PSI - Issue 14

A.N. Savkin et al. / Procedia Structural Integrity 14 (2019) 429–434 Author name / Structural Integrity Procedia 00 (2018) 000–000

431

3

    

    

1

   * *    E K ' 

   n

'

   * r E

*

2

K







,

(1)

    

    

1

   

   n

*

*







'

     * * r E  

2

2

  K



.

(2)

2 ' K

E

Fig. 2. (a) Illustration of combination of Neuber’s rule and Ramberg-Osgood equation; (b) local near-tip stress at some distance from crack tip

Further development of the model is connected with considering the variable nature of the threshold SIF range Δ K th . Below is the equation proposed for calculating the fatigue crack growth rate:

p



max       1           1 th eff c K K K K 

da C K dN

,

n    

eff

q

. (3) The Forman-Mettu formula was used as a basis, which describes the fatigue crack growth rate curve in all three regions. The Paris coefficients C and n are proposed to be determined by the so-called “three tests” method. 3. Material and test schedule The tests were conducted on servohydraulic test system BISS Nano-25 using C(T)-specimens as per ASTM-647. The specimens were cut from aluminum alloy 2024-Т3 A series of three tests under constant amplitude loading were performed with maximum load P max = 2 kN, loading frequency F = 5 Hz and cycle ratios R 1 = 0,1, R 2 = 0,3 and R 3 = 0,5. The asymmetry ratio was varied from 0 to 0.75 for the tests with overloads. The range ΔK was determined by the peak load Δ P of the load history. The Schijve equation U=f(R) was used for considering the crack closure, and the effective SIF was estimated by formula Δ K eff = Δ K*U . The local stress σ* was calculated by formula (2) with using range of SIF. The threshold SIF K th was estimated