Date of Award

Fall 2025

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Subramani Sockalingam

Abstract

Ultrahigh Molecular Weight Polyethylene (UHMWPE) cross-ply [0o/90o] thin film composites are emerging as an effective material for impact applications due to their high axial strength-to-weight ratio and favorable delamination properties. Mode-II interlaminar shear (ILS) fracture-driven delamination has been shown to be a significant energy absorption mechanism. Therefore, accurately characterizing ILS behavior as a function of strain rate in these composites is essential to inform computational models. Since these composites are significantly thinner and weaker in both interlaminar shear and transverse tension than traditional carbon fiber epoxy composites, standard ILS Mode II test methods/specimens such as end-notched flexure (ENF) are expected to exhibit unwanted plastic collapse, and ENF testing at high rates is difficult for even favorable materials. To address this issue, a novel experimental method using a pre-cracked lap shear-type composite (PLSC) specimen bonded to rigid substrates is developed and applied to a range of experimental rates. When subjected to uniaxial tension, the rigid substrates induce simple shear loading on pre-cracked rectangular specimens, resulting in a more controlled variant of typical single lap shear test specimens. Experiments using the PLSC specimen are performed using solid-state extruded UHMWPE Tensylon® HSBD30A at quasi-static (QS) and dynamic test rates.

Experimental results indicate the mechanical response is initially linear, with significant late-stage nonlinearity and sudden ultimate failure, with significant increases in strength and energy release rate as the crack tip relative sliding rate (proportional to the strain rate) increases. For QS experiments performed using a uniaxial testing machine, the traction-separation response is effectively modeled by a trilinear law with an initial interfacial stiffness of 4.708±1.418 MPa/μm, a maximum shear traction of 3.267±0.646 MPa, a Mode II critical energy release rate of 69.6±18.8 J/m2, and a final displacement of 26.09±4.75 μm. For dynamic experiments performed using a tensile split Hopkinson pressure bar, three groups are produced: 3.18±0.94 m/s (low), 6.96±1.11 m/s (intermediate), and 14.06±2.27 m/s (high). Estimating conservatively, these are equivalent to strain rates on the order of 1.6-7.0*104 s-1, but actual strain rates may be on the order of 0.6-2.8*106 s-1. Maximum shear traction increases approximately linearly: 7.34±1.61 MPa, 8.68±1.84 MPa, and 14.63±2.03 MPa for the low, intermediate, and high dynamic rates, respectively, resulting in a 125%-348% increase over QS rate experiments. Energy release rate (GIIC) apparently increases quadratically: 827.7±290.1 J/m2, 1440±475.1 J/m2, and 4528.7±700.1 J/m2, respectively, resulting in a 1089%-6407% increase over QS rate experiments. Final separation (Δuf) values also increase: 142.8±25.4 μm, 207.6±66.0 μm, and 348.2±43.2 μm, respectively. Notably, interfacial stiffness does not appear to significantly change with increasing rate.

Post-fracture images are investigated, and finite element model correlation is performed to confirm the validity of final experimental values. Good agreement is observed in stress-displacement data between experiments and models for both QS and dynamic cases. A mechanistic explanation for the significant rate effects is proposed based on observations in literature, and additional experiments are proposed for further investigation.

Rights

©2025, Frank David Thomas Jr.

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