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In this paper a computational and experimental study to determine the in plane and interlaminar shearing stress-strain response of a composite material is performed. It is known that the above mentioned shear properties can be directly determined from shear and torsion experiments but also can indirectly be determined from bending and tension experiments respectively. The test methods used are the tension of ±45 off-axis woven fabric carbon fibers and epoxy resin matrix and the short beam bending test of the same material. Then calculations with finite elements and some analytical calculations were held for the in-plane and interlaminar shear. Comparison among the results was made. Thus, a main goal which was the combination of three major research methods was attained.

An upper bound of in-plane shear modulus for a general class of unidirectional fibrous polymeric composites of periodic structure is presented in an explicit form, according to linear elasticity approach. The fibers that reinforce the polymeric matrix are continuous, parallel to each other, and thus, no agglomeration occurs. To formulate the fiber arrangement inside the matrix, a reliable geometric body-centered model was adopted. The theoretical predictions were compared with experimental data found in the literature, together with theoretical results obtained from other workers, and a reasonable accordance was observed.

An experimental technique is presented to evaluate mechanical and thermal load-induced microstructural damage, based on the Electron Probe Micro-Analysis (EPMA) principle, by which certain ana­lyti­cal potentialities of Scanning Electron Micro­scopy (SEM) are used. The aim of the study is to apply this technique in the case of metal matrix composites (MMCs). Based on earlier findings it is shown that by this technique the damage-con­trolled micro­­structural integrity distribution ahead of an edge-notch under tension is deter­mined with sufficient reliability. To demonstrate this fact two MMCs were tested. Dog-bone specimens with an edge notch were subjected to slow tension up to their ultimate stress, the loading process was terminated and the EPMA technique was applied. The procedure was also applied after sudden cooling of the specimens by immersion in liquid hydrogen. It is shown that the MMC with larger differences between the elastic and plastic constants of the inclusion and the matrix exhibit increased prone­ness to mechanical and thermal load-induced damage. The findings obtained are discussed on the basis of the dominating com­bined influence of macro-microscopic mechanical and thermo-elastic stress concentration processes on the matrix-particle interfacial fracture strength.

Purpose The purpose of this paper is to investigate the ballistic impact response of square clamped fiber-metal laminates and monolithic plates consisting of different metal alloys using the ANSYS LS-DYNA explicit nonlinear analysis software. The panels are subjected to central normal high velocity ballistic impact by a cylindrical projectile. Design/methodology/approach Using validated finite element models, the influence of the constituent metal alloy on the ballistic resistance of the fiber-metal laminates and the monolithic plates is studied. Six steel alloys are examined, namely, 304 stainless steel, 1010, 1080, 4340, A36 steel and DP 590 dual phase steel. A comparison with the response of GLAss REinforced plates is also implemented. Findings It is found that the ballistic limits of the panels can be substantially affected by the constituent alloy. The stainless steel based panels offer the highest ballistic resistance followed by the A36 steel based panels which in turn have higher ballistic resistance than the 2024-T3 aluminum based panels. The A36 steel based panels have higher ballistic limit than the 1010 steel based panels which in turn have higher ballistic limit than the 1080 steel based panels. The behavior of characteristic impact variables such as the impact load, the absorbed impact energy and the projectile’s displacement during the ballistic impact phenomenon is analyzed. Originality/value The ballistic resistance of the aforementioned steel fiber-metal laminates has not been studied previously. This study contributes to the scientific knowledge concerning the impact response of steel-based fiber-metal laminates and to the construction of impact resistant structures.

In this work, the authors introduce an octahedral body centered model transformed into a nine-layer spherical model, to simulate the periodic microstructure of particulate composites. This model takes into account the vicinity of internal and neighboring particles in the form of their deterministic configurations inside the matrix, along with the concept of interphase on the thermomechanical properties of the overall material. The latter is assumed to be homogeneous and isotropic. Next, by the use of this model, in association with classical elasticity approach, a closed form expression to calculate the thermal expansion coefficient of this category of composites is derived The theoretical predictions were compared with experimental results as well as with theoretical values yielded by formulae derived from other workers and they were found to be in good agreement.

An experimental technique is presented by which mechanical damage in materials can be evaluated. It is based on the Electron Probe Micro-Analysis (EPMA) principle, by which certain analytical possibilities of Scanning Electron Microscopy (SEM) are used. The main scope of the study was to apply this technique for the determination of microcracking damage in edge-cracked metal matrix composites (MMCs). Based on earlier studies this investigation has shown that by this technique the microcracking distribution ahead of an edge-crack under tension can be determined with relatively high reliability. To demonstrate this fact two types of MMCs were investigated. The specimens (shaped as dog-bones with an edge-crack) were subjected to slow tensile loading up to their ultimate stress. At this stress the loading process was terminated and the EPMA technique was applied.

In this study an attempt was made to introduce some novel operational techniques for the characterization of materials’ fracture, assisted by certain approaches of continuum brittle damage mechanics. This was done by using certain variant operational parameters, i.e., initial damage, damage at failure, structural instability number, strain energy dissipation deficiency number, damage process intensity number and failure strain energy efficiency number. The ultimate scope is to characterize the influence of initial damage, on the above parameters. Tensile tests were carried out, taking into account the basic assumptions of continuum damage mechanics allowing analysis of brittle damage to be applied to elastic-brittle solids under low dynamic loading. A polymeric material, commercially known as Plexiglas, was selected, which fulfils satisfactorily, most of the desired theoretical assumptions.

In this paper, the authors introduce a body-centered polyhedral model capable of simulating the periodic structure of particulate composites, in order to estimate the thermal conductivity of a general class of this type of materials. This model takes into account the arrangement of internal and neighboring particles in the form of three distinct deterministic configurations, along with the concept of interphase on the thermal and mechanical properties of the composite, which is assumed to be macroscopically homogeneous and isotropic. Next, by means of this advanced multilayer model, an explicit analytical expression to evaluate the thermal conductivity of this type of composites is derived. The theoretical predictions were compared with the experimental results found in the literature, as well as with the theoretical values yielded by some reliable formulae derived from other workers, and a reasonable agreement was observed.

In this article, the ballistic impact response of square clamped fiber–metal laminates and monolithic plates consisting of different aluminum alloys is investigated using the ANSYS LS-DYNA explicit nonlinear analysis software. The panels are subjected to central normal high velocity ballistic impact by a cylindrical projectile. The implemented finite element models have been validated by comparison with published experimental data concerning GLARE 5 and monolithic 2024-T3 aluminum plates. Using the validated models, the influence of the mechanical properties of the constituent aluminum alloy on the ballistic resistance of the fiber–metal laminates and the monolithic plates is studied. Apart from 2024-T3, the aluminum alloys 2024-T351, 2024-O, 6061-T6, 7039 and 7075-T6 are considered. It is found that the ballistic limits of the panels can be substantially affected by the constituent aluminum alloy. The 7075-T6 aluminum alloy offers the highest ballistic resistance whereas 2024-O aluminum alloy offers the lowest ballistic resistance.

A lower bound of the longitudinal elastic modulus of polymer composite materials reinforced with unidirectional continuous fibres is obtained by means of a Differential Calculus approach. In the mathematical derivations, the concept of interphase between the fibre and matrix was also taken into consideration. The three phases are considered as isotropic. The results obtained from the proposed formula were compared with those arising from some reliable and accurate theoretical models as well as with experimental data found in the literature, and a reasonable agreement was observed.

The present paper deals with the computational study of unidirectional glass fiber reinforced epoxy composites in short-beam bending test and the comparison of the induced results with experimental...

In this work, the effect of moisture absorption on the mechanical properties of particulate composite materials is studied. Moisture absorption constitutes a main parameter affecting the thermomechanical behaviour of composites, since it causes plasticization of the polymer matrix with a concurrent swelling. In the present work, the influence of water absorption on the flexural properties of particle-reinforced composites was thoroughly investigated. It was found that during the process of moisture absorption there exists a variation of the flexural properties closely related to the degradation of the mechanical behaviour of the composite, as well as the percentage amount of moisture absorbed. Experiments were carried out with composite made of epoxy resin reinforced with low-content iron particles. The variation of ultimate stress, breaking strain, deflection, elastic modulus and Poisson ratio due to water absorption was examined.

A theoretical model for the evaluation of the elastic modulus in particulate composites has been developed. The method takes into account the contiguity of the particles and also the existence of an interphase between main phases, which constitutes an important parameter influencing the behavior of a composite material. This layer between the matrix and filler develops different physico-chemical properties from those of the constituent phases and variables along its thickness. The effect of the progressive variation of the elastic modulus of the interphase on the modulus of the composite was estimated by assuming a law of variation. The theoretical values derived from the proposed model were compared with experimental results carried out on iron-particulate-filled epoxy composites and with theoretical values derived from models of other scientists. This comparison was proved to be satisfactory for our proposed model.

Little research has been conducted on emerging adulthood in Greece, although sociodemographic specificities and constraints due to financial crisis are expected to reinforce prolonged transition to adulthood in this Southern European country. Using a person-centered approach, the current study aimed to examine dimensions of emerging adulthood, criteria for adulthood, and identity development among studying youth in association with personal and structural variables (i.e., gender, age, living arrangement, work experience, steady romantic relationship status, and financial adversity). Participants were 814 university students (Mage = 19.9 years). Latent profile analysis yielded five profiles: anxious explorers in-between (29.90%), immature explorers (29.58%), emerging adults (18.30%), adult committers (12.58%), and blocked in transition (9.64%). Significant between-profile differentiation was found regarding all variables except from age and work experience. The results support the existence of emerging adulthood in Greece and indicate diverse paths to adulthood within the rather homogeneous population of studying youth in this country. © 2018 Society for the Study of Emerging Adulthood and SAGE Publications.

Dynamic mechanical analysis (DMA) is a versatile technique that complements the information given by the more traditional thermal analysis techniques such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermal mechanical analysis (TMA). Dynamic constants such as storage modulus, loss modulus, and loss factor are temperature dependent and provide information about interfacial bonding between reinforced fibre and polymer matrix of composite material. To study the above mentioned properties at the glass transition region, for unidirectional fibrous composites reinforced with continuous fibers a reliable model was applied. In particular, the composite material was considered as composed of three phases with the intermediate phase between matrix and fibres, the interphase, to have variable properties depending on those of main phases and the mode of preparation of the overall material. The glass transition temperature is defined as the point at which the specific volume versus temperature curve changes abruptly slope marking the region between rubbery polymer and glassy polymer nature. Hence, the behaviour of unidirectional fibrous composites was investigated at this region. Examination of the glass transition temperature, which constitutes an upper limit for the structurally important glassy region through the loss factor, was performed by its consideration as a combination of glass transition temperature of matrix and interphase.

In this paper, an upper bound of the longitudinal elastic modulus of unidirectional fibrous composites is proposed according to strength of materials approach, on the premise that the fiber is much stiffer than the matrix. In the mathematical derivations, the concept of boundary interphase between fiber and matrix was also taken into account and the main objective of this work is the attainment of an upper bound for the interphase stiffness with respect to fiber concentration by volume. The novel element here is that the authors have not taken into consideration any specific variation law to approximate the interphase modulus. The theoretical results arising from the proposed formula were compared with those obtained from some reliable theoretical models as well as with experimental data found in the literature, and a satisfactory agreement was observed.

In this paper, an icosahedral non – body centered model to simulate the periodic structure of homogeneous particulate composites, by predicting the particle arrangement, is presented. This model has yielded three different variations which correspond at three different deterministic particle configurations. In addition, the concept of boundary interphase between matrix and inclusions was taken into account. Thus, the influence of particle vicinity was examined in parallel with the interphase concept on the thermomechanical properties of the overall material. Next, by the use of this model the authors derived a closed – form expression to estimate the thermal conductivity of this type of composites. To test the validity of the model, the theoretical values arising from the proposed formula were compared with other theoretical predictions obtained from several accurate formulae found in the literature and an adequate accordance was observed.

This article deals with the dynamic response of thin circular clamped GLARE (GLAss REinforced) fiber-metal laminates subjected to low-velocity impact by a lateral hemispherical impactor, striking at the center with constant kinetic energy. The laminates have equal total thickness and consist of GLARE 2A-3/2-0.4, GLARE 2A-4/3-0.238, GLARE 3-3/2-0.4, GLARE 4-3/2-0.317, and GLARE 5-3/2-0.233 standard grades. Three different plate diameters are considered for each GLARE grade. Their dynamic response is predicted by solving previously published differential equations of motion corresponding to a spring-mass modeling of the impact phenomenon. The obtained results are analyzed and compared in order to understand and evaluate the performance of the examined material grades along with the effect of different plate radius. With reference to the radius variation, it is found that it affects substantially the overall impact behavior of a GLARE plate. As far as the examined material grades are concerned, similarities and differences related with their impact behavior are recorded and a comparative evaluation is implemented. Characteristic variables associated with the low-velocity impact response of fiber-metal laminates are discussed and pertinent design recommendations are proposed.

In this work, the thermal expansion coefficient (CTE) of a composite containing spherical particles surrounded by an inhomogeneous interphase embedded in an isotropic matrix is evaluated by means of a new model. The thermomechanical properties of the interphase are formulated as continuous radial functions. It is assumed that this third phase developed between the polymeric matrix and the filler particles contains both areas of absorption interaction in polymer surface layers onto filler particles as well as areas of mechanical imperfections. It can be said that the concept of boundary interphase is a useful tool to describe quantitatively the adhesion efficiency between matrix and particles and that there is an effect of this phase on the thermomechanical properties of the composite. The thickness and volume fraction of this phase were determined from heat capacity measurements for various filler contents. On the other hand, it is assumed that the particle arrangement (distribution) which can be ...

In this work, the stiffness of a composite containing spherical particles surrounded by an inhomogeneous interphase embedded in an isotropic matrix is evaluated. The elastic constants of the interphase are modeled as continuous radial functions. It is assumed that this third phase developed between the polymeric matrix and the filler particles contains both areas of absorption interaction in polymer surface layers onto filler particles and areas of mechanical imperfections.It can be said that the concept of boundary interphase is a useful tool to describe quantitatively the adhesion quality between matrix and particles and that there is an effect of this phase on the thermomechanical properties of the composite. The thickness and volume fraction of this phase were determined from heat capacity measurements for various filler contents.On the other hand, it is assumed that the particle arrangement (distribution) and their interactions should affect the thermomechanical constants of the composite.The...

The present article investigates the analytical and computational derivation of the moduli of continuous fiber-reinforced composites. Theoretical expressions for the longitudinal and transverse elastic moduli and also for the longitudinal and transverse Poisson ratios, which were derived taking into account the concept of boundary interphase between fibers and matrix, were obtained as a function of the fiber volume fraction. The micro-scale model considers that the composite material consists of three phases: the fiber, the matrix, and the interphase. The latter is the part of the polymer matrix lying at a close vicinity to the fiber surface. In this investigation, it was assumed that the interphase is inhomogeneous in nature with varying mechanical properties. Different laws of variation of its elastic modulus and Poisson ratio were taken into account in order to define the overall modulus of the composite. Thermal analysis method had been used for the estimation of the thickness of the interphase. An in...

In this work, the authors present an approximate solution to three point bending equation for a simply supported beam of rectangular section. The deflection of the beam due to a perpendicular point – wise load is estimated for high rates of its curvature. After the deformation of the beam, the shape of its cross sectional area is assumed to remain as is. The proposed solution consists of binomial power series and does not contain elliptic integrals or other special functions. Hence, it may be more appropriate for usual engineering practices. Key words: Simply supported beam, three point bending, large deflection.

In this article, the ballistic impact response of square clamped fiber-metal laminates and monolithic plates consisting of different metal alloys is investigated using the ANSYS LS-DYNA explicit nonlinear analysis software. The panels are subjected to central normal high velocity ballistic impact by a cylindrical projectile. Using validated finite element models, the influence of the mechanical properties of the constituent metal alloy on the ballistic resistance of the fiber-metal laminates and the monolithic plates is studied. Six steel alloys are examined, namely 304 stainless steel, 1010, 1080, 4340, A36 steel and DP 590 dual phase steel. A comparison with the response of GLARE plates is also implemented. It is found that the ballistic limits of the panels can be substantially affected by the constituent alloy. The stainless steel based panels offer the highest ballistic resistance followed by the A36 steel based panels which in turn have higher ballistic resistance than the 2024-T3 aluminum based panels. The A36 steel based panels have higher ballistic limit than the 1010 steel based panels which in turn have higher ballistic limit than the 1080 steel based panels. The behavior of characteristic impact variables during the ballistic impact phenomenon is analyzed.

This article deals with the evaluation of the ballistic resistance of aluminum plates subjected to high velocity impact by a rigid cylindrical projectile. Important impact variables such as the ballistic limit, the ballistic energy and the impact load time history are predicted using the ANSYS LS-DYNA software. A comparison with the ballistic resistance of GLARE 4A fiber-metal laminated plates is also implemented. The time history of the transient impact load of the collision on the aluminum targets is analyzed and useful conclusions are drawn. It is found that the ballistic limit and the ballistic energy of aluminum and GLARE 4A panels increase as their thickness and their areal weight become higher. It is also found that 2024-T3 aluminum plates offer comparable ballistic limit velocities with the GLARE 4A fiber-metal laminates of the same thickness or the same areal weight.

In this paper, a geometric body-centered model to simulate the periodic structure of unidirectional fibrous composites is presented. To this end, three prescribed configurations are introduced to predict in a deterministic manner the arrangement of internal and neighboring fibers inside the matrix. Thus, three different representative volume elements (RVEs) are established. Furthermore, the concept of the interphase has been taken into account, stating that each individual fiber is encircled by a thin layer of variable thermomechanical properties. Next, these three unit cells are transformed in a unified manner to a coaxial multilayer cylinder model. This advanced model includes the influence of fiber contiguity in parallel with the interphase concept on the thermomechanical properties of the overall material. Then, by the use of this model, the authors propose explicit expressions to evaluate the longitudinal and transverse thermal conductivity of this type of composite. The theoretical predictions were compared with experimental results, as well as with theoretical values yielded by some reliable formulae derived from other workers, and a reasonable agreement was found.

In this paper, the thermal conductivity coefficients of unidirectional composites have been estimated. In the present analysis, a model which takes into account the influence of the boundary interphase has been proposed. The thermal conductivity coefficients of the composite were calculated by using this concept to determine the role of the interphase which mainly depends on the quality of adhesion between fibers and matrix. The interphase matter is the part of the polymer matrix lying on the close vicinity of the fiber bounding surface. It has been primarily assumed that the interphase is inhomogeneous with thermophysical properties varying from the fiber surface to the matrix. Five laws of variation have been taken into consideration in order to derive closed form expressions for the thermal conductivity coefficients in which the role of the interphase layer occurs, possessing properties different from those of the constituent phases. A thermal analysis method known in the literature has been adopted in...

The present paper deals with the analytical and computational study of glass reinforced plastic beams in three point bending and the comparison of the induced results with experimental and analytical results. The measurement of the interlaminar shear strength of composite beams, an important design variable in many applications, may be successfully performed by the three point bending test. A Finite Element Analysis (FEA) is adopted throughout the composite beams in order to correlate with experimental and analytical results. From the Finite Element Analysis and the experimental investigation possible crack initiation positions are determined. Finally, the elastic constants of the same material were determined from bending experiments carried out on specimen of large span length to thickness ratio, and compared with those derived from FEA analysis.

In this paper, we will attempt to derive an analytical representation of velocity field for the concept of an internal air flow inside a vortex tube machine of Hilsch-Ranque type. The basic assumption here is that the flux field is assumed to be steady and incompressible.Τhe vector which denotes instantaneous velocity, for the description of the flux field viaLagrangian stand point, will be expressed in a cylindrical coordinate system with orthonormal basis in the center line of the cross section of this vortex tube. We also assume here, that the aforementioned flux field varies only in radial direction, hence velocity vector is actually an one-dimensional damped circular helix trajectory with variable pitch. Key words: Vortex tube, homographic function, vortex lines, helix trajectory.

The extensive use of composites in aerospace, chemical, marine, and structural applications leads to exposure to humidity and water immersion. Hence, there is a need to study the effect of moisture absorption on the mechanical properties of composite materials, especially the matrix dominated properties, such as the interlaminar shear strength (ILSS). The horizontal shear test with a short-beam specimen in three-point-bending is used as a general method of evaluation for the shear properties in fiber-reinforced composites because of its simplicity. In this work, the ILSS of cross-ply glass-epoxy resin composites is determined in seven different fiber directions with short-beam three-point-bending tests, before and after moisture conditioning. It is found that moisture absorption reduces ILSS and stiffness of the examined composites whereas it leads to larger failure deflections. It is also found that the direction of fibers strongly affects the load–deflection response and the ILSS of the dry and conditioned specimens. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

In this paper, an icosahedral non-body-centered model is presented to simulate the periodic structure of a general class of homogeneous particulate composites, by predicting the particle arrangement. This model yielded three different variations, which correspond to three different deterministic particle configurations. In addition, the concept of a boundary interphase between matrix and inclusions was taken into account. In this framework, the influence of particle vicinity on the thermomechanical properties of the overall material was examined in parallel with the concept of boundary interphase. The simultaneous consideration of these two basic influential factors constitutes the novelty of this work. Next, by the use of this advanced model, the authors derived a closed-form expression to estimate the thermal conductivity of this type of composite. To test the validity of the model, the theoretical predictions arising from the proposed formula were compared with experimental data found in the literature, together with theoretical results obtained from several accurate formulae derived from other workers, and an adequate accordance was observed.

The aim of this investigation is to determine the effect of low-filler volume fraction on the elastic modu- lus and the thermal expansion coefficient of particulate com- posites. In the theoretical part, theoretical model valid for low-filler volume fractions is used to evaluate these two magnitudes. In the experimental part, low-percentage filler contents of 3, 5, 7, and 10% are used. The density for these epoxy resin-iron particle composites is also determined. At the same time, an attempt to explain some of the disagree- ments observed between theoretical values and experimen- tal data on a qualitative basis is also made. This attempt is in part assisted by scanning electron microscopy (SEM) obser- vations concerning structural inhomogeneities and fracto- graphical data. The comparison of the theoretical values derived from the present model with experimental results and with theoretical values derived from other workers appears satisfactory in many cases, but in some others the discrepancies among them are considerable. V C 2008 Wiley

In this complex study an attempt was made on the one hand to analyze and understand in a systematic way the exact nature of the formation of certain characteristic energy dissipation-induced fractographic features and patterns/markings revealed by scanning electron microscopy (SEM) in particulate epoxy systems under impact (dynamic) loading conditions, and on the other hand to correlate these patterns and features with relevant crack propagation effects. For this scope a combined approach consisting of a qualitative as well as a semiquantitative analysis was employed. In the qualitative analytical approach it was shown that depending on the actual velocity and direction of crack propagation the above observed fractographic entities can be correlated to certain highly localized energy dissipative processes at front-failures as well as to local inertial molecular mass effects. Depending on the changes in the velocity and direction of propagation, the associated effects may be controlled by two basic processes: the single crack front and the multiple crack front splitting. The first process seemed to be governed by a shear toughness-biased system, whereas the second one used a critical strain energy release rate subcracking mechanism. Under certain conditions both processes may be influenced by inertial molecular effects in promoting the formation of relative-smooth fracture surfaces. The increased presence of particles tends to restrict an increase in the surface roughness due to energy dissipation-induced crack retardation effects. The presence of the notch tends to lower the fracture surface roughness compared to notch-free specimens and also to suppress the occurrence of certain elastic as well as viscoelastic-plastic crack delay effects observed in notch-free specimens in function of particle volume fraction. Based on relevant kinematics-aided modeling and impact energy measurements it seems possible to explain, by a gross semi-quantitative approach, the above particles and notch effects. In this context it seems plausible that the existence of a defect-induced fracturing time spectrum of the propagating crack front, in combination with the `notch-induced shift' behavior of this spectrum, can be valuable for some approximating explanations of the above notch effects and in general the `kinematics' of the surface roughness formation.

The stiffness and strength of a composite material in form of laminate is obtained from the properties of the constituent laminae. The stacking sequence of the laminate affects the mechanical behavior. The interface be- tween different laminae is also an important factor, since it influences the stresses that are developed in the laminate, and hence the strengths. In this study, a theoretical investi- gation of the mechanical behavior of symmetrical laminates made of isotropic layers (Lexan (PCBA) and Plexiglas (PMMA)) was attempted. An analysis based on the lamina- tion theory was performed to determine the stress distribu- tion and strains as well as the elastic constants. Experimental measurements with specimens made of laminates with dif- ferent stacking sequences were carried out. The obtained values were compared with the theoretical values given by the lamination theory and mechanics of materials approach. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3215-3226, 2006

In this work the stiffness and strength of a composite material in the form of an asymmetric laminate are obtained from the properties of the constituent laminae made of isotropic layers, Lexan (PCBA) and Plexiglas (PMMA). The influence of the stacking sequence of the laminate and the interface between the laminae that affects the mechanical properties are investigated. A theoretical analysis based on the lamination theory was performed to determine the stress and strain distribution as well as the elastic constants. Experimental measurements with speci- mens made of asymmetric laminates of various stacking sequences were carried out. The obtained values were com- pared with the theoretical values given by the lamination theory and mechanics of materials approach. © 2006 Wiley

The first objective of the present experimental work is to evaluate the storage and loss moduli of plasticized epoxy polymers over a temperature and frequency range, and to correlate with the ability of the ultrasounds for the characterization of such polymers. The materials used were a series of cold-setting epoxy polymers, plasticized with different amounts of plasticizer. After the investigation of dynamic properties through vibration tests, the velocities of longitudinal and transverse elastic waves, and the acoustic attenuation coefficient a were evaluated and, then, using these quantities, the “dynamic” moduli E′, G′, and ν′ were evaluated. Thereafter, it can be stated that the plasticizer content and measuring frequency range strongly influence the elastic moduli of the material in the sense that the plasticizer tends to reduce, whereas the measuring frequency to increase these. The second objective of this work is to asses and discuss the associated plasticizer-induced microstructural damaging effects by combined methods of ultrasounds, tensile modulus, as well as moisture absorption testing and then to correlate this assessment with the storage moduli behavior. It was shown that, despite the nonneglecting scatter in the experimental data, certain irresistible general trends might be assessed. In this context, it was found that the damage “response” is markedly sensitive to the respective applied “detection” technique and, therefore, the damage evolution data obtained by these different techniques are comparable only under certain circumstances. By comparison between the applied techniques, it seems reasonable to assume that the “moisture” technique due to its inherent microscopic mass transport processes, in the form of H2O molecules diffusion and therefore its ability to “detect” microstructural detects on a much lower scale, should give a better approach to the internal damage phenomena. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3869–3880, 2006

In this paper, the thermal expansion coefficient of particle-reinforced composite was determined using a three-phase theoretical model which consists of the transformation of spatial cubic models simulating a particulate composite into a three sphere model, designating the main phases of a composite material the filler and the matrix, in order to apply the classical theory of elasticity to it. Theoretical values derived from three variations of this model were compared with experimental results derived from thermal experiments carried out on iron particle reinforced epoxy resin as well as with theoretical results obtained by other scientists. In this sense an effort has been made to explain on a qualitative as well as on a semiquantitative methodological basis the possible sources which can contribute to the observed discrepancies between theory and experiment. This methodology was based mainly on certain microscopical observations concerning the structural inhomogeneity as well as on theoretical facts the thermal conductivity and capacity of the composite.

A strength of materials and elasticity ap- proach was used in this study to determine the stiffness of fiber-reinforced composites, by taking into account the con- cept of boundary interphase. Theoretical expressions for longitudinal transverse and shear moduli, as well as for longitudinal Poisson's ratio, were derived by use of this model. Results derived from these expressions were com- pared with others, to observe the discrepancies and evaluate their validity.© 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1578 -1588, 2005

The bending properties of composite materials are often characterized with simply supported beams under concentrated loads. The results from such tests are commonly based on homogeneous beam equations. For laminated materials, however, these formulas must be modified to account for the stacking sequence of the individual plies. The horizontal shear test with a short-beam specimen in three-point bending appears suitable as a general method of evaluation for the shear properties in fiber-reinforced composites because of its simplicity. In the experimental part of this work, the shear strength of unidirectional-glass-fiber-reinforced epoxy resin composites was determined in different fiber directions with the short-beam three-point-bending test. Also, the elastic constants and flexural properties of the same materials were determined from bending experiments carried out on specimens in the 0, 15, 30, 45, 60, 75, and 90° fiber directions with high span–thickness ratios. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 63–74, 2004

The basic objective of the present experimental investigation is to contribute to the development of the ultrasonic non-destructive testing (NDT) method for the improved non-destructive characterisation of concrete. Of all known NDT methods for testing, the most promising seems to be the use of ultrasonics, because through this method several mechanical properties of concrete can be determined. Two approaches seem promising. In the first of them the velocity, whereas in the second one the damage of ultrasonic waves in concrete are used. In this experimental work, destructive and ultrasonic NDT methods are applied in order to study the damage evolution versus the compressive loading of 30 year-old concrete.

In the first part of this complex study the thermomechanical and fractural properties of particle reinforced polymer composites were experimentally obtained. The experimental values for modulus of elasticity, fracture stress, fracture strain and thermal expansion coefficient were compared with those derived from theoretical formulae existing in the literature and also from a theoretical model assuming the existence of an interphase between the two main phases the filler and the matrix. This model was used to obtain theoretical expressions for modulus of elasticity and thermal expansion coefficient. The mechanical properties of the material used in this investigation were determined from tensile experiments carried out with a composite material made of epoxy resin reinforced with iron particles the volume fraction of which varies from 0 to 25% and in some cases up to 40%. To obtain information concerning the thermal expansion coefficient and glass transition temperature of the same material thermomechanical analysis (TMA) measurements were performed. The effects of heating rate and filler content on the glass transition temperature were examined. In the second part of this study an attempt is made to explain on a more phenomenological basis the relative big discrepancies observed between theoretical models and experiments concerning certain strength parameters such as fracture stress and strain presented in the first part. This was possible by assuming a delayed kind of fracture behavior which was simulated by a subcritical crack growth based on the theory of elastic-small yielding fracture mechanics and by an arrest micromechanism. The above simulation in turn was achieved by a procedure of a semiquantitative gross estimation approach which has taken into consideration certain experimental fractographical and microstructural data such as interfacial decohesion features between grain and matrix, grain size and interinclusion spacing, parameters estimated by Scanning Electron Microscopic (SEM) measurements.

The influence of low filler content on some thermomechanical properties of particulate composites was determined. A variant of a previously developed theoretical model valid for low filler contents was used to evaluate the elastic modulus and thermal expansion coefficient of a particulate composite. In the experiments, filler volume fractions of 3, 5, 7 and 10% were used. The density and the velocity of longitudinal waves in these iron particle epoxy resin composites were also determined. An attempt has been made to explain qualitatively some of the discrepancies observed between theoretical models and experimental data. This approach was in part assisted by scanning electron microscopy observation of structural inhomogeneities and fractographic data. The theoretical values derived from the present model agreed satisfactorily with the experimental results and with theoretical values of other workers.