Date of Award

Fall 2021

Publication Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

W. Altenhof

Second Advisor

N. Zamani

Third Advisor

A. Alpas

Keywords

Aluminum, Axial crushing, Axial cutting, Energy absorption, Impact testing, Magnesium

Rights

info:eu-repo/semantics/openAccess

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

The primary objective of this research was to investigate cutting deformation modes, employing higher-bladed cutters (i.e. 6 or more evenly space blades), with an emphasis on superior energy absorbing capabilities in comparison to axial crushing, the current state-of-the-art. A series of test cases involving AA6061 extrusions were identified utilizing analytical models of the steady-state cutting force and mean crushing force and selecting geometries where the ratio of the former force normalized with respect to the latter exceeded unity. Quasi-static testing confirmed that the total energy absorbing capacity could be exceeded while simultaneously reducing the peak force under quasi-static loading for 8 and 10-bladed cutting modes.

The superior energy absorbing capacity was attributed to the onset of a hybrid cutting/clamping deformation mode, caused by interactions between the petalled sidewalls of the extrusion and the outer ring of the cutter. The contribution of this deformation mode towards total energy absorption was concluded to be dominated by the elastic properties of the extruded material since numerous geometric cases of AA6061 extrusions, in both T6 and T4 temper conditions, experienced a consistent (< 10 % variation) gain in the steady-state reaction force.

A comprehensive experimental study consisting of multiple extrusion geometries subjected to 4 to 10-bladed cutting deformation modes was also completed to observe the evolution of fundamental energy dissipating mechanisms, including: the trend of a decreased cutting force-per-blade in higher-bladed cutters and the onset of a localized peak wedge force. The high capacity 8 and 10-bladed cutting deformation modes were also studied at impact velocities up to 32 m/s to characterize the performance of these newly considered energy dissipation modes under elevated impact loading conditions. Velocity dependent force reductions were observed to a critical velocity of 21 m/s, associated with degrading contact forces at the blade/extrusion interface.

Complementary investigations were also conducted for novel AM30 magnesium extrusions, including the crushing-to-cutting comparison, 4 to 10-bladed cutting evolution exercise and dynamic testing up to 18 m/s impact velocities. These extrusions experienced semi-brittle cutting characterized by segmented chip formation in the cutting membrane, in contrast to the continuous membrane observed for aluminum and steel alloys and cracking ahead of the blade tip. Consequently, the load bearing capacity was drastically reduced in comparison to the axial crushing deformation mode. However, crushing of AM30 extrusions was shown to be impractical due to rampant cracking throughout the extrusion, in contrast to the more controlled, semi-brittle behaviour of the extrusions subjected to cutting. Additionally, the AM30 extrusions subjected to cutting were less vulnerable to the diminishing force-per-blade trend and localized peak forces were less prominent. The dynamically measured cutting forces were also higher than their quasi-static counterparts which is contradictory to the findings of traditional materials.

These experimental observations assisted with the derivation of enhanced analytical models of the steady-state cutting force. The newly obtained models accounted for the diminishing force trend, included an extended form to predict the onset and magnitude of cutting/clamping forces and differentiated between ductile and semi-brittle cutting deformation modes, with the consideration for a continuous contact membrane in the former and crack propagation governed by the J-integral in the latter. A procedure to predict the complete force-displacement response for extrusions subjected to axial cutting and hybrid cutting/clamping was also derived and validated utilizing the newly collected data.

Finally, the experimental evidence and improved theoretical modelling tools were utilized to design an actively adaptive cutting device which could alter its geometry prior to an impact, selecting the preferred configuration from a menu of options contained within a single assembly. Such a device could receive, interpret and respond to external stimuli and further utilize the analytical modelling approach to select a configuration which provides the optimal balance between high capacity energy absorption and mitigation of human injury. Furthermore, the proposed device was capable of exceeding and meeting the performance of comparable energy absorbers which implement progressive folding under quasi-static and elevated impact (i.e. > 21 m/s) loading conditions, respectively, via a 12-bladed cutting/clamping deformation mode.

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