PSI - Issue 5

Slobodanka Boljanović et al. / Procedia Structural Integrity 5 (2017) 801 – 808 Slobodanka Boljanović, Stevan Maksimović / Structural Integrity Procedia 00 (2017) 000–000

803

3

2. Fatigue failure assessment

The integrity of engineering structures during service operations can often be endangered by fatigue loadings. Therefore, in the context of fracture mechanics a major research topic is to develop reliable computational models for the crack growth analysis. Thus, Huang and Moan (2007) suggested that the crack propagation behavior can be tackled through the following relationship related to the crack growth rate:

dN da

m C M K )

( = ∆

(1)

where a is crack length, whereas ∆ K and C , m represent the stress intensity factor range and experimentally obtained material parameters, respectively. In the structural durability analysis the influence of variable amplitude stress load is taken into account by means of the fracture mechanics parameter M , defined as follows:

    

−

1 β

− (1 ) (1 ) R R −

− ≤ < 5 R

0

−

β

M

≤ < R

0

0.5

=

(2)

− + (1.05 1.4 0.6 ) 2 R R

−

β

≤ < R

0.5

1

where R is stress ratio, β and β 1 are experimentally obtained parameters by which material properties and environment are taken into account . Furthermore, the fatigue behavior of damaged situations can be quantitatively assessed, after integration of the crack growth rate (Eq. (1)), through the number of loading cyclic to failure, i.e.:

f a

∆ m C M K da ) (

∫

N

=

(3)

a

0

where N is number of loading cycles to failure, a 0 and a f are initial and final crack length, respectively. The residual strength under cyclic loading is estimated through the software program, here developed, in which the numerical integration for complex-valued functions is implemented. Thus, the number of loading cycles is calculated, step-by-step, for appropriate crack increments from initial to final crack length. Such a calculation is performed assuming that the failure occurs when the value of the stress intensity factor reached the fracture toughness.

3. Stress-intensity analysis of the pin-loaded lug under cyclic loading

Fatigue performance of structure is greatly affected by the presence of stress concentrators (holes, notches, cutouts, manufacturing damages and corrosion pits) which serve as nucleation sites for crack initiation, crack growth and even failure. In the context of fracture mechanics, the fatigue behavior of such non-linear stress field zones can theoretically be examined through the stress intensity factor, expressed as follows:

K Y S a π ∆ = ∆

(4)

where ∆ K is the stress intensity factor range, Y represents the corrective factor and ∆ S denotes applied gross/net stress range. Note that, the stress-intensity behavior is here assessed by taking into account the net stress range.