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Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 86-95). Also available on the World Wide Web.

Thesis (M.S.)--Texas State University--San Marcos, 2008. / Vita. Includes bibliographical references (leaves 82-84). Also available on microfilm.

An investigation of the effect of R-ratio on the mode II fatigue delamination of AS4/3501-6 carbon/epoxy composites has been undertaken. Experiments have been performed on end notched cantilever beam specimens over a wide range of R-ratios (-l ≤R ≤0.50). The measured delamination growth rate data have been correlated with the mode II values of strain energy release rate range ∆G[formula omitted]), maximum strain energy release rate (G[formula omitted]) and stress intensity factor range (∆K[formula omitted]). The growth rate is dependent on the R-ratio over the range tested. For a constant level of ∆G[formula omitted], the crack growth rate decreases with increasing R-ratio. A similar trend is observed when the data is plotted as a function of G[formula omitted]. The effect of plotting the growth rate as a function of ∆K[formula omitted] is to produce an R-ratio dependence opposite to that obtained by either the ∆G[formula omitted] or G[formula omitted] approach. For a constant level of ∆K[formula omitted], the crack growth rate increases with increasing R-ratio. Master equations which completely characterize the fatigue behaviour as a function of ∆G[formula omitted] and ∆K[formula omitted] have been derived, based on the observation that the growth rate law exponent, n and constant, A are unique functions of R-ratio. Values for n are surprisingly large and increase with increasing R-ratio whereas values for A decrease with increasing R-ratio. The effect of time-at-load has been considered in an attempt to explain the existence of the R-ratio dependence of the growth rate. The correct trend can be established for the exponent, n but not for the constant, A. Friction between the crack faces, particularly at higher R-ratios, is proposed as a possible explanation for the observed anomaly. Further evidence of a frictional mechanism operating at higher R-ratios has been discovered through a postmortem fracture surface examination. Additional fractographic observations are presented over the entire range of R-ratios tested. In regions subjected to negative R-ratio cycling, there is no evidence of the characteristic mode II hackle features. Instead, loose rounded particles of matrix material are found. An extensive amount of hackling is observed in regions subjected to low positive R-ratio cycles. The extent of hackle damage visibly decreases in areas where higher levels of R-ratio are imposed. A correlation between the general fracture surface morphology and the fatigue data provides support for the hypothesis that energy for delamination is always available in sufficient quantity, and that growth is dependent on the stresses ahead of the crack tip being sufficiently high. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The problem examined is an infinite anisotropic layer with a through crack at arbitrary orientation, subjected to uniform in-plane remote loading. The purpose of this study is to gain a better understanding of several theoretical models for predicting the direction of crack propagation and the level of load causing crack extension, and to present a new model for predicting the critical load. The discussed models are particularly examined in detail with regard to the physical parameters affecting the results. Comparison is made with available experimental results. It is shown that the normal stress ratio theory provides good agreement with experimental crack growth direction, independent of physical parameters, and that the newly proposed traction ratio theory predicts well the critical load causing crack extension. / M.S.

The effect of particle size matching and mismatching on the processability, and the mechanical and physical properties of particulate reinforced composites is investigated in this study. These composites were made from dry powder-powder blends. Polymer and reinforcement materials were chosen, characterized and molded into composite plaques. For the same particle volume fraction (400/0), stiffness was found to increase, in general, as particle size decreased. The intimacy of mixing, stiffness and strength improvements depended upon the reinforcement type. These results were compared with predictions from simple micromechanics models to gain a better understanding of their physical behavior. / Master of Science

The objective of this investigation is to predict the residual strength of notched composite Iaminates with various layups, subjected to low frequency fatigue loading with constant amplitude at room temperature, by using a material modeling approach, fracture and fatigue mechanics and the finite element method (FEM). For simplicity, after thousands of cycles, the geometry of a circular hole of the deformed laminate was categorized as (1) uniformly expanded hole into elliptic shape, (2) crack propagation around the hole transversely. Both types were studied for 12 cases of layups with various proportions of 0, 45, -45 and 90 degree plies. The effect of geometry change during fatigue on residual strength was attributed to the elliptical hole, longitudinal splitting, matrix cracking (reduction moduli of plies), crack propagation and local delamination. Due to the thin through-the-thickness notched laminate, two-dimensional FEM was used and interlaminar stresses were not considered. Reduction of stress concentration is a reason for the increase of the residual strength of the notched laminate. The stress concentration factor decreases while the elliptic hole becomes more slender; that was examined by the FEM. The residual strength and stiffness were determined by the material modeling with moduli reduction and damaged zone, and the numerical result was obtained by FEM. Laminate theory, point stress criterion, polynomial failure criterion, ply discount method, and fatigue and fracture mechanics (Paris' Power Law) were also included in this research. Geometry change and moduli reduction are two major effects that are considered to predict the notched strength. The WN point stress fracture model is adopted for simplicity, instead of the average stress criterion. K_tg that corresponds to the unnotched strength in the normalized stress base curve is used to obtain the characteristic length (d_o). We find that K_tg decreases when the elliptic hole becomes more slender and more moduli are reduced (more plies crack). At the time d_o that is determined from K_tg in the base curve is not necessarily a fixed material constant. The correlation between the fatigue life and the residual strength as predicted by the model and those determined numerically is found within acceptable errors in comparison with the experimental data. / Ph. D.

Aviation and aerospace structural components made of composite laminates due to their internal structure and manufacturing methods contain a number of inter- and intracomponent defects, which size, dispersion and interaction alter significantly the critical compression strain level. While there are a plethora of theoretical and experimental work on the problems stability loss and fracture of composites with internal defects in the scope of classic problems of fracture mechanics, there are few theoretical and numerical analyses available for the nonclassical problems of fracture mechanics of composites compressed along layers with interface cracks. These analyses usually have been considered the simplest problems, where the composite material with pronounced microstructure and interface defects (cracks, delaminations) have been analysed as two-dimensional (2-D) continuum in the condition of plane strain state. In the scope of these analyses only parallel defects have been considered, allowing for the interpenetration of the stress-free crack faces, or assuming so-called interfacial cracks with connected edges. This thesis broadens knowledge in the area of non-classical problems of fracture mechanics. It investigates the effect of interfacial cracks interaction on the critical buckling strain in layered and fibrous composite materials under compressive static loading. The behaviour of composite is analysed on several length-scales, starting from a ply and laminate levels (in 2-D approximation), down to a single-fibre level (a full 3-D model). The statements of the problems are based on the model of piecewise-homogeneous medium model, the most accurate within the framework of the mechanics of deformable bodies as applied to composite materials with pronounced microstructure. All composite constituents are modelled as linear-elastic material, where both isotropic and anisotropic materials are considered depending on the length-scale. It is assumed that the moment of stability loss in the microstructure of materials is treated as the onset of the fracture process. Besides that, the critical strain that corresponds to loss of stability in the microstructure of the composite, either surface or internal instability, must be smaller than the critical strain that corresponds to loss of stability of the entire composite. This project involves parameterised variables, such as the crack size, the crack spacing, the layer volume fraction and the fibre volume fraction. At each length-scale two types of cracks are analysed, namely, cracks with stress-free crack faces and cracks with frictionless Hertzian contact of the crack faces. A number of finite-element models for each length-scale are developed, and are validated analytically and numerically. The models' ability to simulate practical composite structures to a useful degree of accuracy with suitable material properties is discussed. A number of parameters, which quantifies the interfacial crack interaction and crack faces contact interaction phenomena, are introduced and discussed. Qualitative discussion on the crack faces contact zones, post-critical behaviour of composites and crack propagation are presented and discussed. Finally, the subject areas for the future work are outlined.