Showing papers in "Mechanics of Time-dependent Materials in 2000"
TL;DR: In this article, the physical mechanisms responsible for stress-strain relations observed under constant, monotonic, and cyclic loading and especially for the role of moisture bonding in the time-dependent behavior are compared.
Abstract: Paper is a complex structure of composite biological fibers.The behavior of paper is time-dependent with respect to load, moisturecontent, or temperature, whether these control parameters are fixed orvaried in combination. A key question is whether the time-dependentproperties are a consequence of the fiber micro-structure, theinterfiber bond, the fiber distribution in a sheet, or a combination ofthese. Hypotheses for the physical mechanisms responsible forstress-strain relations observed under constant, monotonic, and cyclicloading and, especially, for the role of moisture bonding in thetime-dependent behavior are compared. The moisture accelerated creepphenomenon, due to varying ambient relative humidity, is an importanttime-dependent behavior which creates practical problems such as thewall collapse of stacked cartons in warehouses with non-constanthumidity and control of the paper-making machine. For each type ofloading and ambient control variation, after a discussion of possiblephysical mechanisms inducing the observed response, the mathematicalmodels proposed in the literature are reviewed as tools for the designof both the paper-making process and applications.
123 citations
TL;DR: In this article, the effects of fluids on the deformation and durability of polymeric composites are discussed and an overview on the effect of fluid exposure on composites is provided.
Abstract: This article provides an overview on the effects of fluids on the deformation and durability of polymeric composites. This subject is important for the ever-increasing application of composites in offshore structures, submersibles, and civil infrastructure. Since weight-gain data abound in the literature, it may be useful to recognize that a certain degree of correlation exists between some characteristic features of weight-gain data and the ability of a particular composite to retain its integrity under specified exposures to a certain fluid. In addition, this article provides records of fluid effects on deformation as well as on fluid-affected levels of static fatigue and fatigue lives in some specific circumstances.
98 citations
TL;DR: In this article, a simple method for exploiting data obtained from the initial ramp history via a recursion formula is presented. But the method is limited to the case where the loss of relaxation data during that initial time can be a substantial portion of the total (logarithmically measured) time scale.
Abstract: The response of (linearly) viscoelastic material isapproximately the same for ramp and true step strain histories once aninitial time interval of about ten times the ramp-up time has passed.Because the loss of relaxation data during that initial time can be asubstantial portion of the total (logarithmically measured) time scale,a simple method is documented for exploiting data obtained from theinitial ramp history via a recursion formula. Comparison of dataobtained in ramp and relaxation histories are demonstrated to yield goodresults within the accuracy of the test method.
88 citations
TL;DR: In this paper, the authors present an experimental configuration where polymers can be besubjected to large homogeneous deformations, and the design and operation of the confined compression apparatus is described.
Abstract: Inelastic deformation in polymers under large deformation istypically accompanied by instabilities. These instabilities can beassociated with shear banding, barreling, buckling, etc., and canprevent complete characterization of the constitutive behavior. In thispaper, we present an experimental configuration where polymers can besubjected to large homogeneous deformations. The design and operation ofthe confined compression apparatus is described. Using the apparatus, itis shown that the nonlinear pressure-volume response can be extractedfor pressures as high as 1 GPa. The shear response is also extractedfrom the same experimental data; it is shown that for two polymers,polycarbonate and polymethylmethacrylate, the stress drop observed inuniaxial tests may not represent an intrinsic material property and,further, that by applying high confining pressures plastic deformationmay be suppressed completely.
63 citations
TL;DR: In this article, the dynamic response of steel beams treated with various configurations of constrained viscoelastic layers is considered experimentally and analytically, and numerical predictions are obtained.
Abstract: The dynamic response of steel beams treated with variousconfigurations of constrained viscoelastic layers is consideredexperimentally and analytically. The cantilevered sandwich beams aresubjected to spike inputs and large finite rotations. Frequency domainviscoelastic material properties provided by the manufacturer areconverted to the Prony series time domain expression of the shearrelaxation modulus. A finite element model of the cantilevered beams isimplemented in ABAQUS and numerical predictions are obtained. Naturalfrequencies and damping factors obtained from linear and nonlinear, timedomain finite element analyses are found to be in good agreement withexperimental results.
31 citations
TL;DR: In this paper, it is shown that the velocity fields adjacent to surfaces of maximum friction must satisfy sticking conditions, and that the stress boundary condition, the maximum friction law, may be replaced by the velocityboundary condition.
Abstract: Assuming a rigid viscoplastic material model, it is shown thatthe velocity fields adjacent to surfaces of maximum friction mustsatisfy sticking conditions This means that the stress boundarycondition, the maximum friction law, may be replaced by the velocityboundary condition Axisymmetric flows without rotation and planar flowsare considered Applications of the upper bound theorem are discussed
30 citations
TL;DR: In this paper, an experimental study of the influence of physical aging and temperature on the creep deformation of epoxy resin Master curves for various aging conditions were found to collapse into a single smooth curve corresponding to a reference aging condition after suitable horizontal translation.
Abstract: This paper is concerned with an experimental study of the theinfluence of physical aging and temperature on the creep deformation ofepoxy resin Master curves for various aging conditions were found tocollapse into a single smooth curve corresponding to a reference agingcondition after suitable horizontal translation It is proposed that theresulting curve be designated the ‘master-master curve’ and thecorresponding time scale the ‘reduced-reduced time’ In addition, theamount of horizontal translation, the aging parameter, can be used toestimate the aging process We have also shown that there is one-to-onecorrespondence between the aging parameter and polymer density
29 citations
TL;DR: In this article, the applicability of various pseudo stress and strain models for predicting the nonlinear stress-strain behavior of a lightly filled rubber was explored and parameters were extracted on the basis of relaxation and ramp loading experiments in tension.
Abstract: This paper explores the applicability of various pseudo stressand strain models for predicting the nonlinear stress-strain behavior ofa lightly filled rubber. The objectivity of the different models wasexamined and parameters were extracted on the basis of relaxation andramp loading experiments in tension. The effectiveness of the models forpredicting strip biaxial tension and compression and simple shearresponses was examined. A pseudo stress model based on Cauchy stressprovided the best agreement with experimental results. However,improvements are still needed for modeling the unloading behavior in allstress states and the normal stresses that are induced during simpleshear loading.
27 citations
TL;DR: In this article, the authors introduced a new term in the growth law of one state variable, which has no influence on the modelpredictions but reaches full influence when the flow stress is reached.
Abstract: Almost complete rate independence or negative rate sensitivityand an increase in strength with a concurrent decrease in ductilitytogether with jerky flow are reportedly observed when dynamic strainaging occurs in metals and alloys. These phenomena are sometimesreferred to as the ‘Portevin Le Chatelier’ effect. Most engineeringalloys experience dynamic strain aging in some temperature region andnormal behavior (positive rate sensitivity and a decrease in strengthwith an increase in temperature) is observed outside the strain-agingregion. The ‘unified’ viscoplasticity theory based on overstress (VBO),which models positive rate sensitivity, is modified to reproduce thezero and negative rate sensitivity and the increase in strength withtemperature. This is accomplished through the introduction of a singlenew term in the growth law of one state variable. In the initialquasi-elastic region the new term has no influence on the modelpredictions but reaches full influence when the flow stress is reached.During dynamic strain aging, creep and relaxation behaviors arepredicted to be normal. While there are very few experimental resultsavailable, they are all in agreement with these predictions. An analysisof the properties of this new term and numerical experiments illustratethe capability of the extended VBO model.
25 citations
TL;DR: In this article, the static strength and fatigue life of carbon fiber reinforced plastics (CFRPs) was determined by a test system with fan-shaped grip ends to secure the CFRPstrands.
Abstract: This paper is concerned with the determination of time andtemperature dependent uniaxial static-strength and fatigue life ofunidirectional carbon fiber reinforced plastics (CFRPs). A novel testsystem has been designed with fan-shaped grip ends to secure the CFRPstrands. Constant temperature is maintained in a chamber surrounding thecentral portion of the CFRP strand only. Tensile strengths for carbonfiber reinforced by epoxy resin (CF/Ep) strands under constantstrain-rate (CSR) and fatigue loading were measured at variousloading-rates and temperatures. Corresponding experiments were conductedalso by the conventional split-disk method using the CF/Ep ring as thespecimen. Experimental results from these two methods are compared anddiscussed.
20 citations
TL;DR: In this article, a two-parameter fatigue model, aimed at the prediction of the fatigue behavior of composite materials, was applied to fatigue results available in the literature, concerning graphite/epoxy laminatessubjected to tension-compression loadings.
Abstract: A recent two-parameter fatigue model, aimed at the predictionof the fatigue behaviour of composite materials, was applied to fatigueresults available in the literature, concerning graphite/epoxy laminatessubjected to tension-compression loadings. It was found that the model,successfully applied in the case of tension-tension andcompression-compression fatigue, also works when the sign of stress isreversed during fatigue. However, the constants appearing in theproposed formula must be evaluated anew when the loading mode changesfrom pure tension to tension-compression. The probability of failure infatigue is predicted with reasonable accuracy by the model, which isvery conservative in calculating the residual strength after a givennumber of cycles.
TL;DR: In this article, a computer-simulation of the wetting experiment is done to determine the space and time-dependent MC field in the cross-section of the sample. And the authors conclude that the reason for mechano-sorptive effect (MSE) could be the simultaneous action of permanent load stresses and the transient triaxial sorptive stress state.
Abstract: The mechano-sorptive effect (MSE) can be classified as a veryinteresting, but not yet completely explained phenomenon of woodbehaviour. We therefore decided to try to discover its most relevantmechanisms, by conducting two independent series of measurements:accurate measurements of bending deflections on small, clear specimensof spruce wood in changeable climate conditions; and average moisturecontent (MC) in three parts of the cross-section of unloaded dummysamples. The paper emphasises on the sorptive part of MSE. A computersimulation of the wetting experiment is done to determine the space andtime-dependent MC field in the cross-section of the sample. Using aso-called inverse problem identification method, where the equivalenceof the computed and measured responses of the numerical model and realsample is imposed, we identified unknown material properties. Thesolution of the inverse problem enabled us simultaneously to estimatethe moisture diffusion coefficients and to determine the MC field. Twokinds of boundary conditions were used in simulations. To estimate themagnitude of sorption stresses a numerical analysis using Finite ElementMethod (FEM) was done. The sorptive stresses resulting from thenonhomogeneous MC distribution in the sample were obtained. The decisivefactor is the gradient of moisture content and resulting sorptivestresses must be taken into account because their magnitude is of thesame order as the load induced bending stresses. Our experiments and thecalculations following enabled us to conclude that the reason for MSbehaviour could be the simultaneous action of permanent load stressesand the transient triaxial sorptive stress state.
TL;DR: In this article, anon-linear viscoelastic-viscoplastic analysis based on the Schaperyequations and a Zapas-Crissman functional was performed on an unidirectional graphite-epoxy composite.
Abstract: 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.
TL;DR: In this paper, a finite element simulation was performed to quantify the stress evolutions for different composite geometries: a ceramic fiber embedded within a glass matrix, aspherical particle located at the center of a spherical glass matrix and a dispersion of spherical ceramic particles.
Abstract: Opto-electronics make intensive use of composite materialsbased on amorphous materials, which can be considered as smart materialssince they are capable of high performances in their final state.Particularly, glass-ceramic composites involved in welding operationsfor microelectronics applications are subjected to important thermalstresses during their production, which can deteriorate their propertiesat room temperature, until the failure stage is reached. It is thenessential to be able to predict the evolution of the internal stressesgenerated during the cooling. We have performed finite-elementsimulations in order to quantify the stress evolutions for differentcomposite geometries: a ceramic fiber embedded within a glass matrix, aspherical particle located at the center of a spherical glass matrix,and a dispersion of spherical ceramic particles, this last case beingthe most representative of reality. The thermomechanical modeling of theglassy matrix takes into account its viscoelastic behavior, and theglass transition is described by the decrease during cooling of the freevolume as a function of the temperature history. The combined effect ofthe differential thermal strain during the transition and mechanicalrelaxation of glass on stress evolutions is evidenced. It is shown thatthe consideration of a periodical or random distribution of sphericalceramic particles leads to similar stress profiles.
TL;DR: In this article, a viscoelastic lamination theory is developed assuming that the lamina stiffness matrix is fully complex and frequency dependent, which is not the case for composite laminates.
Abstract: The present work deals with the determination of theviscoelastic properties of composite laminates based on the viscoelasticbehaviour of the single lamina. A viscoelastic lamination theory isdeveloped assuming that the lamina stiffness matrix is fully complex andfrequency dependent.
TL;DR: In this article, the authors investigated the time-dependent failure behavior of transversely loaded composites under various multi-axial loading conditions and provided a good description of the multiaxial yielding behavior of the matrix material with the three-dimensional pressure modified Eyring equation.
Abstract: The time-dependent failure behaviour of transversely loaded composites is investigated, assuming that fracture is matrix dominated. Since the stress and strain state of the matrix in composite structures is complex, the yield and fracture behaviour of a neat epoxy system is investigated under various multi-axial loading conditions. A good description of the multi-axial yielding behaviour of the matrix material is obtained with the three-dimensional pressure modified Eyring equation. The parameters of this three-dimensional yield expression are implemented into a constitutive model, which has been shown to correctly describe the deformation behaviour of polymers under complex loadings. By means of a micromechanical approach, the matrix dominated transverse strength of a unidirectional composite material was investigated. Numerical simulations show that a failure criterion based on maximum strain provides a good description for the rate-dependent transverse strength of unidirectional glass/epoxy composites. Furthermore, such a strain criterion is also able to describe the durability (creep) of transversely loaded unidirectional composites.
TL;DR: In this article, the authors studied the growth of styrene-acrylonitrile copolymer (SAN) as a function of time after a quench (physical aging) at four different temperatures between 22 and 60°C.
Abstract: Craze growth in a styrene-acrylonitrile copolymer (SAN) wasstudied as a function of time after a quench (physical aging) at fourdifferent temperatures between 22 and 60°C. Thecraze growth experiments were performed in stress relaxation conditions.The craze length was found to grow linearly on a logarithmic loadingtime scale. A transition in the logarithmic growth rate was found tooccur on the logarithmic aging time scale. The transition was observedas a change in rate from high to approximately five times slower andoccurred over a relatively narrow range of aging times. The growth ratein the slow growth regime was relatively insensitive to temperature forall four temperatures studied. The transition was found to move toshorter aging times as temperature increases. Although the craze lengthbetween crazes in a sample could differ to a large extent, the growthrates themselves, were found to vary only within a±20% band.
TL;DR: In this paper, an experimental study of slow crack growth behavior of post-consumer recycled high-density polyethylene blended with virgin high density polyethylen copolymer has been done.
Abstract: An experimental study of slow crack growth behavior ofpost-consumer recycled high-density polyethylene blended with virginhigh-density polyethylene copolymer has been done. The study has beenperformed under constant load and in baths of distilled water at 40, 60,80°C. The specimen used is notched with sidegrooves. The test results of crack growth have been analyzed usinglinear fracture mechanics and the rate process theory. The results showthat the resistance to crack growth increases monotonically with theamount of virgin material. The activation energy of the slow crackpropagation is independent of the blend composition and agrees with theenergy required by a known mechanism of pull out of tie molecules fromcrystalline lamellae.
TL;DR: In this paper, life time and failure modes for metallic barssustaining tensile creep are predicted for a single-layer tensile material. And the analysis is based on amodeling of void nucleation and growth in which damage evolution is controlled by two mechanisms of plastic flow in the matrix material.
Abstract: Life time and failure modes are predicted for metallic barssustaining tensile creep. Experimental results show that a ductile or a`brittle' mode of fracture occurs depending respectively on whether thenominal applied stress is large or small. The analysis is based on amodeling of void nucleation and growth in which damage evolution iscontrolled by two mechanisms of plastic flow in the matrix material.Fracture is supposed to occur when the porosity attains a critical valuewhich depends on the mode of fracture considered. Experimental resultsare explained and described in terms of the proposed model.
TL;DR: In this paper, an experimental and a numerical model for the analysis of not only the viscoelastic stress and strain, but also alsodeformations, taking into consideration the dry friction is presented.
Abstract: Moving contact problems in a viscoelastic body with a rigidindentor are often seen in an industrial field. An evaluation of thetime-temperature-dependent stress and strain behavior around a contactregion is required in order to make clear the fundamental mechanism ofthe local fracture and wear on the contact surface of the viscoelasticbody under moving loads with dry friction. No analyses have yet beenpublished about the stress/strain of the viscoelastic moving contactproblem with the dry friction using both an experimental and a numericalmethod. The authors discuss an experimental and a numerical model forthe analysis of not only the viscoelastic stress and strain, but alsodeformations, taking into consideration the dry friction. Animage-processing-aided photoviscoelastic technique is applied foranalyzing the principal stress and strain behavior near the contactregion. Also, a two-dimensional plane stress model which consists of aviscoelastic strip and a rigid sliding cylinder is adopted in a finiteelement analysis of the same problem. The time-dependent stress andstrain and the coefficient of dry friction are successfully evaluated byexperimental and numerical methods.