Issue 37

M. Kurek et alii, Frattura ed Integrità Strutturale, 37 (2016) 221-227; DOI: 10.3221/IGF-ESIS.37.29 223       (8) being   the angle defined by the direction of the maximum normal stress if the above damage accumulation method [3] is applied. An alternative method is that to determine the direction for which the normal stress variance reaches its maximum [5,6]:     dt t T T   0 0 2 0 1       (9) where 0 T is the observation time interval. The criterion proposed by Macha (see Eq. (5)) is here employed on the critical plane, where the determination of the critical plane orientation is performed according to the above damage accumulation method. The weighted factors B and K can be determined by equating Eq.(1) to af  and Eq.(2) to af  : )2 90 cos( cos 2 )2 90 sin( 2 sin cos )2 90 sin( 2 2 2             B B (10)   2 cos 2 2 sin 2 B K   (11) By substituting Eq. (10) in Eq. (11), we get: af af K    2 (12) According to Eq. (12), we can notice that the parameter K is a constant depending on the material fatigue properties. The final step is the calculation of the fatigue strength. For constant amplitude cyclic loading, the fatigue strength is evaluated by using Basquin’s fatigue characteristics ( A and m ) in compliance with the relevant ASTM standard [7]. The formula for strength calculation under cyclic loading is expressed as follows: a eq mA N cal , lg 10    (13) where a eq ,  is the amplitude of the equivalent stress related to the critical plane (Eq.(5)). M ATERIALS EXAMINED atigue test results related to 11 selected construction materials are analysed. According to the ASTM recommendations [7], such results are also used to calculate the regression equation for fully-reversed bending (or uniaxial push-pull): a f m A N    log log   (14) and for fully-reversed torsion: F