PSI - Issue 20

Valeriy Lepov et al. / Procedia Structural Integrity 20 (2019) 24–29 Valeriy Lepov et al / Structural Integrity Procedia 00 (2019) 000 – 000

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been revealed that the increasing hardness of steel heat treatment leads to a significant decrease of impact toughness even at temperature  30-40 Celsius. So the resource of locomotive tires in the extreme environment sharply drops, as shown by Grigoriev and Lepov (2015). Concerning the cold cracking probability of weld joint of high-strength steel structures, it has a more sharp drop at low temperature because of different impact toughness of weld and heat affected zone metal, which is increase under the hydrogen influence, as shown by Lewei Tong et al (2018). The cold brittleness has measured usually by the impact bending test, in hard dislocation moving conditions due to volumetric tensely-deformed state. There are necessary to estimate the influence of impact blow to crack initiation and propagation. The evaluation of top crack velocity, or speed of Reyleigh waves in material, as shown by Ioffe, Nilson and Broberg, is not exceed 90% from the shear waves velocity , what is obtained by Broek (1986). So if only the crack velocity would n’t be close to this value the stress distribution is not changed substantially. If the crack velocity is less than 0.05  c as in case of impact toughness test, the stress distribution should be the same as in the static loading. So this fact allows studying only the lattice hydrogen effect on brittle fracture because the hydrogen in traps hasn’t enough influence on process of deformation during fracture. The Sciuccati (2011) study of impact toughness of hydrogen charged high-strength steel probes shows that the lattice hydrogen decreases the fracture energy and shifts the range of ductile-brittle transition to higher temperature range. This way the impact toughness has drop in the metal of weld joint because of high content of lattice hydrogen in weld metal and low in the heat affected zone, what is revealed by Tong (2018). The hydrogen embrittlement of modern structural steels has a quite similar character as the influence of low temperature on the fracture process. The drop of fracture energy during the impact toughness test was revealed for the mostly structural steels and even for locomotive wheel steel, by Grigoriev and Lepov (2018). In last case the fracture energy drops almost three times at testing temperature -40 Celsius. 3. A damage model for hydrogen embrittlement and low temperature effects To take into account the hydrogen embrittlement effect on metal the scalar damage parameter  was used by Arkhangelskaya et al (2001) with the following kinetic equation: . (1) The function f(C,T,  ) determine the dependence of damage accumulation processes from the hydrogen content, testing temperature and hydrostatic stress. So the failure behavior changes form ductile to brittle at the define values of temperature and hydrogen content. It is known as a cold-brittle point of metal or steel. The dependence like (1) named as logistic curve or Verhulst equation, and expressed the dynamics of dislocation and non-dislocation origin defects at different structural scales. It is known the exact solution of the equation (1) but for constant value of f(C,T,  ) only (for example, at the standard impact toughness test):

1

.

(2)





( , , ) 1) f T C t e   

1 (  

0

The function form could be set on the base of Zhurkov’s activation law by Bartenev (1984):

(3)

   ехр (U 0 –  /kT).

The estimated thermal vibration time is    10 -13 – 10 -14 s for classic metals and 10 -13 s for solid polymers. Activation energy U 0 of bound spontaneous rapture in unstressed crystal lattice defined at   0 and will depends on lattice hydrogen content and temperature close to ductile-brittle transition. In this case, the numerical solution for the equation (1) is necessary, because the exact solution will be incorrect.