PSI - Issue 20

Yakovleva S. P. et al. / Procedia Structural Integrity 20 (2019) 154–160 Yakovleva S. P. et al. / Structural Integrity Procedia 00 (2019) 000 – 000

158

5

3.3. Quantitative analysis of structural damage of spring metal In order to substantiate from the physics standpoint of the nature of the dependences of the working capacity of the KAMAZ vehicle spring on the climatic and road conditions, which were described in paragraph 3.1, statistical regularities of the damage accumulation and its effect on the material properties and destruction processes of the leaf spring during operation in Yakutia have been studied. The structural microdamage accumulation coefficient k characterizes the relative increase in density of the material microdamage during operation was introduced by Zorin (2013):

*

*

N n N n

m

*

i

a

i

*

1

i

i

k

,

(1)

m

i

a

i

1

i

i

2 1 m

1,8

1,8

;

1 ( 1) i

1 (

)

a i

a

m i

i

for

for

,

(2)

i

1

1

m

m

The described method provides a comparison of the microhardness histograms at the various stages of the material operation. In this work, not laboratory specimens were used as the measurement objects, but the spring areas with the different levels of the damage and in the pre-failure zone. This approach made it possible to investigate the damage that occurred in the real operation conditions and to avoid the long-term laboratory tests. Since the initial damage could not be differentiated from the exploitation-induced damage in the case in question, it was agreed to consider as the conditionally initial area the minimally porous section chosen in zone II affected by the weaker stresses. It should be noted that zone I is the pre-failure zone but not the failure zone because the influence of the propagating fatigue crack on this zone can be ignored. This is conditioned by a strong localization of the processes in vertices of the fatigue cracks. In particular, as is shown for the steel of the considered composition by Suchkova et al. (2005), the influence of the fatigue crack on the material structure becomes unnoticeable already at ~2.5 mm from the fracture surface. The calculated values of the coefficient k are presented in Table 1, and the average microhardness values and porosity characteristics are also presented there. According to the results carried out by Yakovleva et al. (2017), the metal in the spring center (zone III) goes through a hardening phase, the metal in zone I is considerably softened by the microscale damages; that was why the zones were arranged by the ascending degree of the damage as zone II, zone III and zone I. It follows from Table that the maximum value of the coefficient k does not correspond to zone I. This discrepancy can be explained as follows. It is known that the microhardness reflects the metal resistance to the indenter forcing-in, dependent on the strain hardening (or softening) degree that characterizes the damage. If we consider the porosity parameters, we see that an intensive generation of the new pores is observed in pre-failure zone I (for the comparison, the volume fraction of the pores in zone III near the fixing center increases mainly due to their increasing dimensions). It is obvious that the multiple fine pores as well as the substructural softening, contribute to the accumulation of the fatigue damage, causing systemic loosening of the steel and changing the structural state of zone I. This manifests itself in the change of the distribution of the microhardness parameter. Verification of a statistical hypothesis of the normal distribution by the chi-squared test for the empirical distribution of the array Н 100 in this zone yielded 2 = 121 at the critical value calculated as 2 cr = 16, i.e., 2 > 2 cr . Therefore, the hypothesis of normal distribution for the metal microhardness in the pre-failure state is rejected. Existence of the multiple pores also makes it more difficult to evaluate the coefficient k correctly in the pre-failure phase.