Issue 42

D. Rozumek et alii, Frattura ed Integrità Strutturale, 42 (2017) 23-29; DOI: 10.3221/IGF-ESIS.42.03 27 Fig. 6 shows experimental test results in the form of diagrams the lengths of fatigue cracks “a” in function of the number of cycles N. During the test analysis it has been observed that fatigue life increases more than 14 times, changing ratio value from R = 0 to R = -1. For specimens subjected to oscillatory loads (R = -1), cracking initiation commenced after 90000 cycles (fatigue failure was formed after 251000 cycles), whereas for R = 0 cracking initiation took place already after 9000 cycles (failure was formed after 17500 cycles). The tests were performed for three specimens at each load level. Fig. 7 presents experimental test results in the form of diagrams showing fatigue crack growth rate da/dN (dc/dN) in function of changes equivalent stress intensity factor range ΔK eq . The increase in fatigue crack growth rate da/dN applies to specimen length (active side), whereas dc/dN concerns specimen depth. Test results for fatigue crack growth rate in function of equivalent stress intensity factor range were described using Paris equation [18]   m eq da dc C K dN dN    (1) a) b) Figure 7 : Comparison of the test results with calculations according to Eq. (1) for specimens under bending with torsion: a) R = -1, b) R = 0. In case of proportional loads (bending with torsion), equivalent stress intensity factor range ΔK eq for mixed cracking mode was computed using the following formula 2 2 2.64 eq I III K K K      (2) A changes range of the stress intensity factor  K I for bending (mode I) and  K III for torsion (mode III) was calculated from [13]   2 0 cos I I K Y a a        (3)   0 sin cos III III K Y a a         (4) where  – range of nominal stresses under bending and torsion. The correction coefficients Y I for bending [19] and Y III for torsion [20] occurring in Eqs. (3) and (4) take the following forms         2 0 0 5 / 20 13 / 7 / I Y a a h a a h      (5)