# Design and Experimental Analyses of Hybrid Piston Rods Used in Hydraulic Cylinders under Axial Load
## 3 Results and Discussion
### Solid Rod  
#### 3.2.1 Buckling Load against Laminates in Solid Hybrid Rod  
Figure 11 shows the change in buckling load for the solid rod in a hydraulic cylinder application as a function of the number of laminate layers used in the CFRP. For both cases 1 and 2 of the solid rod, four specimens with a composite of steel and CFRP or CFRP only, the number of laminate layers was varied between 14 and 60. The buckling load decreased by 4% in case 1 when the number of laminates was increased from 14 to 30. However, for case 2, the decrease in buckling load over the same period is only 0.4% (negligible change), which means that the buckling load remains almost same even when the number of plies is increased. Further down, when the number of laminates is increased from 30 to 46 in both case 1 and case 2, both show a different trend, i.e., for case 1 there is a gradual increase in buckling load up to 16%, while in case 2 there is a decrease in buckling load up to 12%. It is interesting to note that the difference between the buckling load values of case 1 and case 2 is much less. Steel loses buckling strength as its diameter (volume) decreases. However, the addition of CFRP volume compensates for the buckling strength lost by steel without increasing the weight of the piston rod. When the number of laminate layers is further increased in both case 1 and case 2, the rates of change of buckling load are 4% and 7%, respectively, and the difference of buckling load values between the two cases slightly increases. This shows that the buckling load is not greatly affected when a smaller number of laminates are combined with steel (i.e., up to 50% of the total laminates), but when the number of laminate layers laid is higher, the buckling load is greatly affected. These results could be useful in optimizing the buckling load and weight of the piston rod in a hydraulic cylinder application. As shown, the variation in buckling load is minimal at higher values of CFRP volume fraction, and as the CFRP volume fraction increases, the weight of the cylinder decreases. These observed differences in buckling load carrying capacity of CFRP  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \begin{tabular}{c} **Rod** \\ **Type** \\ \end{tabular} & \begin{tabular}{c} **Sample** \\ **Volume \%** \\ \end{tabular} & \begin{tabular}{c} **CFRP** \\ **Values** \\ \end{tabular} & \begin{tabular}{c} **No of Laminate** \\ **Plies** \\ \end{tabular} & \begin{tabular}{c} **Laminate Orientation** \\ **45\({}^{\circ}\)** \\ \end{tabular} & \begin{tabular}{c} **Experimental** \\ **45\({}^{\circ}\)** \\ \end{tabular} & \begin{tabular}{c} **Weight in Kg** \\ **Buckling Load kN** \\ \end{tabular} \\ \hline \begin{tabular}{c} Solid \\ steel rod \\ \end{tabular} & & 0 & 0 & 0 & 0 & 0 & 0 & 291.26 & 3.6 \\ \hline \multirow{6}{*}{\
\begin{tabular}{c} Hybrid \\ Solid \\ \end{tabular} } & \multirow{6}{*}{Case 1} & 25 & 14 & 8 & 2 & 2 & 2 & 182.40 & 2.30 \\  & & 50 & 30 & 18 & 4 & 4 & 4 & 175.54 & 1.38 \\  & & 75 & 46 & 30 & 6 & 6 & 4 & 203.98 & 0.84 \\  & & 100 & 60 & 48 & 4 & 4 & 4 & 195.15 & 0.66 \\ \cline{2-10}  & & 25 & 14 & 4 & 4 & 4 & 2 & 221.63 & 2.32 \\  & & 50 & 30 & 14 & 6 & 6 & 4 & 220.65 & 1.40 \\  & & 75 & 46 & 22 & 10 & 10 & 4 & 194.17 & 0.86 \\  & & 100 & 60 & 44 & 6 & 6 & 4 & 207.90 & 0.68 \\ \hline
\begin{tabular}{c} Hollow \\ steel rod \\ \end{tabular} & \multirow{6}{*}{Case 2} & 25 & 14 & 8 & 2 & 2 & 2 & 175.55 & 1.90 \\  & & 50 & 30 & 18 & 4 & 4 & 4 & 171.62 & 0.98 \\ \cline{2-10} Hybrid & & 100 & 40 & 28 & 4 & 4 & 4 & 169.66 & 0.58 \\ \cline{2-10}  & & 25 & 14 & 4 & 4 & 4 & 2 & 197.12 & 1.92 \\ \cline{2-10}  & & 50 & 30 & 14 & 6 & 6 & 4 & 199.08 & 1.00 \\  & & 100 & 40 & 24 & 6 & 6 & 4 & 197.12 & 0.60 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Experimental data for hybrid (Steel + CFRP) rods.  
wrapped solid bars could be due to the key parameters of CFRP, such as fiber angle and orientation, which affect the buckling strength of the rod and need to be carefully selected in the design of the hybrid (steel + CFRP) rod.