Homogenization of metallic fiber-reinforced composites under stochastic ageing

The main aim of this paper is to present an application of the generalized stochastic perturbation technique to model stochastic ageing processes of the metallic fibre-reinforced periodic composite materials in terms of their effective properties. Those ageing processes are modelled here as two-parametric time series having Gaussian random initial values and time rate, both defined uniquely by their expectations and standard deviations. Computational homogenization procedure is discrete and based on the Finite Element Method program MCCEFF as well as the computer algebra system MAPLE, where the Response Function Method and the stochastic analysis are entirely implemented. This numerical strategy is used to analyze probabilistic moments of the effective elastic tensor of the few metal matrix composites as well as to simulate stochastic ageing of two representative composites - MoSio₂-SiC and Ti-SiC. The approach proposed and results of computations may be further applied in the reliability analysis of metallic or the other composites.     

Fracture behaviour of hybrid glass matrix composites: thermal ageing effects

The thermal aging of a glass matrix composite reinforced by short carbon fibres as well as by ZrO₂ particles (hybrid composite) was investigated at temperatures in the range 500–700 °C for exposure durations of 24 h in air. The mechanical properties of as-received and aged samples were evaluated at room temperature by using the three-point flexure chevron notch technique. The fracture toughness values of as-received specimens were in the range 2.6–6.4 MPa m^1/2. Fracture toughness was affected by the thermal aging conditions. For thermal aging at temperatures <700 °C, degradation of fibre–matrix interfaces occurred and therefore the apparent fracture toughness and flaw tolerant resistance decreased. For the most severe ageing conditions tested (700 °C/24 h), fracture toughness values dropped to 0.4 MPa m^1/2. Significant degradation of the material was detected for this aging condition, mainly characterised by porosity formation in the matrix as a result of softening of the glass and oxidation of the carbon fibres.     

Stochastic modeling of damage evolution in composites under environmental ageing

A mathematical model is proposed for predicting damage evolution and stiffness degradation in composite materials under environmental ageing. Environmental parameters such as moisture, temperature, and ultraviolet (UV) radiation cause hygrothermal loads on the structure, which leads to damage evolution. The present work establishes a constitutive model for treating the damage density as a random variable. A forward stochastic differential equation (FSDE) is proposed to predict the damage density evolution. Damage nucleation and annihilation rates are taken into consideration in terms of Brownian motions. A second-order damage tensor is developed using the solution of the FSDE. The damage tensor is incorporated into the constitutive model for predicting the elastic moduli. Finally, the proposed model is verified against experimental observations under certain hygrothermal conditions.     

Effect of laminate thickness on moisture diffusion of polymer matrix composites in artificial seawater ageing

The influence of laminate thickness of polymer matrix composites on moisture diffusion in seawater immersion, as well as the resulting mechanical property degradation for composites of glass/isopolyester (G/IPE), carbon/isopolyester (C/IPE), glass/vinylester (G/VE) and carbon/vinylester (C/VE), was investigated in this paper. Laminates 3 and 10 mm in thickness, fabricated using the wet hand lay-up technique, were characterized for moisture absorption in artificial seawater medium, and their flexural strength and interlaminar shear strength (ILSS) degradations were studied. Moisture diffusion was observed to be anamolous to the Fick’s law for both 3 and 10 mm thick samples in the later stage of diffusion. Moisture permeability of 10 mm thick samples was two to three order greater than that of 3 mm thick ones, while the time to moisture saturation remained unchanged. With the increase of laminate thickness, moisture saturation increased by 1.4% for C/VE and 7% for G/IPE. The residual flexural strength and ILSS were greater in case of 10 mm thick specimens after 200 days of exposure. SEM examination of the fractured specimens showed greater levels of fibre/matrix debonding in 10 mm thick specimens.     

Mechanical behaviour of Textile Concrete under accelerated ageing conditions

The mechanisms of ageing and environmental degradation involving exposure to cyclic hot/cold temperatures, wetting/drying movements as well as exposure to a carbon dioxide (CO₂)-rich environment, have different effects on the microstructure of interfaces within cementitious composites. This paper presents results of an investigation into changes occurring in fibre pull-out and composite tensile behaviour in Textile Concrete (TC) after exposure to accelerated ageing conditions. The microstructure of the matrix at the fibre/matrix interface, and fibre properties, were found directly to affect the mechanical behaviour at the macro-level. The study illustrated that exposure of TC to a CO₂-rich environment improves the fibre/matrix bond significantly; no major changes were observed in the mechanical behaviour of the composites after exposure to hot/cold and wetting/drying environments.     

Elastic response in wood under moisture content variations: analytic development

Creep evolution of timber structures results from the interaction between mechanical stresses due to different loads and hydric stresses due to moisture content variations. This paper deals with a thermodynamic approach in order to take into account a realistic elastic behavior under moisture content variations. In this context, memory effect, experimentally observed, is introduced employing a mechano-sorptive stress driven by a function dependent of the moisture content variations. This new internal thermodynamic variable enables to define an original separation of the free energy into an instantaneous recoverable part and a stored part during the last drying phase. This energy enables the modeling of the nonreversible strain process during the unloading phase. The locate state method is employed in order to define the thermodynamic function which traduces an indirect hereditary behavior between moisture content history and the stress state in the material.     

Fibre and fibre-surface treatment effects in carbon/aluminium metal matrix composites

A systematic study of the relationship between the microstructure of the interface in C/Al composites and its dependence on variations in squeeze-casting parameters has been undertaken. This research has shown that the amount of Al₄C₃ reaction product at the interface is dependent on the surface structure of the reinforcing fibre and the surface treatment of the fibre. Additionally, the interface shear strength increases with an increase in the amount of reaction product at the interface. An increase in interface shear strength leads to a decrease in composite longitudinal strength. High-resolution electron microscopy and X-ray photoelectron spectroscopy analyses indicate that carbide formation is a conventional two-step process of nucleation and growth. Nucleation occurs preferentially at graphite edge planes on the carbon fibre surface, and growth is restricted along certain matrix planes and directions.     

Heat and moisture transfer in lamina under generalized boundary conditions

An analytic solution is obtained for a problem in the theory of heat and mass transfer under generalized boundary conditions.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 31, No. 2, pp. 347–350, August, 1976.     

Strength and fracture properties of asbestos-cement mortar composites

The strength and fracture properties of random asbestos fibre-reinforced cement mortar composites are reported in this paper. The fibre content varies between 5% and 20% by weight. Both the ultimate tensile strength ( _t) and the modulus of rupture ( _b) increase with increasing fibre-volume fraction. These results are shown to agree satisfactorily with the law of mixtures modified for randomly oriented short fibre-reinforced composites. The critical stress intensity factor (K _c) and the specific work of fracture (R) have been determined using three-point bend edge-notched beams and grooved double-cantilever-beam (DCB) specimens. There is generally good agreement between these two physical quantities estimated from the two testpiece geometries. It is shown that the fibre pull-out mechanism is dominant in the fracture of asbestos cements and that the specific work of fracture can be reasonably well predicted by considering the energies absorbed in both the pull-out and the fibre/matrix interfacial debonding processes.     

Evolution of strength silica optical fibers under various moisture conditions

The reliability and the expected lifetime of optical links are closely related to the chemical action of the water molecules on silica network. However, polymeric coating also contributes largely to the mechanical properties of the fiber. Dynamic fatigue measurements were performed on silica optical fibers, either bare or coated, using two-point bending bench. Water concentration in test atmosphere was 50% relative humidity or very dry atmosphere. When water content is a few ppm, the dynamic fatigue curve, which expresses the breaking strength as a function of stress rate deviates from linearity at high stress rate. The same behaviour is observed in ambient atmosphere for very high testing speeds. The comparison between bare and coated fibers shows that the fatigue behaviour at high stressing rate is influenced by the polymer coating.     

Hygrothermal ageing effects on the static fatigue of glass/epoxy composites

A study of the effects of water ageing on the static fatigue behaviour of unidirectional glass/epoxy composites is presented. The failure mechanisms associated with fatigue damage were investigated under three-point bending loading. Depending on the ageing temperature, two failure features were identified: either fibre microbuckling on the compressive side of the specimen, or progressive cracking on the tensile side. Microbuckling has been found to be related to the reversible plasticization of the epoxy matrix, as measured by dynamic mechanical thermal analysis. On the other hand, tensile failure was associated with an irreversible weakening of the fibres and interfaces at elevated ageing temperatures. Some similarity is identified between damage processes in static and dynamic bending fatigue.     

Non-linear analytical model of composites based on basalt textile reinforced mortar under uniaxial tension

The recent development of inorganic based composites as low-cost materials in reinforced concrete structural strengthening and precast thin-walled components, requires the creation of models that predict the mechanical behaviour of these materials.Textile Reinforced Mortar (TRM) shows complex stress–strain behaviour in tension derived from the heterogeneity of its constituent materials. This complexity is mainly caused by the formation of several cracks in the inorganic matrix. The multiple cracking leads to a decrease in structural stiffness. Due to the severe conditions of the serviceability limit state in structural elements, the prediction of the stress–strain curve is essential for design and calculation purposes. After checking other models, an empirical nonlinear approach, which is based on the crack control expression included in the Eurocode 2, is proposed in this paper.Following this scope, this paper presents an experimental campaign focused on 31 TRM specimens reinforced with four different reinforcing ratios. The results are analysed and satisfactorily contrasted with the presented non-linear approach.     

Micromechanics of transformation fields in ageing linear viscoelastic composites: effects of phase dissolution or precipitation

Transformation fields, in an affine formulation characterizing mechanical behavior, describe a variety of physical phenomena regardless their origin. Different composites, notably geomaterials, present a viscoelastic behavior, which is, in some cases of industrial interest, ageing, i.e. it evolves independently with respect to time and loading time. Here, a general formulation of the micromechanics of prestressed or prestrained composites in Ageing Linear Viscoelasticity (ALV) is presented. Emphasis is put on the estimation of effective transformation fields in ALV. The result generalizes Ageing Linear Thermo- and Poro-Viscoelasticity and it can be used in approaches coping with a phase transformation. Additionally, the results are extended to the case of locally transforming materials due to non-coupled dissolution and/or precipitation of a given (elastic or viscoelastic) phase. The estimations of locally transforming composites can be made with respect to different morphologies. As an application, estimations of the coefficient of thermal expansion of a hydrating alite paste are presented.     

Hybrid effects in composites: conditions for positive or negative effects versus rule-of-mixtures behaviour

A positive or negative hybrid effect in hybrid composites is defined as a positive or negative deviation of a certain mechanical property from the rule-of-mixtures behaviour. The question of hybrid effects is first examined with special hybrids which have been chosen so that the effect of the fibre-matrix interface is minimized. The hybrids examined consisted of two types of carbon fibres with different mechanical properties but similar surface treatments. The results of all the mechanical properties examined (modulus, strength, stress intensity factor, fracture energies) under quasi-static and fast testing conditions do not show any synergism. In view of these results a second hybrid system of E-glass fibre/AS carbon fibre-reinforced epoxy has been chosen. In this system both the mechanical properties of the fibres and the interface which they form with the resin are entirely different. None of the mechanical properties, excluding the fracture energies, show any signs of a hybrid effect. The fracture energy results, however, show the existence of a negative hybrid effect. A theory which sets upper and lower bounds for the hybrid effect is proposed, and the conditions for the occurrence of either a positive or a negative effect are discussed.     

Hygrothermal ageing and fracture of glass fibre-epoxy composites

The effects of hygrothermal ageing upon the failure characteristics and work of fracture of glass fibres in epoxy are described. A collection of data based on the direct observation and measurement of fibre debond length and fibre pull-out length after fracture is displayed in empirical failure diagrams. Similarly, a vast number of experimental measurements of work of fracture is displayed in a three-dimensional diagram where the axes are work of fracture, humidity and ageing time. This information is combined with models of fracture in the construction of a fracture map which is used to interpret hygrothermal ageing phenomena.     

Thermal effects in model specimens under acoustic and convective actions

The temperatures of three model specimens placed in a nonheated air flow under convective and acoustic drying have been measured. It has been shown that in the dry specimens containing no water there occurs an appreciable heating of the internal region of the specimens by the acoustic field. The values of the moistureexchange coefficients of the specimens averaged over the drying period have been found. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 168–173, March–April, 2006.     

铜基复合材料干湿条件下的摩擦学行为

采用粉末冶金技术制备铜基复合材料,在制动压力0. 5～1. 2 MPa 范围内,通过定速摩擦试验机研究了干、湿条件下,速度、压力与材料摩擦磨损性能的关系,结果表明:影响摩擦磨损性能的重要因素在于载荷性质和第三体状态。在干摩擦条件下,处于低摩擦速度范围时,摩擦力的静载荷性质使第三体呈疏松状态,这增加了微凸体间的啮合程度而使摩擦系数处于较高值。随第三体致密性增加,其润滑作用增强,微凸体间的机械啮合程度减弱,使材料的摩擦系数和磨损量降低。在高速摩擦时,微凸体间的冲击作用使处于摩擦表层的硬质颗粒容易发生粉碎性破损而弥散分布,这强化了表面强度而使摩擦系数有所增加。摩擦压力对高速摩擦性能影响明显,原因在于高负荷加剧了摩擦面的变形和损伤程度。湿摩擦条件下,水膜的润滑和流动具有降低摩擦系数和增加磨损率的作用主要体现在低速低压条件下。在高摩擦速度和高压力条件下,水分的高温蒸发与离心作用明显,破坏了水膜的存在条件,从而使材料的摩擦磨损性能与干摩擦状态相近。      

Failure modes of fibre reinforced composites: The effects of strain rate and fibre content

The many aspects of high speed response of fibre reinforced composite materials have received the attention of a large number of investigators. Nevertheless, the understanding of the mechanisms governing failure under high speed loadings remain largely unknown. The effect of rate and fibre content on failure mechanisms was investigated by viewing fractured surfaces of tensile specimens using a scanning electron microscope (SEM). Tensile tests were conducted on a woven glass/epoxy laminate at increasing rates of strain. A second laminate (with random continuous glass reinforcement) was tested in tension at varying fibre volume fractions in order to ascertain the relationship between fibre content and failure mechanisms. The results suggest a brittle tensile failure in fibres of the woven laminate. In addition, the matrix was observed to play a greater role in the failure process as speed was increased, resulting in increased matrix damage and bunch fibre pull-out. The results also indicated that increasing the fibre volume fraction increased the likelihood of a matrix dominated failure mode.