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Title:Mechanical behavior of individual type I collagen fibrils
Author(s):Liu, Julia Hong
Advisor(s):Chasiotis, Ioannis
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
rate dependency
cyclic loading
Abstract:Despite the plethora of studies on the mechanical response of collagen, especially at the molecular scale or at much larger length scales, such as of those of fibers, tendons, and fascicles, there is still limited information about the mechanics of collagen fibrils (50 - 500 nm diameter) that serve as mesoscale building blocks in tissues. In this dissertation research the mechanical behavior of dry individual reconstituted collagen fibrils with different diameters were investigated via monotonic tests at 0.004 s-1, cyclic loading and cyclic loading/recovery experiments, and strain rate tests spanning six decades of strain rates in the range 10-3 – 102 s-1. Experiments were conducted with reconstituted type I calfskin collagen fibrils which were isolated from buffer and after drying were adhesively attached to a microelectromechanical systems (MEMS) device. Experiments were carried out using high-resolution optical microscopy under dry conditions at 20-30% relative humidity (RH) and laboratory temperature. Eight fibrils with diameters of 165±77 nm tested under monotonic loading yielded an average tensile strength of 752±186 MPa, ultimate stretch ratio of 1.3±0.06, and initial stiffness of the stress (σ) vs. stretch ratio (λ) curves, E1 = 5.7±2.3 GPa. These results depended on fibril diameter: fibrils with larger diameters showed increased maximum stretch ratio, λmax, and decreased E1 and decreased stiffness, E2, of the softening regime in the σ - λ curves. Normalizing the applied stress with E1, removed the diameter size effect and provided great consistency in the softening regime of different σ - λ curves. The same process was applied to fibrils tested at nominal strain rates of 10-2 - 102 s-1 showing good agreement between σ/E1 - λ curves obtained at the same strain rate from different fibrils, but also showed a clear increase in E2 with the applied strain rate without a reduction in λ at failure, which implies a gradual linearization of σ-λ curves at higher rates. The mechanical behavior under cyclic loading was studied via experiments in each of the three regimes, with target λmax ~ 1.05 in regime I, λmax ~ 1.25 in regime II, and λmax ~ 1.3 in regime III. In regime I, E1 was unaffected by cycling loading or recovery. The residual strain increased in every cycle, but ~80% of λmax was recovered after resting for 1 hr at zero stress. Regime II was characterized by constant E1, after an initial drop between cycles 1 and 2, a slightly increasing value of E2 in every cycle, and increasing residual strain with cycling. Cycling in regime III also resulted in constant E1 and E2 after an initial reduction between cycles 1 and 2 and increased residual strain with cycle order. The experimental results point out to a process of damage accumulation during cycling, as manifested by the very consistent hysteresis loops and the gradually accumulated residual strain, which however, does not affect the mechanical stiffness of regimes I and II. The latter points out to a cross-link network within the collagen fibril that maintains molecular connectivity, as well as material regions that allow for viscous sliding (supported by the increase in E2 and E2/E1 with applied strain rate) in the softening regime of the σ - λ curves without disrupting the cross-link network. The rapid recovery and restoration of the three-regime shape of the σ - λ curves of collagen fibrils also supports the existence of sacrificial bonds which reform upon recovery that is driven by residual stresses in the fiber.
Issue Date:2016-07-22
Rights Information:Copyright 2016 Julia Liu
Date Available in IDEALS:2016-11-10
Date Deposited:2016-08

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