<p>The light weight of composite materials has attracted interests to improve fuel economy of aircrafts and to extend flight range. The usage of composite materials is increasing in airframes and other parts of aerospace industry. Although most tests on composites are conducted uniaxially, they are subjected to multi-axial loads in real life applications. Hence, there is a need to better understand the complex failure mechanisms in composite structures. More reliable failure theories and damage progression models should be devised. Also, reliable criteria for predicting failure of fiber composite laminates are necessary for rational analysis and design. In this thesis, the behavior of a symmetric composite material under bi-axial loading is studied and the failure of the composite material is predicted by Yeh-Stratton criterion. A MATLAB program is prepared for the study of failure in tubular specimens composed of AS4/3501-6 carbon/epoxy laminates, which were subjected to internal pressure and axial force simultaneously to vary the states of stress. It is shown that the Yeh-Stratton criterion is in a good agreement with the experimental results. Future work may include collection of more accurate and different kind of experimental data on composite materials and modification of the interaction factor B12 value to evaluate its effect on the theoretical prediction by the Yeh-Stratton criterion. </p>

<p> Multiscale simulation is a long standing dream in computational physics and materials. The motivation is natural: each single-scale model has its deficiencies. For example, microscale models such as Molecular Dynamics are limited to size in space and time; macroscale models such as Finite Element Method find difficulty recovering some fundamental physical phenomena such as materials defects. Simulations across scales are challenging because quantities in different scales have distinct properties. Mechanism needs to be harnessed to translate the information. Cross-scale communication is a typical two-way message passing: bottom-up and top down. Bottom-up approach is relatively straightforward, where statistical theory or homogenization is used to collect lower-scale information and interpret it in higher levels. On the other hand, top-down approach requires physical insights. Specifically, in a mechanical system, top-down message passing can be the response of the molecular system when macroscale boundary conditions such as distributed load are enforced. </p><p> In this work, we reveal an intrinsic multiscale structure in solid materials. A &ldquo;supercell&rdquo; is introduced as a cluster of particles. Compare with &ldquo;material point&rdquo; in continuum mechanics, the &ldquo;supercell&rdquo; has internal degrees of freedom, which makes it equivalent to molecular systems. By introducing different force fields, we derive the dynamical equations for the different scales in the structure. The systematic multiscale framework solves the issue of top-down message passing by including quantities from different scales and connecting them in a uniform dynamical framework. We discuss the technical aspects in implementing the theory, i.e. constraints of the variables, integrators and temperature control. Numerical example of phase transition are presented to validate the theory, including bulk Nickel lattice under displacement and traction boundary conditions and Nickel nanowire with traction. Furthermore, based on the developed multiscale theory, we establish a computational model to achieve efficiency in realistic multiscale simulations. The model includes three parts: atomistic region, macro region and transition zone. Atomistic region is where physical details are desired and is simulated by Molecular Dynamics. Macro region only concerns macroscale deformable behaviors of solid materials, which can be calculated by various models depending on the problem of interests. We choose state-based peridynamics in this work as a demonstration. The essential part is the transition zone which is responsible for translating messages across different domains. The &ldquo;supercell&rdquo; developed in the previous theory is employed as a transition element to carry those different messages. With solid theoretic foundation, the cross-scale message translation is clearly characterized. We also construct a filter to solve the issue of high-frequency wave reflection. Examples of 1-D and 2-D wave propagations are presented to demonstrate the procedure of cross-scale transition and the effect of the filter.</p>

Thesis (M.S.)--Lehigh University, 2009. / Adviser: John P. Coulter.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3753. Adviser: M. Taher A. Saif. Includes bibliographical references (leaves 88-94) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2009. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3757. Adviser: Min-Feng Yu. Includes bibliographical references (leaves 93-110). Available on microfilm from Pro Quest Information and Learning.

<p> It is important to understand the dynamic failure behavior of structures subjected to impact loading in order to improve the survivability. Materials under impact are utterly affected by large deformations, high strain-rates, temperature softening and varying stress-states, which finally may lead to failure. It is shown that the impact characteristics are prone to change with several independent factors such as; impact speed, material thickness, and shape and orientation of the impacting object. Validated numerical simulations of impact tests reveal that the failure on ductile metals occur at certain locations of the failure locus that is constructed on a space as a function of all three stress invariants, which indicates that the failure depends profoundly on the state-of-stress. It is shown that existing material models are not always successful enough to cover the whole range of the failure locus and predict the failure. Therefore, it is a common practice to use different sets of material model parameters tuned or calibrated to cover a specific region of the failure loci in an ad hoc manner for practical reasons to match particular test results. Even in that case, specially tuned material properties are not capable of predicting these limited cases if differences in the mesh size and pattern need to be considered. </p><p> In this dissertation a new, generic, thermo-elastic/viscoplastic material model with regularized failure is introduced. The new material model is implemented into a non-linear, explicit dynamics finite element code, LS-DYNA. A von Mises type isotropic, isochoric plasticity is utilized, where isotropic hardening, strain-rate hardening and temperature softening is considered. The model takes adiabatic heating and softening into account due to the plastic work. The constitutive relation is coupled with a new regularized accumulated failure law that is specifically developed to cover a large extent of the failure locus as a function of state-of-stress, strain-rate and temperature. Regularization treatment is implemented to reduce mesh size dependency especially for the problems where softening and failure is involved for the failure prediction. </p><p> Ductile deformation and failure mechanism of 2024-T3/T351 aluminum alloy is investigated experimentally and numerically for quasi-static and dynamic conditions at various temperatures and stress-states. An intelligently contrived test matrix is developed by designing specific test specimens with different geometries that can construct a failure locus as a function of state-of-stress, strain-rate and temperature. An inverse material characterization algorithm is then introduced to generate input data for the new material model. Tabulated inputs of characterized material test results are directly used for both the constitutive and failure treatment of the new material model. Component based specimen tests that are used to characterize the material input properties and full-scale impact tests that are performed at different target thicknesses and impact speeds are used to validate and show the robustness, accuracy and efficiency of the new material model. </p><p> It is shown that the new material model is capable of predicting ballistic limit and failure modes accurately for structures under impact even if the failure mode changes drastically. It is also shown that the new regularization model provides less mesh size dependency. These associated features of the model suggest that the new material model can be used as a promising generic tool for diverse applications of dynamic ductile deformation and failure phenomenon. </p>

<p> To model shrink-fitting in metal components, an analytical model for two long compound cylinders with temperature dependent material properties and interference between them is developed for calculating transient temperatures and stresses. A finite element model is developed for the same geometry which incorporated the temperature dependent material properties. A convergence study is performed on the finite element and analytical model. The finite element model is validated by comparing the approximations of finite element model with the analytical solution. </p><p> In an assembly procedure of fulcrums for bascule bridges, called AP1, the trunnion is shrink-fitted into a hub, followed by shrink fitting the trunnion-hub assembly into the girder of the bridge. In another assembly procedure called AP2, the hub is shrink-fitted into the girder, followed by shrink-fitting the trunnion in the hub-girder assembly. A formal design of experiments (DOE) study is conducted on both AP1 and AP2 using the finite element model to find the influence of geometrical parameters such as radial thickness of the hub, radial interference, and various shrink-fitting methods on the design parameter of overall minimum critical crack length (OMCCL)&mdash;a measure of likelihood of failure by cracking. Using the results of DOE study conducted on both the assembly procedures, AP1 and AP2 are quantitatively compared for the likelihood of fracture during assembly. </p><p> For single-staged shrink-fitting methods, for high and low hub radial thickness to hub inner diameter ratio, assembly procedure AP1 and AP2 are recommended, respectively. For fulcra with low hub radial thickness to hub inner diameter ratio and where staged shrink-fitting methods are used, for AP2, cooling the trunnion in dry-ice/alcohol and heating the girder, and for AP1, cooling the trunnion-hub assembly in dry-ice/alcohol followed by immersion in liquid nitrogen is recommended. For fulcra with high hub radial thickness to hub inner diameter ratio and where staged shrink-fitting methods are used, cooling the components in dry-ice/alcohol and heating the girder is recommended for both AP1 and AP2. </p><p> Due to the limitations of AP2, assembly procedures by heating the girder with heating coils instead of dipping an already stressed trunnion-hub assembly in liquid nitrogen are studied for decreasing the likelihood of failure by cracking and yielding. In an assembly procedure called AP3-A, only the girder is heated to shrink-fit the trunnion-hub assembly in the girder. This assembly procedure AP3-A is found to be infeasible because the girder fails by yielding if heating is expected to be completed in a reasonable amount of time. An alternative assembly procedure called AP3-B is suggested for shrink-fitting where the heating of the girder is combined with cooling the trunnion-hub assembly in dry-ice/alcohol mixture. This assembly procedure AP3-B is found to be feasible. A complete DOE study is conducted on AP3-B to find the influence of parameters like hub radial thickness and radial interference at trunnion-hub interface on the design parameter of overall minimum critical crack length. The design parameter, OMCCL values during the assembly procedure AP3-B are quantitatively compared with the widely used assembly procedures (AP1 single-stage shrink-fitting and AP1 multi-staged shrink fitting). The results of this work suggest that increasing the hub radial thickness decreases the likelihood of fracture significantly. For hubs with large radial thickness, heating the girder combined with cooling the trunnion-hub in dry-ice/alcohol mixture (AP3-B) is recommended but for hubs with low radial thickness, multistage cooling of the trunnion-hub assembly in dry-ice/alcohol mixture followed by dipping in liquid nitrogen (AP1- multistage cooling) is recommended.</p>

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3120. Adviser: Nancy R. Sottos. Includes bibliographical references (leaves 94-99) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3078. Adviser: Nancy R. Sottos. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--Lehigh University, 2007. / Adviser: John P. Coulter.