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

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3

Fig. 1 Histogram of yield strength for S355 steel.

2. Models for estimated S-N curve basing on material hardness

For determining S-N curve several method can be used, like presented in paper Strzelecki and Tomaszewski (2016), Pascual and Meeker (1999) & Castillo et al. (2007). In this study, simples experimental methodology as per ISO 12107 (2003) standard was used for determining the referential S-N characteristics, which will be referred to as model I in the further part of this article. The description of the N number of cycles versus S a stress amplitude relationship is determined using the Basquine Weibull (1961) formula:

) log( a N A m S   .

(4)

) log(

The statistical elaboration of the results for this approach is as follows ISO-12107 (2003):

k



  





 X X Y Y 

i

i

i

m ˆ

1



,

(5)



k







2

 X X

i

i

1



A Y mX ˆ ˆ   .

(6)

Hardness measurement is a non-destructive measurement, and one that is quick to perform. Due to this, numerous publications were made on the subject of the dependency of hardness on yield strength or tensile strength. Empirical relationships describing these functional dependencies may be found, among others, in the following papers: Busby et al. (2005); Khodabakhshi et al. (2015); Roessle and Fatemi (2000); Sanders et al. (1997); Zhang et al. (2011) and were presented in the form of formulas (1), (2) i (3). It must be stressed that hardness is used in Brinall scale, which may be converted from another scale using standards, such as ASTM E-140-02 (2002); PN-H 04357 (1993). When analyzing literature it may be noted that these dependencies feature large correlation with experimental data. The coefficient of determination for these relations is at least 0.9. Due to this fact it is assumed that these interrelations are sufficiently accurate. One of the models presenting the relationship between hardness and fatigue life is the Strzelecki model described in Strzelecki (2014). The main idea behind this model is relating the m slope to the tensile strength value and yield