Date of Award

Fall 2025

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Subramani Sockalingam

Abstract

High performance carbon fibers are widely used as reinforcements in composite material systems for aerospace, automotive, and defense applications. The tensile strength of commercial fibers is significantly less than its theoretical limits. The composite systems are often overdesigned, thus any increase in the fiber tensile strength can yield significant cost and weight savings. Modification of fiber surface treatment (sizing) during manufacturing is a potential route to enhance fiber strength. Single fiber tensile testing at millimeter-scale is typically used to characterize the effect of sizing on the fiber strength. However, the longitudinal tensile failure of a composite system is a result of clustering of single fiber breaks occurring at length-scales (l) on the order of few microns (∼ one fiber radius, rf ) to a few hundred microns (one ineffective length, δ), i.e., rf < l < δ ∼ 20 ∗ rf . It is challenging to directly measure the tensile strength of a single carbon fiber of diameter 5μm at such short gagelengths. Hence, single fiber strength data at millimeter scale is usually extrapolated to determine strength at microscale gagelength using Weibull weakest link statistics. However, this often leads to overprediction in fiber strength that is required as an input for composite strength models. A full fundamental understanding of the strength performance of fibers has been elusive and whether intrinsic strength follows a Weibull distribution remains an open question.

In this study, we investigate the use of the transverse loading method for accessing and testing single IM7 carbon fibers at a range of microscale gagelengths. Transverse loading experiments are performed using indenters of different size at three different fiber starting angles (θ◦ start) to induce varying magnitude and lengths of axial tensile stress (strain) concentration in the fiber. The use of in-situ 2D SEM-DIC experiments is initially explored for characterizing the stress/strain fields developed over the fiber surface. Physical vapor deposition techniques such as sputter coating and pulsed laser deposition are investigated for generating high resolution speckle patterns on the fiber surface, suitable for performing SEM-DIC experiments. However, accurate calibration of SEM drift distortion errors for DIC strain measurement proved to be challenging due to the curved geometry of the fiber, highlighting the need for 3D SEM-DIC and light-weight in-situ tester for stereo imaging. Consequently, SEM-DIC experiments were not pursued further and finite element simulations were employed instead to quantify the stress distributions induced in the fiber during transverse loading.

The transverse loading response of the fiber is studied in terms of the experimental, global (far-field) axial failure stress for the various indenter-starting angle combinations and the locally induced stresses in the fiber-indenter contact region obtained from finite element simulations. SEM images of failed fiber ends retrieved after testing are also used to gain insights into the fiber failure under different loading conditions. Results from transverse loading experiments and simulations indicate that fiber failure may not be governed by axial tension for the different indenters and starting angle combinations investigated. Axial compressive and transverse compressive effects become more pronounced with increasing fiber starting angle for R ∼ 50μm indenter and decreasing tip radius, R < 50μm. Fiber failure in axial tension is expected to occur for relatively large indenters, namely, R ∼ 500μm and R ∼ 100μm at all starting angles, and R ∼ 50μm indenter at 0◦. R ∼ 500μm indenter is found to induce no axial tensile stress concentrations in the fiber-indenter contact region. Therefore, the effective gagelength (leff ) tested for R ∼ 500μm indenter is equivalent to the entire length of the fiber (L = 12 mm) considered for transverse loading. For, R ∼ 100μm and R ∼ 50μm, the axial stress distribution in the fiber at failure consists of a non-uniform stress-concentration region near the fiber-indenter contact zone and a uniform tensile region away from it. Fiber failure can occur in either region depending on the probability of encountering a critical flaw in that region. For a θ◦ start = 0◦, both R ∼ 100μm indenter and R ∼ 50μm indenter exhibit minimal degradation in global axial failure stress compared to the fiber uniaxial tensile strength. Weibull weakest link statistics derived from uniaxial tension experiments are found to accurately described fiber failure for these experiments, suggesting that fiber failure most likely occurred outside the region of localized stresses. At higher starting angles (θ◦ start = 18◦ and 27◦), Weibull analysis of fibers failing under the non-uniform stress state suggest a presence of a different flaw population compared to that characterized using uniaxial tension experiments. Based on the Johnson- Tucker method, the shortest effective gagelength accessed under transverse loading was leff ∼ 25μm with a median fiber strength of ∼ 9.3GPa which is approximately half the value predicted via unimodal Weibull extrapolation (∼ 18GPa) to the same gagelength.

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© 2025, Karan Deepak Shah

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