability of freight vehicles. It should be noted that for vehicle parts, there are practically no studies of cyclic hardening (softening) processes, fatigue failure accumulation in a material at temperatures of climatic cold and their influence on the brittle fracture resistance of materials. The purpose of this work is to study the influence of fatigue damage formed in spring steel under operational affecting typical of the cryolithozone on the level of its resistance to brittle fracture. This formulation of the problem will make it possible to obtain new data that develop the concept of structure damage as a factor that causes the formation and occurrence of the limiting state of the material. Nomenclature n i quantity of the results per specific interval of a microhardness of the conditionally initial state of the material N i total number of microhardness measurements upon controlling the microhardness in the conditionally initial state n i * quantity of the results per specific interval of the microhardness for all the zones N i * total quantity of the microhardness measurements upon controlling microhardness for all three zones i number of the intervals of the microhardness histogram m , m * quantities of the intervals of the microhardness histogram compiled for the conditionally initial state and for the zones, respectively а i , а i * weight coefficients found for each interval of the microhardness within the limits of m , m * of each histogram 2. Study object, procedure and methods The fact of the discrepancy between the test results obtained by simulating damage in the laboratory and the properties of materials with fatigue defects that arose in the material under real operating conditions is known. In this work, spring steel was used as the research material after working in the road and climatic conditions of Yakutia – the destroyed standard main leaf of the KAMAZ truck spring. The leaf was 1675 × 75 × 1 mm in size; chemical composition of the steel is the following: Fe; Si, 1.68; Mn, 0.74; C, 0.63; Cr, 0.14; Ni, 0.09; Cu, 0.11 wt. %. At the moment of the spring failure the machine mileage was ≈ 100000 km, i.e., the failure occurred at the operation stage corresponding to the normal wear of the springs. The crack spread near a front bracket, dividing the spring leaf into long and short fragments (see Fig. 1). Three groups of longitudinal Charpy specimens with the different levels of metal damage were made from the long fragment (based on the fact that the zones near points of spring attachment to a front axle and frame are the most loaded). The specimens marked as group II were made of an intermediate piece between a fracture line and spring center. The operating stresses in this piece are lower compared to the ones