**6. Conclusion**

248 Mechanical Engineering

Crack growth rate da/dN, m/cycle

> K=20 MPam1/2

K=30 MPam1/2

K=15 MPam1/2

Parameter m

WM 2 3.7810-13 3.61 - 1.8810-08 - HAZ 4.0710-13 3.79 - - 1,6110-07

WM 2 3.8510-13 3.88 - 2.0710-07 - HAZ 3.7610-13 3.93 - - 1.1810-06

Table 5. Parameters C, m, Kth and crack growth rate values for all zones of surface welded

Bearing in mind that weld metal consists of two layers (third layer is used for V notch), as referent values of K were taken: K =10 MPa m1/2 for BM, K =15 MPa m1/2 for WM1, K =20 MPa m1/2 for WM2, and K =30 MPa m1/2 for HAZ. It's important that all selected values are within a middle part of the diagram, where Paris law is applied. The crack growth rate in base metal is 3-4 times higher than in both weld metal layers, i.e. the growth of initiated crack will be slower in weld metal layers. This means that for the same value of stress intensity factor rang K, base metal specimen needs less number of cycles of variable

In all three zones of surface weleded joint (WM2, WM1 and HAZ), sample 2 with buffer layer has higher crack growth rate than sample 1, i.e. the growth of initiated crack will be slower in sample 1. This means that for the same value of stress intensity factor rang K, specimen of sample 2 needs less number of cycles of variable amplitude than specimen of sample 1, for the same crack increment. The maximum fatigue crack growth rate is achieved in HAZ for both samples, when stress intensity factor range approaches to plane strain

If a structural component is continuously exposed to variable loads, fatigue crack may initiate and propagate from severe stress raisers if the stress intensity factor range at fatigue

metal (Kth =8 MPa m1/2) is lower than fatigue treshold value Kth in weld metal of both metal. Fatigue treshold value Kth for sample 2 (Kth =8.9 MPa m1/2) is lower than that for sample 1 (Kth =9.5 MPa m1/2). This means that crack in sample 2 will be initiated earlier, i.e.

Values of fatigue threshold and crack growth rates corespond to initiation and propagation energies in impact testing, and in this case, good corelation is achieved (Popovic, 2006). Sample 1 has higher crack initiation energy (20 J) and higher Kth ((Kth =9.5 MPa m1/2 for

*Kth*, is exceeded(Burzic & Adamovic,2008). Fatigue treshold value Kth in base

K=10 MPam1/2

4.4510-13 3.74 1.1110-08 - -

4.6310-13 3.87 1.6510-08 - -

Zone of surface welded joint

WM 1

WM 1

Base

sample 1

sample 2

joints.

fracture toughness.

after less number of cycles, than in sample 1.

threshold,

Fatigue threshold Kth, MPa m1/2

9,5

8,9

Parameter C

amplitude than weld metal specimen, for the same crack increment.

metal BM 8.0 3.3110-11 3.28 6.3110-08

On the base of obtained experimental results and their analysis, the following is concluded:


Surface Welding as a Way of Railway Maintenance 251

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