Date of Award

12-20-2024

Date Published

January 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Yeqing Wang

Keywords

Auxetic;Carbon fiber composites;FEA;LVI;Negative Poisson's ratio

Subject Categories

Engineering | Mechanical Engineering

Abstract

Carbon fiber reinforced polymer (CFRP) matrix composites exhibited superior properties, such as high specific stiffness and specific strength, excellent fatigue and corrosion resistance, and low coefficient of thermal expansion, have been popular across various industries including aerospace, marine, automotive, energy, civil infrastructure, and high-end sports over the past decades. However, one significant drawback of CFRP composites is the susceptibility to low velocity impact damage which, for aircraft, can be results of tool drop, runway debris, and bird strike during takeoff and landing. These impact events could lead to external and internal damages of a CFRP composite structure in various damage modes, such as fiber breakage, delamination, matrix cracking, which will significantly compromise the structural integrity. Introducing auxeticity or negative Poisson’s ratio is one potential solution to mitigate the low velocity impact damage of CFRP composites, which can be achieved by tailoring the layup of an anisotropic composite laminate. Such special layups involve use of nontraditional ply orientations other than the quad layups with ply orientations of 0°, 90°, +45°, -45°, and the effects of such layups on the other apparent mechanical properties are yet to be investigated. This work aims to investigates the impact of introducing in-plane or out-of-plane auxeticity (negative Poisson’s ratio) in carbon fiber reinforced polymer (CFRP) composite laminates to examine their mechanical performance, particularly under tensile, uni-axial bucking and low velocity impact through both experimental tests and numerical simulations. In this work, the in-plane auxetic and counterpart non-auxetic laminates have a layup of [15/65/15/65/15] and [35/60/-5/60/35]. For out-of-plane auxetic and non-auxetic laminates, the layups are [25/-25/25/-25/25] and [50/0/50/0/50], respectively. Tensile tests on laminates with various in-plane and out-of-plane Poisson’s ratios revealed that auxetic layups could significantly alter ultimate tensile strength. Laminates with in-plane negative Poisson’s ratios demonstrated considerably lower tensile strength (51.7% on average) than their Positive Poisson’s ratio counterparts, whereas out-of-plane auxetic laminates showed moderately lowered (23.1% on average) tensile strength than the counterpart non-auxetic laminates. Strain field analysis, supported by digital image correlation and finite element simulations, highlighted stress concentrations near specimen gage sections, influenced by the unbalanced layup designs. Auxetic laminates were found to have a more imminent first ply failure as well as ultimate tensile failure due to transverse strain aggregation. Experimental findings also reveal that auxetic laminates can triple the buckling strength under uniaxial compression compared to non-auxetic counterparts, attributed to the unique effects of negative Poisson’s ratios. A novel monoclinic plate-based approach was used to isolate the influence of auxeticity from changes in the bending stiffness matrix, confirming that auxeticity actively contributes to enhanced buckling strength, though this effect is sensitive to stiffness matrix elements, as well as the loading conditions and plate aspect ratios. Numerical simulations and experimental tests demonstrate that auxetic laminates experience significantly reduced delamination, matrix compressive damage, and fiber tensile damage under low velocity impacts at higher impact energies. The results underscore the potential of auxetic designs to improve the impact resistance and buckling stability of composite laminates but also highlight the need for careful consideration of other mechanical properties, such as tensile, compressive, and bending strengths. The findings provide critical insights for designing advanced composite structures with enhanced performance, leveraging the benefits of negative Poisson’s ratios while managing trade-offs in other structural properties.

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Open Access

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