**5.5 Crack growth rate**

A basic contribution of fracture mechanics in fatigue analysis is the division of fracture process to crack initiation period and the growth period to critical size for fast fracture (Burzic & Adamovic,2008). Fatigue crack growth tests had been performed on the CRACKTRONIC dynamic testing device in FRACTOMAT system, with standard Charpy size specimens, at room temperature, and the ratio R=0.1. A standard 2 mm V notch was located in base metal and in third layer of WM, for the estimation of parameters for BM, WM and HAZ, since initiated crack will propagate through those zones. Crack was initiated from surface (WM) and propagated into HAZ, enabling calculation of crack growth rate *da/dN* and fatigue treshold *Kth*. The results of crack growth resistance parameters, i.e., obtained relationship *da/dN* vs. *K* for base metal, sample 1 and for sample 2 are given in Figure 11 and 12. Parameters *C* and *m* in Paris law, fatigue threshold *Kth* and crack growth rate values are given in Table 5 for all samples as obtained from relationships given in Figures 11 and 12, for corresponding *K* values.

Fig. 10. Diagrams force-time, obtained by instrumented Charpy pendulum for sample 1 and

A basic contribution of fracture mechanics in fatigue analysis is the division of fracture process to crack initiation period and the growth period to critical size for fast fracture (Burzic & Adamovic,2008). Fatigue crack growth tests had been performed on the CRACKTRONIC dynamic testing device in FRACTOMAT system, with standard Charpy size specimens, at room temperature, and the ratio R=0.1. A standard 2 mm V notch was located in base metal and in third layer of WM, for the estimation of parameters for BM, WM and HAZ, since initiated crack will propagate through those zones. Crack was initiated from surface (WM) and propagated into HAZ, enabling calculation of crack growth rate

rate values are given in Table 5 for all samples as obtained from relationships given in

*K* values.

*Kth*. The results of crack growth resistance parameters, i.e.,

*K* for base metal, sample 1 and for sample 2 are given in

*Kth* and crack growth

t, 0C sample 1 sample 2 (BL)



sample 2 (Popovic et al.,2011).

*da/dN* and fatigue treshold

obtained relationship *da/dN* vs.

Figures 11 and 12, for corresponding

Figure 11 and 12. Parameters *C* and *m* in Paris law, fatigue threshold

**5.5 Crack growth rate** 

The behaviour of welded joint and its constituents should affect the change of curve slope in validity part of Paris law. Materials of lower fatigue-crack growth rate have lower slope in the diagram d*a*/d*N vs.* K. For comparison of the properties of surface welded joint constituents the crack growth rates are calculated for different values of stress-intensity factor range K.

Fig. 11. Diagram da/dN vs. K for base metal.

Fig. 12. Diagram da/dN vs. K for sample 1 and sample 2.

Surface Welding as a Way of Railway Maintenance 249

sample 1 i Kth =8.9 MPa m1/2 for sample 2). With comparation of crack propagation energy and crack growth rate, it is hard to establish the precise analogy, as toughness was estimated for the surface weld metal, whereas crack growth rate for each surface welded layer. Generally, buffer layer didn't show slow the initiated crack growth, with aspect of crack growth rate, while this effect is obvious in the case of toughness, i.e. crack propagation

On the base of obtained experimental results and their analysis, the following is concluded: 1. The experimental investigation of surface welded joints with different weld procedures has shown, as expected, significant differences on their performance in terms of mechanical properties. But, in both cases, it was shown, that in spite of poor weldability of high carbon steel, they can be successfully welded. Structural compatibility between deposite metal and base metal was achieved and martensitic layer wasn't formated.

2. The filler material is relevant parameter which affects on deposite layer quality. Work with self-shielded wires is more simple, specially for outdoor applications. Both used wires are on high technological level and can be recomended for reparation of highcarbon steel damaged parts. Final microstructure is the result of different influences: type of filler material, heat input, degree of mixture with previous layer and post heat treatment with subsequent surface layer. It is necessary to know all these factors and also to know the way of affect. Though applied wires are with different alloying concepts, result in both cases is that initial pearlitic moprhology is replaced by final desirable bainitic microstructure. It was shown that, by selecting corresponding

parameters, it is possible to obtain the morphology of the best properties.

3. The maximal hardness level of 350-390 HV is reached in surface welded layers of both samples, with equal hardness of base metal (250-300 HV). The main difference appears in the first deposition layer, where as expected, in sample 2 the hardness is significantly lower (buffer layer). The obtained hardness values ensure simultaneously the improvement of mechanical and wear properties, and in the case of a rail, represents maximal hardness preventing the wheel wear (Popovic et al., 2010). Similar results are obtained by tensile testing. Sample 2 has slightly higher ultimate tensile strength (1360 MPa) than sample 1 (1210 MPa) due to solid solution strengthening by alloying

4. The most improved results are obtained for impact properties. The toughness of base metal is 6-7 times lower than the toughness of weld metal, and more than twice lower than toughness of HAZ. For welding with buffer layer, at -200C, the drop of total impact energy is significant, due to lowering of buffer layer plastic properties at lower temperatures. The transition temperature of this material is above -200C, and it was confirmed by obtained impact toughness results. The use of buffer layer is beneficial for exploatation temperature above -50C. On the contrary, at lower temperatures, buffer layer loses its function and toughess decreases. On the contrary, for sample 1 the change of toughness is continous and without marked drop of toughness. At all tested temperatures, the crack initiation energy is higher than crack propagation energy. This

Obtained HAZ has better structure compare to base metal.

energy (Popovic et al, 2011).

**6. Conclusion** 

elements.


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

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

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 fracture toughness.

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 threshold, *Kth*, is exceeded(Burzic & Adamovic,2008). Fatigue treshold value Kth in base 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. after less number of cycles, than in sample 1.

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 sample 1 i Kth =8.9 MPa m1/2 for sample 2). With comparation of crack propagation energy and crack growth rate, it is hard to establish the precise analogy, as toughness was estimated for the surface weld metal, whereas crack growth rate for each surface welded layer. Generally, buffer layer didn't show slow the initiated crack growth, with aspect of crack growth rate, while this effect is obvious in the case of toughness, i.e. crack propagation energy (Popovic et al, 2011).
