E. Sideridis

The in-plane shear modulus of fibre reinforced composites as defined by the concept of interphase

Abstract In this paper we describe a model to find the approximate equations for determining the in-plane shear modulus of a unidirectional fibre reinforced composite from the constituent material properties. Classical elasticity theory has been applied to the simplified model of a composite unit cell in which the concept of an interphase between fibre and matrix is taken into account. Thus the model considers that the composite material consists of three phases, that is the fibre, the matrix, and the interphase which is the part of the polymer matrix lying close to the fibre surface which possesses different physico-chemical properties from those of the main constituents. Thermal analysis was used for the determination of the thickness and volume fraction of the interphase. The theoretical results are compared with other theoretical expressions and with experimental data. The model introduced in this paper seems to be an improvement for the shear modulus.

Thermal expansion coefficients of fiber composites defined by the concept of the interphase

Abstract The thermal expansion behavior of unidirectional composites has been studied. In this analysis a model which introduces the concept of the boundary interphase has been used. The expansion coefficients of the composite were calculated by using this concept to determine the influence of the interphase which depends on the quality of adhesion between fiber and matrix. In the present paper it has been assumed that the interphase material was inhomogeneous with properties varying from the fiber surface to the matrix. Three laws of variation have been taken into consideration in order to derive expressions for the thermal expansion coefficients in which appeats the role of this interphase layer possessing physico-chemical properties different from those of the constituent phases. A thermal analysis method was used in order to find the extent of the interphase layer. The results are assessed by comparing them with those given by other theories.

A theoretical model for the prediction of thermal expansion behaviour of particulate composites

The thermal expansion coefficient of particle-reinforced polymers was evaluated using a theoretical model which takes into account the adhesion efficiency between the inclusions and the matrix — an important factor affecting the thermomechanical properties of a composite. To measure the adhesion efficiency a boundary interphase, i.e. a layer between the matrix and the fillers having a structure and properties different from those of the constituent phases, was considered. This layer is assumed to have varying properties.To obtain information concerning the properties and extent of the interphase, an experimental study of the thermal behaviour of aluminium-epoxy composites was undertaken. Differential Scanning Calorimetry (DSC) measurements were performed to evaluate heat capacity with respect to temperature. In addition, the effects of different factors, such as heating rate and filler concentration on the glass transition temperature of the composite, were examined. The sudden changes in heat capacity values in the glass transition region were used to estimate the extent of the boundary interphase according to an existing theory.Finally, the values of the thermal expansion coefficient, predicted by this model, were compared with theoretical results obtained by other authors and with experimental results.

Adhesion efficiency and volume fraction of the boundary interphase in metal-filled epoxies

Abstract A model describing the influence of spherical inclusions on the mechanical behaviour of composite materials is presented. Simple theoretical considerations led to the conclusion that the law of mixtures is a good approxiamtion for particle composites. Deviations of the experimental results from the law of mixtures were however verified. For an explanation of these deviations the model was treated in a more realistic way, introducing first a factor of adhesion between matrix and filler and then the concept of interphase. To verify these assumptions a test was established to estimate the degree of adhesion in the particle/composite materials and the factors which were correlated with this phenomenon. For the study of the properties of interphase around the filler, a series of experiments based on differential scanning calorimetry was carried out. From this analysis the volume content of interphase was calculated and hence some important mechanical properties of interphase, were established.

Structural Integrity Studies in Particulate Composites by Means of Thermal Capacity Measurements

Glass-transition temperatures Tg for iron-epoxy composites were deter mined from heat-capacity measurements. The effect of several parameters, such as the heating rate, the filler-volume fraction and the particle diameter on Tg was investigated and conclusions concerning the thickness of the boun dary interphase were derived. Tg -values obtained from TMA-measurements were compared to the respective values obtained from DSC-measurements.

Measurement of the interlaminar shear strength of glass-reinforced plastics of different construction using the off-axis four-point bending test

Abstract In order to measure the interlaminar shear strength of glass-reinforced plastics, an important design variable in many applications of these materials, a method called ‘the off-axis bending method’ was developed. It consists of an offset four-point bending test where two of the forces (P) differ from the other two (Q), the ratio λ = P/Q depending upon the relative positions along the beam. This test appears simple, economic and realistic in many aspects as an interlaminar test method. With this method, in addition to the span length (L) to thickness (t) ratio, the loading factor λ is an equally important parameter and is decisive in determining the mode of failure (i.e. flexural or shear). A bend test-rig was designed to study the effect of these variables in some detail and also to investigate the interlaminar shear strength of glass-reinforced plastics. In the tests the thicknesses varied between 8 and 16 mm, L/t varied between 2·5 and 11, while λ varied between 2 and 10. The specimens were made from polyester resin containing different volume fractions of E-glass fibres. Shear failures have been obtained for small L/t ratios (2–3) and large λ (8–10) and repeatability was good. At progressively larger L/t ratios (or smaller λ) the mode of failure became flexural. The shear loading and the crack formation mechanism was studied using photography.

The dynamic moduli of particulate-filled polymers as defined by the concept of interphase

Abstract A simplified theoretical approach for the prediction of the dynamic moduli of particulate-filled polymers has been developed by considering the concept of interphase. According to this model, an important parameter affecting the behaviour of the composite is the adhesion characterized by the existence of an interphase layer, between filler and matrix, which possesses physico-chemical properties different from those of the constituent phases. Convenient laws of variation were assumed for the storage and loss moduli and for Poissons ratio of the interphase versus the polar radius. The theoretical values were compared to those predicted by other theories and to experimental results obtained from experiments on iron/epoxy composites. The thickness and volume fraction of the interphase were determined by heat capacity measurements according to an existing theory.

Elastic and viscoelastic properties of fibre-reinforced composite materials

Abstract In this paper a model to find the approximate equations for determining the transverse modulus of elasticity of a unidirectional fibre-reinforced composite material from the constituent material properties is described. The classical theory of elasticity was applied to the simplified model of a composite unit-cell. Stress functions were applied to the fibre and matrix with boundary conditions requiring continuity of displacements across the fibre-matrix interface (perfect bonding was assumed). Graphical results are shown for glass-epoxy composites. The numerical results are compared with experimental data. The transverse elastic modulus, derived by applying the model introduced in this paper, seems to explain the experimental results fairly well. In the second part, assuming a viscoelastic behaviour of the matrix, the correspondence principle has been applied to the relationship of the transverse modulus of elasticity and the storage and loss moduli of the composite have been calculated. These theoretical values are compared with the experimental ones.

Study of orthotropic materials: the interphase model and the crack initiation

SummaryIn this paper, a survey on the orthotropic composite materials is given. The model of the interphase, assuming a third phase between fibre and matrix with some different properties, is used for the calculation of the elastic constants of a unidirectional fibre-reinforced epoxy resin composite. Their values are used to determine the stress field at the crack tip and the stress intensity factor of a cracked orthotropic plate. The recently developed Det-criterion of fracture is taken into account to study the crack initiation.

Low-velocity impact response of fiber-metal laminates consisting of different standard GLARE grades

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.