# Al-Foam Compression Tests in Parallel and Serial Concepts
## 1 Introduction  
When a vehicle has movement, it has kinetic energy, which strongly depends on its weight and velocity. At the moment of impact, this energy is converted into impact energy in a few milliseconds. Therefore, the impact force and the deceleration of the vehicle and the passengers are remarkably influenced by the duration time of the collision [1; 2; 3; 4; 5]. That is why crumple zones have indicated roles during a crash. They are designed to extend the duration time of the collision while absorbing the most impact energy possible. This principle would ideally involve long and oversized crumple zones; however, weight optimization and efficient utilization inhibit this possibility. The car body structure is a so-called differing-strength frame, meaning the crumple zones are made of metals with high plasticity, but the cage is of high-strength steel. The latter is intended to prevent the deformation of the zone of passenger and driver, the former has a high deformation capacity, allowing it to absorb the impact energy [6; 7; 8; 9; 10; 11]. Vehicle electrification causes a weight surplus, owing to the mass of the batteries and their safety system. Moreover, modern vehicles contain more driving assistants and entertainment systems, increasing the unloaded weight. The weight surplus not only influences energy inefficiency, but also has a negative effect on the features of an accident. Therefore, weight optimization is one of the most important factors in the vehicle industry. The recently applied high-strength steels are not able to complete the passive safety-related requirements and allow for a lower weight at the same time. Furthermore, not only have the applied materials been modified in the lastdecade, but the applied manufacturing process has, too. Taking into consideration these requirements, the researchers took on the difficult task of investigating and developing the crumple zone structure. A new frame and frame component must be developed which is compact, low-weight, and can absorb as much energy as possible while inhibiting the impact energy conduction in the direction of the cage. To improve the energy absorbing capacity of a thin-walled crumple frame structure without a considerable weight increase, the integration of aluminum foam--as a filler material--is a good, promising method for the vehicle industry. A large number of investigations deal with the positive effect of the foam on the compression features of thin-walled structures [12; 13; 14; 15; 16; 17; 18]. The shell bodies usually have a special notch or thinned cross-sections that allow for predetermined deformation during a collision, while the foam inside of it is loaded by compression. Figure 1 represents a foam-filled thin-walled tube as a simplified crashbox specimens. The foam-filled thin-walled frame components could be considered absorptive structures, meaning several proportions of the compression load are absorbed naturally by the deformation of the outer shell body, and the foam plays a role in improving this energy absorption. If we would like the foam to absorb the same amount of energy, it also requires a special construction, in which the foam is loaded in a radial constrain application. The number of studies about radial constrained foam compression is less than that of normal compression, but most of them report positive results. In the case of radial constrained compression, the lateral strain of the foam is inhibited by the wall of the frame and results in higher compression strength and a higher amount of absorbed energy [19; 20; 21]. The long-term goal of the research is to develop a highly efficient, yet compact crumple crash box based on the principle of the radially inhibited clogging of metal foams. The literature search did not provide any clear results on how to influence the sealing results when using foams of different densities at the same time and this is the subject of this paper.