Behavior of Polyurethane Foam-filled Steel Hat Sections Under Axial Loading: Testing and Simulation

Abstract

Empty hat sections, single and double, made of steel are frequently encountered in automotive body structural components such as front rails, B-Pillar, and rockers of unitized-body cars. These closed-section thin-walled components can play a significant role in terms of impact energy absorption during collisions thereby protecting occupants of vehicles from potential or severe injury. With the need for higher fuel economy and due to stringent emission norms, auto manufacturers are aggressively looking for means to reduce a vehicle's weight either by employing materials lighter than steel (such as aluminum, fiber-reinforced composites, etc.) or by substituting traditional mild steel-based body parts with those made with high strength steel of lower gages, or through a combination of both the strategies mentioned. Reducing gages, however, may lead to structural instability due to early inelastic buckling. This phenomenon of instability provides an opportunity for using polyurethane (PU) foam in structural members. Once a hollow structural member is filled with PU foam, it delays the local buckling mechanism and leads to higher strength of the structure. The current study explores the effect of PU foam-based reinforcement of tubular structures. Here, both empty and PU foam-filled single-hat and double-hat components are tested under quasi-static axial loading in a UTM. The resulting load-displacement responses are compared. Higher energy absorption as well as mean load is observed for foam-filled hat sections. Moreover, for a given total folded length, more folds are observed in foam-filled hat sections as compared to empty hat sections. Experimentally obtained half fold lengths for double-hat sections are compared with analytical predictions of the same for square tubes. Furthermore, quasi-static axial crush behaviors of single- and double-hat section components are predicted through finite element modeling and analysis using the explicit non-linear LS-DYNA code.