مطالعه عددي و مقايسهي انواع مقاطع برفدانه كخ پليمري پر شده با فوم، كاربرد در تصادم

Numerical study an‎d comparison of the different types of foam-filled polymeric Koch fractal sections, application in collision

مهندسي مكانيك

In impact energy absorption systems, the main objectives are to maximize energy absorption, control the crushing process, an‎d minimize the peak force transmitted to critical structural components. In this thesis, the crash behavior of crash boxes with Koch snowflake (fractal) cross-sections an‎d PLA+ polymer shells was numerically investigated, an‎d their performance was compared. Fractal geometries from the zeroth to the second order were considered. In addition to specimens without external reinforcement, configurations surrounded by an external hexagonal wall were also investigated to examine the effect of geometric reinforcement. To further improve energy absorption performance, the effect of internal foam fillers was also eva‎luated using three different materials: aluminum foam, expan‎ded polypropylene (EPP) foam, an‎d phenolic foam. The numerical simulations were carried out in Abaqus using the explicit dynamic solver, which is well suited to impact problems involving complex contact interactions an‎d large deformations. To assess the reliability of the numerical results, the model was validated through comparisons with experimental data an‎d published references. The kinetic-to-internal energy ratio was also examined, an‎d a mesh sensitivity analysis was performed to ensure that the results were not significantly affected by mesh refinement. The crashworthiness performance of the investigated structures was eva‎luated based on absorbed energy (EA), specific energy absorption (SEA), peak crushing force (PCF), mean crushing force (MCF), an‎d crushing force efficiency (CFE). The results show that, among the configurations with an external hexagonal wall, the second-order Koch fractal crash box filled with phenolic foam (KF2-H-Phenolic) provides the most balanced overall crash response. This configuration achieves an absorbed energy of 5,026.8 J an‎d a specific energy absorption of 18.97 J/g, which is the highest SEA obtained among all the investigated configurations. At the same time, it maintains a crushing force efficiency of 72.8%, while the peak crushing force reaches only 57.5 kN. The second-order Koch fractal configuration with an external hexagonal wall an‎d aluminum foam filling (KF2-H-Al) achieves the highest total energy absorption, with an EA of 5,375.1 J, an‎d exhibits a highly stable crushing-force response. However, this improvement comes at the cost of the highest peak crushing force, 62.1 kN, an‎d a shorter crushing stroke. This behavior suggests that a greater portion of the impact energy is absorbed over a shorter deformation range an‎d under higher crushing forces. For the configurations without an external hexagonal wall, the aluminum foam-filled specimen in the KF2 series shows the best overall performance. It provides simultaneous improvements in EA, SEA, an‎d CFE, although these improvements are accompanied by an increase in the peak crushing force. At lower fractal orders, the results indicate that EPP foam can improve energy absorption in some configurations without causing a noticeable increase in the peak crushing force. Therefore, EPP foam can be considered a promising option when a balance between energy absorption an‎d peak-force control is desired.

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