PSI - Issue 20

Yakovleva S. P. et al. / Procedia Structural Integrity 20 (2019) 154–160

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Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2019) 000 – 000

roads, especially in the permafrost zone, decreases in a rainy season, and in early spring and late autumn, a rough layer of ice and snow forms on the road surface, etc. In the study of the impact of the seasonal changes in the profile of the typical for Yakutia dirt road with the gravel and aggregate surface on a vibration frequency of the KAMAZ truck spring, it was found that in autumn the basic frequency increases by almost one and a half times compared to a winter period, and in warm time there are the additional vibrations from small irregularities with the higher frequency (fig. 2). This is consistent with the results of the analysis of spring failures, which showed the increase in their number in the summer months, i.e. the working capacity of springs is more dependent on the road conditions than on the low air temperatures (down to - 45ºC). The obtained results correspond to the data of the work by Kim et al.(2002), in which it was shown that it is the road microprofile that determines a level of the dynamic loads, which make the greatest contribution to the motor vehicle damage.

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Fig. 2. Microprofile (1) and basic harmonic (2) of typical for Yakutia dirt road in winter (a) and autumn (b)

3.2. Chemical composition, microstructure and nature of spring destruction The spring is made of silicon steel with the following chemical composition: Fe; Si, 1.68; Mn, 0.74; C, 0.63; Cr, 0.14; Ni, 0.09; Cu, 0.11 wt. %. The steel microstructure is bainite, martensite, ferrite and a small amount of retained austenite (fig. 3a). A specific feature of the steel structure is the statistically distributed porosity due to technological reasons. In addition to technological factors, the development of the pores in the test spring is conditioned by the strain induced pore formation as pointed by Karzov et al. (1993), Bhat et al. (2011). The favorable factor of the pore formation is the stress state type: the closer the loading to uniaxial, the faster the pores grow as observed by Saedi et al. (2014). The metal of the lower and upper spring surfaces was in a state close to uniaxial, which could facilitate the multiple pore formation. It was noted above that the spring breakdown occurred according to a fatigue mechanism; for confirmation of it the typical surface area of the fatigue fracture with fatigue striations is shown in Fig. 3b. The fracture surface is damaged to varying degrees by corrosion and secondary microcracks.

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Fig. 3. Microstructure of metal (a) and microtexture of fatigue fracture surface of spring (b). Light arrows show pores, black - fatigue striations.