PSI - Issue 4

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

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5

The prediction using Eqns. 1-4 corresponds acceptably to the experimentally determined fracture strain. For long cracks the behavior of all materials is almost identical and it can be observed that the maximum strain decreases due to the geometry function Y ( a / W ). For short cracks, a < a pl , the fracture strain follows a somewhat different trend. In this case the fracture mechanism itself and the hardening behavior are crucial. Compared to other materials the ferrite-martensite shows the smallest fracture strain for small flaws at room temperature. Higher fracture strains may be achieved at elevated temperatures; therefore, the decision whether thermo-mechanical bending is required can be taken using this diagram and the information about the maximum estimated flaw size. In addition, Fig. 1 represents the strain range of the three-point bending process (blue area) during manufacturing and the resulting strains (green area) when a train is passing through the switch. It can be clearly seen that crack lengths up to 10 mm are not critical for static failure during operation.

3. Fatigue design

For cyclic loading the endurance limit is used as a design criterion. In the present work, two different approaches for calculation of the endurable stress are presented; a stress based design and a fracture mechanics approach. The focus is on plain fatigue, excluding issues related to rolling contact fatigue (RCF) in the region of the running tread.

3.1. Stress based design concept

For estimation of the endurance limit under tension/compression loading σ W,zd , the ultimate tensile stress σ UTS is reduced by a fatigue strength factor f W,zd following FKM (2012):

σ

 f

 

.

(5)

W,zd

W,zd

UTS

Furthermore the endurance limit depends on the stress ratio R . Hence, the endurance limit can be calculated by Eq. 6, where K AK ( R ) is the mean stress factor and the Index “0” denotes the endurance limit for a polished specimen under tension/compression,   ( ) w,zd AK a e,0 K R R       . (6)

K AK depends on the stress ratio R , which is calculated from the minimum and maximum stresses in a load cycle,

max min    R .

(7)

For static loading, the endurable stress is limited by the flow stress σ F,0 , i.e., the average of the ultimate tensile stress and the yield stress.







UTS

y





(8)

F,0

2

In general, rails are exposed to bending loads. Therefore, the endurable stress amplitude is higher than in tension/compression. This is considered by the elastic support factor n σ for cyclic loading (Eq. 9) and the plastic support factor η pl factor for static loading (Eq. 10).

n 

  e,0 bending e, 

(9)



   F,0 bending F,

pl  

(10)