G. C. Papanicolaou

On the non-linear viscoelastic behaviour of polymer-matrix composites

The non-linear viscoelastic response of a unidirectional carbon-fibre-reinforced polymer composite has been studied. For the needs of the present study, creep and recovery tests in tension for different stress levels were executed while measurements were made of the creep and recovery strain response of the composite system. For the description of the viscoelastic behaviour of the material, Schaperys non-linear viscoelastic model was used. For the description of the non-linear viscoelastic response of the material and the determination of the non-linear parameters, an analytical method, based on a modified version of Schaperys constitutive relationship where a viscoplastic term was added, has been developed. The method has successfully been applied to the current tests and an estimation of the non-linear parameters was successful. Useful results and conclusions for the applicability of the new method were also extracted.

Prediction of the non-linear viscoelastic response of unidirectional fiber composites

A methodology for predicting the nonlinear viscoelastic behaviour of unidirectional fibre composites is proposed. The prediction, which is based on a modified Schapery formulation, is easy to apply by using a combination of analytical formulations and numerical procedures. In addition a generic function is developed for the description of the stress dependence of the creep-recovery response over the whole stress range examined. All the parameters included in the proposed generic function used for the prediction of the nonlinear viscoelastic behaviour of unidirectional fibre composites, such as the stress threshold of nonlinearity and the ultimate tensile strength of the material, have a clear physical meaning. The accuracy of both the generic function and the numerical technique is checked by creep-recovery tests on 90° unidirectional carbon-fibre/epoxy-matrix composites in which predicted response is compared with measured response. Good-to-reasonable agreement between experimental and analytical results is observed. The method is presented in a generalised manner and could be applicable to a large class of UD fibre composite systems.

Further development of a data reduction method for the nonlinear viscoelastic characterization of FRPs

Abstract According to the well-known Schapery’s formulation, the nonlinear viscoelastic response of any material is controlled by four stress and temperature dependent parameters, g0 g1, g2 and aσ, which reflect the deviation from the linear viscoelastic response. Based on Schapery’s formulation, a new methodology for the separate estimation of the three out of four nonlinear viscoelastic parameters, g0, g1 and aσ, was recently developed by the authors. In the present article, a further development of the previously developed methodology is attempted leading to an analytical estimation of the fourth nonlinear parameter, g2, which additionally includes the viscoplastic response of the system. Thus, a full nonlinear characterization of the composite system under consideration is achieved. The validity of the integrated model was verified through creep-recovery experiments, applied at different stress levels to a unidirectional carbon fibre reinforced polymer.

Thermal stress concentration due to imperfect adhesion in fiber-reinforced composites

Thermal stress and strain analyses have been performed in unidirectional fiber-reinforced composites by considering the concept of an interphase between the fiber and matrix phases. The thermal and elastic material properties of the interphase are assumed to be variable and to depend on the degree of adhesion developed between fiber and matrix. An analytical approach is developed for the solution of the appropriate generalized plane strain steady-state thermoelastic problem. This approach is an extension of the homogeneous subdomain method, requiring appropriate discretization of the problem domain into an appropriate number of subdomains. Application of continuity and compatibility conditions for the temperature field and the thermoelastic stress potential result in a system of linear equations, the numerical solution of which supplies all necessary constants for evaluation of the solution of the problem. Numerical results are given for a wide range of the parameters involved, including material properties and imperfect adhesion coefficients. The introduction of the so-called degree-of-adhesion coefficients gives new insight into the failure mechanisms developed along the fiber/matrix interface. Application to the proposed model of imperfections in the adhesion between fiber and matrix shows very interesting results and explains the failure mechanisms for such materials.

Thermal stresses in fibrous composites incorporating hybrid interphase regions

A micromechanics semi-analytical method has been developed for the determination of thermal stresses in unidirectional fiber-reinforced composites incorporating a hybrid interphase region. The representative volume element consists of a three phase composite structure subjected to a uniform temperature change. An advanced interphase concept is introduced, in which the interphase thickness varies depending on the particular property under consideration. This model involves also imperfect adhesion by immediate softening of material properties. To treat these features, a regenerative piecewise homogeneous formulation has been developed, which accounts for the spatial variation of the material properties within the interphase region in an averaged sense. Global design variables of this formulation involve geometric and material aspects of the composite. Adjustment of these variables yields parametric studies concerning the thermal stress response. Numerical results are illustrated and discussed for a variety of the involved parameters. Thermal stress and strain concentrations occur in a zone surrounding the fiber, although peak values depend on bonding efficiency and overall anisotropy of the material.

On the biocompatibility between TiO2 nanotubes layer and human osteoblasts

Titanium and its alloys are the most popular biomaterials replacing hard tissues in implant surgeries. Clinicians are generally pleased by titanium mechanical properties and non-toxicity performances; on the other hand, there have been reported several cases of titanium implantation failure, phenomenon explained sometimes as non adherence of human tissue to the metallic surface. Yet, researchers reported that titanium surfaces are favorable for osteoblasts adhesion. Therefore, titanium integration into the human body remains an unsolved problem. In the present study, biocompatibility tests were performed on titanium and TiO(2) nanotubes substrates, involving human bone marrow cells. The combination of a newly developed analytical model based on the hybrid interphase concept, applicable to systems consisting of inert materials when in contact with living tissues, together with experimental results, confirmed previous research studies and lead to the conclusion that osteoblasts adhere efficiently to titanium surfaces. However, the present results suggest that osteoblasts strong anchorage at the very first moment of their contact with the metallic material leads to their apoptosis. It is most probable that in several cases this is the reason of failed implantation surgeries involving titanium.

Effect of dispersion of MWCNTs on the static and dynamic mechanical behavior of epoxy matrix nanocomposites

In this investigation, specimens of MWCNT-epoxy nanocomposites were prepared by two different dispersion methods including the use of ultrasonication, and high speed shear mixing. The dispersion degree between MWCNT and polymer resin was analyzed after completing the curing reaction, by scanning electron microscopy. The effect of the nanotubes dispersion achieved on the properties of the manufactured nanocomposite was analyzed through static three point bending tests and dynamic mechanical thermal analysis. Interesting results concerning the dispersion effect of MWCNTs added to the polymer matrix on the storage and loss moduli as well as on tanδ and Tg values of the specimens manufactured by the sonication and high speed shear mixing methods were derived.

Stress analysis of short fiber-reinforced polymers incorporating a hybrid interphase region

The finite element method is used to investigate the load-carrying characteristics and the stress profiles in unidirectional short fiber polymeric composites. The micromechanics composite model incorporates a hybrid interphase region surrounding the fiber. The new interphase concept involves different domains of interaction between different material properties depending on the corresponding properties of primary constituents, volumetric composition and macroscopic characterization of the composite. Thus, the hybrid interphase is not defined within a unique thickness, its outer radius being a function of the material property under consideration. The model describes also imperfect adhesion conditions by immediate softening of material properties. Numerical results are illustrated and discussed for a variety of the involved parameters. These results referring mainly to hoop and radial stresses along the fiber length confirm the intense variation of stress in this area. Parametric studies have been conducted accounting for different degrees of adhesion and shapes of the fiber tip and where it is possible comparisons with existing theories and experimental results taken from literature are illustrated.

Prediction of the residual tensile strengths of carbon-fiber/epoxy laminates with and without interleaves after solid particle erosion

In the present work, the influence of stacking sequence, existence and position of interleaves on the solid particle erosion in carbon-fiber-reinforced epoxy composites (CFRP) was investigated. The erosive wear behavior was studied in a modified sandblasting apparatus at a 90° impact angle. The erosion behavior was considered as a repeated impact procedure (impact fatigue). A semi-empirical approach initially developed for the prediction of the residual strength after single impact was adopted and evaluated in the case of erosion conditions. The model takes into account the inherent material properties, the initial and post-impact tensile strength of the material and the visco-elastic response (mechanical damping) of the non-impacted material. The excellent agreement between theoretical predictions and experimental values corroborated the reliability of this model which may be a useful tool for the prediction of the post impact residual strength in the case of solid particle erosion. Results showed that for impact energy values lower than a characteristic threshold the damage induced does not affect the residual tensile strength after solid particle impact (erosion) of the materials. It was also established that this threshold depends on the orientation of the plies, the existence of interleaves and the energy absorption capacity of the material.

The role of imperfect adhesion on thermal expansivities of transversely isotropic composites with an inhomogeneous interphase

The aim of this article is the prediction of the coefficients of thermal expansion of fiber-reinforced composites consisting of transversely isotropic fibers embedded into an isotropic matrix. The proposed model takes into account the existence of a non-homogeneous interphase lying in the area between the fiber and the matrix while the case of imperfect adhesions between the main constituents is considered and its effect on thermal expansivities is also examined. It was found that in the case of isotropic fibers there is no serious effect of the interphase existence on the overall thermal expansion behavior of the composites. On the contrary, in the case of transversely isotropic fibers, and especially for the transverse thermal expansion coefficient values, there exists a discrepancy between various theoretical predictions derived by other workers and from experimental results. In the latter case, the present model gives a better prediction showing that one of the reasons why different models fail to predict the thermal expansion behavior of fiber composites is ignorance concerning the existence of the interphase area.