PSI - Issue 28

L.R. Botvina et al. / Procedia Structural Integrity 28 (2020) 1686–1693 L.R. Botvina et al. / Structural Integrity Procedia 00 (2019) 000–000

1692

7

const N  13.9 

(6),

 7.17 

const N 

(7),

It was found that γ-curve corresponds with this equation well in surface fracture region – from stress amplitude σ=1300 to 3500 MPa (fig. 8). It is supposed, that it will also be applicable to “fish-eye” zone, but unfortunately, it was not proved due to lack of given data. Therefore, by using such technique it could also be easy to identify the change in fracture mechanism by change in the slope of Basquin’s curve.

Fig. 8. Basquin’s ratio in coordinates N - 1/ σ and N - 1/γ.

4. Discussion and conclusions The proposed method of inverse coordinates is easy to use because of no need in division into subdistributions, and consequently, easy for engineering estimation of probability of fracture because of existence of correlation between probabilistic distributions with mechanical properties of the material. The method can be used for express-analysis of the state of material by the reason of its dependence on non-destruction testing characteristics, such as attenuation coefficient of ultrasonic waves and the local deformation. The research resulted in testing of alternative statistical method, based on using inverse axes in making exponential cumulative distribution functions: • The slope coefficient of cumulative probability distributions of material characteristics plotted in inverse coordinates is an informative parameter associated with the mechanical and physical properties of the material studied; • In microhardness measurements the proposed method provided the γ-parameter to estimate material hardening, attenuation coefficient of ultrasonic waves and local deformation – aspect ratio of grains; • In fatigue testing the method showed its’ sensibility to fracture mechanism by γ-parameter: in ceramic specimens γ linearly depends on stress amplitude, and in SUJ2 steel the same parameter is undergoing the discontinuity at “transition” stress level, at which fracture transforms from fish-eye to surface defects accumulation; • The relation connecting the slope coefficient γ of cumulative probability distributions of cyclic durability to stress amplitude is similar to the Basquin’s equation, which is widely used for describing fatigue curves, and could probably be used for change in fracture mechanism identification.