A.H. Cardon

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.

Characterization and modeling of nonlinear viscoelastic response of PEEK resin and PEEK composites

Abstract The viscoelastic properties of PEEK resin and unidirectional PEEK composite APC-2 were characterized by an accelerated testing procedure based on the time-temperature and time-stress superposition principles. The results obtained by creep and recovery tests are presented, and the constitutive modeling of the nonlinear viscoelastic response is discussed. The stress-dependent viscoelastic responses of the composite were modeled by a special case of Schaperys nonlinear viscoelastic equation with a kernel of the general power law. The relations for the temperature-time scale shift factors and stress-time scale factors are given. With the equations and parameters presented, the linear and nonlinear viscoelastic behaviors of APC-2 composite laminate from ambient temperature up to the glass transition temperature can be described analytically. Comparisons were made on the nonlinear properties of the 15° off-axis laminate, transverse laminate and pure resin. The results agree with the theory that the matrix octahedral shear stress is the main variable determining the nonlinearity of polymers and polymer-based composites.

Durability analysis of polymer matrix composites for structural applications

Abstract Polymer matrix composites have time dependent properties with response times function of the fibre orientations, the volume fraction of the fibres and the stacking sequence for laminates. This viscoelastic–viscoplastic behaviour is also influenced by the type of mechanical loading history and the environmental variations. This time dependent evolution concerns not only the stiffness properties but also the strength characteristics. The durability analysis of such composite systems can be based on strength criteria with time dependent coefficients or on a damage analysis. After an overview of the most important prediction methodologies available today, a combined nonlinear viscoelastic–plastic damage model is described. Results from this model on some specific graphite-epoxy systems are presented for a viscoelastic–viscoplastic behaviour. Extensions for damage development are discussed. Local time dependent effects resulting from the stress transfers between viscoelastic and elastic components are presented in relation to the degree of adhesion between those components

Effects of the environment and curing on the strength of adhesive joints

—The influences of four parameters - pretreatment of the adherend, curing cycle, time before testing, and humidity — on the strength of adhesively-bonded single lap joints were investigated for aluminium 2024-T3 adherend and modified epoxy FM73M adhesive. The importance of conditioning the aluminium surface prior to bonding was confirmed. Furthermore, the pretreatment should be attuned to the environmental conditions under which the joint will be used. We found that roughening the aluminium surface gave a poorer joint strength. Cooling joints in normal environmental conditions after the curing procedure gave satisfactory strengths if a relaxation period of 30 days was allowed before loading. The fitting of the experiments to a statistical distribution was examined to allow use of the experimental results for finite element techniques with probabilistic models. The same statistical behaviour as that for joints submitted to cyclic loading was found for joints submitted to humidity.

The effects of motion on thermoelastic stress analysis

Abstract An indepth examination of the effects of specimen motion on the technique of thermoelastic stress analysis is presented. It is shown that the presence of a non-uniform temperature distribution, coupled with the material motion due to cyclic loading, can greatly corrupt the measured data. This is particularly important when analysing composite materials in which heat is generated due to viscoelastic processes and/or to internal friction at delamination sites. A series of experiments using delaminated composite specimens as well as an externally heated specimen are presented to illustrate this problem and a mathematical model is developed to account for this phenomenon. Methods to remedy this problem are suggested and in the case of symmetrical conditions, it is shown that this effect can be effectively removed by appropriate post-processing of the data.

Prediction of the Residual Structural Integrity of a Polymer Matrix Composite Construction Element

Polymers are time-dependent materials. Polymer matrix compositesbehave as viscoelastic-viscoplastic anisotropic continua. Theirthermomechanical characteristics are not only time-dependent, but arestrongly influenced by the variations of the environmental conditions(temperature, moisture diffusion, radiation, etc.). This concerns notonly the stiffness but also the strength characteristics, related to thedamage development. Due to the composite nature, the number of potentialdamage sources is high and the damage development has to take intoaccount all the interactions between simple mechanical loading modes,with or without, changing environmental conditions. Following anon-linear viscoelastic-viscoplastic analysis, based on the Schaperyequations and a Zapas–Crissman functional, the stiffness degradation isobtained based on short-term creep and creep-recovery measurements atdifferent stress levels and different temperatures.A damagecomponent is added to the viscoelastic-viscoplastic model in order toaccount for damage development and associated strength degradation.Results obtained in the transverse direction of an unidirectionalgraphite-epoxy composite are presented. The extension to completelaminates with different stacking sequences is discussed. An assumptionon the interlaminar behaviour must be introduced in order to take intoaccount the important damage source not considered in classical laminatetheories.

Thermoelastic Stress Analysis of Fibre Reinforced Composite Systems

The thermoelastic effect first analysed by Lord Kelvin is governed by a simple relation between amplitude of temperature and amplitude of the sum of principle stress change as long as adiabatic conditions are maintained. Although valid for isotropic materials it is shown that this simple relation leads to very poor results for anisotropic materials. A non-adiabatic theory taking into account the interlaminar heat transfer shows a better agreement between theory and experiment.

From micro- to macroproperties of polymer based composite systems by integration of the characteristics of the interphase regions

Abstract After an overview and discussion about the amount of information on the interphase region given by classical ‘single number’ characterisation methods such as ILST, pull-out and fragmentation tests, different models of the interphase region considered as classical continuum are analysed. A model is proposed, based on an interphase region with varying properties. By integration of those varying properties over the thickness of the interphase region, from the bulk properties of one phase to those of the other phase, assuming the transverse dimensions as very small, the control parameters, or integrated moduli, of the interphase region are obtained. This model indicates the importance of the gradients of the properties of the interphase region on the stress transfers between the original phases. The application of this model under specific stress states used for the classical mechanical ‘single number’ tests can give insight in the physical meaning of the results of those tests and can give a rational way to compare the scales for the results of those tests.