PSI - Issue 16

Roman Chepil et al. / Procedia Structural Integrity 16 (2019) 211–217 Roman Chepil, Olena Stankevych, Orest Ostash1, Bohdan Klym / Structural Integrity Procedia 00 (2019) 000 – 000

213 3

(0)



y

*     ( ) x

K  

y

y

f

nom

*

1 4.5 / ( x 

d

)

 

,

(2)

*

x d 

and also the initial macrocrack size a i at the initiation stage (Ostash et al. (1999)) and its increment  a at the growth stage (Ostash and Panasyuk (2003)):

ai = d*;  a = d*.

(3)

Fig. 1. Scheme of the stress field in the notch root vicinity (a): 1 – elastic solution; 2 – elastoplastic solution; n – material strain hardening coefficient.

This model provides the estimation of the fatigue macrocrack initiation ( N i ) and propagation ( N p ) periods, and defines the total durability N f of a specimen (structural component) with the stress concentrator of arbitrary configuration based only on the fatigue macrocrack growth rate relationship ( da/dN vs.  K ) (Ostash and Panasyuk (2003), Ostash (2006)):

a

c







N N N N   

( da F K a Y   , , )



,

(4)

f

i

p

i a d 

nom

*

i

*

d

where a с = f (  K fc ) is the macrocrack critical size;  K fc is the cyclic fracture toughness of the material (can be considered as  K fc =  K at da/dN  10 -5 m/cycle); F is the function for analytical description of the fatigue macrocrack growth rates diagram ( da/dN vs.  K ), taking into account the dependencies between SIF range  K and nominal stress range (  nom ), crack length ( a ) in specimens of given geometry ( Y ). A number of experimental methods are developed to determine the parameter d * (Ostash et al. (2001, 2006, 2010), Muravsky et al. (2016). Phase shifting interferometry method (Ostash et al. (2011), Muravsky et al. (2016)) is one of the most effective because provides the direct evaluation of cyclically deformed region near the notch tip (FPZ) due to microplastic deformation. It was established experimentally (Ostash (2006)) that parameter d * is independent or slightly depends on the notch radius  , therefore it may be considered as a material constant under given test conditions. Experimental verification of new procedures of the fatigue life N f prediction using the above referred unified model background for different shape specimens with sharp and blunt notches was shown that the difference between the predicted and experimental values of N f did not exceed 22% (Ostash et al. (2017)). 3. Base notch fatigue curve at the macrocrack initiation stage In many cases, the durability of structural elements is determined mainly by the period of the fatigue macrocrack initiation. For example, it refers to high-loaded elements (engine turbines blades, aircraft landing gear, etc.), made of high-strength materials, when the fatigue macrocrack growth period N p is much smaller than the macrocrack