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The aim of this paper is to present some important practical cases in the analysis of the creep response of unidirectional fiber-reinforced composites. Some of the currently used models are described: the micromechanical model, homogenization technics, the Mori–Tanaka method, and the finite element method (FEM). Each method was analyzed to determine its advantages and disadvantages. Regarding the accuracy of the obtained results, comparisons are made with experimental tests. The methods presented here are applied to carbon-fiber-reinforced composites, but these considerations can also be applied to other types of composite materials. [ABSTRACT FROM AUTHOR]

Polymeric materials have been shown to be rate-dependent materials, that is, their response will vary depending on the conditions to which they are subjected. The present work details the formulation, validation and implementation of a viscoplastic constitutive model with stress, strain, temperature, and relative humidity dependencies aimed to simulate the long-term response of polymeric materials, particularly that of polyester. The model is capable of predicting primary and secondary creep, often observed in geosynthetic materials. Both creep mechanisms can be modelled independently if needed. For calibration, a wide data-set of polyester strap reinforcement creep measurements was used. The validation process was done using parameters for load-, product-, and material-specific scenarios. Load- and product-specific scenarios showed suitable agreement between simulated and measured data. The coupled capabilities of the model are shown via variable temperature and relative humidity boundary conditions. Due to lack of data, temperature and relative humidity dependencies represent idealized scenarios. Simulations of stress-relaxation response for constant rate of strain scenarios are also provided. The proposed formulation is aimed at modelling the mechanical response of reinforced soil structures while accounting for the effect of in-air or in-soil conditions to which reinforcement materials can be exposed to throughout the structure's life-cycle. [ABSTRACT FROM AUTHOR]

Background: A pressure ulcer is common in soft tissue over the greater trochanter (GT) in side-lying position, and sustained tissue deformation induced by the prolonged external force is a primary cause, which can be discussed with soft tissues’ viscoelastic properties (i.e., stress relaxation, creep response). Objects: Using an automated hand-held indentation device, we measured the viscoelastic properties of soft tissue over the hip area, in order to examine how the properties are affected by site with respect to the GT. Methods: Twenty participants (15 males and 5 females) who aged from 21 to 32 were participated. An automated hand-held indentation device was used to measure the stress relaxation time and creep response. Trials were acquired for three different locations with respect to the GT (i.e., right over the GT, 6 cm anterior or posterior to the GT). For each location, five trials were acquired and averaged for data analyses. Results: Soft tissues’ stress relaxation time and creep response were associated with site (F = 23.98, p < 0.005; F = 24.09, p < 0.005; respectively). The stress relaxation time was greatest at posterior gluteal region (19.22 ± 2.49 ms), and followed by anterior region (15.39 ± 2.47 ms) and right over the GT (14.40 ± 3.18 ms). Similarly, creep response was greatest at posterior gluteal region (1.16 ± 0.14), and followed by anterior region (0.95 ± 0.14) and right over the GT (0.89 ± 0.18). Conclusion: Our results showed that the stress relaxation and creep were greatest at the posterior gluteal region and least at right over the GT, indicating that the gluteal soft tissue is more protective to the prolonged external force, when compared to the trochanteric soft tissue. The results suggest that a risk of pressure ulcer over the GT may decrease with slightly posteriorly rotated side-lying position.

The aim of this paper is to present some important practical cases in the analysis of the creep response of unidirectional fiber-reinforced composites. Some of the currently used models are described: the micromechanical model, homogenization technics, the Mori–Tanaka method, and the finite element method (FEM). Each method was analyzed to determine its advantages and disadvantages. Regarding the accuracy of the obtained results, comparisons are made with experimental tests. The methods presented here are applied to carbon-fiber-reinforced composites, but these considerations can also be applied to other types of composite materials.

Creep is eminent in fiber reinforced polymer matrix composite materials given the susceptibility of polymers to constant loading. The response of polymer matrix composites used in retrofitting vital infrastructure to long‐term flexural loadings is investigated to elucidate the effect of creep. The experimental approach consisted of first establishing a benchmark of the quasi‐statically tested flexural properties. A fresh set of samples were then flexural tested for an extended period. Failure was determined to be strain‐driven in both quasi‐static and long‐term testing conditions. The mechanical properties were found to be insensitive to the strain rate (in the low strain regime), while being mildly sensitive to creep loading. The flexural response also showed dependence on the historic creep loading. The creep strain was successfully fitted into Baily‐Norton power law, which allowed for forecasting the creep life based on the critical strain failure criteria. The overall results explicate the interrelation between long‐term loading and fidelity of composites‐based infrastructures. [ABSTRACT FROM AUTHOR]

Breast cancer is the most frequent female malignancy in the world. In this regard, cancer detection by assessing the biomechanical properties of cells is a promising method in oncology. Cell state can be identified by studying viscosity behavior; however, a more complex understanding of cells requires a profound insight into the solidity and fluidity of cells via the characterization of cell viscoelasticity. The present study aimed to compare the viscoelasticity of healthy human breast epithelial cells (MCF-10A) with that of cancerous cells (MCF 7). The experiment included the addition of nano magnetic particles (NMP) to the cell culture environment and placement of the Petri Dishes under a microscope after the completion of primary culture stages and, ultimately, adoption of a magnetic tweezer technique to perform a creep test. A viscoelastic model of cells was suggested with discrete differential equations for both groups of healthy and cancerous cells after obtaining information about cell membrane movements and performing image processes on these data. A comparison of cell stiffness was made under two conditions of static and dynamic. According to the findings, cancerous static stiffness was lower than that of healthy cells by a factor of 3.5. The creep test results showed that MCF 7 cells would exhibit solid-like behavior. At a higher gel point frequency, these cells emerged more solidity compared to their corresponding healthy cells. The obtained results revealed the clear changes in cancerous cells' viscoelastic properties and the potential alterations of their cytoskeleton. • The cancerous cells showed higher resistance to deformation. • Decreased MCF-7 stiffness was observed under static loading processes. • Decreased MCF-7 stiffness was observed under dynamic test. • The creep test in this study resulted in a solid-like behavior of MCF-7 cells. • MCF-7 has higher gel point frequency, these cells emerged more solidity compared to their corresponding healthy cells. [ABSTRACT FROM AUTHOR]

The homogenization theory, used for the study of differential equations with periodic coefficients, with a rapid variation, is used in the paper for the analysis of the creep phenomenon of composite materials, reinforced with fibers. Generally, a polymer composite having a matrix with a viscoelastic response manifests a creep behavior. A good knowledge of mechanical constants allows us to predict the time response under the action of a load, which is important in engineering. The homogenization method is used to determine the engineering constants for a composite reinforced with carbon fibers. The method is applied for the particular case of fiber-reinforced unidirectional composites to obtain the equations that finally offer the required values. The epoxy matrix Fibredux 6376C is reinforced with carbon fibers T800 and the thermoplastic specimens made by APC2 material is reinforced with carbon fibers of the type IM6. The experimental results give a good concordance with the theoretical predictions.

Usually, a polymer composite with a viscoelastic response matrix has a creep behavior. To predict this phenomenon, a good knowledge of the properties and mechanical constants of the material becomes important. Schapery’s equation represents a basic relation to study the nonlinear viscoelastic creep behavior of composite reinforced with carbon fiber (matrix made by polyethrtethrtketone (PEEK) and epoxy resin). The finite element method (FEM) is a classic, well known and powerful tool to determine the overall engineering constants. The method is applied to a fiber one-directional composite for two different applications: carbon fibers T800 reinforcing an epoxy matrix Fibredux 6376C and carbon fibers of the type IM6 reinforcing a thermoplastic material APC2. More cases have been considered. The experimental results provide a validation of the proposed method and a good agreement between theoretical and experimental results.

A study of the mechanical response of bacteria is essential in designing an antibacterial surface for implants and food packaging applications. This research evaluated the mechanical response of Escherichia coli under different loading conditions. Indentation and prolonged creep tests were performed to understand their viscoelastic-plastic response. The results indicate that varying loading rates from 1 μm/s to 5 μm/s show an increase in modulus of 182% and 90%, calculated in the loading and unloading cycles, respectively, and a decrease in adhesion force by 42%. However, on varying loads from 5 nN to 25 nN, nominal change is observed in both modulus and adhesion force. The rupture curve at 100 nN load shows elastic and a small plastic deformation accompanied by a sharp peak indicating the cell wall rupture. The rupture force at the peak was found to be 34.38 ± 5.15 nN, irrespective of the loading rate, making it a failure criterion for bacteria rupture. The creep response of bacteria increases (for 6 s) and then remains constant (for 15 s) with time, indicating that a standard linear solid (SLS) model applies to this behavior. This work attempts to evaluate the mechanical properties of E. coli bacteria focusing on its rupture by contact killing mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

Effects of silica nanoparticle aggregation on the creep behavior of polymer nanocomposites are analyzed. For this purpose, a hierarchical micromechanical model based on the Mori-Tanaka (M-T) scheme is proposed. Formation of interphase region due to the interfacial interaction between the polymer matrix and nanoparticle is incorporated in the modeling. Comparison between the proposed model results considering the viscoelastic interphase shows a good agreement with existing experiment. At the high volume fraction, when the nanoparticles are not well-dispersed into the polymer nanocomposites, it is necessary to consider the viscoelastic interphase together with the nanoparticle aggregation for providing accurate predictions. The results reveal the nanoparticle aggregation affects and degrades the polymer nanocomposite creep resistance. A uniform dispersion of nanoparticles into the matrix leads to a maximum level of the nanocomposite creep resistance. The influences of the nanoparticle volume fraction, diameter and the interphase characteristics on the nanocomposite creep behavior are investigated. [ABSTRACT FROM AUTHOR]

Noninvasive measurement of tissue viscoelastic properties is gaining more attention for screening and diagnostic purposes. Recently, measuring dynamic response of tissue under a constant force has been studied for estimation of tissue viscoelastic properties in terms of retardation times. The essential part of such a test is an instrument that is capable of creating a controlled axial force and is suitable for clinical applications. Such a device should be lightweight, portable, and easy to use for patient studies to capture tissue dynamics under external stress. In this paper, we present the design of an automated compression device for studying the creep response of materials with tissue-like behaviors. The device can be used to apply a ramp-and-hold force excitation for a predetermined duration of time and it houses an ultrasound probe for monitoring the creep response of the underlying tissue. To validate the performance of the device, several creep tests were performed on tissue-mimicking phantoms, and the results were compared against those from a commercial mechanical testing instrument. Using a second-order Kelvin–Voigt model and surface measurement of the forces and displacements, retardation times \boldsymbolT_1 and \boldsymbolT_2 were estimated from each test. These tests showed strong agreement between our automated compression device and the commercial mechanical testing system, with an average relative error of 2.9% and 12.4%, for \boldsymbolT_1 and \boldsymbolT_2, respectively. Also, we present the application of compression device to measure local retardation times for four different phantoms with different size and stiffness. [ABSTRACT FROM AUTHOR]

The long-term viscoelastic behaviour of self-reinforced polypropylene composites (SRPPC) was studied by short-term flexural creep tests at different temperatures. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethylene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures (5, 20, 35°C above the melting temperatures of the matrices) keeping the holding time and pressure constant. The manufactured specimens were subjected to isothermal creep tests at different temperatures ranging from –20 to 80°C under an applied load. The time-temperature superposition principle was verified for the creep data. An Arrhenius type relationship described the shift data obtained from the creep tests. It was found, that with improving consolidation (increasing processing temperature) the creep compliance decreased and good correlation was found between creep compliance and density/peel strength.

In this study, a creep response model for the long-term behavior of interfacial shear stresses induced by intermediate flexural crack of FRP-plated RC beams is developed. A theoretical model based on the bi-linear cohesive zone model for intermediate crack-induced debonding is established, with the unique feature of unifying debonding initiation and growth with time increments. The creep behavior of the RC beam, adhesive layer and FRP plate have been included by considering the time-dependent mechanical properties. The time-dependent stress-deformation relationship caused by creep is referred to as bond-slip law. Consequently, a new interfacial law which combines time-dependent mechanical properties of all retrofitted beam constituents is proposed by considering constant the total energy during the creep response with intermediate crack. Obtained results are in good agreement with those given in literature. A parametric study is carried out to demonstrate the effect of the mechanical properties and thickness variations of FRP, concrete and adhesive on interface debonding. Indeed, the creep behavior, activates the debonding process in long term. Size of the softening zone is reached quickly and the bearing capacity of the retrofitted beam will be relatively affected by the time increase. [ABSTRACT FROM AUTHOR]

Significant levels of creep deformations in structural steel members exposed to high temperatures has been widely observed in experiments; however, inclusion of creep effects in numerical simulations has been limited due to lack of reliable experimental data and corresponding constitutive creep models for structural steel. This paper presents an experimental investigation into temperature-induced creep in low-alloy Q345 steel, which is commonly used in structural members. A set of creep tests is carried out at various stress levels in 300-900°C temperature range. Data from these creep tests indicate that temperature has significant influence on the level of creep deformations in Q345 steel, especially when the temperature in steel exceeds 400°C. The creep strain at a given temperature increases with stress level and the creep strain at a given stress level increases with temperature. [ABSTRACT FROM AUTHOR]

The aim of this paper is to present some important practical cases in the analysis of the creep response of unidirectional fiber-reinforced composites. Some of the currently used models are described: the micromechanical model, homogenization technics, the Mori-Tanaka method, and the finite element method (FEM). Each method was analyzed to determine its advantages and disadvantages. Regarding the accuracy of the obtained results, comparisons are made with experimental tests. The methods presented here are applied to carbon-fiber-reinforced composites, but these considerations can also be applied to other types of composite materials.

Abstract: In the past decades, the use of fibre-reinforced polymer (FRP) for enhancement of strength and stiffness of wood-based structural members has been established as an economical method. New developments have been ongoing to further improve the structural performance of glued-laminate beams. Recently, a novel integral sub-laminated composite, referred to as pre-stressed FRP–wood composite laminate (PWCL), was patented (KarisAllen and Tynes, 2000 ). This system is comprised of pre-stressed high performance fibres, sandwiched and glued within layers of pre-compressed wood strands. The resulting sub-laminated system may be attached to the tension zone of timber/glulam beams. A concern involved with the use of such pre-stressing scheme has been the issue of creep, which could affect the long-term performance of such composite beams. This paper presents the results of a study conducted to investigate the long-term performance of glulam beams reinforced with PWCL sub-laminate. Experimental investigations were conducted to determine the creep parameters of FRP composites and wood species employed in the fabrication of PWCL. The main objective was to develop a finite element model (FEM) to simulate the pre-stressing process and to predict the creep response of an entire reinforced glulam system, including the PWCL, under an externally applied load and constant environmental condition. The FEM was constructed in the Abaqus environment and the residual stress distribution was modeled in a step-wise scheme, corresponding to each step of PWCL and beam fabrication as well as the in situ response of the composite beam. The integrity of the creep model used in the simulation was verified by the experimental results obtained from tests performed on FRP reinforced small-size wood. [Copyright &y& Elsevier]

Abstract: In the past two decades, several attempts have been made to strengthen and stiffen timber beams by reinforcing them with layers of fibre-reinforced polymer (FRP) composites, oriented in various configurations. This paper describes an experimental and numerical investigation into the short-term and creep performance of southern yellow pine (SYP) and Douglas-fir (DF) timber beams strengthened by unidirectional aramid fibre-reinforced polymer sheet (AFRP). The creep parameters of AFRP and plain wood were determined using in-house creep tests. Accordingly, a non-linear finite element model was developed to predict the creep response of the AFRP reinforced timber beams based on the creep characteristics of the individual components. The results of the finite element analysis showed a good agreement with the experimental results conducted on the AFRP reinforced samples. Moreover, the influence of reinforcement on the flexural strength of reinforced beams was also examined. It could be concluded that enhancement of the timber beams with AFRP reinforcements provides improvement to both strength and stiffness, and can also effectively reduce the creep deformation of wooden beams. [Copyright &y& Elsevier]

Usually, a polymer composite with a viscoelastic response matrix has a creep behavior. To predict this phenomenon, a good knowledge of the properties and mechanical constants of the material becomes important. Schapery's equation represents a basic relation to study the nonlinear viscoelastic creep behavior of composite reinforced with carbon fiber (matrix made by polyethrtethrtketone (PEEK) and epoxy resin). The finite element method (FEM) is a classic, well known and powerful tool to determine the overall engineering constants. The method is applied to a fiber one-directional composite for two different applications: carbon fibers T800 reinforcing an epoxy matrix Fibredux 6376C and carbon fibers of the type IM6 reinforcing a thermoplastic material APC2. More cases have been considered. The experimental results provide a validation of the proposed method and a good agreement between theoretical and experimental results. [ABSTRACT FROM AUTHOR]

The homogenization theory, used for the study of differential equations with periodic coefficients, with a rapid variation, is used in the paper for the analysis of the creep phenomenon of composite materials, reinforced with fibers. Generally, a polymer composite having a matrix with a viscoelastic response manifests a creep behavior. A good knowledge of mechanical constants allows us to predict the time response under the action of a load, which is important in engineering. The homogenization method is used to determine the engineering constants for a composite reinforced with carbon fibers. The method is applied for the particular case of fiber-reinforced unidirectional composites to obtain the equations that finally offer the required values. The epoxy matrix Fibredux 6376C is reinforced with carbon fibers T800 and the thermoplastic specimens made by APC2 material is reinforced with carbon fibers of the type IM6. The experimental results give a good concordance with the theoretical predictions. [ABSTRACT FROM AUTHOR]

• Coextruded composites with a core-shell structure were manufactured by coextrusion. • The WPC shell layer improves the creep resistance of the composites. • Since the shell WPC is insensitivity to moisture, the CEC has a more stable creep behavior in a humid environment. • Based on the Findley model, the creep general equation suitable for this composites is derived. Coextruded composites (CEC) with a core-shell structure were manufactured, with laminated veneer lumber (LVL) as the core layer and wood-plastic composites (WPC) as the shell layer. The creep responses of the LVL and CEC at stress levels of 30%, 40%, and 50% of ultimate flexural strength at room temperature were determined using the three-point bending test. The creep parameters of the CEC and LVL were determined by in-house creep testing. The first derivative of creep strain results indicated that the creep performance of the CEC was more stable than that of the separate LVL in an unstable external environment. Three different widely known creep models in the literature, including the Burger model, Findley power law model, and Bailey-Norton model, were used to model the creep behavior of LVL and CEC in this study. The correlation coefficients of all nonlinear fitting curves exceeded 0.9, while the predicted values of the Burger model and Bailey-Norton model were deviated from the experimental values in the later stage of experiment. The creep formula for the LVL and CEC obtained based on the Findley model can be used to predict their long-term creep behavior. These findings have increased the selection range of environmentally friendly load-bearing materials that can be adapted to hot and humid environments, as well as the application of WPC and LVL. [ABSTRACT FROM AUTHOR]

Time-varying viscosity of viscoelastic materials has been found to induce complex rheology behaviors, which cannot be well characterized by the classical viscoelastic models. In this paper, different types of time-varying viscosity, namely, linearly varying viscosity, exponentially varying viscosity, and the proposed power-law viscosity are introduced with the applications to describing experimental data. Subsequently, these time-varying viscosities are embedded into the classical viscoelastic models. The relaxation and creep responses of the modified viscoelastic models are analytically derived and compared with the performance of the corresponding fractional models. The results indicate that the proposed power-law viscosity and the exponentially varying viscosity are capable of characterizing both thixotropy and rheopexy. The modified Maxwell model with power-law viscosity agrees well with the creep and relaxation responses of time-varying materials. It is also found that viscoelastic materials exhibiting thixotropy show faster rheological responses than the materials exhibiting rheopexy. • A novel power-law viscosity is proposed and tested. • Viscoelastic models with time-varying viscosities are established. • The rheological responses of the modified viscoelastic models are derived. • The performances of modified viscoelastic models are compared and verified. [ABSTRACT FROM AUTHOR]

• The effect of gradation on the creep response and CCG is modeled successfully. • C (t)-integral based relation is first time used to estimate CCGR for graded material. • Proposed C (t)-integral calculation is independent of the gradation direction. • Crack growth direction is computed by maximum circumferential stress criterion. • Data transfer and null step analysis are used to maintain history dependency of creep. In this work, a numerical methodology is proposed to predict the creep response and creep crack growth for functionally graded material/plastically graded material in the extended finite element method framework. The stress relaxation and redistribution as a result of creep are incorporated into the analysis of graded material. The C t -integral (which includes all creep stages from small-scale creep to extensive creep) based relation analogous to Paris Law is employed to calculate the creep crack growth rate. The numerical difficulties faced at the time of simulation, such as strain-energy rate density derivative and maintaining plastic/creep irreversibility during crack growth are well addressed in this paper. The strain-energy rate density derivative is evaluated by the surface approximation. The plastic/creep irreversibility is maintained by the appropriate data transfer and null step analysis scheme. The crack growth direction is decided by the maximum circumferential stress criterion. The proposed methodology is first validated with the experimental results for homogeneous material and later on, it is implemented to calculate the crack growth for various cases of gradation. The capabilities of the present scheme are demonstrated by performing the mixed-mode crack growth simulations at the specimen and component level. [ABSTRACT FROM AUTHOR]