Department of Aerospace Engineering and Mechanics

Permanent URI for this community

https://ir.ua.edu/handle/123456789/27

Browse

Browsing Department of Aerospace Engineering and Mechanics by Subject "Applied Mechanics"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Damage evolution in composite materials under environmental ageing: a stochastic model with experimental validation
    (University of Alabama Libraries, 2009) Rahman, Rezwanur; Haque, Anwarul; University of Alabama Tuscaloosa
    This work emphasizes on predicting probability density function of damages or "number density of damage" in graphite/epoxy polymer matrix composite materials (PMC) under hygrothermal aging condition. A coupled Forward-Backward Stochastic Differential Equation (FBSDE) is proposed as a mathematical model to predict number density of damages. The FBSDE consists of damage nucleation and annihilation rate in terms of Brownian motion. The uncertainty in damage nucleation and annihilation rate is noticed by proposing these two terms as "Brownian motion with drift". In order to verify the proposed model, a quantitative analysis was carried out on a limited number of graphite/epoxy specimens manufactured by VARTM process. The specimens were kept in a hygrothermal condition with room temperature cycling. A rigorous quantitative analysis of damages was done by optical microscopic inspection at different stages of aging period. The damages were classified based on the criteria of their size. Finally, the experimentally collected data for number density of damages were verified by using the proposed FBSDE. A detail parametric study was carried out using FBSDE and best possible predicted data was validated with the experimental observation. A reasonable estimation was observed from the model output.
  • Thumbnail Image
    Item
    Investigation of Taylor impact test of isotropic and anisotropic material through geometrical characteristics of specimens
    (University of Alabama Libraries, 2010) Cao, Zhiyi; Barkey, Mark E.; University of Alabama Tuscaloosa
    In this thesis, high strain rate properties of isotropic material (a copper alloy) and anisotropic material (2195-T8 aluminum-lithium alloy) are investigated using Taylor impact tests. Coordinate measuring machines (CMMs) are used to measure the shape of specimens after the deformation. The geometrical data enables us to determine the plastic distributions and the dynamic yield stresses of specimens. A raise in yield strength is found in the copper alloy during the impact. It means that material properties of the copper alloy are sensitive to high strain rate. Yet such phenomenon is not found in the 2195-T8 aluminum-lithium alloy. Based on the uniaxial compression strain state in the barreling regions of the specimens, the dynamic yield stresses in the rolling, transverse and short transverse directions are obtained for the 2195-T8 aluminum-lithium alloy. This enables us to determine the anisotropic coefficients in Hill's criterion and carry out the finite element analysis. The dependencies of fracture are also investigated. It is found that the fracture is sensitive to maximum shear stress, equivalent plastic strain and stress triaxiality.
  • Thumbnail Image
    Item
    Luminescent coating image analysis on a three dimensional grid
    (University of Alabama Libraries, 2010) Esirgemez, Ergin; Hubner, James Paul; University of Alabama Tuscaloosa
    The luminescent photoelastic coating (LPC) technique is a method to measure the full-field strain on three-dimensional (3D) structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding emission intensity for coating is detected via a CCD camera. Images are then processed to find the relative change in emission with respect to camera analyzer position, and subsequently analyzed to determine maximum in-plane shear strain. Image alignment plays a crucial role to obtain accurate measurements, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain related polarization changes due to surface inclination. Image warping methods in the image two-dimensional (2D) coordinate system provides reasonable results for 2D or simple 3D specimens; however, for complex 3D structures with moderate movement or deflection in the field-of-view, the accuracy and efficiency of these methods are not optimal. An alternative approach is to perform the analysis on a 3D grid representation of the structures. This study will research the merit of such an approach and develop the analysis procedures to separate the principal strains on 3D structures. The theoretical results will be compared to experimental data from a 2D and a 3D specimens while assessing the accuracy of the approach.