# Structural Strength Analysis of a Rotary Drum Mower in Transportation Position
## 3 Results and Discussion
### FEA Verification and Validation  
The process for determining that a computational model accurately represents the underlying mathematical model and its solution is known as verification. Validation is the process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model [29]. In order to evaluate the accuracy of the FE model and conduct internal verification, a skewness metric was employed. This metric assesses the deviation of the elements from equilateral cells. A skewness value of 0 indicates perfect cell quality, whereas a value of 1 signifies fully degenerated cells. Specifically, the scale is as follows: 0 (equilateral), >0 to 0.25 (excellent), 0.25 to 0.50 (good), 0.50 to 0.75 (fair), 0.75 to 0.9 (poor), 0.9 to <1 (bad), and 1 (degenerate) [30; 31]. Notably, the FE model exhibited an average skewness metric value of 0.210 \(\pm\) 0.25, which falls within the category of excellent cell quality. A key component of assessing the validity of the strength-based design analysis process is confirming that the FEA stress values obtained through computer simulations accurately reflect the stress values obtained through the experimental analysis setup for real physical operating conditions. The comparison result was assessed using the calculated relative difference ratios in order to determine how closely the FEA simulations matched the machine's actual operating circumstances. Equation (2) was employed to calculate the difference ratios [32]. The difference ratios were defined as the relative differences between the experimental and the FEA values. \[Relative\ Difference\ (\%)=\left[\frac{\sigma_{Experimental}-\sigma_{FEA}}{ \sigma_{Experimental}}\times 100\right] \tag{2}\]  
Here, \(\sigma_{Experimental}\) and \(\sigma_{FEA}\) are defined as von Mises equivalent stress values (in MPa) obtained as a result of experimental analysis and FEM-based analysis, respectively. Accordingly to this, biaxial graphs of the numerical comparisons of experimental and FEM-based stress analysis results for the components coded RDM165-A-004, RDM165-B-003, and RDM165-B-004 are shown in Figure 12.  
It can be noted in the graphs that the relative differences range between 7.577% (RDM165-A-004) and 43.381% (RDM165-B-003) when comparing the equivalent stress values of the analyses carried out for the related components. The average relative difference ratio including all values was calculated as 24.25%. According to the studies including FEA validation that were presented in various scientific fields, the relative difference ratios obtained by comparing experimental/theoretical analyses with FEA results vary [33; 34; 35; 36; 37; 38; 39; 40; 41]. However, there are opinions that the relative difference ratio of a well-established FEA approach should be around 10% at most [42; 43]. In this context, the determined average relative difference value of 24.25% is just a little bit higher than the consensus of 10%. However, it becomes clear that these values are quite low when the absolute numerical values of the comparisons are properly examined. The maximum absolute difference in numbers is approximately 5 MPa. It could be said that, considering the physical conditions in which the experimental work was carried out and the solution approach specific to FEM, the actual numerical absolute difference is quite small. Although the absolute difference is small, the percentage value of the calculated difference is relatively high compared to the general view, since the compared values are also quite small. The following issues were identified as the conclusion of the final assessment for the FEA validation study: unexpected and unpredictable dynamic conditions of machine elements under real operating conditions, limitations in simulating real working conditions, solution approach specific to FEA, and mandatory assumptions which have to be considered in order to handle the limitations within the context of this numerical technique.