Damage Development Mechanisms Research Articles

Performance of Flax/Epoxy Composites Made from Fabrics of Different Structures

Flax fibers have been shown to have comparable mechanical properties to some conventional synthetic fibers. Flax fabrics with different textile structures show differences in resistance against mechanical loads mainly rooted in fabric orientation and the resultant resin impregnation. Thus, in this study, flax fabrics with three different textile structures, fine twill weave, coarse twill weave and unidirectional, were used as reinforcements in an epoxy matrix. The surfaces of the fabrics were chemically treated using an alkaline treatment, and the alterations in fabric crystallinity index (CrI) were determined using X-ray diffraction (XRD). Experimental results confirmed that textile structures and CrI had significant effects on the mechanical properties of composites. Although an increment in CrI, resulting from chemical treatment, always enhanced tensile and flexural properties, it adversely affected damage development once composites were exposed to impact load. In terms of textile structures, unidirectional fabric outperformed woven fabrics in tensile and flexural properties while in impact properties, the latter had a better performance inducing less damage development. Finally, the mechanism of damage development in different composites was discussed in detail using Scanning Electron Microscopy (SEM) images. It is envisaged that the results of this study will provide an insight that will lead to the proper choice of the optimal kind of flax fabric for different applications.

Cellular Mechanisms of Damage and Compensation in the Brain in Cerebral Ischemia: Molecular Proteostasis Control Systems as a Target for Therapy

The relevance of the work is due to the wide spread of cerebrovascular pathology and ischemic strokes among the population. Most studies are devoted to the study of early brain damage in cerebral ischemia, while insufficient attention has been paid to the mechanisms of adaptation and long-term brain disorders. At the same time, delayed death of nerve cells in ischemia is not a predetermined and irreversible process, leaving opportunities for therapeutic intervention. There is a need to search for new data on the molecular and cellular mechanisms of damage development and compensatory processes in brain neurons in the dynamics of cerebral ischemia of varying severity. At the same time, the activation of compensatory mechanisms will reduce the severity of neurodegenerative disorders in the brain, increase the effectiveness of the treatment. An important role in the development of neurodegeneration is played by protein aggregates, which can be localized in nerve cells, in the intercellular space and cerebrospinal fluid. They are formed on the basis of damaged proteins, the violation of the structure of which can be caused by hypoxic and nitrosative stress, inflammatory processes provoked by ischemia, death of neurons. Therefore, an important role in preventing the development of secondary brain damage after ischemia can be played by cellular systems responsible for protein homeostasis, the protein synthesis apparatus, chaperones, the system of autophagy and elimination of damaged proteins ((ubiquitin, proteasome), as well as energy-intensive adaptation processes.

Changes In the Morphology of Neurons of The Parietal Cortex and Hippocampus of Rats in The Dynamics of Step Subtotal Cerebral Ischemia

The study of the brain in norm and pathology is an urgent and promising direction of modern science and, in this regard, a frequent topic of dissertation research. Cerebral ischemia leads to a number of general and local metabolic and functional disorders, the pathogenesis of which is complex, multifaceted and largely. The aim was to change the morphology of neurons in the parietal cortex and hippocampus of rats in the dynamics of stepwise subtotal cerebral ischemia. With stepwise bilateral ligation of both common carotid arteries with an interval of 7 days, negative changes were least pronounced, especially in the hippocampus. The longer the interval between dressings, the more effectively neurons adapt to lack of oxygen, which makes it possible to further study in more detail the dynamics of the mechanisms of damage development and adaptive changes in the brain.

Various mechanisms of myocardial necrosis development due to combat injury as a reason for discussion: a case series

The real clinical practice of the last 8 years in Ukraine indicates the possibility of myocardial injury developing in wounded with combat trauma, including those meeting the criteria for myocardial infarction (predominantly type 2), which differs significantly in the mechanism of development, resulting in numerous difficulties for doctors in providing medical care to such patients (formulation of the diagnosis, choice of treatment tactics). The current recommendations of the European Society of Cardiology for the management of patients with acute coronary syndromes after the adoption of the Fourth Universal Definition of Myocardial Infarction do not consider special approaches to the treatment of patients with myocardial necrosis associated with trauma. The aim of this publication was to demonstrate, using the example of a case series, the relevance of the problem of terminology, diagnosis, and treatment of myocardial infarction caused by a combat injury. In all three clinical cases presented, young and middle-aged patients were diagnosed with myocardial infarction in accordance with the Fourth Universal Definition. In all these cases, there were no atherosclerotic coronary artery lesions, and a type 2 myocardial infarction was assumed due to acute myocardial ischemia as a consequence of the injured coronary artery ligation (case 1); microcirculation disorders in the area of myocardial contusion (case 2); anatomical feature of the coronary artery in the form of its intramural course in the presence of severe sympathicotonia and sinus tachycardia (case 3). Because all three patients had myocardial necrosis associated with trauma of the chest and heart, the question arose of the preference for coding such pathology through ICD-10 as section S00-T88 (injury, poisoning and certain other consequences of external causes), namely S26 (injury of heart), but not as I21 (acute myocardial infarction) from the section I00-I99 (diseases of the circulatory system). Thus, the developed pathology in such patients will be clearly associated with the combat trauma, and their treatment will not be tied to the current Ukrainian Unified Protocol for the Management of Patients with ST-segment Elevation Acute Coronary Syndrome. Conclusions. The presented clinical cases substantiate the planning of clinical trials and the development of recommendations for the management of patients with acute myocardial injury (including myocardial necrosis) and trauma of the heart and coronary arteries, depending on the mechanism of damage development.

Formation of Defects in Two-Dimensional MoS2 in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations.

Production of defects under electron irradiation in a transmission electron microscope (TEM) due to inelastic effects has been reported for various materials, but the microscopic mechanism of damage development in periodic solids through this channel is not fully understood. We employ non-adiabatic Ehrenfest, along with constrained density functional theory molecular dynamics, and simulate defect production in two-dimensional MoS2 under electron beam. We show that when excitations are present in the electronic system, formation of vacancies through ballistic energy transfer is possible at electron energies which are much lower than the knock-on threshold for the ground state. We further carry out TEM experiments on single layers of MoS2 at electron voltages in the range of 20-80 kV and demonstrate that indeed there is an additional channel for defect production. The mechanism involving a combination of the knock-on damage and electronic excitations we propose is relevant to other bulk and nanostructured semiconducting materials.

Study of the Effect of Strain Ageing on Corrosion Failure and Submicrostructure of Low-Carbon Steels for Gas-Distribution Pipelines

Questions are considered of the effect of strain ageing for low-carbon unalloyed steels used for gas distribution pipelines on their corrosion resistance under the action of ground water. A set of mechanical property tests, corrosion tests, and metallographic studies for specimens with a different degree of strain aging accumulation is performed on the basis of a procedure developed. It is shown that for low-carbon unalloyed steels there is an increase in corrosion rate as the degree of strain ageing accumulates. The mechanism of damage development depends considerably on the degree of strain accumulation and is specified by the development of corrosion defects directed over intrusion lines formed during prior deformation and ageing.

Modeling of Moisture Diffusivity of Concrete at Low Temperatures

Moisture diffusivity of saturated concrete varies with decreasing temperature. The variation depends on the amount of ice formed in pores and on the extent of the damage induced by the formation of ice. Ice crystals in the pores decrease the moisture diffusivity of concrete because of the low moisture diffusivity of ice, and the damage attributable to excessive ice formation in the pores increases the diffusivity. An analytical model is developed to characterize the effect of the two opposing processes on the diffusivity of the concrete under low temperatures. Three pore configurations are defined as (1) pores with no ice, (2) pores with ice and no damage, and (3) pores with ice and damage. The effective diffusivity of concrete is a combination of the diffusivities of the three types of porous media, each containing one configuration of pores. In the current study, the three types of porous media are combined by the composite model developed by Hashin and Shtrikman. The volume fraction of each configuration of pores is evaluated by a pore-size distribution based on the adsorption isotherm and a function of freezing temperature in terms of pore sizes. The diffusivities of the three types of porous media are evaluated by considering the ice-formation and damage-development mechanisms. The Druker-Prager plasticity model was used, and pores are assumed to be spherically shaped for evaluating the internal damage. The results of model prediction show that the moisture diffusivity in concrete decreases when the solution is frozen in some of the pores. If the temperature reduces further and causes damage attributable to excessive ice formation, then the diffusivity decreases at a lower rate than that at the same temperature without damage.

Modelling Damage of Composite Laminates with Different Stacking Sequences

This paper considers damage development mechanisms in composite laminates subjected to tensile loading. The continuum damage mechanics is the most widely used approach to capture the non linear behaviour of laminates due to cracking. In this study, a continuum damage model based on ply failure criteria, which is initially proposed by Ladevèze has been extended to cover all plies failures mechanisms using an accurate numerical model to predict the equivalent damage accumulation. However, this model requires a reliable representation of the elementary damage mechanisms which can be produced in the composite laminate. To validate this model, a numerical application has been carried on the cross-ply laminates of type [0n/90m]s..A shear lag model was adapted to calculate the average stress of the 0° and 90° plies. The solution presented is obtained by using finite element analysis which implements progressive failure analysis. The effect of the stacking sequences has been done by varying the thickness of the 90° plies.

Cross-Ply Laminates under Static Three-Point Bending: A Numerical Development Model

This paper considers damage development mechanisms in cross-ply laminates using an accurate numerical model. Under static three points bending, two modes of damage progression in cross-ply laminates are predominated: transverse cracking and delamination. However, this second mode of damage is not accounted in our numerical model. After a general review of experimental approaches of observed behavior of laminates, the focus is laid on predicting laminate behavior based on continuum damage mechanics. In this study, a continuum damage model based on ply failure criteria is presented, which is initially proposed by Ladevèze. To reveal the effect of different stacking sequence of the laminate; such as thickness and the interior or exterior disposition of the 0° and 90° oriented layers in the laminate, an equivalent damage accumulation which cover all ply failure mechanisms has been predicted. However, the solution algorithm using finite element analysis which implements progressive failure analysis is summarized. The results of the numerical computation have been justified by the previous published experimental observations of the authors.

Detection of matrix cracks in composite laminates by using the modal strain energy method

As the foremost mechanism of damage development, matrix cracking is the critical damage found in the early stage of structural failure of composites. This study aims to nondestructively detect matrix cracks in composite laminates by using an experimental modal analysis (EMA). An AS4/PEEK composite was used to fabricate cross-ply [02/9012/02] and quasi-isotropic [(±45/0/90)4] s laminates. The damage in the form of a matrix crack in the laminates was created by using a tensile load. The EMA was conducted on the laminates to obtain the modal displacements before and after damage. The displacements were then employed to compute the modal strain energy and to define the damage index, which is used for detecting matrix cracks. Limited by the mesh points of measurements, we used the differential quadrature method to calculate the partial differentials in the strain energy formula. The results obtained were validated by using the X-ray radiography method and three-point bending tests. The experimental results showed that the damage index well identified the location of breadthwise matrix cracks inside the laminates. However, the resolution of the damage index became poor if the spans of matrix cracks were short or the matrix cracks were scattered over the laminates.

Characterization and evolution of matrix and interface related damage in [0/90] S laminates under tension. Part I: Numerical predictions

This paper considers damage development mechanisms in cross-ply laminates using an accurate numerical method that assumes a Generalized Plane Strain (GPS) state. A 2D Boundary Element Method (BEM) model is generated to investigate the two types of damage progression in a [0/90] S laminate: transverse cracks in the 90° lamina and delamination between both laminae. The model permits the contact between the surfaces of the cracks. The study is carried out in terms of the dependence of the Energy Release Rates (ERR) of the two types of crack on their respective lengths. A special emphasis is put on the mechanisms of the joining of the two aforementioned types of crack, including the study of the distribution of the stresses along the interface between the two plies when the transverse crack is approaching this interface.

Compression Response of 2D Braided Textile Composites: Single Cell and Multiple Cell Micromechanics Based Strength Predictions

This article is concerned with the development of a finite element (FE) based micromechanics model for the prediction of compressive strength and post-peak compression response of 2D triaxial braided carbon fiber polymer matrix composites (2DTBC). This micromechanics based study was carried out on a series of single and multiple representative unit cell (RUC) 3-D FE models. The uniaxial compressive response, including unstable equilibrium paths, was studied using an arc-length method in conjunction with the ABAQUS commercial FE code. In the reported study, explicit account of the braid microstructure (geometry and packing) and the measured inelastic properties of the matrix (the in-situ properties) are accounted for via the use of the FE method. This enables accounting for the different length scales that are present in a 2DTBC. The computational model provides a means to assess the compressive response of 2DTBC and its dependence on various microstructural parameters. The model provides a means to compute the compression strength allowable for a 2DTBC structure. In particular, the dependence of compressive strength on the axial fiber tow properties and axial tow geometrical imperfections is discussed and shown to be significant in capturing the mechanism of damage development. Results are presented for 1, 4, 9, and 16 RUC representations of the 2DTBC, enabling to examine the dependence of compressive strength (or lack thereof) on the size of the region that is modeled. The predicted results are found to compare favorably against experiment.

Braided textile composites under compressive loads: Modeling the response, strength and degradation

This paper discusses the results of a finite element (FE) based micromechanics study of the compressive damage development mechanisms of 2D triaxial braided carbon fiber composites (2DTBC). The micromechanics based study was carried out on a Representative Unit Cell (RUC) size 3D FE model. The uniaxial compressive response was established using an arc-length method in conjunction with the ABAQUS commercial FE code. In this work, explicit account of the braid microstructure (geometry and packing) and the measured inelastic properties of the matrix (the in situ properties) are accounted for via the use of the FE method. This enables accounting for the different length scales that are present in a 2DTBC. This detail is necessary for developing a mechanism based damage prediction capability. The computational model provides a means to assess the compressive strength of 2DTBC and its dependence on various microstructural parameters. In particular, the dependence of compressive strength on the axial fiber tow properties and axial tow geometrical imperfections is discussed and shown to be significant in capturing the mechanism of damage development.

開繊織物複合材料の力学的特性に関する研究(S07-3 複合材料I,S07 メゾスケールにおける先進材料の力学と強度・機能評価)

Woven fabric composites with spread tow may induce better resin impregnation and mechanical properties than the conventional woven architectures. However, the mechanism of damage development of woven fabric composites with spread tow has not been investigated. To investigate the effect of spread tow on the damage development, we have developed the numerical simulation of the mechanical behavior based on damage mechanics. We have prepared two numerical models with a distribution of volume fraction in a strand, and an effect of the volume fraction in a strand on the damage development has been investigated. The numerical results indicate that the location of occurrence and the type of propagation of transverse cracks are quite different. Furthermore, the mechanical behaviors of woven fabric laminates with three layers with distributions of micro volume fractions in fiber bundles have also been analyzed. In this paper, the numerical results of the damage states are described.

Impact fatigue properties of glass fiber-reinforced thermoplastics

The impact fatigue properties of three kinds of glass-fiber or glass-bead reinforced polymers were studied through uni-axial and cyclic indentation tests with acoustic emission (AE) measurements and visco-elastic properties measurements. The detailed failure mechanisms and damage development mechanisms were investigated with acoustic velocity measurements and optical microscope (OM) observations. Prior to fatigue tests, the materials were characterized by basic mechanical tests such as tensile tests, indentation tests, and drop weight tests. In performing fatigue tests, the authors paid special attention to the effect of interval times between loading on the fatigue properties of those materials. It was found that the numbers of cycles to failure were strongly dependent on the duration of the interval time. In the case of uni-axial fatigue tests, the interval times resulted in greater numbers of cycles to failure. On the other hand, in cyclic indentation tests, the interval times resulted in smaller numbers of cycles to crack initiation and ultimate failure. The results of AE measurements were in good agreement with the results of cycles to failure in fatigue tests and the test conditions. Attention was also paid to the relationship between the size of the damage zones and interval times between loading. The difference in the cycles to failure due to the different loading modes and interval times were understood by the differences in the mechanisms of damage zone development, where breakage of glass, micro-voiding, change in the orientation of glass fibers, and plastic deformation of matrix occurred. The degree of damage localization had a strong effect on the fatigue life. The difference in the damage development mechanisms were found to be caused by the difference in the elastic response of the specimens due to the different loading mode (uni-axial or indentation) and the interval time.

Comparison of in situ damage assessment in unbalanced fabric composite and model laminate of planar (one-directional) crimping

In order to develop a basic understanding of damage and failure mechanisms in fabric (woven) composites, a comparative study has been undertaken of in situ damage development in two composite systems. The two composite systems, an unbalanced (6:1) five-harness satin (5HS) fabric composite and a model laminate with planar (one-directional) fiber crimping, were tested in tension. The in situ damage observations were made by means of a portable load frame placed on an optical microscope stage. The damage in the unbalanced fabric initiated in the form of yarn interface cracks and then developed further as intra-yarn delamination before failure. In the case of the model laminate, the damage first appeared as cracks in transverse laminae and then further developed as lamina interface cracks. The fiber breakage that was observed in the wavy region of the model laminate appeared to be limited to the free-edge region of the specimens. Stress analyses of the two composite systems need to be performed in order to understand the damage development modes. The damage in the model laminate is expected to serve as a basis for understanding damage development mechanisms in woven composites.

Microstructure, Damage and Resistance during Electromigration Life-Testing of Al-Cu Interconnects

AbstractUnpassivated 1.4 mm long lines of Al-4wt.%Cu metallization have been successively imaged (by scanning electron microscopy) and electromigration stressed until failure. The resistance of lines, evolution of line microstructure and the development of electromigration damage are thus discontinuously recorded through the accelerated life-testing (260°C, 2 × 1010 A m-2). Correlations are made among microstructure evolution, electromigration damage development and line resistance. The probable mechanisms of damage development are discussed.

Microstructure and the Development of Electromigration Damage in Narrow Interconnects

The microstructure in narrow (1.1, 1.5 and 2.1 μm) unpassivated lines of Al-4wt.% Cu is found to be ‘near-bamboo’, with Al 2 Cu grains being a significant feature correlated with thermal hillocking and with the development of damage on electromigration. The development of damage is shown to be closely related to the median time to failure, with its initiation being at a rate proportional to the square of the current density. The mechanisms of damage development are discussed, with particular reference to near-bamboo, two-phase microstructures.

Fretting fatigue of continuous carbon fibre reinforced polymer composites

Continuous carbon fibre reinforced epoxy resin laminates were exposed to a fatigue loading. Simultaneously, cylindrical metallic pins with flat and polished front surfaces were pressed against the specimen surfaces with an apparent contact pressure between 10 and 23 MPa. The peak-to-peak oscillation width in the contact region amounted to about 700 μm. Fatigue life of the composite could drastically be reduced by this additional fretting component, if load bearing 0° layers were damaged by fretting. This paper studies the mechanisms of damage development in carbon fibre reinforced laminates under fretting fatigue conditions and proposes a quantitative measure for the degree of fretting fatigue damage. The influence of the loading conditions (contact load, counterpart material and slip amplitude) is investigated.

Dislocational pile-up—grain boundary interaction at elevated temperature

The nucleation and growth of grain boundary voids on the grain boundary facets, transverse to the direction of applied load, is one of the widely accepted mechanisms of damage development and material failure at high temperature. The aim of this work is the analysis of certain sources of stress concentration on the grain boundary which lead to time dependent high stress gradients. Specifically, the interaction of a dislocational pile-up with the grain boundary at high temperature is studied in order to model a variety of practical possible grain defects which can be approached on the basis of the theory of continuously distributed dislocations. Assuming the existence of the pile-up before the temperature rises, one investigates the time dependent stress field along the grain boundary, stress relaxation parameters and critical time. The diffusive absorption of the dislocations from the pile-up as well as the dislocation redistribution in the pile-up versus time are represented. This data leads to the estimation of possible grain boundary void nucleation, material damping due to diffusive matter redistribution and evaluation of plastic energy dissipation.