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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at the attachment points; therefore the specimens of group II were taken as conditionally initial. The specimens from the zone near a fixation of the spring center were designated as group III, from the prefracture zone (near the fracture) as group I. Manufactured Charpy specimens were preliminarily used to study the structure and micromechanical properties.

Fig. 1. General view of the destroyed half of the main leaf of of KAMAZ Trucks spring. I, II, III – Charpy specimens cutting zones. Arrows point to the fracture lines.

For the study of the damage, the parameters of the microhardness (for a description of the microdamage) and porosity (for the description of the mesoscale damage) were used as it is given in the review of Volegov et al. (2016). Since the initial damage could not be differentiated from the exploitation-induced damage in the case in question, it was agreed to consider as conditionally initial area the minimally porous area chosen in zone II affected by weaker stresses. Microhardness Н 100 was measured on a PMT-3 instrument with an indenter load of 0.98 N . The sample volume for each zone of the spring was about 1000 indents. The statistical analysis was conducted in Excel. In addition, the microscale structural damage accumulation coefficient k was found for the specimens from all three groups in accordance with the methodology proposed by Zorin (2013) which uses an array of Н 100 values. This method compares of the microhardness histograms at the various stages of the material damages. The pore characteristics were determined on three 2x1.4 mm 2 areas observed under a Neophot-32 optic microscope. According to their cross section, the pores were tentatively divided into fine (up to 20 mm) and coarse (the largest of the pores observed did not exceed 40 mm in size). The volume fractions of fine and large pores are indicated as V fine and V crs , respectively, whereas the total porosity is indicated as V total . On an Amsler RKP 450 Zwick impact testing machine the impact-bending tests of the Charpy specimens were conducted at 20, -20 and -60 о C. To answer the question of how a certain level of impact toughness is achieved in a material, the fracture micromechanisms and features of the fine structure of the fracture surfaces of impact samples were determined in accordance with the well-known principles of fractographic analysis, described by McEvily (2002), Botvina (2008) and in the book Fractography of Modern Engineering Materials (1993). The research was performed using ТМ 3030 Hitachi и JEOL JSM-6480LV scanning electron microscopes. 3. Results and discussion 3.1. Microstructure and quantitative analysis of structural damage to spring steel The studied spring underwent fatigue failure, as illustrated in Fig. 2 a . The process of spring destruction is considered by Yakovleva et al. (2017). The most important property of spring steels is high resistance to small plastic deformations, since residual deformation is prohibited in the elastic elements of the vehicle suspension. The necessary parameters are achieved by alloying using silicon and manganese, which affect the elastic limit, as well as by strain hardening and thermal processing with martensitic transformation and subsequent tempering. In the microstructure of the studied steel, bainite, martensite, ferrite and a small amount of retained austenite can be traced (Fig. 2 b ). Porosity is also observed due to technological reasons and pore formation induced by the strain, which is shown by Ermakov (2017), Karzov (1993) and Bhat (2011). The stress state at the upper and lower surfaces of the springs approaches to uniaxial one, which could favor pore formation, since the closer the loading is to uniaxial, the higher the pore growth rate, which is shown by Saedi et al. (2014). The high resistance of spring steels to small plastic deformations should exclude the appearance of inelastic