## 3 Results
### Optimization of the Material Used
#### 3.3.2 Aluminium Alloy EN AW-6060 T6  
Aluminium alloy suitable for the construction of the frame of the alternative SkidCar system was chosen (EN AW-6060 T6 designation), according to CSN EN 573 [38], with the chemical designation AlMgSiO. The T6 label indicates the heat treatment of the alloy, in this case dissolving, annealing and artificial ageing. The aluminium alloy EN AW-6060 T6 is sufficiently machinable, and has sufficient strength and hardness for use on the frame of the alternative SkidCar system. Not least, its corrosion resistance is sufficient and can be increased by anodic oxidation (by anodizing). The weldability of this alloy is good; it can be welded by MIG or TIG alternating current methods. The strength of this material is lower than that of the currently used steel, so the profile of the crossbars has been changed from PR OBD--80 \(\times\) 60 \(\times\) 5 to PR OBD--100 \(\times\) 80 \(\times\) 6. As the width of the proposed profiles is larger, it is recommended to change the width of the mounts on the experimental car, which sit on the upper surface of the crossbars, to the same width, i.e., 80 mm. Welds usually have a lower strength than the surrounding material, so it is advisable to subject the weldment to dissolving, annealing and curing, to obtain the original strength in the weld area. The basic parameters of the material are shown in Table 5.  
The allowable stress of the structurally optimized frame for aluminium alloy EN AW-6060 T6 is determined according to the Formula (1) \(\sigma_{\text{{D}OV}}\) = 50 MPa. The modified frame structure model was subjected to a geometrically and materially non-linear strength analysis (Figure 16). The execution procedure is similar to that of the original frame design. To obtain the Load Factor value for loss of stability, tangents to the elastic and plastic part of the curve were created again, and the value of their intersection was read. The value \(L_{\text{{F}st}}\) was concerning this case (3.50). Figure 17 shows the mechanical stress in the critical area (point 53 719,) for the entered value of the load used for the GMNA analysis. According to the GMNA analysis, the frame has not exceeded the maximum permissible stress. From this point of view, the aluminium alloy frame complies with the requirement. The value of the permissible Load Factor (\(L_{\text{{F}D}}\) = 1.02) calculated according to relation 2 is greater than 1; therefore, this concept of aluminium alloy frame fits from the perspective of strength. The maximum permissible load can only increase by 2% for compliance with the limit conditions. The structure is conformed to the limits of plasticity and stability and the maximum allowable stress.Within the material optimization, two materials were proposed from which the frame of the alternative SkidCar system can be manufactured. The first was considered as stronger steel S355J0. Since this steel is stronger than the S235 JR steel from which the original ASC frame solution was designed, it resulted in even more significant weight savings (Table 6). \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Ultimate tensile strength (MPa) & 170 \\ Yield strength (MPa) & 140 \\ Density (kg.m\({}^{-3}\)) & 2700 \\ Ductility (\%) & 10 \\ Poisson’s ratio (-) & 0.33 \\ Young’s modulus (MPa) & 70,000 \\ \hline \end{tabular}
\end{table} 
Table 5: Basic parameters of aluminium EN AW-6060 T6 [38]. Figure 16: Von Mises’s stress distribution in the optimized ASC crossbar made of aluminium material. Figure 17: Load characteristics of the node 53 719. The second alternative material for the SC frame was chosen as an aluminium alloy, with the designation EN AW-6060 T6. This alloy has lower strength characteristics than the original material. It was therefore necessary to use larger PR OBD profiles 100 \(\times\) 80 \(\times\) 6 with a greater thickness than the crossbars. However, even so, the greatest weight savings were made.