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This paper evaluates the ability of progressive damage analysis (PDA) finite element (FE) models to predict transverse matrix cracks in unidirectional composites. The results of the analyses are compared to closed-form linear elastic fracture mechanics (LEFM) solutions. Matrix cracks in fiber-reinforced composite materials subjected to mode I and mode II loading are studied using continuum damage mechanics and zero-thickness cohesive zone modeling approaches. The FE models used in this study are built parametrically so as to investigate several model input variables and the limits associated with matching the upper-bound LEFM solutions. Specifically, the sensitivity of the PDA FE model results to changes in strength and element size are investigated.

This 'Damage Tolerance Assessment Handbook' consists of two volumes: Volume 1 introduces the damage tolerance concept with a historical perspective followed by the fundamentals of fracture mechanics and fatigue crack propagation. Various fracture criteria and crack growth rules are studied. Volume 2 treats exclusively the subject of damage tolerance evaluation of airframes. Damage Tolerance, Fracture Mechanics, Crack initiation, Fracture Toughness, Stress Intensity Factor, Residual Strength, Crack Propagation, Fatigue, Inspection.

Some Mumford-Shah functionals are revisited as perturbed area functionals in connection with brittle damage mechanics. We find minimizers "on paper" for the classical Mumford-Shah functional for some particular two dimensional domains and boundary conditions. These solutions raise the possibility of validating experimentally the energetic model of crack appearance. Two models of brittle damage and fracture are proposed after; in the one of these models the crack belongs to the set of integral varifolds. We have felt the necessity to start the paper with a preliminary section concerning classical results in equilibrium of a cracked elastic body reviewed in the context of Sobolev spaces with respect to a measure. Further information at http://irmi.epfl.ch/cag/buliga_bfrac.html .

Recent advances in the damage mechanics of brittle solids have made is possible to calculate stress-strain curves for the non-linear source regime of underground nuclear explosions where the rock is being actively fractured. It has been shown that the damage-based rheology can explain the anomalously broad source pulses observed in the free field of explosions in granite. It also offers a physical explanation for why such pulse broadening was not observed in laboratory experiments in terms of the scaling of rock strength with the size of preexisting fractures. In this report we discuss how damage mechanics may be used to modify the Mueller-Murphy source model to explicitly include the fracture distribution in the explacement medium thereby providing a physical interpretation of source parameters which specify the width of the pressure pulse at the elastic radius, and the elastic radius itself. These parameters are currently evaluated empirically using calibration shots of known yield for each site. The resultant improvement of our understanding of the relation between the source medium and seismic coupling at high frequencies is especially important in view of the recent trend toward the use of higher frequency regional phases for yield estimation and discrimination allowed by improved seismic accessibility to test sites.

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The work performed in the reporting period has been focused on completion of Tasks 1.2and 1.3 related to Synergistic Damage Mechanics and Tasks 2.2 and 2.4 related to Peridynamics, as described in the project proposal. The activities related to Task 1.2 concern growth and instability of initiated cracks in the environment of the disordered fiber distribution, and Task 1.3 evaluates the effect of these cracks on the response of the composite to imposed impulses. The activities related to Task 2 cover the new peridynamic model for elasticity and fracture at an interface in a multi-phase composites.

In most manufacturing processes for polymer matrix composites (PMCs) one starts with dry bundles of fibers. On resin infusion, the initially closed-pack fibers are spread out to the degree dictated by the intended fiber volume fraction, resulting in disordered fiber distributions and defects such as voids in the matrix. Pores, manufacturing defects, holes and round notches are locations where fatigue cracks may initiate or arrest, depending on the loading conditions. We introduce a new peridynamic model for materials with pores based on an Intermediate Homogenization Model (IHM). We investigate how the new model compares with the classical homogenization approach for dynamic elasticity as well as for crack and damage evolution. This model will allow us to model pores, voids, manufacturing defects in a composite material without having to represent the exact shape and size of such defects, which would require a very costly, fine discretization for computing the solution.

The focus of the research conducted under Grant No. 91-0106 (a two year effort) was to identifying the fundamental mechanisms of fatigue damage that occur in fiber-reinforced ceramics. Several new findings were made during the research effort: (1) the fatigue life of fiber-reinforced ceramics decreased markedly during high frequency fatigue loading, (2) fiber-reinforced ceramics undergo significant internal heating during cyclic loading, (3) because of frictional wear along the fiber-matrix interface, the frictional shear stress in fiber-reinforced ceramics decreases sharply under cyclic loading. Based upon insight gained from the analytical and experimental parts of the investigation, we developed a novel approach to estimate the level of frictional shear stress that exists along the fiber-matrix interface during fatigue. Since this technique allows confirmation of other techniques for estimating frictional shear stress (e.g., fiber pushout technique developed by Marshall at Rockwell Science Center). Moreover, it is the only approach that allows determination of the in-situ change in frictional shear stress during cyclic loading (note that the level of frictional shear stress controls many mechanical properties such as strength, toughness and mechanical damping as well as thermophysical properties such as thermal diffusivity). The analysis that was developed to estimate frictional shear stress can also be used to understand the relationship between composite microstructure and cyclic energy dissipation in fiber-reinforced ceramics.

An empirically based mathematical model and computer code capable of predicting cumulative damage effects on fatigue crack growth in engine disks has been developed for Waspaloy (PWA 1007) and IN100 (PWA 1073). The computing software segregates a complex loading mission, applies a multi-parameter interpolation procedure to determine cycle-by-cycle crack growth, and performs a specimen life calculation. The model was developed from a broad matrix of tests of both materials, and a number of auxiliary investigations were conducted to examine the range of applicability of the model. (Author)

In this paper the scalability of a computationally intensive, multi-scale theory exhibiting progressive damage in a 3-D woven composite is investigated. The theory is based on evolving some fundamental damage modes in a representative volume element (RVE) of the micro-structure of a 3-D woven composite. The evolving damage modes affect the local stresses in the composite's actual microstructure and eventually the overall stresses in the composite. This situation is considered in the RVE via a transformation field analysis (TFA). Since the model is computationally intensive, it benefited by having its computations made parallel. This was accomplished by coupling the model to LS-DYNA3D, an explicit Lagrangian finite element code. The coupled RVE-TFALSDYNA3D code enabled the study of weave-level damage progression in the 3-D woven composites under general impact conditions and could be a useful design tool. In the present work, the scalability of the coupling between the RVE-TFA and LS-DYNA3D codes is examined and results are presented.

The behavior of plain woven fabric composites is studied using three-dimensional finite elements which allows detailed modeling of the geometric complexities and spatial material variations within the fabric. Damages in the composite constituents viz. yarn and pure matrix are modeled on a continuum basis and related to their material constitutive behavior. The 3D constitutive laws describing pure matrix and yarn behavior are developed using a damage mechanics based approach with the dissipated energy density as the damage parameter. The strain energy dissipation (SED) concept is employed to describe the damage state and current stiffnesses of the weave constituents. A progressive failure analysis of plain woven fabrics subjected to tension and in-plane shear is carried out considering both geometric and material nonlinearities. The initiation and progression of damage within the fabric is investigated and the significant damage mechanisms outlined.

This article is from Chemistry (Weinheim an Der Bergstrasse, Germany) , volume 20 . Abstract AlkB repair enzymes are important nonheme iron enzymes that catalyse the demethylation of alkylated DNA bases in humans, which is a vital reaction in the body that heals externally damaged DNA bases. Its mechanism is currently controversial and in order to resolve the catalytic mechanism of these enzymes, a quantum mechanics/molecular mechanics (QM/MM) study was performed on the demethylation of the N1-methyladenine fragment by AlkB repair enzymes. Firstly, the initial modelling identified the oxygen binding site of the enzyme. Secondly, the oxygen activation mechanism was investigated and a novel pathway was found, whereby the catalytically active iron(IV)–oxo intermediate in the catalytic cycle undergoes an initial isomerisation assisted by an Arg residue in the substrate binding pocket, which then brings the oxo group in close contact with the methyl group of the alkylated DNA base. This enables a subsequent rate-determining hydrogen-atom abstraction on competitive σ-and π-pathways on a quintet spin-state surface. These findings give evidence of different locations of the oxygen and substrate binding channels in the enzyme and the origin of the separation of the oxygen-bound intermediates in the catalytic cycle from substrate. Our studies are compared with small model complexes and the effect of protein and environment on the kinetics and mechanism is explained.

This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research on the damage assessment of structures. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology. Introduction The Air Force Office of Scientific Research (AFOSR) is the fundamental basic research component of the Air Force Research Laboratory.

Ionizing radiation on optical fibers induces large transmission losses. The permanence and intensity of the damage depends on the concentration of germanium and phosphorus. The sensitive structure(s) may act as electron or hole traps when illuminated with gamma rays (1.2 meV) or ultraviolet (4.9 eV). The process of emptying traps by heat and refilling with UV (254 nm) can be done repeatedly. These traps re-populated by UV are the same as the transmission loss traps due to gammas. Certain TL traps cannot be re-populated with UV. The nonrefillable TL traps exist in those glasses which have the GeO2/P2O5 ratios smaller than 4. Higher ratio glasses exhibit one refillable peak which bears an inverse relationship to germanium content. The TL traps have a depth of about 0.85 eV. Pure silica fibers display a permanent thermoluminescence. The presence of these traps is not detected by transmission measurements. Based on UV thermoluminescence, transmission, and UV fluorescence measurements, it is suggested that Ge(+2) plays an important role in the damage mechanism. (Author)

The interpolative hyperbolic sine model for mission-mix crack growth was used to predict the propagation histories and cycles-to-failure for two specimen geometries (compact tension and surface crack) at two temperatures (1000 F and 1200 F). Results were encouraging with an average prediction error of less than 15%. (Author)

Rate mechanisms govern the mechanical behavior of construction materials under three conditions: 1) at high homologous temperatures; 2) when subjected to short term loading that is quasi-static, impulsive, or vibratory; and 3) when subjected to sustained (or creep) loading over long times. Examples of advanced construction materials satisfying one or more of these conditions include: Polymer matrix structural composites such as FRPs; Single-ply roofing membranes made of elastomers; c) Polycrystalline ice in cold regions engineering; and d) Cementitious composites such as high-strength concretes and FRCs. The deformation and progressive failure of such rate sensitive materials is governed by three primary mechanisms: flow, distributed cracking, and localized crack propagation. The interaction of these mechanisms gives rise to complex mechanical behavior on the macro-scale. The characterization of these mechanisms and their interaction is therefore of fundamental importance. The objective of this research is to develop an understanding of and characterize the rate and damage processes and their interaction in advanced construction materials. This report presents: a multiaxial differential flow model; a rate and pressure sensitive anisotropic damage model; and, specification for testing materials.

High temperature and long duration applications of monolithic ceramics can place their failure mode in the creep rupture regime. A previous model advanced by the authors described a methodology by which the creep rupture life of a loaded component can be predicted. That model was based on the life fraction damage accumulation rule in association with the modified Monkman-Grant creep ripture criterion However, that model did not take into account the deteriorating state of the material due to creep damage (e.g., cavitation) as time elapsed. In addition, the material creep parameters used in that life prediction methodology, were based on uniaxial creep curves displaying primary and secondary creep behavior, with no tertiary regime. The objective of this paper is to present a creep life prediction methodology based on a modified form of the Kachanov-Rabotnov continuum damage mechanics (CDM) theory. In this theory, the uniaxial creep rate is described in terms of stress, temperature, time, and the current state of material damage. This scalar damage state parameter is basically an abstract measure of the current state of material damage due to creep deformation. The damage rate is assumed to vary with stress, temperature, time, and the current state of damage itself. Multiaxial creep and creep rupture formulations of the CDM approach are presented in this paper. Parameter estimation methodologies based on nonlinear regression analysis are also described for both, isothermal constant stress states and anisothermal variable stress conditions This creep life prediction methodology was preliminarily added to the integrated design code CARES/Creep (Ceramics Analysis and Reliability Evaluation of Structures/Creep), which is a postprocessor program to commercially available finite element analysis (FEA) packages. Two examples, showing comparisons between experimental and predicted creep lives of ceramic specimens, are used to demonstrate the viability of this methodology and the CARES/Creep program.

The report documents a 12-month program conducted to study cumulative structural damage in solid rocket propellant. Tests conducted and experimental data obtained therefrom are described, and conclusions drawn from the study are presented. The theory of fracture in viscoelastic materials provides the analytical expressions for evaluating damage to propellant during loading histories comparable to those which might be experienced by rocket motor grains. Poker chip and biaxial strip tensile tests were used to simulate the states of stress typically occurring in actual motors. Simple constant rate tests were used to evaluate the constants necessary to apply the fracture equations to histories involving sequences of loadings.

The evolution of damage and strategies for assessing damage when it has occurred in structural materials is the primary focus of this research program. The research effort is collaborative one on mesomechanics called 'Damage in 2D and 3D Microstructures' (AFOSR-89-0423). What distinguishes this effort from others is the attempt to perform incisive model experiments to assist in evaluating stochastic models for constitutive laws (elasticity and conductivity) mechanical response plasticity and hardness and associated damage processes. The theoretical models are designed to be elegant and efficient in the use of computing resources as opposed to brute force procedures which handle systems with different scales by scaling up the size of the computer. The essence of this work is to explore sample size microstructure relationships via stochastic models which reflect the same types of variability present in the response of real materials. This leads directly to concerns with effective properties of materials. Thus, research integrates the effects of microstructure and preferred orientation on the effective tensorial properties of polycrystalline materials.... Microstructure, Damage, Brittle Fracture, Failure, Mechanics, Stochastic, Elasticity, Two-dimensional.

This report describes a preliminary investigation of the applicability of fractal geometry to damage modeling. Microstructural heterogeneity, both the size distribution and spatial distribution of microstructural features, can be modeled simply and compactly with a fractal dimension. The ultimate goal of this research is the development of an alternative to continuum damage and micromechanics models. The scaling nature of fractal geometry may aid the development of models that connect microscale damage with global mechanical response. An assumed fractal size distribution of microcracks in a brittle solid was used to derive the Weibull distribution for strength, and a relationship between the Weibull shape parameter and the fractal dimension of the flaw distribution was obtained. Published data on the strength of glass fibers were consistent with a fractal flaw distribution. Stable damage evolution in tougher materials with fractal microstructures was also considered. Although preliminary results are promising, further work is necessary to develop the concepts of fractal damage more fully. Damage Mechanics, Micromechanics, Composite Materials, Fractal Geometry.

This report results from a contract tasking University of Cassino as follows. The contractor shall investigate temperature and strain rate effects on ductile damage processes performed at room and elevated temperature on alloys having aerospace applications using continuum damage mechanics. The results will be used to validate their model and characterize various alloys having U.S. Air Force interests for applications in aging aircraft and corrosion.

A method for predicting isothermal plastic fatigue life of a composite lamina is presented in which both fibers and matrix are isotropic materials. In general, the fatigue resistances of the matrix, fibers, and interfacial material must be known in order to predict composite fatigue life. Composite fatigue life is predicted using only the matrix fatigue resistance due to inelasticity micromechanisms. The effect of the fiber orientation on loading direction is accounted for while predicting composite life. The application is currently limited to isothermal cases where the internal thermal stresses that might arise from thermal strain mismatch between fibers and matrix are negligible. The theory is formulated to predict the fatigue life of a composite lamina under either load or strain control. It is applied currently to predict the life of tungsten-copper composite lamina at 260 C under tension-tension load control. The calculated life of the lamina is in good agreement with available composite low cycle fatigue data.

Three multiaxial isothermal continuum damage mechanics models for creep, fatigue, and creep/fatigue interaction of a unidirectional metal matrix composite volume element are presented, only one of which will be discussed in depth. Each model is phenomenological and stress based, with varying degrees of complexity to accurately predict the initiation and propagation of intergranular and transgranular defects over a wide range of loading conditions. The development of these models is founded on the definition of an initially transversely isotropic fatigue limit surface, static fracture surface, normalized stress amplitude function and isochronous creep damage failure surface, from which both fatigue and creep damage evolutionary laws can be obtained. The anisotropy of each model is defined through physically meaningful invariants reflecting the local stress and material orientation. All three transversely isotropic models have been shown, when taken to their isotropic limit, to directly simplify to previously developed and validated creep and fatigue continuum damage theories. Results of a nondimensional parametric study illustrate (1) the flexibility of the present formulation when attempting to characterize a large class of composite materials, and (2) its ability to predict anticipated qualitative trends in the fatigue behavior of unidirectional metal matrix composites. Additionally, the potential for the inclusion of various micromechanical effects (e.g., fiber/matrix bond strength, fiber volume fraction, etc.), into the phenomenological anisotropic parameters is noted, as well as a detailed discussion regarding the necessary exploratory and characterization experiments needed to utilize the featured damage theories.

A consistent and systematic theory is developed for the analysis of damage mechanisms in metal matrix composites (MMCs). The theory involves both analytical and experimental procedures for the quantification of certain types of damage in MMCs. The analytical model involves both overall and local approaches to characterize damage in these materials. A series of experiments are conducted for micro- and macro-structural characterization of the MMC. Experiments were conducted on a titanium aluminide SiC-reinforced metal matrix composite. Center-cracked plates with laminate layups of (0/90)s, and (+/-45)s were tested under uniaxial tension. In order to investigate and model damage evolution, each of the specimens is loaded to different load levels ranging from fracture load down to 70% of the fracture load. Scanning electron microscopy (SEM) is performed on representative cross-sections of the damaged specimens. The measured crack densities are then used to define the damage parameters that are used in the theoretical model.... Metal matrix composites, Scanning electron microscopy, Damage parameters, Plasticity, Finite elements.

This paper examines the use of a continuum damage model to predict strength and size effects in notched carbon-epoxy laminates. The effects of size and the development of a fracture process zone before final failure are identified in an experimental program. The continuum damage model is described and the resulting predictions of size effects are compared with alternative approaches: the point stress and the inherent flaw models, the Linear-Elastic Fracture Mechanics approach, and the strength of materials approach. The results indicate that the continuum damage model is the most accurate technique to predict size effects in composites. Furthermore, the continuum damage model does not require any calibration and it is applicable to general geometries and boundary conditions.

This paper describes the development of input properties for a continuum damage mechanics based material model, Mat 58, within LS-DYNA(Registered Trademark) to simulate the response of a graphite-Kevlar(Registered Trademark) hybrid plain weave fabric. A limited set of material characterization tests were performed on the hybrid graphite-Kevlar(Registered Trademark) fabric. Simple finite element models were executed in LS-DYNA(Registered Trademark) to simulate the material characterization tests and to verify the Mat 58 material model. Once verified, the Mat 58 model was used in finite element models of two composite energy absorbers: a conical-shaped design, designated the \"conusoid,\" fabricated of four layers of hybrid graphite-Kevlar(Registered Trademark) fabric; and, a sinusoidal-shaped foam sandwich design, designated the \"sinusoid,\" fabricated of the same hybrid fabric face sheets with a foam core. Dynamic crush tests were performed on components of the two energy absorbers, which were designed to limit average vertical accelerations to 25- to 40-g, to minimize peak crush loads, and to generate relatively long crush stroke values under dynamic loading conditions. Finite element models of the two energy absorbers utilized the Mat 58 model that had been verified through material characterization testing. Excellent predictions of the dynamic crushing response were obtained.

The present work is concerned with development of innovative damage mechanics models for constitutive modeling of modern composite materials. Special attention is focused on micromechanical damage theories to explain and model behavior of composites based on micromechanics and micro-geometry. In particular, basic studies are performed on the notion of isotropic and anisotropic damage variables in continuum damage mechanics. Furthermore, a micromechanical damage model is presented for uniaxially reinforced composites and is analytically derived based on micromechanical bi-material arc crack solutions. Finally, a novel three-dimensional statistical micromechanical theory is proposed to investigate the nonlinear behavior of microcrack-weakened brittle solids. The theory is fundamentally different from existing effective medium theories and deterministic microcrack interaction method.

The major aircraft structural situation considered in this research is that of a thin skin stiffened by orthogonal frames and stringers, with the possibility of a lap joint. Effects on this situation considered in this research are: (1) impact by foreign objects, (2) fatigue damage due to repetitive loading, and (3) corrosion damage. Various theoretical models, pertaining to the response of the subject aircraft structure to these effects, have been considered: (1) the Finite Element Alternating Method (FEAM) applied to a single crack, (2) FEAM applied to Multiple Site Damage (MSD), and others. An analysis is also presented which pertains to the repair of cracks by application of a composite patch using adhesive.

This final progress report summarizes the accomplishments of the three tasks conducted during the contract period. Task I dealt with mechanisms of damage under static and fatigue loading of composite laminates and laid the physical basis for the modeling efforts that followed in Task 2 and Task 3. Task 2 focused on the formulation of stiffness-damage relationships and provided a procedure for assessment of material coefficients experimentally as well as by computational simulation. Finally, Task 3 treated evolution of damage. A model was developed that predicted the increase of transverse crack density with cycles of a constant amplitude. The overall outcome of the project was a methodology for durability assessment of composite materials, in accordance with the goals and objectives set in the proposal.

Presently there are many opportunities for the application of ceramic materials at elevated temperatures. In the near future ceramic materials are expected to supplant high temperature metal alloys in a number of applications. It thus becomes essential to develop a capability to predict the time-dependent response of these materials. The creep rupture phenomenon is discussed, and a time-dependent reliability model is outlined that integrates continuum damage mechanics principles and Weibull analysis. Several features of the model are presented in a qualitative fashion, including predictions of both reliability and hazard rate. In addition, a comparison of the continuum and the microstructural kinetic equations highlights a strong resemblance in the two approaches.

Manufacturing defects, like voids/pores in the matrix and weak planes at the matrix-fiber interfaces, make the material softer and weaker. Implementing such initial damage into the computational model is critical for predicting the performance and the failure mode of the fiber-reinforced composite (FRCs). Previously, we introduced a new Intermediate Homogenization Peridynamic approach (IH-PD model) for failure in multiphase materials. We plan to apply this IH-PD model for the implementing of initial damage (provided by the SDM approach) and modeling of crack growth and coalescence, through full failure of FRCs. In this quarter, we have tested the new IH-PD model against existing experimental results for fracture in a Functional Graded Material (FGM) plate. In the next quarter, we will implement and test the new IH-PD model for anisotropic multiphase material, such as FRCs.

An experimental study of the R-curve behaviour of an Alumina ceramic by in-situ SEM has yielded detailed information on the extent of the R-curve in this material as well as on the actual physical mechanisms responsible for the toughness increase. These have been identified as both ligamentary bridging due to crack bifurcation and bridging by wedged grains between the crack faces which dissipates energy by friction. Simple modelling calculations show that these mechanisms can account for the 75% increase in toughness observed using the double torsion technique. Further, the feasibility of Plaster of paris as a model ceramic material for damage mechanics investigations has been examined and extensive mechanical property characterization has been carried out. Keywords: Ceramics; Alumina; Fracture; R-curve; Scanning electron microscopy; In-situ observations; Damage mechanics; Uniaxial and Hydrostatic compression; Plaster of paris.

This research in support of the Air Force Research Laboratory Materials and Manufacturing Directorate was conducted at Wright-Patterson AFB, Ohio from 1 August 2007 through 15 October 2009. The objective of this task was to develop methods to better predict damage initiation, such as cracking, in superalloys under engine representative conditions. This report details work that focuses on characterizing the material microstructure of single crystal nickel-based superalloys, which is important for damage initiation in these materials. Additionally, this report details work that involves in situ mechanical testing of miniature specimens within the SEM as well as the setup of a miniature test specimen apparatus.

This research in support of the Air Force Research Laboratory Materials and Manufacturing Directorate was conducted at Wright-Patterson AFB, Ohio from 1 August 2007 through 15 October 2009. The objective of this task was to develop methods to better predict damage initiation, such as cracking, in superalloys under engine representative conditions. This report details work that focuses on characterizing the material microstructure of single crystal nickel-based superalloys, which is important for damage initiation in these materials. Additionally, this report details work that involves in situ mechanical testing of miniature specimens within the SEM as well as the setup of a miniature test specimen apparatus.

This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research on the damage assessment of structures. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.

This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research on the damage assessment of structures. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.

This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research on the damage assessment of structures. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.

Book Source: Digital Library of India Item 2015.193252 dc.contributor.author: Sankar Dhar dc.date.accessioned: 2015-07-08T02:24:43Z dc.date.available: 2015-07-08T02:24:43Z dc.date.digitalpublicationdate: 2005-08-27 dc.identifier.barcode: 1990010091602 dc.identifier.origpath: /rawdataupload/upload/0091/602 dc.identifier.copyno: 1 dc.identifier.uri: http://www.new.dli.ernet.in/handle/2015/193252 dc.description.scannerno: 14 dc.description.scanningcentre: IIIT, Allahabad dc.description.main: 1 dc.description.tagged: 0 dc.description.totalpages: 164 dc.format.mimetype: application/pdf dc.language.iso: English dc.publisher: Indian Institute Of Technology Kanpur dc.rights: Out_of_copyright dc.source.library: Indian Institute Of Technology Kanpur dc.subject.classification: Technology dc.subject.classification: Engineering. Technology In General dc.subject.classification: Mechanical Engineering In General. Nuclear Technology. Electrical Engineering. Machinery dc.title: A Continuum Damage Mechanics Model For Ductile Fracture

Book Source: Digital Library of India Item 2015.193252 dc.contributor.author: Sankar Dhar dc.date.accessioned: 2015-07-08T02:24:43Z dc.date.available: 2015-07-08T02:24:43Z dc.date.digitalpublicationdate: 2005-08-27 dc.identifier.barcode: 1990010091602 dc.identifier.origpath: /rawdataupload/upload/0091/602 dc.identifier.copyno: 1 dc.identifier.uri: http://www.new.dli.ernet.in/handle/2015/193252 dc.description.scannerno: 14 dc.description.scanningcentre: IIIT, Allahabad dc.description.main: 1 dc.description.tagged: 0 dc.description.totalpages: 164 dc.format.mimetype: application/pdf dc.language.iso: English dc.publisher: Indian Institute Of Technology Kanpur dc.rights: Out_of_copyright dc.source.library: Indian Institute Of Technology Kanpur dc.subject.classification: Technology dc.subject.classification: Engineering. Technology In General dc.subject.classification: Mechanical Engineering In General. Nuclear Technology. Electrical Engineering. Machinery dc.title: A Continuum Damage Mechanics Model For Ductile Fracture

A new model is proposed that represents the kinematics of kink-band formation and propagation within the framework of a mesoscale continuum damage mechanics (CDM) model. The model uses the recently proposed deformation gradient decomposition approach to represent a kink band as a displacement jump via a cohesive interface that is embedded in an elastic bulk material. The model is capable of representing the combination of matrix failure in the frame of a misaligned fiber and instability due to shear nonlinearity. In contrast to conventional linear or bilinear strain softening laws used in most mesoscale CDM models for longitudinal compression, the constitutive response of the proposed model includes features predicted by detailed micromechanical models. These features include: 1) the rotational kinematics of the kink band, 2) an instability when the peak load is reached, and 3) a nonzero plateau stress under large strains.

A new model is proposed that represents the kinematics of kink-band formation and propagation within the framework of a mesoscale continuum damage mechanics (CDM) model. The model uses the recently proposed deformation gradient decomposition approach to represent a kink band as a displacement jump via a cohesive interface that is embedded in an elastic bulk material. The model is capable of representing the combination of matrix failure in the frame of a misaligned fiber and instability due to shear nonlinearity. In contrast to conventional linear or bilinear strain softening laws used in most mesoscale CDM models for longitudinal compression, the constitutive response of the proposed model includes features predicted by detailed micromechanical models. These features include: 1) the rotational kinematics of the kink band, 2) an instability when the peak load is reached, and 3) a nonzero plateau stress under large strains.

No abstract available

The work performed in the first three-month period has been focused on Task 1.1 and Task 2.1 described in the project proposal. The activities related to Task 1.1 are a computational micromechanics failure analysis of a representative volume element containing disordered fiber distributions. Task 2.1 is concerned with damage evolution in a peridynamic model of poroelastic materials. Initial results for both tasks are reported and the ongoing work is outlined.

The development of microcrack damage in brittle in solids in compression is analyzed, using a simple model. The model is developed from recent detailed analysis of the initiation, propagation and linkage of microfractures from pre-existing cracks, voids, or other in-homogeneities. It describes the evolution of damage with strain and from it the stress-strain response, and criteria for failure, can be established. The results are used to construct failure surfaces in stress space which combine information about brittle failure with data describing the onset of plastic yielding. Such failure surfaces are constructed for a number of rocks and are compared with previously published experimental data. Brittle failure, Damage mechanics, Microfractures, Compressive failure.

This report results from a contract tasking University of Cassino as follows: The contractor shall investigate the behavior of ductile metals under high strain rates at various temperatures. The contractor shall produce a new advanced model for predicting ductile failure in metals using continuum mechanics and solid state equations.

A method is presented to represent the large-deformation kinematics of intraply matrix cracks and delaminations in continuum damage mechanics (CDM) constitutive material models. The method involves the additive decomposition of the deformation gradient tensor into 'crack' and 'bulk material' components. The response of the intact bulk material is represented by a reduced deformation gradient tensor, and the opening of an embedded cohesive interface is represented by a normalized cohesive displacement-jump vector. The rotation of the embedded interface is tracked as the material deforms and as the crack opens. The distribution of the total local deformation between the bulk material and the cohesive interface components is determined by minimizing the difference between the cohesive stress and the bulk material stress projected onto the cohesive interface. The improvements to the accuracy of CDM models that incorporate the presented method over existing approaches are demonstrated for a single element subjected to simple shear deformation and for a finite element model of a unidirectional open-hole tension specimen. The material model is implemented as a VUMAT user subroutine for the Abaqus/Explicit finite element software. The presented deformation gradient decomposition method reduces the artificial load transfer across matrix cracks subjected to large shearing deformations, and avoids the spurious secondary failure modes that often occur in analyses based on conventional progressive damage models.

The efforts to provide a penetration code called PEN4 version 10 is documented for calculation of projectile and target states for the impact of 2024-T3 aluminum, R sub B 90 1018 steel projectiles and icy meteoroids onto 2024-T3 aluminum plates at impact velocities from 0 to 16 km/s. PEN4 determines whether a plate is perforated by calculating the state of fragmentation of projectile and first plate. Depth of penetration into the second to n sup th plate by fragments resulting from first plate perforation is determined by multiple cratering. The results from applications are given.

Progressive failure is a crucial concern when using laminated composites in structural design. Therefore the ability to model damage and predict the life of laminated composites is vital. The purpose of this research was to experimentally verify the application of the continuum damage model, a progressive failure theory utilizing continuum damage mechanics, to a toughened material system. Damage due to tension-tension fatigue was documented for the IM7/5260 composite laminates. Crack density and delamination surface area were used to calculate matrix cracking and delamination internal state variables, respectively, to predict stiffness loss. A damage dependent finite element code qualitatively predicted trends in transverse matrix cracking, axial splits and local stress-strain distributions for notched quasi-isotropic laminates. The predictions were similar to the experimental data and it was concluded that the continuum damage model provided a good prediction of stiffness loss while qualitatively predicting damage growth in notched laminates.

The work performed in the reporting period has been focused on completion of Task 1.1, and continuation of Task 1.2 and Task 2.2 described in the project proposal. The activities related to Task 1.1 are formation of cracks in a computational micromechanics failure analysis of a representative volume element containing disordered fiber distributions and Task 1.2 concerns growth and instability of initiated cracks in the environment of the disordered fiber distributions. The activities related lo Task 2 cover modeling of interface bonds between different phases in a composite.

A fracture mechanics based approach to quantify the influence of initial corrosion damage on structural integrity is described. This approach assumes that corrosion can be approximated by a geometric structural change consisting of a general thickness reduction combined with a localized stress concentration. These two parameters enable quantification of the damage and the application of fracture mechanics principles to corrosion. Several numerical examples of this concept are discussed and a test program, combined with inspection capabilities, is introduced.