PSI - Issue 57

Boris Spak et al. / Procedia Structural Integrity 57 (2024) 445–451 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

449

5

certain mechanical properties, e. g. ultimate tensile strength R m , or preferably through cyclic material testing in a constant strain amplitude regime with a strain ratio of R ε = -1. The latter method was used in this study, with specimen manufactured and testing procedure as well as parameter identification carried out in compliance with the German guideline SEP 1240 (2006). Table 3 provides the cyclic material properties of the material in the as received state. Table 3. Cyclic material properties of EN AW-7475 T761 in the as received state, obtained from constant strain amplitude testing.

Material parameter EN AW-7475 T761

σ f ' in MPa

b

ε f '

c

K’ in MPa

n’

1819.45

-0,2023

0,2969

-0,7556

963.10

0.1631

A brief summary of required equations to perform a fatigue life estimation with the LSA is given hereafter. The cyclically stabilized stress strain relationship is commonly described according to Ramberg-Osgood (1943):

1/ ' n  

  

  

= +



.

(1)

'

E K

T he memory effects and Masing’s law (1926) are taken into account through:

1/ ' n  



   

  

2

 = 

+

.

(2)

2 ' E K

From a loading sequence, by counting closed hysteresis loop, the damage contribution of each hysteresis loop can be computed by employing a damage parameter, e. g. proposed by Smith, Watson & Topper (1970): ( ) E P a m a SWT   = +    . (3) It is worth mentioning that other damage parameter exist, such as P RAM described in FKM Guideline nonlinear (FKM nonlinear, 2019). P RAM differ from P SWT by introducing an additional mean stress sensitivity factor. According to the guideline FKM Guideline nonlinear, an aluminium wrought alloy such as the EN AW-7475 T761 with R m = 470 MPa exhibits a mean stress sensitivity factor close to unity. Therefore, P SWT and P RAM can be assumed to yield analogous fatigue estimation results. The strain Wöhler curve is derived from experimental results under constant strain amplitudes and R ε = -1:

' 

( ) N 2

( ) c f N

b

f

' 2 

a 

=



+



.

(4)

f

f

E

Finally, the damage parameter curve is require to compare the computed damage from hysteresis counting with the strain Wöhler curve of the material: ( ) ( ) b c f f f b f f SWT E N N P +    +  = ' 2 ' ' 2 2 2    . (5)

The linear damage accumulation hypothesis is used to compute the total number of cycles to crack initiation, with failure assumed to occur at D = 1: