PSI - Issue 5

Przemyslaw Strzelecki et al. / Procedia Structural Integrity 5 (2017) 832–839 Przemyslaw Strzelecki et al. / Structural Integrity Procedia 00 (2017) 000 – 000

835

4

strength value, as well as the fatigue limit and the limit number of cycles. The relationships used in this method are as follows:

   

   

6

log 10

N

Sy

m



,

(7)

     S 0.9

   

S

y

log

e

10



   

   

S S

u y

,

(8)

N



400

Sy

) log( ) log( 0 N A m S e   .

(9)

The formula (8) was determined based on 52 fatigue characteristics. The value of the N 0 inflexion point in this method is assumed as 10 6 cycles. The determination of this relationship was described in the paper Strzelecki (2014). For the structural elements, the slope is determined based on the following formula from Strzelecki (2014):

       

       

3 6

10 log 10

m

,

(10)



k

S

3

log

S

en

1

   

   

m e S S N  

m

0

.

(11)

3

3

10

Formula (1) and (3) are to be applied to determine tensile strength and yield strength, accordingly. The S en value (fatigue strength at for N k cycles for notched elements) is to be divided by ( K t ) stress concentrator factor for the case when K t · S e <1.1 S y . Otherwise, FITNET procedure, described by Kocak et al. (2006) is to be followed. The functions employed in this method are based on the material hardness value. This approach will be hereinafter referred to as model II. Schematic representation of this model is presented in Fig. 2 a). The scope of applicability of this approach is from ~10 3 to ~10 9 cycles for steel materials and aluminium casts. Another analysed approach is the Bandar and others model, which was accurately presented in the papers Bandara et al. (2016), (2015). The main purpose of this model is to describe, through a single equation, a low, high and giga cycle range, i.e. from 1 to 10 9 cycles, depending on the material hardness. It was assumed that the slope in the high cycle range is 5 (1/0.2), which is a deterministic approach, owing to the fact of significant dispersion of this value Strzelecki et al. (2015) is to be followed. The base number of cycles is differentiated depending on hardness. The relationships used in this method are as follows:

   

   

   

   

 N N S S N   0.2 0.2 GCF e GCF  k

 N N S S   0.2 GCF GCF k





0.2



e

GCF

,

(12)

S

 N B





a

0.2

0.2



