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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calculation model employed) must be obtained. Such data are not available at the design and construction stage, when the designed element is merely a concept. For this purpose, analytical models are used for estimating the S-N fatigue characteristics. These models are based on strength, which is most often derived from standards or manufacturer specifications. This value may deviate from actual properties of the material. In order to illustrate the possible dispersions of strength values, Fig. 1. presents a histogram of yield strength for S355 steel, which was manufactured in the Czech Republic, according to Melcher et al. (2004). As per PN-EN 10025-2 (2007) standard, the yield strength value for this steel should be 355 MPa at minimum. From the normal distribution, estimated based on experimental data for 1089 specimens, we may calculate that this value will be smaller for over 6% products. Due to the above, it seems justified to use strength values obtained through experimental means. For that purpose we may use the measurement of material hardness, and, with the use of the formulas presented below, convert it to tensile strength ( S u ) according to Roessle and Fatemi (2000): Su = 0.0012·HB2+3.3·HB, (1)

or according Bandara et al. (2015): Su =3.4·HB,

(2)

and yield strength ( S y ) according Busby et al. (2005), Khodabakhshi et al. (2015), Sanders et al. (1997) and Zhang et al. (2011): Sy = 3·HB. (3)

Nomenclature A

constant from Basquin equation B or N up number of cycles to failure at the first knee point b inverse slope of the S-N curve equal 1/ m HB Brinell hardness HRB Rockwell hardness in B scale HRC Rockwell hardness in C scale HV Vikers hardness K t stress concentrator factor m slope of the S-N curve m’ slope of the S-N curve in the GCF region N number of cycles N GCF number of cycles to failure in the GCF region N k number of cycles to failure at the second knee point N Sy

number of cycles to failure at stress amplitude equal 0.9 S y

S a S e

stress amplitude

fatigue strength for N k cycles (fatigue limit) fatigue strength for N k cycles for notched elements

S en

S GCF

fatigue strength at GCF region ultimate tensile strength

S u S y

yield strength

X Y

log( S a )

log( N ) ͞ mean value

̂

estimated value