With the traditional fracture mechanics approaches, an initial crack and self-similar progression of cracks are assumed. In this treatise, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in fracture mechanics as Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and lower surface connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of fracture is determined by the critical energy release rate. The material between two adjacent laminae of a laminated composite structure or the material between the adherend and the adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The proper work-conjugacy relations between the stress and deformation measures are identified for the interfacial surface theory. In the principle of virtual work, the interfacial cohesive-decohesive tractions are conjugate to the displacement jumps across the upper and lower surfaces. A finite deformation kinematics theory is developed for the description of the upper and lower surface such that the deformation measures are invariant with respect to superposed rigid body translation and rotation. Various mechanical softening constitutive laws thermodynamically consistent with damage mechanics are postulated that relate the interfacial tractions to the displacement jump. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. In addition, interfacial constitutive laws are developed to describe the contact-friction behavior. Interface elements applicable to two dimensional and three dimensional analyses are formulated for the analyses of contact, friction, and delamination problems. The consistent form of the interface element internal force vector and the tangent stiffness matrix are considered in the formulation. We investigate computational issues related to interfacial interpenetration, mesh sensitivity, the number of integrations points and the integration scheme, mathematical form of the softening constitutive law, and the convergence characteristics of the nonlinear solution procedure when cohesive-decohesive constitutive laws are used. To demonstrate the predictive capability of the interface finite element formulation, steadystate crack growth is simulated for quasi-static loading of various fracture test configurations loaded under Mode I, Mode II, Mode III, and mixed-mode loading. The finite element results are in agreement with the analytical results available in the literature and those developed in this work. A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. The methodology is validated with respect to experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts. Very good agreement between the progressive failure analysis and the experiments is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures in the postbuckling response of the panels. In addition, ideas concerning the implementation of a fatigue model incorporated with a cohesive zone model are discussed. / Ph. D.

Laminated composite structures have started to play a very significant role in today's aircraft industry. The application of composite materials has now gone beyond the borders of aircraft design and has entered into such fields as automotive, athletics and recreational equipment, etc. The light weight and high specific strength of composite material helps design vehicles with higher fuel efficiency and longevity. In order to understand the influence of design parameters related to the use of composite materials in these applications, a proper study of the laminated composite structures requires a complete failure analysis, which includes both initiation and propagation of damage. In this work a progressive failure methodology is developed and implemented in the commercial Finite Element software package, Abaqus. Out of the numerous failure criteria available in the literature to study damage initiation and propagation in unidirectional fiber reinforced composites, Puck and Schurmann's failure criteria have been chosen due to their ability to predict results close to those observed experimentally. Key features of the Puck and Schurmann's failure criteria for three-dimensional deformations of unidirectional fiber reinforced composites have been summarized. Failure modes in the matrix and the fiber are considered separately. The failure criteria are simplified for plane stress deformations. Whereas the failure plane can be analytically identified for plane stress deformations, a numerical search algorithm is needed for three-dimensional problems. Subsequent to the initiation of matrix failure, elastic moduli are degraded and values of these degradation parameters and fracture plane angles are found by using a Continuum Damage Mechanics (CDM) approach. It is found that the assumption that the material response remains transversely isotropic even after the matrix failure has initiated requires the degradation of the transverse Poisson's ratio. The Puck and Schurmann's failure criteria and the material degradation process have been implemented as a User Defined Field (USDFLD) subroutine in Abaqus. The implementation has been verified by analytically computing results for simple loadings and comparing them with predictions from using the USDFLD in Abaqus. Subsequently, both two- and three-dimensional problems of more realistic geometries and loadings have been analyzed and computed results compared with either experimental findings or results available in the literature. Major contributions of the work include identifying the degradation parameter for the transverse Poisson's ratio in terms of the matrix degradation parameter for the matrix failure in compression, development of the USDFLD based on Puck and Schurmann's failure criteria, implementing the USDFLD in the commercial finite element software, Abaqus, and verifying that results computing using the USDFLD for various laminates and loadings agree with those from either the analytical solution of the problem or those available in the literature. / Ph. D.

The appeal of hybrid composites is the ability to create materials with properties which normally do not coexist such as high specific strength, stiffness, and toughness. One possible application for hybrid composites is as backplate materials in layered armor. Fiber reinforced composites have been used as backplate materials due to their potential to absorb more energy than monolithic materials at similar to lower weights through microfragmentation of the fiber, matrix, and fiber-matrix interface. Composite backplates are traditionally constructed from graphite or glass fiber reinforced epoxy composites. However, continuous alumina fiber-reinforced aluminum metal matrix composites (MMCs) have superior specific transverse and specific shear properties than epoxy composites. Unlike the epoxy composites, MMCs have the ability to absorb additional energy through plastic deformation of the metal matrix. Although, these enhanced properties may make continuous alumina reinforced MMCs advantageous for use as backplate materials, they still exhibit a low failure strain and therefore have low toughness. One possible solution to improve their energy absorption capabilities while maintaining the high specific stiffness and strength properties of continuous reinforced MMCs is through hybridization. To increase the strain to failure and energy absorption capability of a continuous alumina reinforced Nextel" MMC, it is laminated with a high failure strain Saffil® discontinuous alumina fiber layer. Uniaxial tensile testing of hybrid composites with varying Nextel" to Saffil® reinforcement ratios resulted in composites with non-catastrophic tensile failures and an increased strain to failure than the single reinforcement Nextel" MMC. The tensile behavior of six hybrid continuous and discontinuous alumina fiber reinforced MMCs are reported, as well as a description of the mechanics behind their unique behavior. Additionally, a study on the effects of fiber damage induced during processing is performed to obtain accurate as-processed fiber properties and improve single reinforced laminate strength predictions. A stochastic damage evolution model is used to predict failure of the continuous Nextel" fabric composite which is then applied to a finite element model to predict the progressive failure of two of the hybrid laminates. / Ph. D.

Long linear earthwork assets constructed in high-plasticity overconsolidated clay are known to be deteriorating due to long-term effects of wetting and drying stress cycles as a result of seasonal weather patterns. These stress cycles can lead to shallow first-time failures due to the mobilisation of post-peak strength and progressive failure. Design requirements of new earthworks and management of existing assets requires improved understanding of this critical mechanism; seasonal ratcheting. Incremental model development and validation to allow investigation of multiple inter-related strength deterioration mechanisms of cut slope behaviour in high-plasticity overconsolidated clay slopes has been presented. Initially, the mechanism of seasonal ratcheting has been considered independently and a numerical modelling approach considering unsaturated behaviour has been validated against physical modelling data. Using the validated model, the effects of slope geometry, design parameter selection and design life have been considered. Following this, an approach to allow undrained unloading of soil, stress relief, excess pore water pressure dissipation, seasonal ratcheting and progressive failure with wetting and drying boundary conditions has been considered. Hydrogeological property deterioration and the potential implications of climate change have been explored using the model. In both cases the serviceable life of cut slopes is shown to reduce significantly in the numerical analyses. Finally, a model capable of capturing hydrogeological behaviour of a real cut slope in London Clay has been developed and validated against long-term field monitored data. Using the validated model, a climate change impact assessment for the case study slope has been performed. The numerical analyses performed have indicated that seasonal ratcheting can explain shallow first-time failures in high-plasticity overconsolidated clay slopes and that the rate of deterioration of such assets will accelerate if current climate change projections are representative of future weather.

The aim of this thesis is to investigate buckling, post-buckling behaviors and failure characteristics of composite laminated plates under compressive loading with the help of finite element method and experiments. In the finite element analyses, eigen value extraction method is used to determine the critical buckling loads and nonlinear Riks and Newton-Raphson methods are employed to obtain post-buckling behaviors and failure loads. The effects of geometric imperfection amplitude on buckling and post-buckling are discussed. Buckling load, post buckling loaddisplacement relations, out of plane displacements and end shortening of the plates are determined numerically. Furthermore, the numerical results are compared with experimental findings for two different laminates made of woven fabric and unidirectional tapes where buckling, post-buckling behavior and structural failure of laminated plates were determined. The comparisons show that there is a good agreement between numerical and experimental results obtained for buckling load and post-buckling range. However, 15 % - 22 % differences are predicted between the experimental and numerical results for failure of laminates made of woven fabric whereas the laminates with uni-directional tapes show good agreement.

The classical continuum theory is not capable of predicting failure without an external crack growth criteria and treats the interface having zero thickness. Alternatively, a nonlocal continuum theory referred to as peridynamic theory eliminates these shortcomings by utilizing formulation that uses displacements, rather than derivatives of displacements, and including material failure in its constitutive relations through the response functions. This study presents a new response function as part of the peridynamic theory to include thermal loading. Furthermore, an efficient numerical algorithm is presented for solution of peridynamic equations. Solution method relies on the discretization of peridynamic equations at collocation points resulting in a set of ordinary differential equations with respect to time. These differential equations are then integrated using explicit methods. In order to improve numerical efficiency of the computations, spatial partitioning is introduced through uniform grids as arrays of linked lists. Furthermore, the domain of interest is divided into subunits each of which is assigned to a specific processor to utilize parallel processing using OpenMP. In order to obtain the static solutions, the adaptive dynamic relaxation method is developed for the solution of peridynamic equations. Furthermore, an approach to couple peridynamic theory and finite element analysis is introduced to take advantage of their salient features. The regions in which failure is expected are modeled using peridynamics while the remaining regions are modeled utilizing finite element method. Finally, the present solution method is utilized for damage prediction of many problems subjected to mechanical, thermal and buckling loads.

The Saint Lawrence Lowlands in eastern Canada contain extensive deposits of marine soils deposited in post-glacial seas during and following the retreat of the most recent continental glacier. These marine soils include silt and clay deposits known collectively as Champlain clay. When the pore fluid in these marine deposits has changed over time to a lower salinity, the clay can become very sensitive, or demonstrate substantial strength loss after reaching the peak strength with sufficient strain under undrained load conditions. Sensitive clay soils are subject to a peculiar type of very large landslide that typically involves great extents of nearly horizontal ground, usually occurring suddenly and without warning. These landslides tend to be described as “retrogressive” in the literature and practice, implying that they develop as a series of successive small failures that advance rearward until a final stable position is reached. The work of this thesis is organized into four different themes, with an overall objective of understanding the hazard and risk associated with large landslides in sensitive clay to linear infrastructure such as railways. The first theme, documented in Chapter 2, develops a number of spatial relationships between specific physiographic and geologic features and landslide occurrence or absence, as determined through air photo analysis and a review of the literature. The second theme, documented in Chapter 3, presents the construction of a digital database of large landslides in sensitive clay in eastern Canada, for the purposes of studying landslide susceptibility, hazard and risk. The third theme, documented in Chapters 4 and 5, presents and defends a novel mechanical model for development of these large landslides. This model suggests the landslides develop progressively, rather than retrogressively, and the science of fracture mechanics is employed to substantiate the model. The fourth theme, documented in Chapters 6 and 7, synthesizes the findings of the earlier themes and presents a methodology for estimating landslide susceptibility in Champlain clay. That approach is then extended to develop an understanding of the hazard. The concluding chapter extends that work to present an initial appreciation of landslide risk to railways. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2009-04-23 13:22:19.53

<p>Carbon-fiber reinforced polymers (CFRPs) are often used in the aerospace and automotive  industries for their high strength-to-weight ratios and corrosion resistance. A new class of  composites – known as Prepreg Platelet Molded Composites (PPMCs) – offers further  advantageous such as high forming capabilities with modest compromises in strength and stiffness.  One such property of PPMCs that have garnered interest over the years is their apparent  insensitivity to notches. Previous studies have researched the effect of specimen size and platelet  length on its effect on the open-hole performance of PPMCs. Research however has focused on  thinner samples with smaller hole sizes and neglected thicker samples with larger holes.  Additionally, while platelet sizes have been investigated for unnotched samples, platelet width on  notched samples is less clear from the literature. The present thesis offers some investigations to  aid in filling this knowledge gap. </p> <p><br></p> <p>The objective of this work is to study two parameters that could influence the performance of PPMCs under open-hole tension. First, thick (7.6 mm) specimens are subjected to large hole  sizes (up to 19.08 mm) to investigate their behavior in comparison to the smaller sample sizes  previously investigated in the literature. Through-thickness DIC measurements are taken to  investigate strain gradients in these thicker specimens. Second, various platelet widths are tested  to research their influence on notch insensitivity of open-hole tensile PPMC specimens. Lastly, a  finite element based continuum damage mechanics model is implemented to predict macro-level  structural properties using only material properties of the parent prepreg. It is found that large holes  in thick samples increase notch sensitivity compared to other samples of similar diameter-to-width  ratios. Narrower platelets were found to produce higher unnotched strengths, while wider platelets  offered more notch insensitivity. Lastly, the finite element model developed was found to  qualitatively replicate features and failure modes that are exhibited by PPMCs, though strength  predictions became inaccurate at larger specimen sizes. Recommendations are made for future  work on the basis of these findings.   </p>

This work presents a detailed predictive modeling methodology for comprehensive crashworthiness analysis of advanced jet engine forward sections, containing hybrid and composite structures, when subjected to soft impact. Bird strike onto the fan assembly is chosen as the impact event to be studied. The aim is to develop a numerical methodology capable of accurately capturing the full range of multifaceted damage in hybrid and composite structures as they evolve throughout the forward section of a propulsion system. Effective strategies are developed within an explicit finite element framework for modeling a bird, an engine forward section, intra-ply and inter-ply composite damage, and hybrid structural failure. The accuracy of each approach and their numerical modeling considerations are thoroughly investigated. These techniques are then combined to form the full crashworthiness methodology. It is demonstrated that the complete methodology effectively captures progressive hybrid fan blade fracture, leading edge de-bonding, composite casing delamination, and other significant progressive damage effects caused by direct impact and subsequent engine component interactions. The full damage prognosis capabilities demonstrated by this approach encompass aspects which have remained mainly unaddressed in soft impact analysis. A methodology for assessing the complete extent of impact damage for advanced structural engine designs represents a breakthrough that can contribute greatly to the rapid development of these systems in the future. / Master of Science

In this study, a progressive failure analysis is used to investigate leakage in internally pressurized non-circular composite cylinders. This type of approach accounts for the localized loss of stiffness when material failure occurs at some location in a structure by degrading the local material elastic properties by a certain factor. The manner in which this degradation of material properties takes place depends on the failure modes, which are determined by the application of a failure criterion. The finite-element code STAGS, which has the capability to perform progressive failure analysis using different degradation schemes and failure criteria, is utilized to analyze laboratory scale, graphite-epoxy, elliptical cylinders with quasi-isotropic, circumferentially-stiff, and axially-stiff material orthotropies. The results are divided into two parts. The first part shows that leakage, which is assumed to develop if there is material failure in every layer at some axial and circumferential location within the cylinder, does not occur without failure of fibers. Moreover before fibers begin to fail, only matrix tensile failures, or matrix cracking, takes place, and at least one layer in all three cylinders studied remain uncracked, preventing the formation of a leakage path. That determination is corroborated by the use of different degradation schemes and various failure criteria. Among the degradation schemes investigated are the degradation of different engineering properties, the use of various degradation factors, the recursive or non-recursive degradation of the engineering properties, and the degradation of material properties using different computational approaches. The failure criteria used in the analysis include the noninteractive maximum stress criterion and the interactive Hashin and Tsai-Wu criteria. The second part of the results shows that leakage occurs due to a combination of matrix tensile and compressive, fiber tensile and compressive, and inplane shear failure modes in all three cylinders. Leakage develops after a relatively low amount of fiber damage, at about the same pressure for three material orthotropies, and at approximately the same location. / Master of Science