Browsing by Subject "Fracture toughness"

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A numerical study on mechanical properties of low-density two-dimensional networks of crosslinked long fibers
(2023-04-03) Mane, Soham Manohar; Huang, Rui, doctor of civil and environmental engineering; Liechti, K. M.; Kyriakides, Stelios; Rausch, Manuel; Bonnecaze, Roger
In this dissertation, we study mechanical properties of low-density two-dimensional (2D) networks by finite element methods. Fiber-based materials are prevalent in nature and in engineering applications. To understand the relationship between the effective mechanical properties and the underlying microstructures, we consider a variety of periodic and random 2D networks of crosslinked long fibers. The linearly elastic properties of periodic 2D networks (e.g., square, triangular and Kagome) are well understood. However, for low-density networks, cooperative buckling of the fiber segments can take place at small strains, leading to nonlinear, anisotropic elastic behaviors. A transition from stretch to bending and then back to stretch dominated deformation is predicted for the Kagome and triangular networks. For random 2D networks, the stress-strain behavior is statistically isotropic and slightly nonlinear under uniaxial tension, dominated by stretch of the fibers aligned closely to the loading direction. Meanwhile, stochastic buckling occurs continuously in the random networks, leading to significant lateral contraction. Consequently, while the effective Young’s modulus follows a nearly linear scaling with respect to the relative density, the effective Poisson’s ratio exhibits a transition from stretch to bending dominated mode as the relative density decreases. The comparison between the periodic and random 2D networks highlights the profound effects of the network topology on the effective elastic properties. Furthermore, we study the strength of 2D periodic networks. First we present the elastic beam models to predict the effective tensile strength of the rotated square, triangular and Kagome networks. Next we conduct finite element analyses to simulate the damage initiation and progression in the periodic 2D networks assuming elastic-brittle fibers. For the Kagome networks subject to uniaxial tension in the y-direction, four different failure modes (including post-buckling modes) are predicted, depending on the relative density and the fiber strength. The elastic beam model does not consider the nonlinear elastic behavior due to buckling and thus generally overestimates the tensile strength. Moreover, for Kagome networks consisting of many unit cells, the effective tensile strength depends on the boundary conditions, and the presence of a crack-like defect could reduce the strength considerably.
Interactions between chemical alteration, fracture mechanics, and fluid flow in hydrothermal systems
(2018-08-16) Callahan, Owen A. (Owen Anders); Eichhubl, Peter; Barnes, Jaime D; Behr, Whitney M; Davatzes, Nicholas C; Olson, Jon E; Stockli, Daniel F
The hydromechanical properties of fault zones reflect evolving feedback between chemical, hydrological, and mechanical processes. These processes are evident in differences in fault zone architecture and the mineralogical, textural, and mechanical properties of the constituent parts. In this study, I quantify each of these attributes and explore feedback pathways evident in the Dixie Valley-Stillwater fault zone, Nevada, USA. I conducted 1) double-torsion load-relaxation tests to measure mode-I fracture toughness (KIC) and subcritical fracture growth index (SCI) in ambient and aqueous conditions, 2) uniaxial testing to measure unconfined compressive strength (UCS) and static elastic parameters, 3) mineralogical and textural characterization of altered and damaged rock, and 4) field observations focused on the role of alteration in fault zone evolution. The first investigation explored the impact of alteration on fracture mechanical properties of exhumed alteration assemblages, including: fumarole-related acid-sulfate alteration and silicification, silicification in an epithermal environment, quartz-kaolinite-carbonate alteration in an intermediate depth system, and calcite-chlorite-hematite alteration. The second investigation examined the impact of physiochemical conditions on fracture growth in silicified rocks. Environments included: ambient air, deionized water, dilute HCl, NaOH, and NaCl solutions, and deionized water at elevated temperature. The third investigation employed field observations to assess the impact of alteration on fault evolution. The results from these complimentary investigations show that fault-proximal weakening or strengthening are strongly influenced by hydrothermal processes. Silicification is associated with increased KIC, SCI, UCS, and brittleness, producing fault cores as strong or stronger than adjacent damage zone material. Calcite-chlorite-hematite assemblages containing abundant unsealed microfractures are approximately six times weaker than silicified rocks. All measures of strength increase when sealing of microfractures surpasses ~85%. SCI in silicified rocks is reduced in aqueous environments, with >60% reduction in alkaline solutions, suggesting that physiochemical conditions in hydrothermal systems may facilitate fracture growth. Field observations support the importance of alteration and precipitation in fault zone development; silicification and precipitation-strengthening contribute to thick fault cores, whereas damage and alteration-weakening promote strain localization. Together, results from these investigations highlight the important and underappreciated role of hydrothermal processes in the development of hydromechanical properties in fault zones.
Phenomenological constitutive modeling and numerical analysis of fracture toughness for shape memory alloys
(2022-05-02) Alsawalhi, Mohammed Yousuf; Landis, Chad M.; Foster, John T; Ravi-Chandar, Krishnaswamy Ravi; Mear, Mark E; Kyriakides, Stelios
Nickel titanium (NiTi) alloys possess unique characteristics that provide them the ability to recover large mechanical strains up to 8%. Pseudoelasticity and the shape memory effect are phenomena associated with SMA behavior. Shape recovery is driven by thermomechanical loading/unloading during the martensitic phase transformation. NiTi behavior also exhibits the property of asymmetry in transformation stress and transformation strain between the tension and compression responses as a result of forward and reverse phase transformations, as well as the reorientation and detwinning of the martensite phase. Furthermore, the process of heat generation during phase transformation near a crack tip effects the local temperature variations and thus the fracture toughness of the material. A new thermomechanical constitutive modeling approach for shape memory alloys (SMAs) that undergo a martensite to austenite phase transformation is presented. The novelty of this new formulation is that a single transformation surface is implemented in order to capture the main aspects of SMAs including forward transformation, reverse transformation, and martensite reorientation. Specific forms for the transformation surface and the transformation potential are devised and results for the behaviors captured by the model are provided for a range of thermomechanical loadings. The validity of the model is assessed with experimental studies of complex thermomechanical proportional and non proportional load paths at different temperatures using numerical simulations. The phenomenological constitutive model is implemented in finite element calculations and applied to the pseudoelastic and shape memory effects of a beam in pure bending. Fracture analysis is implemented within finite element computations to model the toughening due to the austenite to martensite phase transformation and martensite reorientation during steady mode I crack growth. Several dimensionless parameters relating the thermomechanical parameters of the constitutive model, the crack growth velocity, and the prevailing sample temperature are identified and applied to study the thermomechanical crack tip fields and the toughening enhancement due to the forward and reverse phase transformations in the vicinity of the crack tip. The first part of this dissertation involves validation of the model by comparisons of numerical simulations with experimental data and by developing consistent tangent moduli and applying the model to simple structural analysis of pure beam bending. First, uniaxial tensile and compressive stress-strain responses are simulated at four different temperatures: below the martensite finish temperature, between the martensite start and austenite start temperatures, between the austenite start and austenite finish temperatures, and above the austenite finish temperature. The numerical model reproduces the major aspects of the experimental measurements including the stress and strain levels. The transformation stress and transformation strain asymmetry between the tensile and compressive responses is also implemented in the model. The second problem investigates the performance of the model for a NiTi tube under a square axial-shear strain load path. The asymmetric model outperforms the symmetric model by reproducing the main features observed in the experiments. However, there is a notable difference in the magnitudes of stresses, mainly the shear stress, due to the anisotropy of the SMA material which is not accounted for in this model. The third problem examines the behavior of the constitutive model for tension-torsion of SMA wires for temperatures at the martensite and austenite phases. Again, the asymmetric model performs better than the symmetric model in terms of fitting the model response to the experimental measurements. The exclusion of anisotropy from the constitutive model has noticeable impact on the axial strain behavior at high temperatures. Lastly, the final problem investigates the pseudoelastic and shape memory behaviors of a beam under pure bending. The analysis in each case captures the moment-curvature and the temperature-curvature responses, as well as the axial stress distribution through the cross-section of the beam. The asymmetric model produced asymmetry in the axial stress distribution that fits the behavior of real SMAs. The second part of this dissertation involves fracture computations to analyze the toughening due to the stress-induced martensitic transformation and martensite reorientation during steady mode I crack growth. First, analyses are performed on the sizes and shapes of the various transformed zones near the crack tip for a range of temperatures analyzed. Secondly, the uniaxial stress-strain response is impacted by the thermomechanical parameters in the constitutive model which results in a relatively strong dependence of the transformation toughening on the material parameters. Next, numerical simulations are used to illustrate the effects of crack growth speed and heat capacity on the toughening. Finally, different sample temperatures show the strong impact on the toughness enhancement due to phase transformation. The last part of this dissertation discusses different approaches for material modeling, including different formulations associated with the transformation potentials and the associated integration routines. The first approach introduces a new internal variable that is a function of the other two in an attempt to control the pure shear stress-strain response as being a mixture between the tensile and compressive responses. The second approach introduces two stress invariants that are a linear or non-linear combination of the strain invariant. Here the objective is to control how fast the strain invariant goes towards uniaxial tension in a pure shear loading in order to allow the pure shear response to be a controlled mixture between the tensile and compressive responses as opposed to having similar behavior to the tensile response. The last approach for the integration algorithm utilizes a classical elastic prediction-transformation correction return mapping. This method simplifies the number of unknowns solved in the integration routine to just one. Therefore, a 1-D Newton-Raphson (NR) scheme is used which allows for more robust numerical calculations.