PSI - Issue 37

A. Vshivkov et al. / Procedia Structural Integrity 37 (2022) 570–575 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

573

4

Fig. 3. The kinetic diagram of fatigue fracture

Fig. 4. Mechanical hysteresis of the cyclic tension-compression deformation

4. Stored energy According to the first law of thermodynamics the part of mechanical work during plastic deformation is absorbed by the material, while the other is dissipated as heat. Thus, the stored energy is calculated as the difference between the plastic work and the heat dissipated into the environment. ( ) ( ) ( ) stored p E t W t Q t = − (1)

The heat dissipation:

i t

( )

( ) H t dt = 

Q t

(2)

0

where H(t) – heat dissipation rate registered by heat flux sensor (W). Numerical calculation of the work of plastic deformation (trapezoidal method):

i t

( )

( ) F t dx = 

p W t

(3)

0

where F(t) – load, x(t) – displacement registered by contact extensometer. As a result of experimental data processing the time and crack length dependences of stored energy were obtained. The crack length dependence of energy stored per cycle is presented on figure 5. We can see that the rate of stored energy per cycle slows down at the final stage of deformation. This observation is in a good agreement with the thermodynamic theory of strength. In a sense, the internal energy stored in material as a result of deformation is measure of damage, and fracture occurs at the moment when the internal energy of a critical value accumulates in the representative volume of material.