PSI - Issue 4

Stefan Kolitsch et al. / Procedia Structural Integrity 4 (2017) 95–105 Stefan Kolitsch/ Structural Integrity Procedia 00 (2017) 000 – 000

97 3

σ 0 σ a

yield stress in the Ramberg-Osgood hardening law [MPa]

load amplitude [MPa]

σ a ( R )

endurable stress amplitude depending on the stress ratio [MPa]

stress in the outer fibre due to bending [MPa]

 b

σ e,0 endurance limit for polished specimen under tension and compression [MPa] σ e,bend endurance limit for polished specimen under bending [MPa] σ e,bend,skin endurance limit under bending and consideration of the surface roughness [MPa] σ F,0 flow stress for polished specimen under tension and compression [MPa] σ F,bending flow stress for polished specimen under bending [MPa]  UTS ultimate tensile strength from experiment [MPa] σ WK fatigue strength for design [MPa] σ W,zd fatigue strength for tension and compression [MPa]  y yield stress from experiment [MPa]  angular position along the semi-elliptical crack front [°] W height of the specimen [mm] Y ( a / W ) geometry factor [-]

2. Failure assessment in the bending process

In order to obtain the required curvature, the switch blade is formed in a three-point bending process. For small bending radii during manufacturing, the strains in the outer fiber of a switch blade can be 100 times higher than in track when a train is passing over the rail. General rail standards prescribe a minimum fracture strain of 8-9% proven by tensile tests. The occurrence of small flaws will lead to a significant decrease of the allowable maximum strain in the outer fiber depending on the fracture behaviour of the material. In order to design a limiting curve for the allowable strain under static loading, different materials have been tested at room temperature using bending specimens with constant specimen height W and different crack lengths a . The main results are summarized below; for details the reader is referred to Kolitsch et al. (Kolitsch 2017). There exist three regimes characterizing the dependence of the fracture strain on the flaw size. For long cracks the concept of linear elastic fracture mechanics (LEFM) is valid and the stress intensity factor K I can be used as a failure criterion. For physically short cracks the J-integral of elastic-plastic fracture mechanics (EPFM) is valid and for still shorter cracks the strain from the tensile test can be used as design criterion. Hence, the failure strain in the outer fiber can be calculated by Eq. 1, where a pl specifies the transition between LEFM and EPFM.

      W a E Y a K Ic

                

for

0

  



f

0

  



n

      

      

      

      

n

1



1





n

2

   

      0

W a Y a J c

E f n

n

1



0  





 

0

ε

ε

for





  













(1)

f

0

0

f

f,exp

n

2

  2 1 

n

2

    

2

      

ε

ε

for

f 



f,exp

f,exp