Modelling damage in composite materials has played an important role in designing composite structures. Although numerical models for the progressive damage in laminated composites (e.g. transverse cracking, delamination and fibre breakage) have been developed in the literature, there is still a need for further improvement. This thesis aimed at developing damage models suitable for predicting intra-laminar and inter-laminar damage behaviour in fibre-reinforced composite materials. Several approaches such as fracture mechanics and continuum damage mechanics have been adopted for constructing the damage model. Meso-macro-mechanics analysis was performed to gain an insight into the entire damage process up to the final failure of the composite laminate under various conditions. Cohesive elements were placed in the finite element model to simulate the initiation and propagation of matrix crack and delamination in cross-ply laminates. This helped to understand the direct interactions between damage modes, i.e. whether one damage mode would initiate the other damage mode. The formation of a single matrix crack and its propagation across the layer thickness was also revealed. A new cohesive zone/interface element model was developed to consider the effect of through-thickness compressive stress on mode II fracture resistance by introducing friction into the constitutive law of the conventional cohesive zone model. Application of the model to practical problem in composite laminates shows that this model can simulate delamination failure more accurately than the cohesive element in ABAQUS.Damage models based on continuum damage mechanics were proposed for predicting intra-laminar damage and interlaminar damage. Five intra-laminar failure modes, fibre tension, fibre compression, matrix tension, matrix compression and shear failure, were modelled. Damage initiation was predicted based on stress/strain failure criteria and damage evolution law was based on fracture energy dissipation. The nonlinear shear behaviour of the material was considered as well. These models have been implemented into ABAQUS via a user-defined material subroutine and validated against experimental/numerical results available in the literature. The issue related to numerical implementation, e.g. convergence in the softening regime, was also addressed. Numerical simulation of the indentation test on filament-wound pipe was finally conducted and damages generated in the pipe were predicted using the above developed damage models. The predictions show an excellent agreement with experimental observations including load/indentation responses and multiple delaminations shape and size. Attempt was made to detect damage-induced leakage path in the pipe after indentation.

The problem of damage accumulation in laminated composite materials hasreceived much attention due to their widespread application in the aerospace, automotive,civil, and sports industries. In the aerospace industry, composites are usedto make light weight and efficient structural components. In the Boeing 787, forexample, more than 50% of the structure is made of composite materials. Althoughthere have been significant developments in analyzing cross-ply laminates, none ofthe present approaches provides reasonable predictions for multidirectional laminatesin which intralaminar cracks may form in multiple orientations. Nevertheless, theprediction of damage accumulation and its effect on structural performance is a verydifficult problem due to complexity of the cracking processes.This study presents a synergistic damage mechanics (SDM) methodology to analyzedamage behavior in multidirectional composite laminates with intralaminarcracks in plies of multiple orientations. SDM combines the strengths of micro-damagemechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to these cracks. The micromechanics is performed on a representativeunit cell using a three-dimensional finite element analysis to calculate thecrack opening displacement accounting for the influence of the surrounding plies, theso-called constraint effect. This information is then incorporated in the CDM formulationdealing with laminates containing cracks in different ply orientations through a `constraint parameter'. Following CDM, a separate damage mode is defined for eachtype of crack and the expressions for engineering moduli of the damaged laminateare then derived in terms of crack density and the constraint parameter. The SDMmethodology is implemented for [0m/±θn/0m/2]s laminates containing cracks in ±θplies. It is then extended to [0m/±θn/90r]s and [0m/90r/±θn]s laminates with cracksadditionally in the 90°-plies. The predictions agree well with published experimentaldata as well as independent FE computations. Limited parametric studies areperformed to show usability of SDM for more general laminates.To predict the initiation and growth of intralaminar cracks, an energy basedmodel is proposed in which these cracks initiate and multiply when the work requiredto form new set of cracks exceeds a laminate dependent critical energy release rate.The approach requires determination of average crack opening and sliding displacementsat varying crack spacing. This task is performed through a suitable 3-D FEanalysis. In case of off-axis ply cracking, a mixed mode fracture criterion is utilized,where the critical energy release rates in normal and shear modes are determinedby fitting the damage model with the experimental data for a reference laminate.The predictions from the model for [0/± θ4/01/2]s and [0/90/ ± 45]s laminates showremarkable agreement with the experimental results.The methodology and the results covered in this dissertation will be of interest tomechanics of materials researchers as well as to engineers in industry where compositematerials for structural applications are of interest.

No description available.

The problem of damage accumulation in laminated composite materials hasreceived much attention due to their widespread application in the aerospace, automotive,civil, and sports industries. In the aerospace industry, composites are usedto make light weight and efficient structural components. In the Boeing 787, forexample, more than 50% of the structure is made of composite materials. Althoughthere have been significant developments in analyzing cross-ply laminates, none ofthe present approaches provides reasonable predictions for multidirectional laminatesin which intralaminar cracks may form in multiple orientations. Nevertheless, theprediction of damage accumulation and its effect on structural performance is a verydifficult problem due to complexity of the cracking processes.This study presents a synergistic damage mechanics (SDM) methodology to analyzedamage behavior in multidirectional composite laminates with intralaminarcracks in plies of multiple orientations. SDM combines the strengths of micro-damagemechanics (MDM) and continuum damage mechanics (CDM) in predicting the stiffness degradation due to these cracks. The micromechanics is performed on a representativeunit cell using a three-dimensional finite element analysis to calculate thecrack opening displacement accounting for the influence of the surrounding plies, theso-called constraint effect. This information is then incorporated in the CDM formulationdealing with laminates containing cracks in different ply orientations through a `constraint parameter'. Following CDM, a separate damage mode is defined for eachtype of crack and the expressions for engineering moduli of the damaged laminateare then derived in terms of crack density and the constraint parameter. The SDMmethodology is implemented for [0m/±θn/0m/2]s laminates containing cracks in ±θplies. It is then extended to [0m/±θn/90r]s and [0m/90r/±θn]s laminates with cracksadditionally in the 90°-plies. The predictions agree well with published experimentaldata as well as independent FE computations. Limited parametric studies areperformed to show usability of SDM for more general laminates.To predict the initiation and growth of intralaminar cracks, an energy basedmodel is proposed in which these cracks initiate and multiply when the work requiredto form new set of cracks exceeds a laminate dependent critical energy release rate.The approach requires determination of average crack opening and sliding displacementsat varying crack spacing. This task is performed through a suitable 3-D FEanalysis. In case of off-axis ply cracking, a mixed mode fracture criterion is utilized,where the critical energy release rates in normal and shear modes are determinedby fitting the damage model with the experimental data for a reference laminate.The predictions from the model for [0/± θ4/01/2]s and [0/90/ ± 45]s laminates showremarkable agreement with the experimental results.The methodology and the results covered in this dissertation will be of interest tomechanics of materials researchers as well as to engineers in industry where compositematerials for structural applications are of interest.

No description available.

Several critical issues in the mechanics of the interface between the fibers and matrix in ceramic matrix composites (CMCs) are studied. The first issue is the competition between crack deflection and penetration at the fiber/matrix interface. When a matrix crack, the first fracture mode in a CMC, reaches the interface, two different crack modes are possible; crack deflection along the interface and crack penetration into the fibers. A criterion based on strain energy release rates is developed to determine the crack propagation at the interface. The Axisymmetric Damage Model (ADM), a newly-developed numerical technique, is used to obtain the strain energy in the cracked composite. The results are compared with a commonly-used analytic solution provided by He and Hutchinson (HH), and also with experimental data on a limited basis. The second issue is the stress distribution near the debond/sliding interface. If the interface is weak enough for the main matrix crack to deflect and form a debond/sliding zone, then the stress distribution around the sliding interface is of interest because it provides insight into further cracking modes, i.e. multiple matrix cracking or possibly fiber failure. The stress distributions are obtained by the ADM and compared to a simple shear-lag model in which a constant sliding resistance is assumed. The results show that the matrix axial stress, which is responsible for further matrix cracking, is accurately predicted by the shear-lag model. Finally, the third issue is multiple matrix cracking. We present a theory to predict the stress/strain relations and unload/reload hysteresis behavior during the evolution of multiple matrix cracking. The random spacings between the matrix cracks as well as the crack interactions are taken into account in the model. The procedure to obtain the interfacial sliding resistance, thermal residual stress, and matrix flaw distribution from the experimental stress/strain data is discussed. The results are compared to a commonly-used approach in which uniform crack spacings are assumed. Overall, we have considered various crack modes in the fiber-reinforced CMCs; from a single matrix crack to multiple matrix cracking, and have suggested models to predict the microscopic crack behavior and to evaluate the macroscopic stress/strain relations. The damage tolerance or toughening due to the inelastic strains caused by matrix cracking phenomenon is the key issue of this study, and the interfacial mechanics in conjunction with the crack behavior is the main issue discussed here. The models can be used to interpret experimental data such as micrographs of crack surface or extent of crack damage, and stress/strain curves, and in general the models can be used as guidelines to design tougher composites. / Ph. D.

The research presented in this thesis is all based on the impedance method for structural health monitoring. The impedance method is an electro-mechanical technique which utilizes a single piezoelectric transducer as both a sensor and actuator. Due to the high frequencies of excitation used for the method, the sensing area for damage detection can be very localized. Previous work has shown that wave propagation can be added to systems already equipped with hardware for impedance-based structural health monitoring. The work in this thesis shows what happens under varying temperature conditions for a structure being monitored with wave propagation. A technique to compensate for temperature fluctuations is also presented. The work presented here is an initial study to directly correlate the actual amount of damage in a composite specimen with a damage metric indicated by impedance-based structural health monitoring. Two different damage mechanisms are examined: transverse matrix cracking and edge delamination. With both composite defects, a sample is interrogated with the impedance method before and after damage is introduced. The exact amount of damage in each specimen is found using radiography and compared with the health monitoring results. Traditional impedance techniques require the use of a bulky and expensive impedance analyzer. With the trend of structural health monitoring moving towards unobtrusive sensors which can be permanently placed on a structure, an impedance analyzer does not lend itself to these small, low power consuming requirements. In this thesis, an initial attempt to miniaturize the hardware is described. A prototype impedance-based structural health monitoring system, incorporating wireless based communications, is fabricated and validated with experimental testing on a number of different structures. The first steps towards a complete self-contained, robust structural health monitoring sensor are presented. / 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

Laminated carbon/epoxy composite structures are increasingly used in the aerospace industries. Low weight, elastic tailoring, and high durability make the composite materials well suited for replacement of conventional metallic structures. However the difficulty to capture structural failure phenomena is a significant barrier to more extensive use of laminated composites. Predictions are challenging because matrix (resin) dominated failure mechanisms such as delaminations and matrix cracking contribute to the structural failure in addition to fiber-dominated failures. A key to rigorous failure predictions for composites is availability of measurements to quantify structural model parameters including matrix-dominated stress-strain relations and failure criteria. Novel techniques for measurement of nonlinear interlaminar constitutive properties in tape composites have been recently developed at Georgia Institute of Technology. Development of methods for accurate predictions of failure in carbon/epoxy tape laminate configurations with complex lay-ups is the main focus of this work. Failures through delamination and matrix cracking are considered. The first objective of this effort is to implement nonlinear interlaminar shear stress-strain relations for IM7/8552 carbon/epoxy tape in ABAQUS finite element models and validate structural delamination failure predictions with tests. Test data for composite configurations with wavy fibers confirm that nonlinear interlaminar shear stress-strain response enables accurate failure prediction. The problem of the presence of porosity and its influence on failure was noted. The second objective is to assess the ability to simulate initiation and propagation of matrix-ply cracking. Failure models for IM7/8552 carbon/epoxy tape open-hole tensile coupons are built and validated.

No description available.