Effects of temperature and stress on creep properties of ethylene tetrafluoroethylene (ETFE) foils for transparent buildings

Creep properties of ethylene tetrafluoroethylene (ETFE) foils are indispensable for evaluating serviceability limit state, especially under high temperature and high stress. This paper concerned temperature and stress effects on creep properties of ETFE foils with experimental and theoretical studies. Experimental results showed that dimensionless stress effect on creep properties could be higher than that of temperature effect. A unified equation incorporating temperature, stress and time based on experimental results was determined and could be utilized to calculate the stress limits and long-term creep strains. The stress limits in response to creep strain of 10% were less than 5 MPa, 4 MPa and 3 MPa for temperature ranges of 40–50 °C, 50–60 °C and 70–80 °C, respectively. The long-term creep strain of ETFE foils under 40 °C was 5.96% concerning 50-year working time.Master curves of ETFE foils were evaluated considering time-temperature superposition (TTSP) and time-stress superposition (TSSP). Long-term creep strains with these master curves were identified and compared with experimental creep strains. It is found that TTSP could be a little underestimation of creep strains while TSSP could overestimate creep strains to some extent. Moreover, the maximum creep strain difference was only 0.48%, which justified the feasibility and suitability of using the unified equation to predict creep strains of ETFE foils.     

Tensile mechanical properties of basalt fiber reinforced polymer composite under varying strain rates and temperatures

High-strength woven fabrics and polymers are ideal materials for use in structural and aerospace systems. It is very important to characterize their mechanical properties under extreme conditions such as varying temperatures, impact and ballistic loadings. In this present work, the effects of strain rate and temperature on the tensile properties of basalt fiber reinforced polymer (BFRP) were investigated. These composites were fabricated using vacuum assisted resin infusion (VARI). Dynamic tensile tests of BFRP coupons were conducted at strain rates ranging from 19 to 133 s⁻¹ using a servo-hydraulic high-rate testing system. Additionally, effect of temperature ranging from −25 to 100 °C was studied at the strain rate of 19 s⁻¹. The failure behaviors of BFRP were recorded by a Phantom v7.3 high speed camera and analyzed using digital image correlation (DIC). The results showed that tensile strength, toughness and maximum strain increased 45.5%, 17.3% and 12.9%, respectively, as strain rate increased from 19 to 133 s⁻¹. Moreover, tensile strength was independent of varying temperature up to 50 °C but decreased at 100 °C, which may be caused by the softening of epoxy matrix and weakening of interfaces between fibers and matrix when the glass transition temperature was exceeded.     

Hot tensile fracture characteristics and constitutive modelling of polyether-ether-ketone (PEEK)

The effects of strain rate and deformation temperature on the deformation behaviors of polyether-ether-ketone (PEEK) were studied by uniaxial tensile tests with the temperature range of 23–150 °C and strain rate of 0.01–1 s⁻¹. The effects of deformation temperature and strain rate on the hot tensile deformation behavior and fracture characteristics were investigated by scanning electron microscope (SEM) and discussed in detail. SEM experimental results suggest that fracture morphology is not strain rate sensitive but temperature sensitive. Based on the tensile results, the Johnson-Cook and modified Johnson-Cook constitutive models were established for PEEK. Furthermore, a comparative study has been made on the accuracy and effectiveness of the developed models to predict the flow stress. The results show that the original Johnson-Cook model reflects the deformation behavior more accurately throughout the entire test temperature and strain rate range under uniaxial tensile conditions.     

Effect of strain rate on the dynamic tensile behaviour of UHMWPE fibre laminates

Ultra-high molecular weight polyethylene (UHMWPE) fibre has great potential for strengthening structures against impact or blast loads. A quantitative characterization of the mechanical properties of UHMWPE fibres at varying strain rates is necessary to achieve reliable structural design. Quasi-static and high-speed tensile tests were performed to investigate the unidirectional tensile properties of UHMWPE fibre laminates over a wide range of strain rates from 0.0013 to 163.78 s⁻¹. Quasi-static tensile tests of UHMWPE fibre laminates were conducted at thicknesses ranging from 1.76 mm to 5.19 mm. Weibull analysis was conducted to investigate the scatter of the test data. The failure mechanism and modes of the UHMWPE fibre laminates observed during the test are discussed. The test results indicate that the mechanical properties of the UHMWPE fibre laminate are not sensitive to thickness, whereas the strength and the modulus of elasticity increase with strain rate. It is concluded that the distinct failure modes at low and high strain rates partially contribute to the tensile strength of the UHMWPE fibre laminates. A series of empirical formulae for the dynamic increase factor (DIF) of the material strength and modulus of elasticity are also derived for better representation of the effect of strain rate on the mechanical properties of UHMWPE fibre laminates.     

Influence of chemical crosslinking on the creep behavior of ultra-high molecular weight polyethylene fibers

In this study, the effect of chemical crosslinking on the creep behavior of high-strength fibers, obtained by gel-spinning and subsequent hot-drawing of ultra-high molecular weight polyethylene (UHMWPE), is examined. In the first part of the paper, the general aspects of the creep behavior of these fibers are discussed. The second part deals with UHMWPE fibers that are crosslinked by means of a) chlorosulfonation and b) dicumyl peroxide treatment followed by UV irradiation. The latter technique leads to an improvement of the creep resistance of the UHMWPE fibers without affecting their high tensile strengths. In spite of the fact that the network formation is fairly high, the creep cannot be completely removed. The results indicate that the creep process in UHMWPE fibers is associated with a deformation mechanism in the crystalline regions of the fiber, which are not affected by chemical crosslinking.     

Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Temperature and variable strain rate sensitivity in recycled HDPE

The recycling of post-consumer plastics and their utilization as raw materials to develop value-added products has become an important goal worldwide. The present work is concerned with the thermo-mechanical analysis of recycled high-density polyethylene (HDPE) under uniaxial tensile loading. The main focus is to propose a one-dimensional phenomenological model able to describe the influence of temperature and strain rate on the mechanical behavior. Tensile tests were performed over a wide range of temperatures (from 25°C to 100°C). Each experiment was performed under controlled strain rate varying from 7.25 × 10⁻⁵ s⁻¹ to 7.25 × 10⁻³ s⁻¹ in steps. It is shown that only one tensile test performed at three different temperatures is necessary to fully identify experimentally all material parameters that arise in the theory. Thus, with this experimental procedure, the number of tests used to evaluate the mechanical properties of recycled HDPE is significantly reduced. The experiments are compared with the model predictions and show good agreement.     

Characterization of tension set behavior of a silicone rubber at different loads and temperatures via digital image correlation

This work aims to determine the tensile set behavior of a silicone rubber under different stress magnitudes and temperatures through digital image correlation implemented in an improved creep experimental set-up. Creep-recovery strains were measured with time at 20, 40 and 60 °C under tensile strengths of 98.1, 196.2 and 394.3 kPa, respectively. The behavior of creep and recovery strain with time at the different stress magnitudes and temperatures was successfully obtained by the experiments. The corresponding elastic and viscous components of the material for each condition were determined from the results. Overall, all obtained creep behaviors matched with the behavior of a four-element model of creep-recovery. The increase of temperature generated an increase of creep compliance at the three loads, but the increase of tensile load produced a decrease of creep compliance for the three temperatures. The strain was not recovered entirely in any case for the test time stated.     

Temperature-dependent ratcheting of PTFE gaskets under cyclic compressive loads with small stress amplitude

Uniaxial stress-controlled ratcheting experiments on PTFE gaskets under cyclic compressive loads with small stress amplitude were performed. The effect of temperature on the deformation behavior was considered. Results showed that the compressive modulus decreases rapidly when the temperature increases from 100 °C to 200 °C. Compressive ratcheting deformation with cycles increase significantly with the increases of temperature. The ratcheting deformations at 100 °C, 150 °C and 200 °C are nearly two, three and five times that at room temperature, respectively. Most of ratcheting deformation mainly occurs during the first 20 cycles because the subsequent ratcheting rate and strain range are small and much less than those in the previous cycles. The accumulated deformation under cyclic loads with small stress amplitude is relatively approach to the static compressive creep with the same peak stress. Therefore, the accumulated deformation with time of PTFE gaskets obtained by cyclic compression with small stress amplitude can be estimated by the corresponding static creep deformation with good accuracy under the approximate stress rate and the same temperature, especially at room temperature.     

Evaluation of Mechanical Properties of Epoxy/Nanoclay/Multi-Walled Carbon Nanotube Nanocomposites using Taguchi Method

This paper reports the improvement of the mechanical properties of epoxy/nanoclay/multi-walled carbon nanotube (MWNT) nanocomposites prepared by the solution casting method for a range of pre-cure temperatures (room temperature, 50, and 70 °C), cure temperature (120, 130, and 140 °C), nanoclay content (0.5, 1.0, 1.5 wt%) and content of MWNT (0.2, 0.6, 1.0 wt%) for three levels. The influence of these parameters on the mechanical properties of epoxy/nanoclay/MWNT has been investigated using Taguchi's experimental design. The output measured responses are the tensile properties (tensile modulus, tensile strength and strain at break), impact strength and fracture toughness. From the Analysis of Mean (ANOM) and Analysis of Variance (ANOVA), MWNT content, pre-cure temperature and cure temperature had the most significant effects for the impact strength with contribution percentages of 38%, 28% and 23% respectively. However, for the fracture toughness and strain at break, the enhancements of properties come from the nanoclay content (59%), MWNT content (18%) and pre-cure temperature (23%). While the improvement in tensile strength was influenced by nanoclay and MWNT content, the cure temperature has a stronger effect on the tensile modulus. In this respect, Taguchi method points to the Taguchi method, in this way, points to the dominant parameters and gives the optimum parameter settings for each mechanical property. Confirmation experiments were performed with the optimum parameter settings and the mechanical properties were measured compared with the predicted results.     

Mass and momentum transfer in polymers. I. Effects of equilibrium vapor sorption on tensile creep of an amorphous polymer

Diffusion and tensile creep measurements were made for systems of poly(-n-butyl methacrylate) (PBMA) and sorbed ethanol, MEK, or benzene at 23°C. Rates of penetration of an inert spherical indenter into PBMA also were investigated and compared with the tensile creep behavior of the polymer. Creep measurements for various volume fractions of penetrant sorbed at equilibrium revealed that master curves, resulting from a time-concentration superposition procedure, could be constructed for each penetrant. At long times, these master curves, particularly that for ethanol, show deviations from the corresponding time-temperature superposition master curve. These deviations are interpreted in terms of probable long-range entanglement coupling governed in part by the partially specific nature of polymer-penetrant interactions. Parameters calculated by a free-volume theory, describing both diffusion and tensile-creep data, indicate that MEK is a more efficient plasticizing agent than the other penetrants and requires less local free volume for diffusion. Analysis in terms of the free volume concept was not attempted for the case of ethanol, where specific polymer-penetrant interactions are more important.     

Mechanical properties of anion exchange membranes by combination of tensile stress–strain tests and dynamic mechanical analysis

The thermomechanical properties of anion exchange polymers based on polysulfone (PSU) quaternized with trimethylamine (TMA) or 1,4‐diazabicyclo[2.2.2]octane (DABCO) and containing hydroxide or chloride anions by tensile stress–strain tests and dynamic mechanical analysis (DMA) have been determined. The reported mechanical properties included the Young's modulus, tensile strength, and elongation at break from tensile tests and the storage and loss modulus and glass transition temperature from DMA. The anion exchange membranes behaved as stiff polymers with Young's modulus in the order of 1 GPa, relatively with high strength (about 30 MPa) and low elongation at break (around 10%) was observed. Tensile tests were also made with membranes exchanged with hydrogen‐carbonate and carbonate anions to control the absence of important carbonation of the OH form. The glass transition temperatures were of the order of 150 °C (PSU‐TMA) or 200 °C (PSU‐DABCO) for the hydroxide form, confirmed by differential scanning calorimetry; they increase further by about 50 K, when hydroxide ions are replaced by chloride. This result and the increase of the storage modulus could be interpreted by the higher hydration of hydroxide ions and the plasticizing effect of water, which reduced the Van der Waals interactions between the macromolecular chains. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1180–1187     

Effect of surface treatment with potassium permanganate on ultra-high molecular weight polyethylene fiber reinforced natural rubber composites

The effects of surface treatment using potassium permanganate on ultra-high molecular weight polyethylene (UHMWPE) fibers reinforced natural rubber (NR) composites were investigated. The results showed the surface roughness and the oxygen-containing groups on the surface of the modified fibers were effectively increased. The NR matrix composites were prepared with as-received and modified UHMWPE fibers added 0–6 wt%. The treated fibers increased the modulus and tensile stress at a given elongation. The tear strength increased with increasing fiber mass fraction, attained maximum values at 4 wt%. The hardness of composites exhibited continuous increase with increasing the fiber content. The dynamic mechanical tests showed that the storage modulus and the tangent of the loss angle were decreased in the modified UHMWPE fibers/NR composites. Several micro-fibrillations between the treated fiber and NR matrix were observed, which meant the interfacial adhesion strength was improved.     

Constitutive model calibration of the time and temperature-dependent behavior of high density polyethylene

HDPE is commonly used in pipelines and piping for industrial and societal infrastructure. Like most polymers, HDPE's mechanical properties are sensitive to temperature and show time dependent properties. The temperature effect on both the short and long term compressive and tensile behavior of HDPE, in a combined manner, have not been investigated thoroughly in the past. Especially the constitutive behavior of HDPE, incorporating temperature effects on its long and short term behavior, could be essential when designing such infrastructural components. Hence, the temperature effect on the short and long term response in tension and compression of HDPE is investigated in this study. The short term tensile and compressive stress-strain behavior at 23, 40, 60, and 80 °C were obtained through experiments at constant displacement rate and temperature. Tensile and compressive stress relaxation (e.g. long term) behavior at 23, 40, 50, 60, 70, and 80 °C were investigated through stress relaxation tests. The experimental results from the short term tests showed that both the tensile and compression moduli and yield strength of HDPE decrease linearly with the increase in temperature. It is also shown from the long term test that relaxation modulus in tension and compression are highly dependent on temperature. Based on the experimental results, the constitutive three network model (TNM) was calibrated and implemented in a FEA model, which was then validated through a three point bending (3 PB) relaxation test with a prescribed temperature profile. The FEA model and the calibrated model results agree markedly well with the experimental results, which indicates that the model can be used reliably to predict the temperature dependent short and long term behavior of HDPE in design and analysis of HDPE components.     

Mechanical and thermal properties of dragline silk from the spider Nephila clavipes

Dragline silk from the spider, Nephila clavipes, was characterized by thermal analysis (TGA, DSC, DMA), computational modeling, scanning electron microscopy and by quasi-static as well as high rates of strain. Thermal stability to about 230°C was observed by TGA, two transitions by DMA, ?75°C, representative of localized motion in the amorphous domain, and a main chain motion associated with partial melt at 210°C. Tensile tests indicated average initial modulus, ultimate tensile strength and ultimate tensile strain of 22 GPa, 1.1 GPa and 9%, respectively. The corresponding properties of the best fibers tested were 60 GPa, 2.9 GPa and 11%, respectively. High strain rates (>50,000%/sec) indicated similar mechanical properties to the average values indicated above. Microscopy showed compressive and tensile strains to failure of 34%. Computational modeling yielded a crystal modulus of 200 GPa.     

Surface modification of ultra‐high molecular weight polyethylene fibers by chromic acid

Ultra‐high molecular weight polyethylene (UHMWPE) fibers were modified by chromic acid. The effects of surface modification were evaluated with Fourier transform infrared spectroscopy (FTIR), X‐ray photoelectron spectroscopy (XPS), contact angle measurement, and scanning electron microscope (SEM). The results showed that both the content of O‐containing functional groups and surface roughness of modified fibers increased. The polar groups on the modified fiber surface decreased the contact angles with water and ethylene glycol, as evidenced by contact angle measurement. The tensile test results showed the strength and the elongation at break of UHMWPE fibers decreased but the modulus increased after chromic acid modification. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Effects of ultrahigh molecular weight polyethylene and mould temperature on morphological evolution of isotactic polypropylene at micro-injection moulding condition

The effects of ultrahigh molecular weight polyethylene (UHMWPE) and mould temperature (T_mould) on an isotactic polypropylene (iPP) matrix moulded via micro-injection were investigated via polarized light microscopy, scanning electron microscopy, differential scanning calorimetry, wide-angle X-ray diffraction and small-angle X-ray scattering. Results showed that the complex viscosity of the system increased significantly when the UHMWPE content was more than 5%; however, this viscosity decreased when the UHMWPE content was less than 5%. In addition, the addition of UHMWPE increased the onset of crystallisation temperature and the relative crystallinity of the β-form crystals in micro-injection moulded specimens. Moreover, the UHMWPE phase induced the formation of fan-shaped β crystals in iPP/UHMWPE blends. When mould temperature was 50 °C, the degree of orientation of microparts increased and the crystalline structures were highly compact. However, the relative crystallinity of the β-phase form (K_β) was lower than those prepared at 130 °C T_mould. Most importantly, well-oriented, bundle-like β crystals have been discovered for the first time in 5 wt.% UHMWPE/iPP blends obtained at 130 °C T_mould owing to the “orientation-maintenance” and “shear-amplification” effects of UHMWPE.     

Mechanical and tribological performance of UHMWPE influenced by temperature change

Water-lubricated surface bearing components experience boundary and mixed lubrication during operation. The lack of lubrication induces temperature increase, affecting the properties of the component. Ultra-high molecular weight polyethylene (UHMWPE) is commonly used for these applications and the influence of the temperature on the mechanical and tribological performance has not been clearly identified. This study evaluates the wear resistance and hardness of UHMWPE with the temperature increase in a range of 20 °C–60 °C. An important reduction of hardness and wear resistance was observed in this interval. The wear rate increased 94.8% when the temperature changed from 20 °C to 50 °C. The wear resistance decreased more rapidly than the hardness when the temperature was increased. The correlation between hardness and wear rate is less consistent when the hardness value was below 4.12 (Hv_0.3), reported at 40 °C. Plastic deformation and adhesion were highly enhanced with temperature.