Nuri Akkas

Nuri Akkas
Middle East Technical University | METU · Department of Engineering Sciences

BS, MS, PhD

About

116
Publications
3,963
Reads
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1,737
Citations
Additional affiliations
September 2001 - May 2003
Baskent University
Position
  • Professor (Full)
September 1995 - present
Middle East Technical University
Position
  • Professor (Full)
September 1985 - August 1990
Ankara University
Position
  • Professor (Full)
Education
August 1968 - December 1970
Clemson University
Field of study
  • Engineering Mechanics
August 1967 - July 1968
Clemson University
Field of study
  • Civil Engineering
August 1962 - June 1967
Robert College
Field of study
  • Civil Engineering

Publications

Publications (116)
Article
In March 2000, the Lisbon European Council set the goal of becoming by 2010 “the most competitive and dynamic knowledge-based economy in the world, capable of sustainable economic growth with more and better jobs and greater social cohesion”. This was called the Lisbon Strategy. The project of creating a European Research Area (ERA) was endorsed as...
Conference Paper
Full-text available
Reasons for the everlasting complex oscillatory behavior of human postural sway have been explored by constructing the phase-plane representation of the Center of Pressure data recorded experimentally. For this purpose, 275-seconds time records are received from a healthy subject while standing still in an upright posture. A Sensorial threshold has...
Article
In this research, a conventional propylene glycol-based unsaturated polyester is synthesized and modified with diphenylmethane-4,40-diisocyanate (MDI), which leads to the formation of a hybrid polymer network (HPN). The effect of this modification on the mechanical properties is studied experimentally. The optimum molar ratio of MDI to unsaturated...
Article
Full-text available
In this experimental study, the goal was to test the sufficiency of actual fixation plates in zygomatic complex fractures and the efficiency of a modified plate at the zygomaticofrontal suture in a suitable model, which was designed for biomechanical study. To address this issue, a zygomatic fracture model produced by using a cadaveric cranium was...
Article
The possibility of using xylan, as an agricultural by-product, for production of composite films in combinations with wheat gluten was investigated. Different levels of xylan (0-40% w/w) were incorporated into wheat gluten to form biodegradable composite films. Films were prepared at pH 4 and 11, and dried at either uncontrolled or controlled condi...
Article
The mechanical and physical properties of glycerol-plasticized wheat gluten films dried at different temperatures (20, 50, and 80 degrees C) and relative humidities (35 and 70% RH) were investigated. Dispersion of wheat gluten was prepared at pH 11 in aqueous solution. Films were obtained by casting the wheat gluten suspension, followed by solvent...
Article
One of the most important tasks in clinical practice is to determine accurately the forces and displacements of the various anatomical structural components of the human leg, especially when subjected to impact loading. This paper therefore develops a computer simulation which describes the biodynamic response of the human lower extremity for reali...
Article
In this study, the mechanical properties of two different permanent soft lining materials and their bonding to poly(methyl methacrylate) (PMMA) were compared. Both of the soft liners were heat-cured commercial materials. The polymerization was carried out by conventional methods suggested by manufacturer, and the curing was done at the temperature...
Article
In this study, the snap-through buckling behaviour of axisymmetric shells, subjected to axisymmetric horizontal peripheral load or displacement for various shell parameters and various boundary conditions, is investigated. Results obtained seem not to have been reported previously. An application of peripheral displacement type of loading is seen i...
Article
Dispersions of wheat gluten were prepared at different pHs (4, 6 and 11) in the presence or absence of ethanol. Films were obtained after casting suspensions followed by evaporation of the solvent. Films prepared at pH 11 were significantly stronger than the films prepared at pH 4 and 6 with tensile strengths of approximately 7 vs. 2 MPa. With incr...
Article
In this paper, based on the characterizations of human knee-joint anatomical structures and reports of the literature and experiments, a biomechanical model of the human knee-joint elastic articulate contact is developed under the conditions of sampling the human knee-joints. This model is believed to be a powerful tool for functional analysis of t...
Article
This work covers studies on systems with car tire regrind used as the toughening phase in epoxy resins. Different surface modification techniques were used to improve the compatibility at the between the rubber particles and the epoxy matrix. Silane coupling agents, plasma surface modification and acrylic acid/benzoylperoxide were used to enhance t...
Article
Determination of the fracture resistance of various restorative materials in Class II approximal slot restorations has not been studied. This study evaluated the effects of retention grooves and different restorative materials in Class II approximal slot restorations. To explore the possibilities for further research, the probable effects of prepar...
Article
In this paper, based on the characterizations of human knee-joint anatomical structures and reports from the literature and experiments, a biomechanical model of the human knee-joint elastic articulate contact is developed and its analytical solution forms is established according to Hertz's elastic contact theory. These solutions include the maxim...
Article
Transient response of ellipsoidal shells submerged in an acoustic medium subjected to a concentrated Heaviside load at the apex is studied numerically using improved transmitting boundaries. By employing the rigorous Residual Variable Method, the second derivative in the wave equation with respect to the spatial variable extending to infinity is el...
Article
Unlabelled: Comparative data on the mechanical properties of epidural catheters used clinically are not available. We performed a controlled laboratory investigation to assess the mechanical performance of three different intact or traumatized catheter types (Polyurethane, clear nylon, and radiopaque nylon catheters, designed for 18-gauge Tuohy ne...
Article
Selenium, being an essential mineral in the mammalian diet, is important in providing protection against oxidative damage. Numerous invitro studies of selenium compounds reveal a very high correlation between catalytic activity of selenium compounds and toxicity. The present study was designed to investigate the effects of dietary selenium on the b...
Article
A spherical cavity in an infinite, elastic medium with and without a shell embedment is subjected to axisymmetric, non-torsional surface loads in the radial and meridional directions. The so-called Residual Variable Method (RVM) is used to obtain exact, closed-form solutions of the wave propagation problems. Some representative numerical results ar...
Article
The purpose of this work is to present some numerical results on nonlinear, snap-through buckling of shallow spherical shells made of laminated composite materials under general static loading. Isotropic shells are covered as a special case of the general problem. A special-purpose finite-difference computer program has been developed. Moreover, th...
Article
In this paper, based on the previous work of S. Turgut Tumer et al, a human knee articulate mathematical model on femur-tibia-patella 3-segments is established by introducing patella-femur joint. This model includes both rolling and slipping motions of femur-tibia joint and femur-patella joint. It can be employed in simulating the biodynamic respon...
Article
In this paper, a set of anatomically based constraint conditions of the human knee joint, consisting of three moving body segments (femur-patella- tibia) and two articulations (tibia-femoral joint and patella-femoral joint), is developed. For the purpose of studying that, the four independent coordinate systems are established on the pelvis, the fe...
Article
The thermal and mechanical properties of poly(methyl methacrylate) prepared at different curing times were studied using DSC, TGA, tensile, and three-point bending test methods. The molecular weights of the polymer samples were determined from the viscosity measurements. The curing time applied for two different commercial materials, manufactured f...
Article
Naturally occurring, biocompatible, and biodegradable polyhydroxybutyrate-co-hydroxyvalerate (PHBV), and synthetic, non-degrading polyhydroxyethylmethacrylate (PHEMA) membranes were prepared and their mechanical properties were studied. Their performances were compared with the interpenetrating networks (IPN) prepared by photopolymerization of HEMA...
Article
A three-dimensional finite element computer analysis of multistorey building structures, made of pierced shear walls of open and/or closed cross-sections and flat plates, is presented. The computer program developed for this purpose provides a special and powerful mesh generation subroutine. A graphic program is also developed to prepare the data i...
Article
It has been suggested that the vibration transmission across a fracture is affected by the stages of healing of the fracture callus. This study aims to correlate the change in vibration transmission with mechanical stiffness of the callus measured by three-point bending. The right tibiae of male, three-month old local albino rabbits were osteotomiz...
Chapter
The response of filament-wound composite tubes under pure torsional loading has been investigated. The tubes, produced using the wet winding technique, are made of E-Glass, S2-Glass, Aramid and Carbon/Epoxy with three lamination sequences and three different helix-winding angles. Experiments have been performed by applying twisting moment at the fr...
Article
The aim of this controlled study is to investigate the effect of heparin on osteoporosis initiation and of calcitonin and tamoxifen on the progress of osteoporosis induced by heparin through biomechanical means and, thus, to assist in clinical usage of these drugs. 32 four-month-old female New Zealand white rabbits were divided into four different...
Article
The effects of microsurgical and medical treatments on reduction of adhesions in surgically traumatized flexor tendons of rabbits are quantified in this study. The effects of the mentioned techniques were investigated for the following 4 groups: (1) neither primary sheath repair nor aprotinin application was done, (2) primary sheath repair was done...
Article
The present study was designed to investigate and compare the effects of dietary selenium (Se) and vitamin E on some physiological parameters and histological changes in liver, heart, and skin tissues, as well as the blood parameters and the related enzymes. Both sex young rabbits were fed with deficient (9.8 micrograms/kg diet), adequate (225 micr...
Article
It is generally agreed that combined deficiency of selenium and vitamin E leads to several abnormalities including Kashin-Beck disease which is an endemic and chronic degenerative osteoarthrosis. The abnormalities can be reversed by the administration of various forms of selenium and vitamin E. The present study was designed to investigate the effe...
Article
The solution is obtained for the transient response to a nontorsional surface load of an infinite, isotropic, elastic medium containing a cylindrical cavity with and without a thin elastic shell embedment using the Residual Variable Method The main advantage of the present approach is that it eliminates the computational problems arising in the exi...
Article
The dynamics of shells of arbitrary geometry submerged in an acoustic medium has attracted attention for many decades. Only a finite region of the finite fluid domain can be modelled in the numerical analysis. The artificial surface encompassing the finite region is called the truncation surface, on which the transmitting boundary condition is used...
Article
In a previous ultrastructural study, the benefit of a single high dose of vitamin D3 on fracture healing in a healthy animal model was demonstrated. This study examined the biomechanical consequences of applying a single high dose of vitamin D3 in a healthy rabbit model subsequent to femoral fracture. The fracture load, the values of energy absorbe...
Article
Full-text available
The experimental conditions for the synthesis of sub-micrometre, spherical particles of calcium hydroxyapatite [Ca10(PO4)6(OH)2] (HA) and tricalcium phosphate [Ca3(PO4)2] (TCP) are investigated through chemical coprecipitation from the aqueous solutions of calcium nitrate and di-ammonium hydrogen phosphate salts. The precipitation process employed...
Article
External skeletal fixation is an alternative method of treatment to conventional therapy and open surgical procedures in pelvis fractures. The appropriate type of frame and the configuration of the connecting bars of these fixators are under investigation. In the Gazi type pelvis external fixator (GPEF) which has been developed, a 70 degree angulat...
Article
In a previous ultrastructural study, the benefit of a single high dose of vitamin D3 on fracture healing in a healthy animal model was demonstrated. This study examined the biomechanical consequences of applying a single high dose of vitamin D3 in a healthy rabbit model subsequent to femoral fracture. The fracture load, the values of energy absorbe...
Article
Female New Zealand white rabbits were subjected to heparin treatment to induce osteoporosis and to observe the effects of calcitonin and tamoxifen treatments on the initiation of osteoporosis. The aim of this study is to investigate zinc-calcium interaction as well as the alterations of plasma biochemical parameters in these animals and, thus, to a...
Article
The dynamic behaviour of a Toda particle is investigated numerically. The system is a discrete spring-mass system and it oscillates in a plane two-dimensionally under the action of a harmonic excitation. Periodic and chaotic motions are shown to be possible in the parameter space. Numerical methods are used to obtain the time histories, the Poincar...
Article
Animal cells acquire a spherical shape just before the divi-sion. During cleavage the mother spherical cell is divided into two daughter cells and the total volume remains constant. According to the so-called 'contractile ring theory,' the mother cell is constricted by an equatorial ring. In view of the numerical results obtained from previous biom...
Article
In the second part of this two-part paper, numerical solution of the proposed model is provided for two cases. Namely, a) the cortex stiffness around the equatorial region is higher than that of around the polar region, b) the cortex stiffness is lower around the equatorial region than that around the polar region. The numerical results show that t...
Article
Full-text available
Porous calcium hydroxyapatite (CHA) ceramics are biocompatible and present osteoconductive properties. These ceramics are widely used in orthopaedic surgery; however, it is not yet known whether they have some adverse effects on bone and bone marrow healing. Our previous radiological study revealed possible local porosis at the adjacent sites of th...
Article
Forced vibrations of nonlinear lattices, consisting of one or two particles vibrating one dimensionally, are investigated. Harmonic response, bifurcations and chaotic behaviour are considered. We use the Toda potential, the Morse potential and a third “combined” potential as the interaction potentials between the adjacent particles in the models. T...
Article
Full-text available
In this paper, a two-dimensional anatomically-based dynamic model of the human lower limb, consisting of the thigh and shank segments, is introduced. The hip joint is modelled as a hinge whereas a previously reported anatomical model is incorporated for the knee joint. In addition to the knee joint ligaments, the model includes the effects of five...
Book
The NATO Advanced Study Institute on Biomechanics of Active Movement and Division of Cells was held September 19-29, 1993 in Istanbul and the Proceedings are presented in this volume. Sixty-eight scientists from sixteen countries attended. Prof. J. Bereiter-Hahn of Goethe-Universitat, Frankfurt, Germany, Prof. A.K. Harris of the University of North...
Chapter
Cytokinesis is the division of the cytoplasm and the plasma membrane into two. Nuclear division and cytokinesis take a relatively short time to happen in animal cells in which cytokinesis takes place by means of furrowing. Here we consider the cleavage of the animal cells only. Although the basic division mechanisms appear to be similar in all anim...
Article
We consider a partial differential equation in spherical (cylindrical) coordinates describing a dynamic process in an infinite medium with an inner spherical (cylindrical) boundary. If an analytical solution is not possible to obtain, then one resorts to numerical techniques. In this case it becomes necessary to discretize the infinite domain even...
Article
The classical wave equation in spherical coordinates is expressed in terms of a residual potential applying the Residual Variable Method. This method essentially eliminates the second derivative of the potential with respect to the radial coordinate from the wave equation. Thus, the dynamic pressure distribution on the surface of a spherical cavity...
Article
This paper investigates the dynamic behaviour of a shallow, viscoelastic, spherical shell under a harmonic excitation. The time evolutions of the response of the corresponding nonlinear dynamical system are described by the phase portraits and the bifurcation of the parameter dependent system is studied numerically so as to identify qualitative cha...
Book
Cytomechanics is the application of the classical principles of mechanics in cell biology. It is an applied science concerned with the description and evaluation of mechanical properties of cells and their organelles as well as of the forces exerted by them. Thus, this topic needs a truly interdisciplinary approach, and accordingly this volume give...
Chapter
Aneurysms are pathological, localized. blood-filled dilatations of the blood vessels. Their origin may be congenital, traumatic. arteriosclerotic or infectious (Photon et al., 1977). The congenital (saccular) aneurysms, which make up over 90% of intracranial aneurysms, are generally found in and about the circle of Willis and especially at bifurcat...
Chapter
Mechanics is the science of forces and motions. When employed in its classical connotation, as opposed to statistical or quantum mechanics, it may be defined as that discipline which investigates the macroscopic response of systems to the action of forces and/or couples. The methods used are analytical, numerical and experimental. Systems considere...
Article
The classical cylindrical diffusion equation is expressed in terms of a residual concentration applying the Residual Variable Method. Thus, it becomes possible to eliminate the second derivative of the concentration with respect to the radial coordinate from the diffusion equation. This, in turn, leads to the advantage that the diffusion on the sur...
Book
There are virtually hundreds of life scientists publishing hundreds of papers a year on numerous aspects of the cell cycle. The following are few of the topics covered: cell membrane organization, membrane components, cytoskeleton and associated proteins, cell motility, actin in dividing cells, surface modulating assemblies, microfilaments, microtu...
Chapter
Biology is the study of all living things that inha bit the universe. Mechanics, when employed in its classical connotation, as opposed to statistical or quantum mechanis, may be defined as that discipline which investigates the macroscopic response of systems to the action of forces or couples. The methods used are analytical, numerical and experi...
Article
The mechanical behavior of fetal dura mater was investigated by means of a biaxial tension test designed to simulate the constraints imposed on the membrane by the cranial bones. The experimental results are compared with the theoretical results obtained by using two published strain energy functions: one defined by Mooney and Rivlin (MR) and the o...
Article
Full-text available
Seismic hazard maps for Turkey are developed by utilizing the current probabilistic procedures. The maps contain probabilistic estimates of the maximum MSK intensity and the maximum horizontal peak ground acceleration for the return periods of 225, 475 and 10,000 years. A modified and updated catalogue for the large and damaging earthquakes affecti...
Article
We consider a partial differential equation in spherical coordinates which describes a dynamic process going on in an infinite medium with an inner spherical boundary. If an analytical solution to the problem is not possible to obtain then one would have to resort to numerical techniques. It becomes necessary to discretize the infinite domain even...
Article
A fully coupled finite-element model is presented to simulate geothermal reservoirs in terms of the unknown variables of displacement, pore pressure and temperature using an elasto-plastic stress-strain relationship. The mathematical model combines the simultaneous mechanisms of subsidence, mass flow and heat flow through porous media by utilizing...
Chapter
Realistically developed theoretical models of human joints play a significant role in understanding both normal and abnormal joint functions, as well as improving biodynamic response of multi-segmented total-human-body models. A substantial difficulty in theoretical modeling of human joints arises from the fact that the number of unknowns are usual...
Article
A biomechanical model of virus-to-cell and cell-to-cell fusion is presented. Virus and the cells are modelled as initially spherical membranes of nonlinear elastic material that undergo large deformations. The membranes are connected by a cytoplasmic bridge which expands, resulting in the formation of a larger single sphere. In virus-to-cell fusion...
Article
The effects of non-linear fluid-structure interaction on the dynamic buckling of structures are investigated. In particular the structural buckling characteristics are studied for the case of a strong shock wave propagating through a fluid medium striking a structure. Non-linear terms are retained for both fluid and structural systems. A one-dimens...
Chapter
The development of fluid finite element is presented. Earthquake response analysis of a submerged thick cylindrical shell is studied. The response of the same shell in vacuo is also investigated. The effect of the fluid-shell interaction on the displacements, stresses and the base moment of the shell is discussed. It was noted that the presence of...
Chapter
The theoretical preliminaries needed for the analysis of fluid-structure interaction are presented. After the derivation of the wave equation which governs the response of an acoustic fluid, the effect of the surrounding fluid on the response of a spring-mass system is studied. A submerged rigid cylinder, which may be considered as the model of a b...
Article
The paper presents the views of the authors on the parameters to be considered in the assessment of earthquake resistance of existing buildings. Among these parameters the following are discussed: type of building, quality of material used, age and expected lifetime of building, workmanship and quality control, morphology and architectural and stru...
Article
This paper, to the authors' best knowledge, presents the first attempt on the understanding of the biomechanics of hernial sac formation. First, a brief survey of the selected etiological factors and their related theories on hernia is given. Next, the results of some preliminary tensile tests conducted on normal and sac peritoneum are discussed. T...
Article
A dynamic approximate theory capable of predicting high-frequency behavior of cylindrical thermoelastic bars is developed using a new theory. The cross section of the bar has an arbitrary shape and contains an arbitrary number of holes. The approximate theory is valid for all of the deformation modes such as flexural, longitudinal, torsional, etc....
Article
In order to show and illustrate the power of the theory proposed in Part I [see the summary above (Zbl 0541.73008)] for the dynamic behavior of thermoelastic bars, the theory is applied to bars with circular and rectangular cross sections. In these applications, the order of the theory m is chosen to be 3. The general equations of the approximate t...
Article
The nonlinear mechanical behavior of fetal dura mater was tested experimentally and compared to two published nonlinear material strain energy functions, the Mooney-Rivlin and the Skalak, Tozeren, Zarda, and Chien (STZC). The STZC constitutive relations best fit the behavior of the dura mater and were used to describe quantitatively its stiffness....
Article
A mathematical dynamic model of the two-dimensional representation of the knee joint is presented. The profiles of the joint surfaces are determined from X-ray films and they are represented by polynomials. The joint ligaments are modelled as nonlinear elastic springs of realistic stiffness properties. Nonlinear equations of motion coupled with non...
Article
The classical spherical diffusion equation is expressed in terms of a residual concentration. Thus, it becomes possible to eliminate the second derivative of the concentration with respect to the radial coordinate from the diffusion equation. This, in turn, leads to the advantage that diffusion on the surface of a spherical cavity can be studied by...
Article
The cell membrane is modeled as a spherical membrane of non-linear, viscoelastic material. The membrane undergoes large deformations under the action of a contractile ring force in its equatorial plane. The numerical results are compared with the experimental results available in the literature and the results of the previous elastic models. The co...
Article
If it is assumed that, during its forced vibrations which lead to dynamic axisymmetric snap-through buckling, a dynamically loaded spherical cap passes through one of its corresponding static, unstable equilibrium configurations, then it is possible to estimate the dynamic axisymmetric snap-through buckling load from a simple static analysis only,...
Article
Ultrastructure of animal cells is presented in detail. The role of this ultrastructure in passively regulating the cell membrane stiffness change during cleavage is pointed out. There is a network of microfilaments in the submembraneous layer which is randomly oriented at the onset of cleavage. This network is closely associated with the plasma mem...
Article
A theoretical and experimental investigation was undertaken to study wave propagation in an exponential rod composed of aluminum when subjected to impact by steel spheres with diameters from 3·175 to 50·8 mm at velocities up to about 60 m/s. Strain measurements were effected at three gage stations and force inputs were obtained in some cases by san...
Article
The transient response of a spherical shell in an acoustic medium is studied. The exact solution is obtained by expressing the classical spherical wave equation in terms of a residual potential. Approximate solutions are obtained from the use of eight functions; namely, doubly asymptotic approximations, δ-sequence functions, and spherical wave appr...
Article
The material properties of the cell membrane are discussed. Various theories concerning the mechanism of cytokinesis in animal cells are presented. The currently accepted mechanism is that of active muscle-like contraction of the furrow base itself. A mathematical model is developed based on this theory. The cell membrane is modelled as a spherical...
Article
We present a model of cell division of a spherical cell which relates the deformation of the cell and the cell membrane to the forces in the contractile ring or furrow region of a dividing cell. The spherical cell of a sea urchin egg is considered as a spherical nonlinear elastic membrane enclosing an incompressible fluid. The results of the analys...
Article
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Article
The elasticity matrix of a general-purpose finite element program, SAPIV, is modified in such a way that it becomes possible to idealize water as a structural finite element with zero shear modulus. Using the modified version of SAPIV, several solid-fluid interaction problems are solved. The numerical solutions are compared with the available analy...
Article
The dynamic behavior of a spherical membrane, made of Mooney material and subjected to a uniform inflating step-pressure, is studied. Its phase-planes and nondimensional radius versus time curves are plotted for different values of the material constants. The conditions for dynamic snapout instability are discussed. The relations between the static...
Article
In this paper, two areas are investigated: first, the effect of pulse duration on skull fracture and brain injury and, second, under what conditions a scaling can be considered between subhuman primates and human. The skull is modeled as a spherical sandwich shell representing the general structure of the skull bones as being the compact inner and...
Article
An adiabatic gas-filled bubble in a viscous, incompressible liquid is subjected to a tension wave in the form of a step function in time. The governing equation is solved numerically. It is shown that there exists a dynamic tension threshold, similar to the Blake threshold in the static case, beyond which the bubble in the liquid will grow infinite...
Article
The dynamic response to ground motion of hemispherical shells in a fluid, medium is studied numerically. In the analysis, linear thin shell theory is used and the fluid is assumed to be compressible and inviscid. The effect of the duration of the ground motion on the dynamic response is studied using two forcing functions, one with a very short dur...
Article
A ring-stiffened spherical shell is submerged in an acoustic medium. The shell is thin and elastic. The acoustic medium is inviscid, irrotational and compressible. The center of mass of the shell is subjected to a translational acceleration which is an arbitrary function of time. The absolute displacements of the shell are expressed in terms of the...
Article
A thin, elastic and ring-stiffened spherical shell accelerates in an acoustic medium which is inviscid, irrotational and compressible. The transient response of the fluid-shell system is studied numerically. The effect of the translation of the center of mass of the shell on the radiated fluid pressure is discussed. Two acceleration functions with...
Article
The asymmetric dynamic behavior of clamped shallow spherical shells under a uniform step pressure of infinite duration is investigated. The solution of a linear eigenvalue problem yields the bifurcation paths and also the lower bound for the asymmetric dynamic snap-through buckling pressure. The asymmetric dynamic response of shells with a shape im...

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