Cyclic Uniaxial Tensile Tests Research Articles

Experimental Study on Force Sensitivity of the Conductivity of Carbon Nanotubes-Modified Epoxy Resins.

The addition of a conductive material into polymer improves its mechanical properties, electrical properties and thermal conductivity and bestows it with good self-sensing and self-adjusting properties. In this study, carbon nanotubes-modified epoxy resins (CNTs-EP) were successfully prepared with good dispersion through the combined methods of three roller rolling, ultrasonic processing and adding surfactant. Tests were conducted to evaluate the resistivity of unloaded modified epoxy resins with different mixing amounts of carbon nanotubes (CNTs), to determine the conductive percolation threshold. On the basis of the test results, a series of monotonic and cyclic uniaxial tensile tests were then conducted to investigate the force sensitivity of the conductivity of epoxy resins modified with different mixing amounts of CNTs. The relationship between the stress and the resistivity under various mixing amounts was studied, indicating that the resistance response could play a good warning role on the damage of the modified polymer material.

Constitutive modeling of randomly oriented electrospun nanofibrous membranes

In this paper, a simple phenomenological model describing the macroscopic mechanical response of electrospun nanofibrous structures is proposed. Motivated by the experimental observation, the model development starts from the description of membrane response at fiber scale in order to capture individual fiber response and irreversible inter-fiber interactions using hyperelastic and large strain elasto-plastic frameworks, respectively. The macroscopic response is subsequently obtained by integrating the fiber responses in all possible fiber orientations. The efficiency of the proposed model is assessed using experimental data of PVDF electrospun nanofibrous membranes. It is found that the model is qualitatively in good agreement with uniaxial monotonic and cyclic tensile loading tests. Two other deformation modes, i.e., equibiaxial extension and pure shear (planar extension), are simulated to further evaluate the model responses. Finally, the deformation-induced fiber re-orientation is investigated for different modes of deformations.

Continuous cyclic deformations of a Ni/W film studied by synchrotron X-ray diffraction

In this paper, a method to study cyclic deformations is used in a polycrystalline Ni thin film 600nm thick with a W cap-layer of 20nm. The Ni/W bilayer film was deposited onto compliant polyimide substrate allowing the film to be subjected to a relatively large tensile applied stress range during loading-unloading of the film/substrate composite. Film deformations were in situ measured both by synchrotron X-ray diffraction and digital image correlation during continuous cyclic uniaxial tensile test. This work focuses on the characterization of the deformation mechanisms during the first cycles of loading-unloading tensile test. It is found that up to a strain of 0.5% corresponding to an applied stress of 1.1GPa the elastic strain of the Ni film follows precisely the strain applied to the substrate, but for larger applied stress the elastic strain of the metallic film, though completely reversible, is smaller than the substrate true strain. The results open out onto characteristic strain-strain hysteresis curves, indicative of the very first step of fatigue processes in the Ni/W thin film.

The effects of cyclic tensile and stress-relaxation tests on porcine skin.

When a living tissue is subjected to cyclic stretching, the stress-strain curves show a shift down with the increase in the number of cycles until stabilization. This phenomenon is referred to in the literature as a preconditioning and is performed to obtain repeatable and predictable measurements. Preconditioning has been routinely performed in skin tissue tests; however, its effects on the mechanical properties of the material such as viscoelastic response, tangent modulus, sensitivity to strain rate, the stress relaxation rate, etc….remain unclear. In addition, various physical interpretations of this phenomenon have been proposed and there is no general agreement on its origin at the microscopic or mesoscopic scales. The purpose of this study was to investigate the effect of the cyclical stretching and the stress-relaxation tests on the mechanical properties of the porcine skin. Cyclic uniaxial tensile tests at large and constant strain were performed on different skin samples. The change in the reaction force, and skin's tangent modulus as a function of the number of cycles, as well as the strain rate effect on the mechanical behavior of skin samples after cycling were investigated. Stress-relaxation tests were also performed on skin samples. The change in the reaction force as a function of relaxation time and the strain rate effect on the mechanical behavior of skin samples after the stress-relaxation were investigated. The mechanical behavior of a skin sample under stress-relaxation test was modeled using a combination of hyperelasticity and viscoelasticity. Overall, the results showed that the mechanical behavior of the skin was strongly influenced by cycling and stress relaxation tests. Indeed, it was observed that the skin's resistance decreased by about half for two hours of cycling; the tangent modulus degraded by nearly 30% and skin samples became insensitive to the strain rates and accumulated progressively an inelastic deformation over time during cycling. Finally, the hysteresis loops became very narrow at the end of cycling and after relaxation process.

Thermomechanical analysis of polymeric foams subjected to cyclic loading: Anelasticity, self-heating and strain-induced crystallization

The present study investigates the thermomechanical behavior of TPU foams. Different densities were tested, including the compact state, which ensures a relevant characterization of the relative effect of the void volume fraction. A series of cyclic uniaxial tensile tests was carried out at different loading rates and different specimen densities. The effects of the density and the loading conditions on the softening, the residual strain and the hysteresis have been characterized. The thermal responses exhibit numerous particularities. First, a threshold effect in terms of the density on the self-heating has been highlighted. Second, entropic effects are weighted by energetic effects (internal energy variations) during the deformation. Typical changes in the thermal response highlight that strain-induced crystallization (SIC) and crystallite melting occur during the deformation. The characteristic stretches of this phenomenon evolve with the maximum stretch applied, which increases the residual stretch, and the number of cycles, which induces the softening. Decreasing the density decreases the crystallinity: the volume of crystallizing matter is lower and cell wholes become more and more thin, which decreases the mobility of the molecular chains. These effects cannot be predicted from the mechanical responses and the present study provides therefore information of importance to better understand and model the effects of the density and the loading conditions on the thermomechanical behavior of TPU foams.

Mechanical properties of fluorinated ethylene propylene (FEP) foils in use for neutrino detector project

The use of fluorinated ethylene propylene (FEP) foils as engineering materials for aerospace, solar thermal collector and neutrino detector applications has attracted considerable attention in recent decades. Mechanical properties are indispensable for analyzing corresponding structural behavior to meet the demands of safety and serviceability. In this paper, uniaxial tensile tests taking into account loading speeds, uniaxial tensile cyclic tests in terms of stress amplitude and loading cycles and creep tests considering loading stress and time were carried out to characterize mechanical properties. For uniaxial tensile properties, elastic modulus, yield stress, breaking strength and elongation were analyzed in detail. It is found that these mechanical properties except breaking elongation increased with loading speeds and that mechanical properties obtained in transverse direction were more sensitive than those obtained in machine direction. For cyclic properties, elastic modulus and ratcheting strain tended to be stable after certain cycles, demonstrating that cyclic elastic moduli were more suitable for analyzing structural behavior than those obtained in uniaxial tensile experiments. For creep properties, apparent strain at 6 MPa suggested that special attention was necessary for analyzing structural behavior if maximum stress was larger than 6 MPa. In general, this study could provide useful observations and values for understanding mechanical properties of FEP foils.

Mechanical characterization of a short fiber-reinforced polymer at room temperature: experimental setups evaluated by an optical measurement system

Composite materials are of great interest for industrial applications because of their outstanding properties. Each composite material has its own characteristics due to the large number of possible combinations of matrix and filler. As a result of their compounding, composites usually show a complex material behavior. This work is focused on the experimental testing of a short fiber-reinforced thermoplastic composite at room temperature. The characteristic behavior of this material class is often based on a superposition of typical material effects. The predicted characteristic material properties such as elasto-plasticity, damage and anisotropy of the investigated material are obtained from results of cyclic uniaxial tensile tests at constant strain rate. Concerning the manufacturing process as well as industrial applications, the experimental investigations are extended to multiaxial loading situations. Therefore, the composite material is examined with a setup close to a deep-drawing process, the Nakajima test (Nakazima et al. in Study on the formability of steel sheets. Yawate Technical Report No. 264, pp 8517–8530, 1968). The evaluation of the experimental investigations is provided by an optical analysis system using a digital image correlation software. Finally, based on the results of the uniaxial tensile tests, a one-dimensional macroscopic model is introduced and first results of the simulation are provided.

Factors affecting the mechanical behavior of collagen hydrogels for skin tissue engineering

The effect of the production factors yielding a functional dermal substitute was investigated by means of monotonic and cyclic uniaxial tensile tests, as well as electron microscopy visualizations. The role of (i) plastic compression, (ii) product incubation, and (iii) cell permanence in the collagenous matrix in order to achieve a skin-like behavior were characterized in terms of material and structural stiffness, in-plane kinematics, and cyclic response, as well as pore size and network density. The plastic compression resulted in a denser and stiffer material, while no corresponding change was observed in the behavior of the entire structure. This was related to the progressive reduction in product thickness and amount of excess water, rather than to formation of new crosslinks between fibers. Contrary, irrespective of the presence of human fibroblasts, the product incubation induced both material and structural stiffening, indicating the formation of a denser network. These results were confirmed by similar evolutions in the construct in-plane kinematics and cyclic stress reduction. Finally, comparison of constructs incubated in different culture media indicated a determinant contribution of the biochemical environment, rather than of the seeded cells, to the achieved mechanical properties. The observed features are relevant in terms of mechanical biocompatibility of the implant and might direct future optimizations of the production process in order to rapidly attain the desired mechanical properties.

Fading memory of deformation history in carbon black-filled thermoplastic elastomers

Observations are reported on a carbon black–reinforced thermoplastic elastomer in multistep uniaxial tensile cyclic tests with a mixed deformation program (oscillations between maximum elongation ratios kmax and various minimum stresses σmin with kmax monotonically increasing with number of cycles n). Fading memory of deformation history is demonstrated: when specimens are subjected to two loading programs that differ along the first n −1 cycles of deformation and coincide afterwards, their stress–strain diagrams become identical starting from the nth cycle. A constitutive model is developed in cyclic viscoplasticity with finite deformations, and its adjustable parameters are found by fitting the observations. Ability of the stress–strain relations to describe the fading memory phenomenon and to predict the mechanical response of polymer composites in multi-step cyclic tests with large strains is confirmed by numerical simulation.

Characterization of short fiber reinforced polymers

AbstractThis contribution presents a possibility how to characterize composite materials with respect to experimental testing. Furthermore a modeling approach is presented based on the experimental results.Composite materials, like short fiber reinforced polymers, are of great interest because of their good properties for industrial applications. In this work a polybutylene terephthalate (PBT) reinforced by short glass fibers (GF) is characterized. An overview of the properties of the investigated material which are obtained from results of cyclic uniaxial tensile tests under monotonous and cyclic loading at a constant strain rate is given. The short fiber reinforced PBT shows elasto‐plastic material behavior and damage. The complete strain information in all directions is obtained by an optical area analysis over the whole surface using a digital image correlation (DIC) software. The optical sytem uses four cameras to realize a 360°‐3D‐measurement by a two‐sided stereography [2]. Based on the results of the experimental investigations with uniaxial tensile tests, a one‐dimensional macroscopic model to describe the material behavior is introduced and the results of the simulation are provided. The modeling description takes into consideration the observed behavior like elasto‐plasticity and damage. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cyclic tensile response of Mo-27 at% Re and Mo-0.3 at% Si solid solution alloys

Stress-controlled uniaxial cyclic tensile tests were conducted on binary Mo-27at% Re and Mo-0.3at% Si solid solutions as a function of temperature and compared against the previously reported cyclic response of pure Mo. The Mo-27at% Re alloy with a recrystallized grain size of ~30µm was evaluated in the temperature range 25°C–800°C at R=0.1 and stress range that was 80% of the ultimate tensile strength (UTS); a peak in fatigue life was observed between 300°C and 500°C. The decrease in fatigue life at the higher temperatures of 700°C and 800°C is attributed to dynamic strain aging. Transmission electron microscopy of the cyclically-deformed alloy revealed parallel bands of dislocation at room temperature that transitioned to a uniform cell structure at 500°C and back to orthogonal planar arrays at 800°C. The as-extruded Mo-0.3at% Si alloy was evaluated from 25°C to 1200°C and showed superior fatigue life and ratcheting strain resistance as compared to pure Mo and the Mo-27at% Re alloy (within the temperature range where data were available for comparison). The superior resistance is attributed to the high density of dislocations within the material in this mostly unrecrystallized state rather than Si in solid solution. Above 800°C, the ratcheting strain increases and fatigue life decreases rapidly with increasing temperature and is associated with dynamic recovery.

ランニング動作におけるスポーツ用スパッツの関節トルク評価に関する研究

In this study, knee joint torque generated by sports spats in running was estimated by numerical simulation. The fabric materials for sports spats show anisotropic stress strain relations and stress softening in mechanical properties. Therefore, an anisotropic hyperelastic model was applied to numerical simulation. Uniaxial monotonic tensile tests and uniaxial cyclic tensile tests were carried out to estimate material parameters of an anisotropic hyperelastic model. Then, we supposed that the strain of sports spats on human body during running was reproduced using 3-dimensional computer graphic model. As a result, stress and strain distribution were calculated, and knee joint torque produced by sports spats during running was calculated. From the numerical result, joint torque caused by sports spats depended on bending angle of knee.

Influence of Predeformation on Mechanical Behavior of Hastelloy C-276

The accurate prediction of springback is a subject of major concern during the sheet metal forming. In this study, the recovery behavior of Hastelloy C-276 sheet under different amounts of pre-strain has been investigated by uniaxial cyclic tensile tests. The total springback during unloading could be separated into a linear springback and a nonlinear springback. The percentage of nonlinear recovery to the total recovery increased as the pre-strain increased. Both unloading and reloading elastic moduli decreased as the pre-strain increased, which affected the springback phenomenon significantly. Nonlinear recovery could be explained by the movement of dislocations in the reverse direction. The decrease of unloading elastic modulus is mainly related with the dislocation motion and the mobile dislocation density.

Stress-relaxation and fatigue behaviour of synthetic brow-suspension materials

Ptosis describes a low position of the upper eyelid. When this condition is due to poor function of the levator palpebrae superioris muscle, responsible for raising the lid, “brow-suspension” ptosis correction is usually performed, which involves internally attaching the malpositioned eyelid to the forehead musculature using brow-suspension materials. In service, such materials are exposed to both rapid tensile loading and unloading sequences during blinking, and a more sustained tensile strain during extended periods of closure. In this study, various mechanical tests were conducted to characterise and compare some of commonly-used synthetic brow-suspension materials (Prolene®, Supramid Extra® II, Silicone rods (Visitec® Seiff frontalis suspension set) and Mersilene® mesh) for their time-dependent response. At a given constant tensile strain or load, all of the brow-suspension materials exhibited stress-relaxation or creep, with Prolene® having a statistically different relaxation or creep ratio as compared with those of others. Uniaxial tensile cyclic tests through preconditioning and fatigue tests demonstrated drastically different time-dependent response amongst the various materials. Although the tests generated hysteresis force–strain loops for all materials, the mechanical properties such as the number of cycles required to reach the steady-state, the reduction in the peak force, and the cyclic energy dissipation varied considerably. To reach the steady-state, Prolene® and the silicone rod required the greatest and the least number of cycles, respectively. Furthermore, the fatigue tests at physiologically relevant conditions (15% strain controlled at 6.5Hz) demonstrated that the reduction in the peak force during 100,000 cycles ranged from 15% to 58%, with Prolene® and the silicone rod exhibiting the greatest and the least value, respectively.Many factors need to be considered to select the most suitable brow-suspension material for ptosis correction. These novel data on the mechanical time-dependent performance could therefore help to guide clinicians in their decision-making process for optimal surgical outcome.

Characterization of plantaris tendon constructs for ankle ligament reconstruction.

Many techniques have been described for lateral ligament reconstruction. One frequently overlooked autograft option is the plantaris tendon, potentially due to the paucity of data on its mechanical characteristics. This study examined the structural properties of double and quadruple plantaris tendon constructs. Plantaris tendons were harvested from 35 fresh-frozen human cadaver specimens (mean age, 66 years [range, 43-89 years]; 17 female, 13 male). The tendon ends were sutured in a running locking technique and then woven onto a template board to create double or quadruple graft constructs with a 20-mm functional length. If additional tendon length remained, a single 40-mm specimen was isolated to provide tissue material properties. Structural properties were calculated from the results of cyclic and failure uniaxial tensile tests. Quadruple-strand constructs had a tensile strength of 205.8 ± 68.2 N and a stiffness of 133.1 ± 46.3 N/mm. Single strands had a tensile strength of 66.9 ± 26.3 N and a stiffness of 43.8 ± 14.7 N/mm. Material properties were similar to a prior study. The average maximum tensile strength for the quadrupled plantaris grafts exceeded the strength of the intact anterior talofibular ligament of 139 to 161 N; therefore, the quadruple plantaris construct may be a viable autograft for foot and ankle ligament reconstruction. The tensile strength of the plantaris tendon is comparable to, or stronger than, other grafts already in use and offers a donor site that may result in negligible loss of strength.

Influence of the mechanical stress and the filler content on the hydrostatic compression behaviour of natural rubber

The behaviour of natural rubber (NR) compounds under mechanical stress is often reported in literature. An important and widely discussed effect that occurs is the Mullins effect. During the first loading cycles in a tensile test for example, a stress-softening effect is observed. This and other effects on the mechanical behaviour are investigated for different rubber materials with and without different types of fillers and filler contents. Besides, the hydrostatic compression behaviour is affected by the type and content of filler as well, which is shown for an NR with and without waxes and different contents of carbon black (CB) in this contribution. In contrast to the Mullins effect, there is no dependence of the number of loading cycles on the volumetric behaviour determined in hydrostatic compression tests. Furthermore, the influence of the previous stress-softening due to mechanical stress on the compression behaviour is elaborated. Cyclic uniaxial tensile tests are performed to realize the stress-softening in the rubber materials. The subsequent compression tests are compared to compression tests without any pre-stretching to determine the influence of previous mechanical loading on the compression behaviour of natural rubber with different filler contents.

Unusual mechanical response of carbon black-filled thermoplastic elastomers

Observations are reported on carbon black-filled thermoplastic elastomer in multi-step uniaxial tensile cyclic tests with various strain rates at room temperature. Experimental data reveal several unusual features of stress–strain diagrams: (i) fading memory of deformation history in cyclic tests with increasing maximum elongation ratios, (ii) transitions from simple to mixed to inverse relaxation and creep in specimens subjected to stretching and subsequent retraction down to various minimum stresses, and (iii) strain rate-induced acceleration of inverse relaxation and creep. A constitutive model is developed for the time- and rate-dependent response of polymer composites under an arbitrary three-dimensional deformation with finite strains. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. Ability of the constitutive equations to describe the mechanical behavior of thermoplastic-elastomer composites under cyclic deformation and to predict the observed phenomena is confirmed by numerical simulation.

Filler effects on the thermomechanical response of stretched rubbers

This paper deals with the calorimetric analysis of deformation processes in filled styrene-butadiene rubbers. More especially, the study focuses on the effects of the addition of carbon black fillers on the calorimetric response of “demullinized” SBR. Temperature variations are measured by infrared thermography during cyclic uniaxial tensile tests at ambient temperature. Heat sources11The term heat source is used in this paper to mean the heat power being produced or absorbed. produced or absorbed by the material due to deformation processes are deduced from temperature fields by using the heat diffusion equation. First, the results show that no mechanical (intrinsic) dissipation is detected for weakly filled SBR, meaning that the heat produced and absorbed over one mechanical cycle is the same whatever the stretch ratio reached. Second, the mechanical dissipation in highly filled SBR is significant. The quantitative analysis carried out highlights the fact that it increases quasi-linearly with the stretch ratio. Finally, a simplified framework is proposed to discuss the identification of the heat sources, in particular the mechanical dissipation.

Effect of crystalline structure on the mechanical response of polypropylene under cyclic deformation

Abstract To evaluate the influence of crystalline structure on the mechanical behavior of polypropylene (PP), uniaxial tensile cyclic tests with a mixed program (oscillations between fixed maximum strains and the zero minimum stress) were performed on isotactic PP (iPP) manufactured by the Ziegler-Natta catalysis method, metallocene catalyzed PP (mPP), and annealed mPP. Although the stress-strain diagrams of iPP and mPP under tension are quite similar, their responses under unloading differ markedly. The residual strain (measured as the strain under retraction down to the zero stress) of iPP strongly exceeds that of non-annealed mPP, and annealing of mPP increases this difference. To rationalize these findings, constitutive equations are developed in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers, and adjustable parameters in the stress-strain relations are found by fitting the observations. The ability of the model to describe the observed phenomenon and to predict the mechanical response in multi-cycle tensile tests, with various deformation programs, is demonstrated by numerical simulation.

Fading memory of loading history in polypropylene and a polypropylene/clay nanocomposite

Experimental data are reported for neat polypropylene and a polypropylene/clay nanocomposite in multistep uniaxial cyclic tensile tests at room temperature (oscillations between maximum strains and the zero minimum stress, with the maximum strains increasing monotonically with number of cycles). The fading memory of deformation history is observed: when two test specimens are subjected to loading programs that differ during the first n − 1 cycles and coincide afterwards, their stress–strain diagrams become identical starting from the nth cycle. Constitutive equations are developed in the cyclic viscoelasticity and viscoplasticity of nanocomposites with semicrystalline matrices, and the adjustable parameters of the stress–strain relations are determined by fitting them to experimental data. The ability of the model to predict the fading memory phenomenon is confirmed by numerical simulations.