Changes in mechanical behavior during fatigue of semicrystalline thermoplastics

The tensile fatigue behavior of two engineering thermoplastics (polyacetal and nylon_6,6) were studied by measuring changes in the dynamic viscoelastic response together with changes in potential energy density, strain energy density, and irreversible work. The results show that both stress softening and hardening can occur in controlled load cyclic conditions. At high stress levels and/or frequencies, both the polyacetal and nylon_6,6 show evidence of thermal softening as characterized by changes in their dynamic viscoelastic properties and decrease in storage modulus with corresponding increases in loss modulus and loss tangent. This effect is supported by observed decreases in the overall crystallinity as measured in DSC experiments. At lower stress levels (the mechanically dominated region), all results indicate that, although fatigue crack propagation (FCP) is one of the mechanisms governing the fatigue life, its contribution is minor and crack initiation time constitutes the majority of the fatigue life. Also, during the initiation stage, both materials become less viscoelastic and more elastic. This phenomenon is evidenced by overall reductions in the loss modulus, loss tangent, and irreversible work densities while the storage modulus is maintained. © 1995 John Wiley & Sons, Inc.     

Generating Line Spectra from Experimental Responses. III. Interconversion between Relaxation and Retardation Behavior

Abstract

An algorithm is described which allows relaxation line spectra to be interconverted into retardation line spectra, and vice versa. The first line spectrum is generated from a given experimental response such as the relaxation modulus, the storage modulus, or the loss modulus, or from a response such as the creep compliance, storage compliance, or loss compliance. The interconversion algorithm is here applied to the standard linear solid and liquid models, and to a 32-line spectrum simulating the behavior of an entangled polymer.     

Effect of test frequency and water content on localized crack-tip heating in nylon-6,6

Fatigue crack propagation (FCP) tests were conducted at various frequencies on nylon-6,6 specimens equilibrated over a range of moisture levels to determine the crack growth rates and the crack-tip temperatures as a function of water content. Frequency-sensitivity was correlated with the amount of crack-tip heating taking place, with crack-tip temperature being found to depend strongly on the estimated loss compliance, D″, of the material. The frequency-sensitivity of FCP in nylon was seen also to be affected by mean stress, suggesting that creep processes are often significant in FCP of nylon.     

Overload effect on the fatigue crack propagation of PC/ABS alloy

The effect of single overload within an otherwise constant amplitude loading sequence on the fatigue crack propagation (FCP) behavior of the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length of the specimen. Optical and scanning electron microscopes are used to observe the features of crack surface and the process of crack tip deformation. The overload waveform has slight effect, while the overload ratio has great effect on the crack growth retardation. A small crack increment is produced during overloading. The crack growth rate reduces quickly, and then increases gradually until it reaches the steady crack growth rate level when the loading recovers to normal constant amplitude fatigue loads. Porous or dimple features govern the fatigue crack surfaces.     

Dynamic mechanical studies of oriented p-oxybenzoate (POB)-poly(ethylene terephthalate) (PET) copolyester films

The effects of orientation on the dynamic mechanical properties of p-oxybenzoate/poly(ethylene terephthalate) (POB/PET (60/40)) copolyesters were studied using films prepared by extrusion drawing and by uniaxially drawing sections of compression moulded sheets. The extrusion drawn sample ED-1 showed a higher degree of orientation than uniaxially drawn samples U-2S and U-8S. The loss tangent curves for these oriented samples are characterized by a β-relaxation at 62°C and a high temperature (′) relaxation. The position of the ′-relaxation peak is dependent on the degree of orientation. Samples U-8S (f_H = 0.05), U-2S (_H = 0.06) and ED-1 (_H = 0.21), have their ′ relaxation peak at 135, 140 and 156°C, respectively. The dynamic storage modulus (E′) below 50°C increases with the degree of orientation. The highly oriented sample ED-1 maintains its high storage modulus (4 GPa) over a larger temperature range (25–125°C).     

Inorganic–organic hybrid materials with zirconium oxoclusters as protective coatings on aluminium alloys

Inorganic–organic hybrid materials are attracting a strong scientific interest mainly for their outstanding inherent mechanical and thermal properties, which can be traced back to the intimate coupling of both inorganic and organic components. By carefully choosing the experimental parameters used for their synthesis, chemically and thermally stable acrylate-based hybrid material embedding the zirconium oxocluster Zr₄O₄(OMc)₁₂, where OMcCH₂C(CH₃)C(O)O, can be deposited as UV-cured films on aluminium alloys.

In particular, the molar ratios between the oxocluster and the monomer, the polymerisation time, the amount of photo-initiator and the deposition conditions, by using an home-made spray-coating equipment, were optimised in order to obtain the best performing layers in terms of transparency and hardness to coat aluminium alloy (AA1050, AA6060 and AA2024) sheets. Furthermore, it was also evaluated whether the hybrid coatings behave as barrier to corrosion.

Several coated samples were prepared and characterised. Environmental scanning electronic microscopy (ESEM) and scratch test were used to investigate the morphology of the films and to evaluate their scratch resistance, respectively. Electrochemical impedance spectroscopy (EIS) was performed in order to evaluate if the coatings actually protect the metallic substrate from corrosion.

In order to measure shear storage modulus (G′) and loss modulus (G″) of the materials used for coatings, bulk samples were also obtained by UV-curing of the precursors solution. Dynamical mechanical thermal analysis (DMTA) was performed in shear mode on cured disks of both the hybrid materials and pristine polymer for comparison. The values of T_g were read off as the temperatures of peak of loss modulus. The length and mass of all the samples were measured before and after the DMTA analysis, so that the shrinkage of the materials in that temperature range was exactly evaluated.     

Rejuvenation and physical ageing of a polycarbonate film subjected to finite tensile strains

Differential storage and loss tensile moduli, E′ and E″, were determined intermittently at 10 Hz on specimens of an annealed polycarbonate film during stress relaxation at static tensile strains from 1.2 to 6.25% at 50°C. It was found that E′ and 1/E″ decrease when a static strain is applied but thereafter they increase progressively with time. These changes, which increase with the applied strain until it becomes 4%, are attributed primarily to a rejuvenation of a specimen (an increase in segmental mobility) followed by physical ageing (a progressive decrease in segmental mobility). Measurements at a static strain of 3% at six temperatures from 30°C to 130°C showed, among other things, that the rates of increase of E′ and decrease of E″ are sensibly independent of temperature up to 110°C.     

In-situ measurement of viscoelastic properties of fresh cement paste by a microrheology analyzer

A microrheology analyzer was adapted to in-situ follow the development of viscoelastic properties of fresh cement pastes (FCPs) for the first time. It enables a non-disturbing measurement on the FCPs through monitoring the mean square displacement of cement particles, which gives an insight into the elastic and viscous properties of materials from a microstructural point of view. Various parameters including elastic index, macroscopic viscosity index, storage modulus, loss modulus and Maxwell parameters were obtained to quantitatively analyze the viscoelastic properties of FCPs. Results indicate that these parameters show a progressive increase with time at first and then stay stable. The incorporation of superplasticizer significantly decreases these parameters and their growth rates. Moreover, superplasticizer could evidently weaken the elastic feature of the FCP due to its effects of improving the dispersion of cement grains and retarding cement hydration. The effects of superplasticizer are more pronounced at lower water to cement ratio.     

Evaluation of fatigue lifetime and elucidation of fatigue mechanism in plasticized poly(vinyl chloride) in terms of dynamic viscoelasticity

The fatigue behavior of plasticized poly(vinyl chloride) was investigated by means of a tension–compression-type fatigue apparatus. Complex elastic modulus and mechanical loss tangent were obtained continuously with time under the conditions of constant ambient temperature as a function of imposed strain amplitude. Brittle failure was observed under the conditions of low ambient temperatures and small strain amplitudes, or forced convection of air, whereas thermal failure was observed under the conditions of high ambient temperatures, or large strain amplitudes and natural convection of air. In the case of brittle failure, the dynamic storage modulus E′ exhibited a maximum and the loss tangent tanδ exhibited a minimum on approaching the point of failure. In the case of thermal failure, E′ decreased and tanδ increased monotonously until the onset of thermal failure. It was found that failure occurs when the effective energy loss reaches a certain magnitude depending on an ambient temperature. The fatigue criterion was represented schematically from a standpoint of self-heating. When the heat generation rate of the specimen under cyclic staining is larger than that of the heat transfer to the surroundings, thermal failure takes place. In this case, the specimen temperature increases up to a limiting constant temperature corresponding to the α-absorption temperature. When the heat generation rate is nearly equal to that of the heat transfer to the surroundings, the specimen temperature does not change appreciably and brittle failure takes place.     

Study of Hybridized Kenaf/Palf-Reinforced Hdpe Composites by Dynamic Mechanical Analysis

This article presents an experimental study on the dynamic mechanical property of hybridized Kenaf/PALF-reinforced HDPE composites. Variation in storage modulus (E′), loss modulus (E″) and damping parameter (tan δ) with fiber loading and variation in fiber length were investigated. The concept of hybridization was also discussed as it affects the dynamic properties. Initial storage modulus (E′) of all hybrids at different fibre ratios have been enormously improved compared to pure HDPE, and dependence of modulus on cellulose content of natural fibres was very clear. A lower percentage of PALF is required for hybridization with kenaf bast fibre to achieve a positive hybridization effect. Adequate hybridization could impart higher impact strength to the composite. The dynamic modulus curve showed an increase in the E′ value with increase in operating temperature up to about 130°C and is at a maximum at higher fibre loading. At lower temperatures, 60% of fibre loading had reduced the loss modulus peak of the pure HDPE. At temperature range of 30 to 65°C, incorporation of the fibres helped reduce the E″ peak of the matrix. Increasing the fibre content of the hybrids raised the damping peak with temperature. In addition, there is an increase in storage modulus with increased fibre length at room temperature up to about 65°C. Above this temperature, variation in fibre length became irrelevant up to the less viscous point of the matrix. A marginal difference in loss modulus with variation in fibre length was observed, no difference could be seen in the case of loss tangent (tan delta) in regard to variation in fibre length.     

Dynamic moduli change of aluminum hydroxide sol during sol-gel transition

Aluminum hydroxide sol was prepared by the sol-gel method. Dynamic moduli such as storage and loss moduli were measured to investigate the relative dominance of elastic and viscous contributions to the viscoelastic response of aluminum hydroxide sol during sol-gel transition. The loss modulus, a measure of viscous response. is larger than the storage modulus, a measure of elastic response, for the sols of low particle concentrations. But at a high particle concentration above 15.3 wt%, the storage modulus is found to be larger than the loss modulus. This inversion from a viscous to an elastic response with the increase of particle concentration is attributable to the formation of gel structures by the aggregation of dispersed particles     

Morphology and viscoelastic properties of melt‐spun HDPE/hydrotalcite nanocomposite fibers

Viscoelastic properties of nanocomposite fibers of high density polyethylene (HDPE) and organically modified hydrotalcite were studied. Neat and nanofilled HDPE fibers (with nanofiller content between 0.5 and 3 wt%) were produced by melt spinning and hot‐drawing at different draw ratios up to 20. Effect of temperature on storage modulus, loss modulus, and creep compliance were compared. Rising nanofiller content and/or drawing ratio accounted for an increase in storage modulus in the glassy (i.e., below the γ transition at −100°C) as well as in the rubbery state of non‐crystalline regions. The α relaxation temperature read‐off for the maximum of the loss modulus peak ranged from 20 to 60°C being dependent on frequency, filler content and draw ratio. Sumita model was successfully applied to evaluate the effective volume fraction of the dispersed phase; maximum fraction of immobilized matrix was observed for the composite with 1 wt% of nanofiller. Creep behavior was evaluated by fitting experimental data with the Burgers model. The addition of a small amount of well‐dispersed hydrotalcite (0.5–1 wt%) had a beneficial effect on the creep resistance of drawn fibers at room temperature as well as at 70°C. TEM analysis evidenced a good dispersion of 0.5% nanofiller in as‐spun fibers and improved interfacial adhesion after drawing. The best mechanical properties were observed for the composition with 1 wt% of hydrotalcite, due to combined effects of nanofiller reinforcement and stiffening produced by hot drawing. POLYM. COMPOS., 288–298, 2016. © 2014 Society of Plastics Engineers     

Investigation on the dynamic melt rheological properties of polypropylene/wood flour composites

The rheological behavior of polypropylene/wood flour (WF) composite was investigated at constant temperature over a wide range of frequencies using a mechanical compact rheometer operated in the dynamic mode. The effect of WF content, particle size, and coupling agent on melt rheological properties were investigated. The melt rheological data in terms of complex viscosity (η*), storage modulus (G′), loss modulus (G″), and loss tangent (tan δ) were studied and compared for different samples. It was found that complex viscosity increases with increasing wood content and coupling agent. Compatibilization using coupling agent increased both storage modulus and loss modulus, but the variation of storage modulus is more. By increasing wood content storage modulus increases. Complex viscosity, storage modulus, and loss modulus showed a minimum value by increasing of wood particle size. Tan δ decreases with increasing of wood content. Cole–Cole plot indicated that relaxation process changes with addition WF, coupling agent, and using different mesh size of wood. The Han plots revealed the sensitivity of rheological properties with composition at constant temperature. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Fatigue crack propagation of rubber-toughened epoxies

Epoxies containing epoxy-terminated butadiene acrylonitrile rubber (ETBN) or amino-terminated butadiene acrylonitrile rubber (ATBN) were prepared and studied in terms of fatigue crack propagation (FCP) resistance and toughening mechanisms. Rubber incorporation improves both impact and FCP resistance, but results in slightly lower Young's modulus and T_g As T_g increases, the degree of toughening decreases. Rubber-induced shear yielding of the epoxy matrix is believed to be the dominant toughening mechanism. Decreasing fatigue resistance with increasing cyclic frequency is observed for both neat and rubber-toughened epoxies. This result may be explained by the inability of these materials to undergo possible beneficial effects of hysteretic heating. FCP resistance is linearly proportional to M_c^1/2, where M_c is the apparent molecular weight between crosslinks determined on the rubber-toughened material. FCP resistance also increases with increasing static fracture toughness K_IC. ATBN-toughened epoxies demonstrated better fatigue resistance than ETBN-toughened systems.     

Physical Characterization of a Commercial Suspoemulsion as a Reference for the Development of Suspoemulsions

Viscoelasticity, low shear flow, microstructure, and physical stability of a commercial suspoemulsion pesticide are studied to serve as a point of reference for the development of new suspoemulsions. Creep compliance tests allow the zero‐shear rate viscosity, η₀, to be calculated. As expected, the values of η₀ decrease with temperature. The storage modulus is higher than the loss modulus until a crossover frequency, ω*, which is located at the lower frequencies studied. The dynamic parameters decrease with higher temperature and ω* increases in agreement with a faster relaxation mechanism. Multiple light scattering predicts the occurrence of a creaming destabilization process, whose kinetics depends on temperature. Rheological measurements are demonstrated to be a powerful tool to assist in the prediction of destabilization processes together with the multiple light scattering technique.     

Rheological study of crosslinking and gelation in chlorobutyl elastomer systems

The multiple waveform rheological technique was used to study the crosslinking behaviour of a filled elastomeric system at various temperatures. The gel point could be precisely determined from a single experiment at each temperature using this technique. At the instant of gelation, the storage (G′) and loss (G″) moduli scale with frequency in an identical manner, i.e. ωⁿ. The relaxation exponent, n was found to be approximately constant (0.14) in the temperature range studied. Reported values of n for chemically crosslinking systems are typically higher; the low value in this case can be attributed to the high molecular weight of the prepolymer and the presence of filler. The gel times at various temperatures were used to calculate an apparent activation energy of the curing reaction, which was found to be approximately 92 kJ mol⁻¹.     

轮胎橡胶材料动态力学研究

采用德国 GABO公司500N EPLEXOR动态粘弹谱仪在拉伸和剪切模式下对轮胎橡胶材料进行温度/频率扫描,研究结果表明,在动态温度/频率扫描情况下,频率一定,随着温度升高,胶料的储能模量、损耗模量和损耗因子逐渐降低;而当温度固定时,随着扫描频率加快,储能模量、损耗模量和损耗因子的整体变化趋势是上升的。此外,在移位因子拟合时,Gauss拟合移位因子的准确度比WLF方程拟合移位因子的准确度更高,WLF方程在高温区拟合效果比低温区更佳,并且采用Gauss拟合的移位因子拓宽得到的胶料粘弹主曲线频率范围与DMA测试仪自动拓展的胶料粘弹主曲线的频率范围相比更宽。     

PVA/Nano-SiO_2薄膜的制备表、征及性能研究

采用熔融法制备了聚乙烯醇(PVA)/纳米二氧化硅(nano-SiO2)薄膜。利用FTIR、DMA﹑AFM﹑水煮、DMSO煮等方法对薄膜进行了表征,并探讨了nano-SiO2对PVA薄膜的改性机理。结果表明:nano-SiO2的加入使PVA/nano-SiO2薄膜的储能模量和损耗模量均明显提高,其玻璃化转变温度随着nano-SiO2含量的增加先降低后升高;PVA/nano-SiO2薄膜中存在着由羟基脱水而产生的化学交联结构,这种结构影响着薄膜的各种性能。     

PTFE基复合材料动态力学性能的研究

将聚四氟乙烯(PTFE)和几种添加物采用机械共混、冷压、烧结成型的方法制备了PTFE基复合材料。用粘弹分析仪测试了复合材料的动态力学性能,得到了损耗因子、储能模量及损耗模量随温度变化的曲线,用扫描电子显微镜观察了PTFE与添加物的结合状况。结果表明,PTFE基复合材料的储能模量随添加物含量的增加而增大：加入聚苯硫醚(PPS)的PTFE基复合材料的损耗因子曲线只出现一个明显的峰,峰值变大;而加入聚苯酯、聚全氟(乙烯／丙烯)共聚物、聚醚醚酮后可使PTFE基复合材料的损耗因子峰值变小,当含量在某一范围时,复合材料的损耗因子曲线出现双峰,此时可拓宽复合材料的有效阻尼温域;随着石墨、MoS2含量的增加,PTFE基复合材料的储能模量提高,损耗因子峰值变小。     

Phase transition in swollen gels

Summary The dynamic mechanical behaviour of 4% aqueous solutions and networks of poly (N, N-diethylacrylamide) and copolymers of diethylacrylamide with sodium methacrylate (MNa) (molar ratio x_MNA=0–0.05) swollen in water was measured in the temperature range 20–80 °C. With increasing temperature, at T_c polymer chains collapse from random coil to more compact globular conformations. While in the region of coil conformations (T > T_c) the mechanical behaviour of solutions has a liquid-like character (the loss modulus G″ is higher than the storage modulus G′ for a constant frequency ω=1 Hz), in the region of globular conformations (T > T_c) a heterogeneous physical network is built in solutions, and the mechanical behaviour has a solid-like character (G′ > G″). In networks the collapse is reflected in an increase of storage modulus G′; the magnitude of this increase decreases with ionization. The dependence of the loss modulus G″ on the temperature of solutions and networks allows us to conclude that the magnitude of losses in the collapsed state is affected rather by x_MNa (the modulus G″ increases with increasing ionization) than by the heterogeneous structure of the samples.