# Al-Foam Compression Tests in Parallel and Serial Concepts
## 6 Results
### Parallel Compression Scenario  
For this test, the foams SP-LD2 and SP-HD2 were used. However, contrary to our expectations, the stress-strain curve did not totally add up mathematically, but rather, remarkable changes occurred. In the first 20% of the strain, the real stress of the compression stayed under the mathematical summation; however, in the second half of the plateau and the densification zone, the curve closely approached the theory summation. As hypothesized, the denser foam was predominant, and the lighter foam was only present as a mechanical alloy and it had a positive effect on the test results. Due to the parallel arrangement, the obtained values can be interpreted as a common element, as the foams were compressed simultaneously. The first local maximum of the curve occurred at 3.99% deflection at 2.014 MPa. This was followed by a plateau section at a higher location than in the previous test, with an average value of 2.764 MPa. No significant deviation in the runout uniformity of the plateau section was observed, and the section was nicely uniform. During the first 20% of the strain, the lighter foam had no significant effect on the test, but after this, it improved the performance of the compression. The densification zone started at 63% strain. The higher stress values naturally meant a higher amount of energy absorbed during the test. The diagram obtained during the test is shown in Figure 8.  
The values determined from the data collected during the test are summarized in Table 3. It can be clearly stated that too large a difference in density would have a negative effect on the results of the measurement, which is why it was important not to choose foams that differed in density too much. Similarly, a large proportion of the additional components are present in the alloyed metals. Choosing an extremely low percentage \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Label** & \(\sigma_{\rm max}\) [MPa] & \(\sigma_{\rm pl}\) [MPa] & \(\sigma_{\rm pl}\) [MPa] & \(\epsilon_{\rm D}\) [\%] & \(\epsilon_{\rm abs\_plateau}\) [M]/m\({}^{3}\)] & \(\epsilon_{\rm abs\_total}\) [M]/m\({}^{3}\)] \\ \hline SP-LD1 (0.22 g/cm\({}^{3}\)) & 1.315 & 1.062 & 1.044 \({}^{*}\) & 70 & 0.76 & 2.653 \\ \hline SP-HD1 (0.36 g/cm\({}^{3}\)) & 2.046 & 1.445 & 1.386 \({}^{*}\) & 73 & 1.25 & 3.996 \\ \hline \hline \end{tabular}  
* Calculated plateau stress with the Formula (3).  
\end{table} 
Table 2: Results of single compressions (own collaboration of authors). Figure 8: Stress–strain diagram of parallel scenario up to 80% strain (own collaboration of authors). First, the SP-LD3 and the SP-HD3 labeled specimens were used where the lighter was placed closer to the crosshead, while in the second step, the order was changed. The linear-elastic section reaches its first local maximum at 1.949% strain where the stress was 0.564 MPa. It is interesting that both layouts (parallel and serial) used the same foam, but showed a different curve occurrence. At up to 30% deformation, only the low-density foam was compressed, then at 35%, it started to harden. This appears as a rising section in the plateau area. After that, the denser foam started to bulk and created the second plane of the plateau section. Another interesting phenomenon is the beginning of the thickening zone. At nearly 80% deformation, the actual hardening of the foam structure begins. This means that the ideal energy absorption range was extended. It is important to mention the regression before the hardening zone, which can be explained by the physical unification of the foam with two densities. Based on the diagram, we analyzed the plateau section in two parts. The average plateau value in the first section was 0.69 MPa and in the second section 2.43 MPa. Table 4 collects the values of the test. The snapshots clearly show the order of the deformation in Figure 11. By changing the order of the foams, similar results were obtained. Figure 10 clearly shows the two levels of plateau sections. A sharp transition between the two may be observed.