The deformed sections and the pattern of deformations from both test and CAE are compared in the Figure-12.7. It can be observed that both the specimens collapsed in a consistent manner with regular repeatable lobes with mixed modes involving inextensional and
extensional modes. The specimens are without any triggers and targeted to collapse in their
natural folding modes. The test and CAE curves agree with each other to a reasonable extent at their peaks and also through the crushing process. The first comparator for the effectiveness of the sections in consideration for energy absorption is initial peak force Pmax
needed for initiation of the collapse process. The specimens of Hexagon specimen collapsed with higher peak force at 343kN than that of Octagon due the differences in packaging of the corner elements in the edges which in Octagon is unfavourable and so is the lower initiation force. Although the main deformation mode of repeatable collapse was initiated at the contact point, a late instability due the elasto-plastic stress wave propagation caused the octagon specimen to deviate from regular crushing process at the end as indicated in the Figure 12.7.
The second comparative parameter which is indicative of crushing progression along the specimen is the dynamic crush
δ
c.
The test results for the hexagon and octagon specimens indicates the simillar progression of the deformation process and agree to a good extent with the corresponding CAE values. The similarity is in crushing process and its progression should provide simillar energy absorbtion which is the case as the specimens are designed to simillar dimensions. The Crush energy Edyn absorbed stands at about 14kJ and bothChapter 12: Results and Discussions
test and CAE provides close predictiveness in a reliable way with reasonable accuracy. The CAE models were able to predict and describe the plastic behaviour with inclusion of strain rate effects. The predictiveness of both static and dynamic behaviours of the crush process is at higher note, mainly due to good material model description through eloborate testing and incorporation of the test data within material models of Ls-Dyna.
The results conformance between test and CAE provided foundational step upon which further investigations were performed. More experiments were conducted numerically across the intended sections with selcted materials ranging from low end HA3 steel to high end Boron- steel along with Alumnium-AA6063. The numerical experiments were done under same loading condtions of 15m/s amd with same boudary conditions. The results so obtained are discussed further.
The intended specimens with sections of Square, Hexagon, Octagon and the newly developed 12-edge are subjected to dynamic crush tests at 15m/s with different materials of steel and Aluminium as mentioned before and their responses are extracted in a similar fashion. The fundamental characteristic of the crush process is as in the Figure-12.8. The force oscillation characterises the crushing process within the package domain in terms of crush. The velocity of impact invokes the strain rate loading on the specimens while an appropriate material stress-strain curve is selected corresponding to the loading rate to describe the state of stress in the plastic collapse process.
The force-displacements curves (Figure-12.8) indicate the dominance of 12-edged section over rest of the specimens with highest peak forces and shortest dynamic crush. The design of 12-edge section with corner angle of 950 provides an ideal corner element and collapse of that element provides highest energy absorption. With its tight packaging of edges the lobes collapse with shortest wavelength and there less collapse. The data curves of the peak crush force Pmax and dynamic crush
δ
c as in Figure-12.9 provides the overlay with other sections with different materials.Chapter 12: Results and Discussions
The sections of Octagon and Hexagon performed similar to each other for the same reason as discussed previously. The Square section which performed inferior in the quasi-static crush tests performed better in the dynamic crush. The favourable corner angle closer to 90deg in
the square sections gets attenuated by the higher strain rate effects during the crushing process. The sections with Aluminium collapsed in a similar fashion as that of Quasi-static tests due to insensitiveness of Aluminium to the rate effects which makes a unique material choice for energy absorption where energy absorption at constant force without getting affected by loading rate is desired.
The dynamic crush responses indicate very low crush for 12– edge sections across the material range indicative of it continuous dominant response. As the strength of the material increases the dynamic crush reduces indicative of increase in corner wall stiffness and there by higher force to initiate collapse. Square Section with lowest number of corners performed lowest in the ladder.
Chapter 12: Results and Discussions
The energy responses in term of Dynamic crush energy and percentage of Initial kinetic energy is an important parameter in characterising the performance of the thin walled sections. The energy absorbing capability of the sections decides the choice of the sections for designers to develop the crash compatible structure. The responses in Figure-12.10 indicates again the dominant performance of the 12-edge sections across all material ranges and has indicated to absorb about 70% of initial kinetic energy with the current design space and dimensions. This is about 52 % higher than convention square sections, 37% higher than Hexa and 32% higher than Octagon sections with same material, thickness and packaging spaces. The trends indicate increase in the energy absorption with material strength due to stiffness enhancements and higher force requirements for collapse initiation and progression.