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About this book

This book contains 31 papers presented at the symposium on "Recent Advances in Composite Materials" which was organized in honor of Professor Stephanos A. Paipetis. The symposium took place at Democritus University of Thrace, in Xanthi, Greece on June 12-14, 2003. The book is a tribute to Stephanos A. Paipetis, a pioneer of composite materials, in recognition of his continuous, original diversified and outstanding contributions for half a century. The book consists of invited papers written by leading experts in the field. It contains original contributions concerning the latest developments in composite materials. It covers a wide range of subjects including experimental characterization, analytical modeling and applications of composite materials. The papers are arranged in the following six sections: General concepts, stress and failure analysis, mechanical properties, metal matrix composites, structural analysis and applications of composite materials. The first section on general concepts contains seven papers dealing with composites through the pursuit of the consilience among them, computation and mechatronic automation of multiphysics research, a theory of anisotropic scattering, wave propagation, multi-material composite wedges, a three-dimensional finite element analysis around broken fibers and an in situ assessment of the micromechanics of large scale bridging in ceramic composites.

Table of Contents

Frontmatter

General Concepts

Frontmatter

From Composites to Consilience

Since the Ionian Times (Thales of Miletus) thinkers strived to reduce all knowledge to certain essential parts. The explosion of knowledge in recent times resulted in overspecialization. A call for interdisciplinarity and even an holistic approach to addressing scientific problems is ever present. Attempts to linking all knowledge, from natural sciences to social sciences and humanities should not ignore the historical fact that the social environment is of paramount importance for human conscious actions.

Y. F. Missirlis

Computational and Mechatronic Automation of Multiphysics Research for Structural and Material Systems

A review of NRL’s application of an automation of research paradigm is presented. The rampant computational technology evolution along with automated experimentation are utilized as the means for developing a characterization methodology of continuum systems under the action of multidimensional multiphysics fields. Computationally controlled robotic, multiple degree-of-freedom mechatronic systems are used to greatly accelerate the rate of performing mechanical experiments along loading paths embedded in multidimensional loading spaces. The massive data collections are utilized for the inexpensive and accurate determination of bulk material non-linear constitutive behaviour models as a consequence of multi field generalized loading through parameter identification/estimation inverse approach methodologies. Developments in computational simulation technologies that utilize advances in visualization and automated program synthesis have made possible inexpensive and fast characterization analysis of complex structures for design and qualification needs. The example of how this computational and mechatronic infrastructure is used to create conceptual models for describing and predicting material performance is demonstrated in the case of virtual wind tunnel environment.

J. G. Michopoulos

A First Attempt Towards a Theory of Anisotropic Scattering

Integral representations for the scattered field, as well as for the scattering amplitude, for scalar scattering in a completely anisotropic space are derived. These results are obtained via Green’s integral formula, the derivation of the fundamental solution for the anisotropic Helmholtz equation and standard asymptotic analysis.

G. Dassios, K. Karadima

Wave Propagation in Composites

The present work deals with dispersion and attenuation of elastic waves propagating in particulate and fiber reinforced composite materials. An iterative effective medium approximation based on single scattering considerations, for the quantitative estimation of wave dispersion and attenuation is applied. The single scattering problem is solved numerically by means of an advanced boundary element code valid for two and three-dimensional elastodynamic analysis. Representative numerical results concerning the frequency dependent behavior of the wave velocity and attenuation coefficient of various types of particulate and fiber reinforced composites are presented. The obtained results are compared to those taken either experimentally of numerically by other investigators.

J. T. Verbis, S. V. Tsinopoulos, J. P. Agnantiaris, D. Polyzos

The Validity of the Variable Separable Method in Special Loading Cases in Composite Multimaterial Wedges

The problem of a multi-material composite wedge under a normal and sheer loading at its external faces is considered with a variable separable solution. The special cases of the infinite isotropic wedge, the two materials wedge and the multiple-materials wedge under shear and normal loading of the order r−1 and T−2 of the radial coordinate r, are studied.

J. H. Stabouloglou, E. E. Theotokoglou

Three-Dimensional Finite Element Analysis Around Broken Fibers in FRPs

A finite element computational procedure is developed for the linear elastic analysis of a single broken fiber in unidirectional fiber-reinforced composites. Depending on the packing sequence and symmetry conditions, a slice is modeled including the segments of the broken fiber and unbroken, along with the epoxy matrix material surrounding the fibers. Considering the volumetric composition as independent variable, the computational model may treat hexagonal or square packing sequences, and any material combination. Numerical results are presented and compared with experimental and numerical ones available in the literature. Stress response predicted by the present method seems to behave in accordance with the available solutions.

P. A. Kakavas, C. C. Kiritsi, N. K. Anifantis

In Situ Assessment of the Micromechanics of Large Scale Bridging in Ceramic Composites

The macro- and micro-mechanics of Large Scale Bridging in SiC/MAS-L composites were assessed by means of tensile testing with in situ LRM probing of Double Edge Notch specimens. The macromechanical behaviour was analyzed in terms of bridging stresses. A dedicated Raman calibration curve was established for a fibre extracted from the composite; the curve served in the transformation of wavenumber shifts collected from bridging fibres in the composite to bridging strain. Bridging strain profiles were established along the notched ligament of the specimen and their shape was discussed in conjunction with local notch effects. The failure strain was not attained within the exposed length of the fibres but within the embedded length. The individual contributions of intact fibres and pull-out fibres were distinguished and discussed in the text.

K. Dassios, C. Galiotis, V. Kostopoulos, M. Steen

Stress and Failure Analysis

Frontmatter

Durability Analysis of Polymer Matrix Composites: 1978–2003 — Results and Needs

Polymer matrix composites exhibit some local and global time dependent behaviour due to the viscoelastic nature of the matrix in interaction with the elastic nature of the reinforcement. As usual this viscoelastic behaviour is also influenced by the environmental conditions: especially temperature changes and moisture uptake and diffusion. Internal transformations in the polymer matrix, known as physical and/or chemical ageing have also to be taken into account. For reliable applications of polymer matrix composites in structural components it is necessary to have a prediction of the stiffness and strength evolutions function of time, environmental changes and a complex loading history. The analysis of the stress-strain relations allows us to obtain the stiffness evolution. For the strength evolution we can use a limit criterion with time and environmental dependent characteristics, but it is also possible to predict damage initiation and development related to the loading history. After an overview of different methods we introduce a nonlinear viscoelastic-viscoplastic model for stiffness prediction under quasi static loading conditions with corrections for interactions between different loading modes. Finally we will discuss the prediction of the strength evolution and we suggest a follow-up of the damage development by a full field optical method able to observe the heterogeneisation and localisation of the strain field. Predictions of stiffness and strength evolutions give us the basis of a complete durability analysis for those polymer matrix composite based structural components.

A. H. Cardon

Experimental and Numerical Analysis of Matrix Cracking in Brittle Composites

Inorganic Phosphate cement (IPC) is a cementitious material developed at the “Vrije Universiteit Brussel” which can be reinforced with E-glass fibres. In order to improve its application in civil engineering constructions like wall and roof panels, the constitutive behaviour of IPC should be fully understood and accurately modelled. Using a stochastic cracking theory, the averaged matrix crack distance can be calculated as a function of the applied stress. This theory is experimentally verified using a stereomicroscope equipped with a video camera and image acquisition board. It is shown that the experimentally obtained value of the average crack distance as a function the applied stress correlates quite well with the theoretically obtained value.

D. Van Hemelrijck, H. Cuypers, J. Wastiels, G. Kalogiannakis, W. P. De Wilde

Delamination Detection in Laminated Composites Using Lamb Waves

This investigation examines the application of low frequency Lamb waves for the detection of delaminations in thick composite laminates. Surface mounted piezoelectric devices were excited with a tone burst of few cycles generating a stress wave that propagates along the structure. Experiments were carried out on composite beam specimens where wave propagation distances over 2 m were achieved and artificially induced delaminations as small as 1 cm2 were successfully identified. The resonance spectrum method, which is based on the study of spectra obtained by forced mechanical resonance of samples using sine-sweep excitation, was used for measuring the Ao Lamb mode phase velocity. Finite element analyses of wave generation and propagation in wide laminated plates are also presented. The feasibility of employing piezoelectric devices for the development of smart structures, where a small and lightweight transducer system design is required, has been demonstrated.

C. Soutis, S. H. Díaz Valdés

Perforation Investigations of Zylon Sheets

This investigation is concerned with the determination of the ballistic resistance of sheets of Zylon, a polymeric PBO fiber manufactured by Toyobo Company. The current application is intended to ascertain whether this material is suitable for body armor. Ten layers of this substance were stitched together, tightly emplaced in a specially constructed holder and subjected to impact by standard 9 mm NATO bullets. Resistance to perforation is represented by the bulk deformation of the sheets, as well as fiber damage. Thus, the ballistic limit of this fabric combination depended significantly on the impact position relative to the edges of the steel frame. The magnitude of this quantity for impact near the center was found to be of the order of 350 ± 10 m/s.

W. Goldsmith, E. Pineda

Strain Redistribution in Composite Laminates Resulting from Off-Axis Ply Cracking

The initial mode of damage in multidirectional composites is the accumulation of matrix cracks in the off—axis plies. This kind of damage develops until reaching a so-called saturation point. Several other damage modes, such as delamination, may appear concurrently or just upon termination of the matrix cracking process due to joining and growth of these cracks. Remote Laser Raman Spectroscopy (ReRaM) has been employed to monitor the local strains in cracked cross—ply composites. The experiment involves the incorporation of an aramid (Kevlar 49®) fibre Raman strain sensor into the 0° ply and near the 0°/θ° interface of glass—fibre reinforced epoxy resin laminates. This type of laminate is transparent due to the matching of the refractive indices of glass fibres and epoxy resin. Thus, the changes in the longitudinal strain in the 0° ply, caused by transverse cracking in the θ° ply, are quantified.In this work, the results obtained from [0/90/0], and [0/45/0]t laminates that have been produced under identical conditions, are reported. The strain magnification in the 0° ply caused by 90° or 45° cracking are measured at different levels of loading. A relationship between the values of strain magnification and the distance between the fibre and the 0°/θ° interface has been derived. Finally, important conclusions concerning the mechanisms of strain redistribution as well as the conditions of crack growth and crack—crack interaction are drawn from this work.

D. G. Katerelos, J. Parthenios, C. Galiotis

Mechanical Properties

Frontmatter

Mechanical Properties of Nanocomposite Multilayers

This work s concerned with the mechanical properties of nanocomposite metal/ceramic multilayers. Three thin film nanocomposite systems, namely Al/Al203, Ti/TiN and Cr/C, have been investigated. The nanocomposites were synthesized on Si (100) wafers by using an electron-beam physical vapor deposition method. The microstructure of the films was characterized by transmission electron microscopy. The deposited metal layers Al, Ti and Cr possessed fcc, hcp and bcc structures, respectively, where the ceramic TiN layers had fcc structure and the Al2O3 and C layers were amorphous. The mechanical properties of the nanocomposite thin films were studied by conducting nanoindentation and microhardness experiments. The nanosize of the components was found to affect significantly the mechanical properties that were dominated by the thickness of the metal layer. The hardness response in all three systems can be described as a function of the metal layer thickness by a Hall-Petch relationship. A hardness increase was observed up to a 40 ma, 5 nm, and 20 nm layer thickness for the Al, Ti, and Cr layers, respectively. The effects by the ceramic phase were found to be significant only in films with metal layer thickness at or just before the critical thickness.

Z. Q. Qi, X. Nie, E. I. Meletis

Ultrasonic Stiffness Matrix Measurements of Oxide/Oxide Composites

The stiffness matrix of Al2O3/ Al2O3 composites has been non-destructively evaluated using ultrasonics. The method is based on the propagation velocities of sound waves in the material. These were estimated experimentally using a custom pulser-receiver setup which allows control of the angle of the incident pulse on the sample, while the latter is immersed in a water bath. The recording of the time delay of the longitudinal and transverse wave components as they travel through the composite allows the determination of their respective propagation velocities within the composite, as a function of the incident angle for a given recording plane..The derivation of the elastic constants in order to reproduce the stiffness matrix of the composite is an inverse wave propagation problem described by the Christoffel equation. All 9 elastic constants are derived assuming an orthotropic medium. The derived elastic constants are compared to experimental data acquired destructively.

A. Paipetis, V. Kostopoulos

Investigation of the Mechanical Characteristics of Fibre Reinforced Photopolymers

The effect of the addition of fibre mat reinforcements on the mechanical properties of specimens fabricated by the laser solidification process using two different photopolymers, an acrylic-and an epoxy based photo-curable resin, has been studied. Comparisons of the mechanical properties between pure-polymer specimens and their fibre-reinforced counterparts were made, by subjecting the parts to tensile tests. It was found that the fibre-reinforced specimens yielded higher measured values of elastic modulus and ultimate tensile strength.

D. E. Karalekas

Correlation Between Micromechanical and Macromechanical Properties of Composite Materials Based on a New Interphase Concept

In the present investigation, a novel approach of the interphase concept is introduced according which the interphase volume fraction represents the percentage of the bulk matrix surrounding the inclusions in which a specific matrix property is strongly affected by the existence of the reinforcement, while the interphase thickness represents the maximum radial distance from the inclusion boundary at which this property varies. This means that both the interphase volume fraction and the interphase thickness are not simple geometrical/structural concepts but they are property-dependent, that is, their values depend on the property considered at the time. Such an interphase is the so-called “hybrid interphase”. Based on this new interphase concept, a micromechanics semi-analytical model has been developed for the determination of stresses developed in the interfacl area of unidirectional fiber — reinforced composites incorporating a hybrid interphase region..Macromechanical material characterization and predictions of interfacial stress distribution indicate the effect of the hybrid interphase, while a satisfactory agreement between theoretical predictions and respective experimental results was found.

G. C. Papanicolaou, N. K. Anifantis

Microfibrillar Reinforced Composites — A New Concept for the Preparation of Stronger, Biodegradable Implants

In contrast to the classical composites, microfibrillar reinforced composites are not prepared via melt blending of the matrix and the reinforcing material. In fact, the reinforcing elements of this composite, the microfibrils, are created during processing. This advantage allows the manufacturing of a full biodegradable composite material with improved mechanical properties. Basing on a blend of two common biodegradable polymers, polylactide and polyglycolide, this new composite shows promising values under ambient conditions.

K. Friedrich, J. Hoffmann, M. Evstatiev, S. Fakirov

Long Term Behaviour of Continuous Fiber Oxide/Oxide Composites Under Thermal Exposure

The present work deals with the mechanical characterization of a new Al2O3/Al2O3 composite material and its durability performance during the exposure at high temperature environment that simulates the working conditions of a gas turbine. Stiffness and strength variation versus the exposure duration at 1100 °C are identified by means of tensile tests and ultrasonic stiffness matrix measurements. More precisely, a new characterization technique based on the inversion of phase velocity data gained by immersive ultrasonic tests, was used for the determination of the complete stiffness matrix of the oxide/oxide composite, and anisotropic damage functions was introduced in the case the material experiences long term high temperature exposure.

V. Kostopoulos, D. E. Vlachos

Metal Matrix Composites

Frontmatter

On a Relationship Between COD and Strain Energy Density in a Modern MMC

A relationship between the critical value of Crack Opening Displacement and the mean value of dilatational strain energy density, T v , computed along crack-lips, is indicated in the present work. Namely, T v takes a maximum value at the moment of crack initiation. This relationship does not hold when the crack tends to be parallel to loading axis and shear stresses dominate over normal ones. Experimental evidence from a metal matrix composite supports the present conclusions.

N. P. Andrianopoulos

Dependence of Fracture Characteristics on the Thickness of the Specimen in Case of Metal Matrix Composites: An Experimental Study

The dependence of various fracture characteristics on the thickness of the specimen is studied experimentally in case of a particulate Metal Matrix Composite. Both intact and cracked specimens are considered. The Double Edge Notched Tensile specimen is used for the experimental program, in the case of pre-cracked specimens. The ductility and fracture strength appear to be increasing functions of the thickness in case of intact specimens. On the other hand, the critical J-integral and the critical Crack Tip Opening Displacement are found to increase in an almost linear manner with thickness for relatively small specimen thickness. However, the dependence becomes non-linear as the thickness increases further, leading finally to a maximum value. Dependence of the above quantities on the direction of the specimen with respect to the rolling axis is also detected, indicating the importance of the plastically induced anisotropy due to the manufacturing process.

S. K. Kourkoulis

Theoretical Evaluation of Strength Degradation During Monotonic and Cyclic Loading in Unidirectional Metal Matrix Composites

By implementing the concept of critical fibre length into a modified version of the Law of Mixtures and considering the effect of the thermal residual stresses, the monotonic strength of unidirectional MMCs is predicted. The above is also used as a reference value for strength degradation in the case of cyclic loading. Herein, the conditions of constraint effect and flow resistance degradation are taken as the major, strength controlling, damage mechanisms. The approach is then executed for two kinds of titanium matrices and the commonly used SCS-6 fibre. The approach allows the study of the above composite system in terms of the mechanical properties of the constituent phases and the fibre volume fraction.

C. A. Rodopoulos

Structural Analysis

Frontmatter

Damage Tolerant Composite Joint Design

Damage initiation and progressive fracture of adhesively bonded graphite/epoxy composite joints are investigated under various loading conditions. Load induced damage in both the adhesive bond and the adjoining laminate is considered. An integrated computer code is utilized for the simulation of composite structural degradation. The damage progression sequence and structural fracture resistance during different degradation stages are evaluated in detail. Results indicate that adhesive properties have a significant effect on structural durability even when damage initiation/progression is not in the adhesive bond. Design implications with regard to damage tolerance of adhesively bonded joints are examined. Influence of the type of loading and the choice of adhesive on damage initiation and progression for adhesively bonded composite joints are investigated.

C. C. Chamis, L. Minnetyan

Core Failure of Sandwich Beams

Core failure modes were studied in composite sandwich beams under three-point bending and in cantilever beams under end loading. The beams consisted of unidirectional carbon/epoxy face sheets and a variety of core materials, including aluminum honeycomb, PVC closed-cell foams, polyurethane foam and balsa wood. The constituent materials were fully characterized and in the case of the core materials, failure envelopes were developed for biaxial states of stress. Deformation and failure mechanisms were studied experimentally by means of moiré gratings and birefringent coatings. Results were obtained for stress (strain) distributions in the linear and nonlinear/plastic range of the core, critical failure loads and their dependence on geometrical dimensions, material parameters and loading conditions.

I. M. Daniel, E. E. Gdoutos, J. L. Abot, K.-A. Wang

Buckling of Long Sandwich Cylindrical Shells Under External Pressure

The paper deals with the theoretical prediction of buckling loads for sandwich long cylindrical shells with laminated facings and foam core. The loading is a uniform hydrostatic pressure, which means that the loading remains normal to the deflected surface during the buckling process. Several fiber materials are used in the laminated facings. The materials are: Boron/Epoxy, Graphite/Epoxy and Kevlar/Epoxy laminates with 0° orientation with respect to the hoop direction. These various materials are employed to provide comparative data that can be used in design. Shell theory results are generated with and without accounting for the transverse shear effect. Moreover, results based on three-dimensional elasticity are also generated for comparison purposes. The effect of the ratio of radius to thickness is assessed.

G. A. Kardomateas, G. J. Simitses

Applications

Frontmatter

Design and Manufacturing of High Performance Structural Components by Resin Transfer Moulding

The ability to reduce the manufacturing and operating cost of high performance structural components and vehicles governs the competitiveness in the market. The use of RTM technologies in Aerospace applications gives added potential in recent days due to the combined benefit of weight — cost reduction. Complicated components could be easily manufactured by one injection, provided that the mould has been well designed and the positioning of gate ports accurately defined by flow simulation. In the present work a sequenced methodology was established for the development of an aircraft structural stiffened skin. A mould with inserted heating elements and preformed reinforcements were used for the manufacturing of component The experimental Investigation lead to various techniques for eliminating lack of wetting and improving the quality of final product.

Z. P. Marioli-Riga

High Friction, Low Wear Composites Based on Fibre Reinforced Ceramics

Carbon fibre reinforced ceramic matrix composites manufactured via the liquid silicon infiltration (LSI) route have a high potential for tribological applications, such as brake disks and pads. Manufacturing costs can be reduced by using simplified manufacturing techniques, such as hot pressing and short fibres without deleterious effects on tribological behaviour. Graphite modifications and novel ceramic coating techniques developed by DLR have been proven to increase the thermal conductivity of these C/C-SiC materials, so that a considerable improvement in wear and friction performance under high energy conditions can be achieved. These developments mean that a series manufactured brake disk which lasts the lifetime of a vehicle is feasible in the near future.

R. Kochendörfer

Design and Experimental Analysis of Modular Pedestrian Bridge with Concrete Deck and IPC Truss Girder

In the present work the design and experimental analysis of a pedestrian bridge is presented. The actual bridge consists of a supporting truss system built with IPC sandwich panels and a horizontal concrete deck. The IPC is a kind of ceramic which is reinforced with glass fibers. The connection between the sandwich panels the truss system and the concrete deck is done using specially designed steel elements. The aim of this work is the examination of the behavior of the prototype that is constructed and evaluated via static and dynamic tests. The results of the experimental analysis are necessary for the final evaluation of the structure before the construction of bigger bridges and in order to have an overview of the critical areas of the structure that have to be reconsidered during the design phase of a different structure.

G. Giannopoulos, J. Vantomme, J. Wastiels, L. Taerwe

Strengthening of Historic Structures with Advanced Composites: A Review

The unique advantages offered by FRP materials have made them ideal candidates for increasing the strength, stiffness and ductility characteristics of historic structures, especially in cases where traditional strengthening materials and techniques have proven inadequate and/or inapplicable. In this paper, which serves as an introduction to the papers for the special theme session on “Strengthening of Historic Structures with FRP”, the author presents a short overview of FRP applications to historic structures made of masonry, timber, metals (cast iron, steel) and concrete, with emphasis on concepts and key case studies.

T. C. Triantafillou

Effective Strengthening of Reinforced Concrete Beams with Anchored FRPs

The beneficial effect of a composite material anchorage system (CMAS) on the ductility and stiffness of reinforced concrete (RC) beams strengthened with Carbon (CFRP) or Glass (GFRP) sheets is investigated The system consists of a tuft of high-strength glass fibers, anchored into the lateral faces of the reinforced concrete beam by penetrating the accompanied GFRP or CFRP “U”-shaped strips (U-strips) used to prevent delamination of the FRP sheets that are placed in the lower face of the beam for strengthening. Experimental analysis of RC beams strengthened with one GFRP sheet and anchored at its ends by GFRP U-strips shows an increase of the ultimate bending capacity as well as a ductility enhancement due to the CMAS. Similarly, a CFRP sheet on the bottom face of the beam anchored at its ends by CFRP U-strips with the aid of CMAS provides, not only a significant strength increase, but also, its failure mode becomes ductile without considerable variation of the beam stiffness. This behavior is experimentally verified by measuring the deflection and deformation of the FRPs and of the concrete beam at the middle point of its bottom and upper faces.

C. B. Demakos, D. P. Koutsoukos

On the Optimum Design of Fibre Reinforced Concrete — Steel Composite Columns

This paper presents a procedure for the optimum design of fibre reinforced concrete — filled steel tube columns. It takes into account the effects of confinement of concrete and its steel fibre reinforcement. The constraints of the optimum design, prescribed by the strength and stability requirements, are adopted from the provisions of modern structural codes. A computer program for the analysis has been developed, which emphasizes the dimensioning of the composite column and identification of the optimum quantity of fibres in matrix material. The paper concludes with two representative numerical examples, which show that in-filled composite columns can be designed both economically and effectively using the proposed methodology.

G. D. Hatzigeorgiou, D. E. Beskos

Contact Demamination of Composite Patches Repairing Metal Structures

The increasing demand for life extension of both military and civilian aircrafts leaded to significant advances in repair technology of cracked metallic structures. Thus composite patch repair of metallic structures became a rapidly grown technology in the field of aerospace. However, because of the different nature and properties of the material that form a repair (metals composites adhesives), side effects like debonding due to high stress concentration in the vicinity of the crack or thermal residual stresses because of different thermal expansion coefficients of the adherents may occur.In this paper the important phenomenon of delamination is examined taking into account the possibility of frictionless contact that appears between laminates. Buckling delamination is examined in particular, because in-place compressive loading is a typical and most damaging type of delamination. To that end, the concavity of initial imperfections is important in connection with contact. Two types of contact can be distinguished, local and global. This is because delamination can be thought both as a plate system, in the macro-behavior, but also as a crack near the delamination front. The global contact appears as the laminates try to interpenetrate. The local contact relates to the impending interpenetration of crack — tips (negative kI stress intensity factors). Both local and global contact conditions were implemented in a FEM code using non-linear plate theory. An example of thin film delaminations is examined.

G. Tsamasphyros

Backmatter

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