Creep Of Composites Research Articles

Numerical modelling of the creep behaviour of GFRP sandwich panels using the Carrera Unified Formulation and Composite Creep Modelling

Sandwich panels often need to be designed to sustain significant permanent loads, raising the need to accurately account for long-term creep deformation. However, the accurate prediction of creep in sandwich panels is not trivial, particularly owing to their composite multi-layered nature, the multitude of possible face and core material combinations, and the possible existence of through-thickness shear reinforcement (ribs/webs). This paper presents numerical investigations on the creep behaviour of composite sandwich panels produced by vacuum infusion with glass-fibre reinforced polymer (GFRP) faces and ribs, and polyurethane (PUR) and polyethylene terephthalate (PET) foam cores. Carrera Unified Formulation (CUF) is implemented, for the first time using 1D elements with an equivalent single layer (ESL) methodology, to model the creep response of simple and ribbed panels by adopting a Composite Creep Modelling (CCM) approach. Previous experimental results from creep tests carried out on such sandwich panels and their constituent materials are used to obtain time-dependent constitutive relations for the materials in various layers and validate the numerical results. Additionally, results from analytical beam models using Timoshenko beam theory (TBT) with multi-layered sections are used to further validate the numerical outputs obtained with CUF. The developed numerical models were able to predict the experimental creep behaviour of the full-scale sandwich panels with reasonable accuracy. Differences observed between the CUF and TBT models mainly stem from the inherent approximations concerning the shear correction factors used with TBT, which contrast with the solutions provided by CUF, where such factors do not need to be considered when higher degrees of approximation are used.

Time-dependent and energy dissipation effects on the electro-mechanical response of PZTs

In this study, we formulate a constitutive model for coupled thermo-electro-mechanical behavior of piezoelectric ceramics, i.e. lead zirconate titanate (PZT) that takes into account time-dependent behavior and heat generation due to energy dissipation from the electro-mechanical response. Experimental studies show that PZTs dissipate energy when subjected to cyclic electric fields. The dissipated energy is, in some part, converted into heat and raises the temperature of the PZTs. The hysteretic dielectric and strain responses are also dependent on the amplitude and frequency of the applied electric field. The aim of this study is to investigate the energy dissipation and time-dependent effects on the electro-mechanical responses of polarized PZTs. The thermodynamics of irreversible processes for modeling dissipative material responses is adopted to incorporate the time-dependent electromechanical coupling effect. A Gibbs free energy is defined in order to account for the different time-dependent and dissipation behaviors in piezoelectric materials under combined electro-mechanical stimuli. Experimental data on polarized PZTs and PZT based composites at applied electric field magnitudes lower than the coercive electric field, are used to examine the model. The constitutive equations along with the energy equations are solved numerically and used to predict the creep, hysteretic and heat generation responses of PZTs and composites at different frequencies and cyclic electric fields.

Creep of Sandwich Panels with Longitudinal Reinforcement Ribs for Civil Engineering Applications: Experiments and Composite Creep Modeling

AbstractComposite sandwich panels with reinforcement ribs or webs are being increasingly considered for structural applications in civil engineering. Such panels are prone to creep when subjected to significant permanent loads and therefore the effects of this phenomenon must be duly accounted for in their design. Data regarding the viscoelasticity of sandwich panels and their constituent materials is still scarce and often cannot be directly used for creep predictions in design practice. To address this issue, this paper presents the experimental assessment and the analytical modeling of the viscoelastic response of two types of sandwich panels, with and without reinforcement ribs. Panels comprising glass fiber–reinforced polymer (GFRP) faces, cores of polyurethane (PU) foam, and longitudinal GFRP ribs are considered. The ribs increased the flexural strength and stiffness of the panels by a factor of two, while providing a threefold reduction in their creep compliance. A composite creep modeling (CCM) ap...

Elastic and viscoelastic behaviour of sandwich panels with glass-fibre reinforced polymer faces and polyethylene terephthalate foam core

This paper presents experimental and analytical investigations about the elastic and viscoelastic (creep) behaviour of sandwich panels made of glass-fibre reinforced polymer faces and a polyethylene terephthalate foam core, produced by vacuum infusion for civil engineering structural applications. First, the elastic response of the panels’ constituent materials (glass-fibre reinforced polymer and polyethylene terephthalate) in tension, compression and shear was experimentally assessed; shear tests on the foam were carried out using a novel test method, the diagonal tension shear test. The creep behaviour in shear of the polyethylene terephthalate foam was evaluated for different load levels. The effective flexural properties of the full-scale sandwich panels as well as their flexural behaviour up to failure were experimentally assessed. Flexural creep and subsequent recovery experiments were also conducted for different load levels, to characterise the viscoelastic behaviour of the full-scale sandwich panels. Creep deformations of the polyethylene terephthalate foam and of the sandwich panels were found to be significantly lower than those corresponding to polyurethane foam and balsa wood reported in the literature; unrecoverable viscoelastic deformations were observed in the full-scale panels. In the analytical study, the creep response of the panels was modelled using Findley’s power law and the composite creep modelling approach. The composite creep modelling predictions were reasonably accurate and allowed assessing the relative contributions of bending and shear deformations to the total sandwich panel creep deflections.

Circular Functions Based Comprehensive Analysis of Plastic Creep Deformations in the Fiber Reinforced Composites

Analytically based model is presented for behavioral analysis of the plastic deformations in the reinforced materials using the circular (trigonometric) functions. The analytical method is proposed to predict creep behavior of the fibrous composites based on basic and constitutive equations under a tensile axial stress. New insight of the work is to predict some important behaviors of the creeping matrix. In the present model, the prediction of the behaviors is simpler than the available methods. Principal creep strain rate behaviors are very noteworthy for designing the fibrous composites in the creeping composites. Analysis of the mentioned parameter behavior in the reinforced materials is necessary to analyze failure, fracture, and fatigue studies in the creep of the short fiber composites. Shuttles, spaceships, turbine blades and discs, and nozzle guide vanes are commonly subjected to the creep effects. Also, predicting the creep behavior is significant to design the optoelectronic and photonic advanced composites with optical fibers. As a result, the uniform behavior with constant gradient is seen in the principal creep strain rate behavior, and also creep rupture may happen at the fiber end. Finally, good agreements are found through comparing the obtained analytical and FEM results.

Creep-fatigue behaviour of aluminum alloy-based metal matrix composite

Metal Matrix Composite (MMC) represents a valuable option as structural material for different type of structures and components. Despite this they struggle to become widely adopted due to expensive manufacturing process and complex microstructural behaviour. When subjected to cyclic load conditions the structural response of MMC is not trivial, and becomes even more difficult when high temperature load is involved. Different failure mechanisms would happen and they are originated by the different material properties between the fibre and surrounding matrix. Among all, the mismatch of thermal expansion coefficient is recognized to be the dominant one. The significantly differing coefficients of thermal expansion between ceramic and metal give rise to micro thermal stresses, which enhance the initiation of matrix micro cracks. Their performance under varying load and high temperature is complex, and hence it is difficult to have a clear understanding of the structural responses, especially when fatigue and creep damages become the main failures of MMCs. To improve current understanding of the relationship between creep fatigue interaction of MMCs, the history of thermal and mechanical loading, and the creep dwell period, a highly accurate but robust direct simulation technique on the basis of the Linear Matching Method (LMM) framework has been proposed in this paper, and been applied to model the fatigue and creep behaviour of MMCs. A homogenised FE model is considered in all analyses, which consist of continuous silicon carbide fibres embedded in a square 2024T3 aluminium alloy matrix array. Various factors that affect creep and fatigue behaviours of composites are analysed and discussed, including effects of the applied load level, dwell period and temperature on the MMC's performance. The effects of reversed plasticity on stress relaxation and creep deformation of MMC are investigated, and the behaviours of cyclically enhanced creep and elastic follow-up are presented. A detailed study of the creep ratchetting mechanism is also performed with the concentration on the impact of temperature and different loading conditions. The accuracy of the proposed method has been verified by detailed incremental finite element analyses using the commercial finite element solver Abaqus. Such verifications further improve the understanding of the failure mechanisms identified and discussed in this work.

Effect of Concrete Age and Creep on the Behavior of Concrete-Filled Steel Tube Columns

The influence of concrete age and creep on the ultimate axial loading capacity of concrete-filled steel tube (CFST) columns is experimentally and numerically investigated. After validation of numerical models, a parametric study is conducted and the results are used to formulate empirical formulas for predicting the ultimate axial load-bearing capacity of the columns. Formulas are also proposed for predicting both the composite creep and aging coefficients of the CFST columns, which consider the confinement action of steel tubes on concrete. Then, the proposed formulas are validated independently by comparing their predictions with existing test results performed by other researchers. The comparisons show that the empirical formulas have the potential to be used in the practical design of CFST columns.

Calculation of Constrained Stress in Expansive Mortar with a Composite Creep Model

The creep phenomenon of hardening cement paste mixed with an expansive additive was modeled by considering the creep performance of hydration products of cement and expansive additive. A new composite model that is appropriate for particle conditions is proposed by considering the balance of the hydration products of cement and expansive additive and the stress redistribution phenomenon of hydration products newly generated by the progress of hydration. The creep of mortar and concrete mixed with the expansive additive was evaluated using a composite model of the paste and aggregate. Under the assumption that the modeled creep deformation is proportional to the stress and the gel volume of the hydration products, which allows the law of superposition to be applied, the distribution stress was predicted by applying the step-by-step method at each time increment. By predicting the maximum tensile stress applied to an inner steel ring through a creep analysis based on the measured deformation of the inner steel ring, it is possible to predict the stress progression with age to some degree.

High-temperature plastic deformation of spark plasma sintered boron carbide-based composites: The case study of B4C–SiC with/without graphite (g)

B4C-based composites are considered to be a practical option in order to overcome the major limitation of pure boron carbide ceramics, their poor sinterability, without a major degradation of mechanical properties. This work is focused on the analysis and comparison of the high-temperature creep of silicon carbide-reinforced B4C composites sintered with or without graphite. The mechanical tests, together with microstructural characterization, will allow clarifying the micromechanisms into play and will give insight into the capability of these composites to optimise the mechanical resistance of these materials. The results have shown that the presence of graphite plays a key role on the plasticity of B4C-based composites. In the absence of graphite, silicon carbide-reinforced B4C composites have similar plastic behaviour as pure boron carbide polycrystalline ceramics.

Creep relaxation and fully reversible creep of foam core sandwich composites in seawater

AbstractFoam core sandwich composites were subjected to (i) creep to failure, (ii) cyclic creep-relaxation and (iii) fully reversible cyclic creep loading in seawater in order to mimic an actual ship hull’s service lifetime scenario. The results indicate a strong dependence of lifetime on the mode of loading. A significant reduction in the overall life was observed under cyclic creep as compared with the conventional creep to failure. Creep relaxation (R=1) tests were performed at loading-relaxation periods of 24/24, 24/12, 24/6, 12/12 and 6/6 h, while the fully reversible (R=-1) creep tests were conducted at loading-reversed loading times of 36/36, 24/24, 12/12, 6/6, and 3/3 h. The results suggest that creep-relaxation lifetime characteristics depend predominantly on the relaxation time as opposed to loading times, i.e. longer relaxation periods lead to shorter life. Whereas, fully reversible creep appears to be dependent upon the number of reversals whereby, life is observed to reduce as the number of reversals increase. These significant observations are explained in terms of various possible paths to interface cell wall collapse. Modes of failure were predominantly indentation and core compression in the vicinity of the loading site.

How interface size, density, and viscosity affect creep and relaxation functions of matrix-interface composites: a micromechanical study

Matrix-inclusion composites are known to exhibit interaction among the inclusions. When it comes to the special case of inclusions in form of flat interfaces, interaction among interfaces would be clearly expected, but the two-dimensional nature of interfaces implies quite surprising interaction properties. This is the motivation to analyze how interaction among two different classes of microscopic interfaces manifests itself in macroscopic creep and relaxation functions of matrix-interface composites. To this end, we analyze composites made of a linear elastic solid matrix hosting parallel interfaces, and we consider that creep and relaxation of such composites result from micro-sliding within adsorbed fluid layers filling the interfaces. The latter idea was recently elaborated in the framework of continuum micromechanics, exploiting eigenstress homogenization schemes, see Shahidi et al. (Eur J Mech A Solids 45:41–58, 2014). After a rather simple mathematical exercise, it becomes obvious that creep functions do not reflect any interface interaction. Mathematical derivation of relaxation functions, however, turns out to be much more challenging because of pronounced interface interaction. Based on a careful selection of solution methods, including Laplace transforms and the method of non-dimensionalization, we analytically derive a closed-form expression of the relaxation functions, which provides the sought insight into interface interaction. The seeming paradox that no interface interaction can be identified from creep functions, while interface interaction manifests itself very clearly in the relaxation functions of matrix-interface materials, is finally resolved based on stress and strain average rules for interfaced composites. They clarify that uniform stress boundary conditions lead to a direct external control of average stress and strain states in the solid matrix, and this prevents interaction among interfaces. Under uniform strain boundary conditions, in turn, interfacial dislocations do influence the average stress and strain states in the solid matrix, and this results in pronounced interface interaction.

Rate dependent plastic deformation analysis of creeping short fiber composites using the virtual fiber method in the non-reinforced regions

In this research paper, analysis of time-dependent plastic deformation in non-reinforced regions of creeping short fiber composites is carried out under axial tensile stress using the virtual fiber method. As an important application of the present method, shuttles and spaceships, turbine blades and discs are usually subjected to the creep effects. So, analysis of the creep phenomenon is necessary and important in various industries. The nature of the stress distributions is investigated using the equilibrium and constitutive equations, compatibility relations, and incompressibility condition with various boundary conditions, and geometric relations. Analytical and finite element methods are used to predict the stress distributions in the steady state creep of the short fiber composites. Also, the creep analysis (nodal solution) is done by FEM to predict the possible creep rupture, in which, debonding at the interface of the matrix and fiber may happen due to the maximum tensile axial stress values at the interface. In addition to similarities of the present method and experimental results [34], a good agreement of the stress distributions between the present analytical solutions and the FEM results is found. As a result, the short fiber composites with large non-reinforced regions in the axial direction are proper because it increases the factor of safety for design.

A micromechanical creep model for stress analysis of non-reinforced regions of short fiber composites using imaginary fiber technique

Stress behavior in short fiber composites under axial tensile stress is predicted based on well-behaved displacement rates in the steady state creep using imaginary fiber technique. This analysis is performed without using shear-lag model and other complex theories in non-reinforced regions of the short fiber composites. Direct analytical method (DAM) is presented to obtain the composite creep strain rate and stress behavior. Because of many applications of silicon carbide fiber SiC/Al6061 composites, stress analysis of this composite is done. Good agreements are found between the obtained present analytical and finite element method (FEM) results. Some important applications of the present comprehensive method are in the fields of the safe composite design and control of creeping composites in order to prevent the creep rupture. Also, present method is simple and accurate, unlike costly, difficult and time-consuming experimental methods.

On the Creep Fatigue Behavior of Metal Matrix Composites

The mechanical behaviour of Metal Matrix Composites (MMCs) subjected to a high temperature and cyclic load condition is difficult to understand. The significantly differing coefficients of thermal expansion between ceramic and metal give rise to micro thermal stresses. Their performance under varying load and high temperature is complex and inconsistent, where fatigue and creep damages become the main failures of MMCs. To improve current understanding of the relationship between creep fatigue interaction of MMCs, the history of thermal and mechanical loading, and the creep dwell period, a highly accurate but robust direct simulation technique on the basis of the Linear Matching Method (LMM) framework has been proposed in this paper, and been applied to model the fatigue and creep behaviour of MMCs. A homogenised FE model is considered in all analyses, which consist of continuous silicon carbide fibres embedded in a square 2024T3 aluminium alloy matrix array. Various factors that affect creep and fatigue behaviours of composites are analysed and discussed, including effects of the applied load level, dwell period and temperature on the MMC's performance. The effects of reversed plasticity on stress relaxation and creep deformation of MMC are investigated, and the behaviours of cyclically enhanced creep and elastic follow-up are presented. The applicability and accuracy of the proposed direct method has also been verified by the detailed step-by-step analysis via Abaqus.

Temperature dependence of creep and recovery behaviors of polymer composites filled with chemically reduced graphene oxide

Resistance to time-dependent plastic deformation of polymer composite materials is a crucial requirement in their application for long-term durability and reliability. Herein, creep and recovery behaviors of polystyrene (PS) composites filled with various loadings of chemically reduced graphene oxide (CRGO) were investigated at different environment temperatures. As expected, incorporation of CRGO into PS polymer increases the thermal stability, glass transition temperature and elastic modulus, although the tensile strength of the composite has a slight decrease. It was found that the creep deformation and strain rate of PS polymer reduce with decreasing temperature and with increasing loading of CRGO. A significant improvement in the recovered strain of PS was also obtained after the presence of CRGO. Based on the analytical modelings (Burger’s model and Weibull distribution function) and experimental results, the role of CRGO on improving the creep and recovery performance of thermoplastics was proposed and discussed.

A displacement based model to determine the steady state creep strain rate of short fiber composites

Abstract A novel analytical method is developed for predicting steady state creep of short fiber composites using shear-lag theory, imaginary fiber technique and polynomial displacement functions. Also, this method employs equilibrium and constitutive equations. Polynomial displacement method (PDM) is a new insight for analysis of plasticity and elasticity problems which can be used as a simple, exact and general method. PDM is more accurate than the available methods. Important novelties of the PDM are determination of unknowns such as shear stresses and displacement rates on top of the fiber analytically. In this paper, all unknowns are obtained by well-behaved polynomial displacement functions. These functions satisfy incompressibility and boundary conditions. In spite of the previous researches, strain rates and stresses are analytically obtained by PDM without non-analytical assumptions. Also, suitable agreements are found among present analytical method, numerical (FEM) and available published results.

Creep Behavior Study for Carbon Fiber Nano-Composites

The surface modification of carbon nanotubes (CNTs) has been recently observed to influence the distribution of CNTs in epoxy resin and the mechanical properties and electrical conductivities of these CNTs. Accordingly, the treatment of CNTs to with organic acids to oxidize them generates functional groups on the surface of CNTs. This investigation studies the consequent enhancement of the mechanical properties and electrical conductivities of CNTs. The influence of adding various proportions of CNTs to the epoxy resin on the mechanical properties and electrical conductivities of the composites thus formed is investigated, and the strength of the material is tested at different temperatures.The test results also indicate that mechanical strength and electrical conductivity increase with the amount of CNTs added to the composites. Different coefficients of expansion of the matrix, fiber and CNTs, are such that overexpansion of the matrix at high temperature results in cracking in it.Moreover, the creep behaviors of carbon fiber (CF) /epoxy resin thermosetting composites and CNTs/CF/ epoxy resin composites were tested and analyzed at different stresses, orientations of fiber, temperatures and humidities. The creep exhibits only two stages-primary creep and steady-state creep. The effects of creep stress, creep time, and humidity on the creep of composites that contain various proportion of CNTs were investigated at various temperatures.Additionally, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain even at a high temperature of 55°C. Possible room temperature creep mechanisms have been proposed and discussed. With increasing number of creep tests, the creep strain decreased due to strain hardening which occurred during creep. Creep strain is believed to increase with applied stress, creep time, humidity, temperature and degree of the angle θ between the orientation of fiber and the direction of the applied stress.Finally, the test results of creep strain of CF/epoxy resin composites and CNTs/CF/epoxy resin composites tested under various conditions can be smoothly fitted by the fitting curves of Findley power law.

Time–temperature and stress dependent behaviors of composites made from recycled polypropylene and rubberwood flour

The effects of time, temperature, and stress on the flexural creep of composites from recycled polypropylene (rPP) and rubberwood flour (RWF) were experimentally investigated and numerically modeled. Creep of rPP/RWF composites increased with an increase of time, temperature, and stress. A critical temperature of rPP composites containing 44.5wt% RWF is 65°C. Burger, Power law, and HRZ models fit the creep profiles well in general, but at high temperature and stress levels the Power law and HRZ models performed poorly. However, the HRZ model interpolated creep well across the applied stresses, or across the temperatures. The time–temperature superposition (TTS) and time–stress superposition (TSS) principles were used to model long-term creep. The master curves from TTS and TSS principles were in good agreement with each other. They predicted that the lifetime limitation by long-term creep exceeds 10years for 15MPa stress at 25°C. All these results pertain to a specific formulation of rPP/RWF composites.

Effects of fiber size on short-term creep behavior of wood fiber/HDPE composites

When natural fiber-thermoplastic composites are used in long-term loading applications, investigating creep behavior is essential. The creep behavior of high-density polyethylene (HDPE)-based composites reinforced with four sizes of wood fibers (WFs) (120–80, 80–40, 40–20, and 20–10 mesh) was investigated. The instantaneous deformation and creep strain of all WF/HDPE composites increased at a fixed loading level when the temperature was increased incrementally from 25 to 85°C. At a constant loading level, composites containing the larger-sized WFs had better creep resistance than those containing smaller-sized fibers at all measured temperatures. The creep properties of composites with smaller-sized WFs were more temperature-dependent than those with larger-sized WFs. Two creep models (Burger's model and Findley's power law model) were used to fit the measured creep data. A time–temperature superposition principle calculation was attempted for long-term creep prediction. The Findley model fitted the composite creep curves better than the four-element Burger's model. From the predicted creep response of the WF/HDPE composites, two groups of small fibers (120–80 and 80–40 mesh) had the lowest creep resistance over long periods of time at the reference temperature of 25°C. The largest WFs (10–20 mesh) provided the best composite creep resistance. POLYM. ENG. SCI., 55:693–700, 2015. © 2014 Society of Plastics Engineers

Creep behavior of AS41 alloy matrix nano-composites

Nano-alumina reinforced AS41 magnesium alloy matrix composites with 2wt% and 5wt% Al2O3 were fabricated by combined stir casting and ultrasonic processing in order to distribute the nano-alumina particles uniformly in the melt. Indentation creep tests were performed on the nano-composites and the as cast AS41 alloy at four different temperatures of 448K, 473K, 498K, and 523K and three different stresses of 109.2MPa, 124.8MPa and 140.4MPa. The creep curves obtained in these tests show that creep resistance increased with the addition of nano-Al2O3 into the magnesium alloy, and nano-composites with 5wt% Al2O3 exhibited highest creep resistance. The values of obtained stress exponent range from 3 to 3.5, 3.55 to 6, and 4 to 6.5 for the as-cast AS41 alloy, and AS41/2wt% Al2O3, and AS41/5wt% Al2O3 composites, respectively. Thus, dislocation creep is the dominant creep mechanism. The calculated activation energies for as-cast AS41, AS41/2wt% Al2O3 and AS41/5wt% Al2O3 are approximately 66.17kJ/mol, 83.77kJ/mol, and 88.3kJ/mol, respectively. The activation energy values for the creep of composites are close to that of the pipe diffusion (92kJ/mol) of magnesium.