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Title:Limiting factors of the tensile strength of aramid fibers
Author(s):Sahin, Korhan
Director of Research:Chasiotis, Ioannis
Doctoral Committee Chair(s):Chasiotis, Ioannis
Doctoral Committee Member(s):Chew, Huck-Beng; Geubelle, Philippe H.; Lambros, John; Sottos, Nancy R.
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Kevlar aramid fiber
Fiber skin
Fiber core
Size effect
Strain rate effect
Shear strength
Abstract:The correlation between the evolution of crystallite orientation in aramid fibers during loading and their mechanical and failure behavior were investigated. Three types of as-spun aramid fibers and a heat-treated type were employed with initial distributions of crystallite orientations between 16.7º and 9.7º with respect to the fiber axis. These directly correlated with the initial moduli that were between 66 GPa and 119 GPa, with no correlation between the initial crystallite orientation distribution and the tensile strength values that ranged between 3.5 and 4.0 GPa. Cyclic loading of individual, 10 mm long, as-spun filaments increased their initial moduli, all converging to 100 GPa for all fiber types when cycled to 90% of their respective tensile strength values. This modulus value (100 GPa) corresponds to a stable crystallite orientation distribution of 11.6º. On the other hand, the initial unloading modulus of all fiber types when loaded to 90% of their tensile strength converged to ~165 GPa which approaches the theoretical modulus of 220 GPa for monopolymer aramids. This limit value of the unloading modulus also signifies the tightest crystalline domain orientation distribution of 6.6º with respect to the fiber axis. However, this orientation distribution is not retained upon unloading. On the other hand, as-fabricated, post-spun heat-treated fibers had a much higher initial modulus of 120 GPa, and an initial unloading modulus of 170 GPa after mechanical cycling to 90% of their tensile strength value, corresponding to 5.8º domain orientation distribution. In all aramid types, mechanical cycling increased the initial modulus by as much as 54% while leaving the tensile strength of each fiber type unaffected in the narrow range of the aforementioned initial values. Thus, the limiting orientation distribution of ~6º emerges as the controlling factor in tensile failure of this class of fibers. Tension tests conducted at different strain rates showed that the permanent orientation of crystalline domains at high strains/stresses scales inversely with the applied strain rate. Notably, at strain rates of 0.2-0.3 s -1 both as-spun and heat-treated fibers were linearly elastic until failure. A hypothesis that the fiber tensile strength is controlled by preexisting defects was tested by examining the scaling of the tensile strength with the fiber gauge length for fibers with lengths in the range of 200 µm to 10 mm. Prior works that were limited to fiber gauge lengths longer than 2 mm, have been inconclusive due to large data scatter for short fibers. Controlled tests conducted in this dissertation research with dedicated test apparatuses for small scale experimentation, demonstrated a constant tensile strength for gauge lengths as short as 200 µm, thus, implying that failure does not obey weakest link statistics that are descriptive of critical flaw-induced failure initiation. Notably, in short gauge length fibers (200 µm) of all aramid types failure initiation occurred near the skincore interface, followed by extrusion of the fiber core from the skin. Thus, the microstructural differentiation between the fiber core and the skin presents a likely limiting factor in tensile strength of aramid fibers. Finally, the shear strength of the fiber core was measured for the first time with novel experiments that were designed and implemented with the aid of surface micromachined Microelectromechanical Systems (MEMS) devices. Edge notches were milled out in individual fibers using a focused ion beam to generate a zone of uniform shear stress along the fiber, when the latter was subjected to uniaxial tension. The optimum specimen design and specimen geometry were guided by a finite element analysis to shape the notch tip such that the stress singularity is eliminated and a uniform shear dominant plane is achieved. These unique but also challenging experiments were carried out on aramid fibers with the lowest orientation of 16.7º resulting in average shear strength of 85±7.6 MPa. In conclusion, this dissertation research established new experimental tools and methods to investigate the origin of failure initiation in aramid fibers manufactured under different conditions. A limiting orientation distribution angle was established for all aramid grades, including those that were subjected to heat treatment, while the skin-core interface was identified as the weak interface where failure may take place.
Issue Date:2017-12-08
Rights Information:Copyright 2017 Korhan Sahin
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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