## 3 Results and Discussion
#### VCCT-Based Fatigue Crack Growth  
The consequence of crack extension (G) during the fracture per unit process of a newly created fracture surface is implemented in the FE computation of field variable at the nodes to approximate the values of the interior element by interpolation of nodal values. The  
Figure 8: Crack length vs. cycle numbers by crack gauge and optical measurement. Figure 9: Fatigue crack growth rate for both steels. element edge alignment with the crack in the finite element method analysis provides vital benefits in the numerical modelling of crack propagation with constant amplitude, high cycle fatigue. The analysis is run for the same increments after the unloading step has to crack through the thickness. Figure 10 presents the distribution of stress in terms of the von Mises and deformation shape in the FE model for symmetrically loaded crack configurations with sharp crack. Furthermore, the anticipated small-scale yield elastic singularity governed by stresses at distances from the notch tip characterise the condition in which linear elastic SIFs form a valuable measure of the intensity of the surrounding elastic field. Calculation from the test starts after initial crack length and the crack growth increment of 0.2 mm as recommended by IIW [33]. The post-processing results for the cycle count are obtained from the simulation. The crack growth curve at stress ratio R = 0 for S355 with 150 MPa and 100 MPa was applied for S960 steel, the comparison of the results with the experiment is presented in Figures 11 and 12, respectively. Table 3 depicts the percentage error based on the load cycle at a crack length of 17 mm of both the experimental and numerical results. The FCG based on the strain energy release rate for VCCT presents a better performance close to the experiment value of load cycle with corresponding crack trend in S355 steel than that of the S960 steel. Although, fracture mechanics is more explicit, which is accomplished with a crack growth test with pre-cracked standard specimens to get the crack behaviour. However, other material qualities such as yield strength, tensile strength, and modulus of elasticity may have an influence on metal fatigue strength. Figure 11: Fatigue crack growth of S355 steel with applied stress of 150 MPa at stress ratio R = 0. Figure 10: Three-dimensional simulation of stress distributions and deformation. The value of the stress ratio R has a very strong effect on the crack growth rate, the simulation of 3D FEM model with a stress ratio R = 0.1 and applied CAL of 62.5 MPa for S355 and S960 steel performed. The predicted crack length plot as a function of load cycle counts is presented in Figure 13. Furthermore, Table 4 displays the percentage error based on the number of load cycles at the crack length of 17 mm for both steels. Figure 12: Fatigue crack growth of S960 steel with applied stress of 100 MPa at stress ratio R = 0. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**S960 Steel Load Cycle (N)**} & \multicolumn{1}{c}{**S355 Steel Load Cycle (N)**} \\ \hline Experiment & 120,000 & 275,000 \\ VCCT-based FEM & 114,873 & 239,679 \\ Percentage error & 4.3\% & 12.8\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of number of cycle at 17 mm crack length for the experiment and simulation. Figure 13: Comparison of FCG at CAL of 62.5 MPa, R = 0.1 for S355 and S960. Although the steel properties are quite different, many works of literature dealt with the effect of fatigue crack properties of very high strength steels. The fatigue crack initiation stage of high-strength low alloys steel such as S960 withstands a longer load cycle. This was the reason why steel S960 performed at 100 MPa and 150 MPa is applied for S355 steel. Hence, the computation shows a faster crack propagation rate in the case of the S960 steel [34; 35]. However, the fatigue crack resulted in exponential growth between the steels compared with the same stress ratios.