PSI - Issue 20

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

159

6

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

Table 1. Microdamage accumulation coefficient, average microhardness and metal porosity characteristics in zones I, II and III. Parameter I zone II zone III zone k 1.95 1.89 2.07 Н 100, MPa 3590 3720 3796 V total = V fine + V crs /quantity 1.9 / 1348 1.8 /1081 2.2 /1021 V fine / quantity 1.2 / 1258 0.9 / 980 0.8 / 857 V crs / quantity 0.7 / 90 0.9 / 101 1.4 / 164 If we compare the data for the most loaded zones I and III of the spring leaf, we see that at the lower values of the coefficient k and volume fraction of the total porosity V total the failure occurred in the zone characterized by the lower value of the microhardness due to the softening; a deviation of the law of the microhardness distribution from the Gaussian law; significant prevalence of the fine pores (by ≈47%); and the much smaller quantity of the coarse pores (by ≈45%). 3.4. Impact toughness of fatigue-damaged spring metal According to the test results (Fig. 4), the spring metal shows the fairly high fracture toughness at 20 o C despite the structural damages. With the temperature dropping, the decrease in the impact toughness of the metal in zones II and III is very monotonic. In pre-failure zone I the temperature dependence of the metal impact toughness is somewhat different: the toughness values sharply decrease during the temperature change from 20 to – 20 o С and then vary insignificantly, remaining on par with KCV – 60 of the other zones. This shows that zone I is more sensitive to the low temperatures than the others. Considering the ambiguous dependence of KCV on the structural microdamage accumulation coefficient k and the fact that the most significant differences of the metal structure in the three zones are observed for the porosity, it will be legitimate to say that the mesodamages have the decisive influence on the metal resistance to the brittle fracture. An explanation of this character of the effect of the multiple pores on the resistance of the spring metal to the crack formation requires the detailed microfractographic analysis to identify micromechanisms and kinetics of the fracture.

Fig. 4. Temperature dependences of metal impact toughness in different spring zones

4. Conclusion 1. Using the example of the destroyed spring of the KAMAZ truck, the statistical regularities of the silicon steel fatigue damage progressing in the time have been studied, moreover, the data have been obtained from the empirical parameters of the actual material operation in the conditions of the North. Two main types of the