Issue 42

Correia et alii, Frattura ed Integrità Strutturale, 42 (2017) 136-146; DOI: 10.3221/IGF-ESIS.42.15

This procedure considers that the fatigue crack growth (FCG) data pertain to a random sample where all * i da dN       are independent and there are no run-outs or suspended tests for the entire range of   * K  . Furthermore, the linear model

 *       da dN

is the estimative (estimator) for the values of FCG

for the Paris relation can be rewritten as Eq. (4) shows, where

i

rates.

 *

* dN           da

  C m K *

*

(4)

i

i

The characterization parameters of the statistical analysis, such as the variance and the standard deviation must be defined. Regarding the variance of the log-normal distribution, it is constant and maximum likelihood estimators of * C and * m are, respectively, defined by Eqs. (5) and (6).

*

da      

     * K da

*

* dN C m   i

k

k





  * m K        dN

*

*





i

(5)

k

k





i

i

1

1

*                      * i da da dN dN

      * * i K K   



k



i

1

*



m

(6)

      * * i K K   

2



k



i

1

* da

*

da dN      

dN      

,   * K  is the average value of   * i K 

where

is the average values of

and k is the total number of

i

* da

dN      

and   * K  are determined as shown in Eq. (7).

readings during the test by specimen. The average values

*

da k       dN

     * K da k dN   ;  i

*

k

k





  K

*

 

  

i

(7)

i

i

*

da dN      

where   * i K 

represents the computed during the test of the stress intensity factor ranges and

represents the

i

readings during the test of the FCG rates. Concerning the standard deviation of the normal distribution for

  log K  , it is defined through the Eq. (8).

 *                      * i i da da dN dN



k i



1



S

(8)



k

2

Aiming the definition of a design FCG curve, rectilinear confident bands were defined as Eq. (9) shows, where  is an integer.

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