PSI - Issue 33

Sabrina Vantadori et al. / Procedia Structural Integrity 33 (2021) 773–780 S. Vantadori, C. Ronchei, D. Scorza, A. Zanichelli / Structural Integrity Procedia 00 (2021) 000–000

776

4

Then, the fatigue endurance condition is reached when eq,a  equates the fatigue strength 1 af ,   . Finally, for finite life tests, the number of loading cycles to failure, cal N , can be determined by replacing the fatigue strengths 1 af ,   and 1 af ,   in Eq. (2) with the Basquin relationships for both fully reversed normal and shear stresses. In particular, the value of cal N can be worked out by solving the following equation through an iterative procedure.

2

2

m*



2

1

m

m





0 *

   

   

   

 



  

  

  

N

N

N

1

af ,



2

2

cal

cal

(4)

N

C









  



 

1

eq ,a

eq ,a

a

af ,



N

N

N



1

0

0

af ,

cal



m and 0 N being the slope of the S-N curve and the reference number of loading cycles under fully reversed normal stress, respectively, and * m and 0 * N being the slope of the S-N curve and the reference number of loading cycles under fully reversed shear stress, respectively. 3. Criterion validation by experimental data The stress-based fatigue criterion presented in Section 2 is hereafter applied to some high/medium fatigue tests available in the literature, both at infinite (Cengiz, (2012)) and finite life (Tovo et al. (2014)). 3.1. Materials and experimental tests Regarding the infinite life fatigue tests reported in Cengiz, (2012), a commercial DCI identified as EN-GJS-700-2 was investigated. The material microstructure is characterised by spheroidal graphite nodules (black circles in Fig. 1(a)) uniformly distributed in a pearlitic matrix.

Fig. 1. Microstructure of: (a) DCI EN-GJS-700-2; (b) DCI EN-GJS-400-18.

The specimens, with a cylindrical shape and a diameter of the gauge section equal to 7 mm for tensile tests and 16 mm for torsional and biaxial tests, were cut from a large section of the investigated DCI. Such specimens were subjected to both uniaxial and biaxial cyclic loading with constant amplitude, under high-cycle fatigue regime. More precisely, fully reversed tensile and torsional loading (that is, 1 R   ) were considered, whereas the biaxial fatigue tests were performed under a pulsating compression together with an alternating torsion with a phase shift  equal to 90°. As a fracture criterion was considered the appearance of a visible crack during testing, whereas run-out tests were stopped at a number of loading cycles equal to 10 7 cycles. The details of the experimental loading conditions are reported in Vantadori et al. (2021).