Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100° C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

Stress measurement in alumina scales on high temperature alloys using X-ray stress evaluation and laser Raman spectroscopy

The effect of silicon and titanium on the spallation resistance of alumina scales grown on NiCrAlY-type alloys has been investigated using model alloys with different additions of Si or Ti. For this purpose cyclic oxidation experiments have been carried out at temperatures between 950 and 1100 degrees C. After various times stresses in selected Si-doped samples have been determined by X-ray stress evaluation (XSE) at ambient temperature. The compressive stresses in the scales have been found to increase with an increasing oxidation time tending to become constant for long times. The development of stress is affected by the presence of Si. Laser Raman spectroscopy (LRS) has been calibrated for strain measurement using XSE results. Then LRS has been applied for strain measurement at higher temperatures.     

Viscoelastic behavior of polyvinylcyclohexane

The tensile stress relaxation master curve for polyvinylcyclohexane (completely hydrogenated polystyrene) has been measured. Direct relaxation experiments were carried out at several temperatures above the glass transition temperature over the rather long time range of four orders of magnitude. This long time span was realized by calculating the modulus during the period when a constant small strain rate was applied to the sample as well as during the usual constant strain interval. A computer solution to the Boltzmann superposition equation allowed data from these two regions to be joined into a smooth curve representing E(t), a parameter indicative of an instantaneous strain experiment. The measured T_i was found to be 143°C; T_g is expected to fall within several degrees of this temperature. This result is apparently at odds with a previously reported T_g value of 120°C. More importantly, the maximum value of the negative slope of the stress relaxation master curve of polyvinylcyclohexane in the primary transition region was only slightly different from that for polystyrene. This observation clearly indicates that the molecular factors which result in the highly coupled nature of the primary transition in polystyrene are not strongly dependent upon any side-chain π–π interactions which might be present in polystyrene.     

Rheo-optical studies of high polymers. XIV. Study of the deformation mechanism in polymer blends of polypropylene with ethylene–propylene rubber

To clarify the deformation mechanism in polyblends of polypropylene with ethylene–propylene rubber having different compositions, simultaneous measurements of the infrared dichroism with stress and strain under a constant rate of strain of 1.64%/min have been carried out. The orientation function of the crystallographic c axis of polypropylene in the blends has been obtained as a function of strain ranging from 0 to 20% and of polypropylene content ranging from 0.3 to 1.0. These results have been compared with the temperature dependences of the dynamic Young's modulus and of the loss modulus, as well as of stress–strain curves for the same blends. The modulus data analyzed by Kerner's equation reveal the occurrence of phase inversion at polypropylene contents higher than about 0.5, and this is supported by the infrared dichroism data. The strong effect of quenching on crystalline structure and mechanical properties of pure polypropylene has also been elucidated.     

Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

Time-dependent temperature distribution of graphite-tube atomizers

The time behavior of the temperatures of cylindrical graphite atomizers have been measured as a function of the distance from the tube center. From these measurements the evolution of the axial temperature distribution during a ramp-hold-step was evaluated. Starting at a ramp rate of 200°C s^?1, deviations of the distribution from the equilibrium function has been observed. For ramp rates above 1000°C s^?1, a nearly constant temperature over the largest part of the tube has been found during the ramp step when the heating started at ambient temperature. The transition of this distribution into the equilibrium function can be described satisfactorily by solving the differential equation for heat conduction.     

Viscoelastic properties of Nafion at elevated temperature and humidity

Tensile stress–strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0–100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ～ 0.3. At temperatures ≥90 °C, hydrated membranes are stiffer than dry membranes. Stress‐relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 10³–10⁴ s. Water absorption slows down stress‐relaxation rates. At high water activity, the maximum stress relaxation time was >10⁵ s at all temperatures. No maximum relaxation time was seen at T ≤ 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >10⁵ s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 10³–10⁴ s. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 11–24, 2009.     

Dynamic mechanical and crystal-strain measurements on ultrahigh modulus oriented polyoxymethylene: A revised value for the true crystal modulus

Dynamic mechanical measurements on ultrahigh modulus polyoxymethylene have been undertaken over the temperature range ?150 to 20°C. Measurements of the longitudinal crystal modulus have also been made by studying changes in the (009) reflection with load, over a similar range of temperatures. The dynamic Young's modulus at 5 Hz reaches a value at low temperatures of 64.5 GPa for the most highly oriented sample. The crystal modulus at low temperatures is 105 GPa, which is almost twice the previously reported room-temperature value.     

Tensile properties of ultradrawn polyethylene

High-density polyethylene filaments prepared by a solid-state deformation in an Instron capillary rheometer show unusually high crystal orientation, chain extension, axial modulus, and ultimate tensile strength. The Young's modulus and ultimate tensile strength have been determined from stress–strain curves. Gripping of this high modulus polyethylene has been a problem heretofore, but the measurement of ultimate tensile strength has now been made feasible by a special gripping procedure. Tensile moduli show an increase with sample preparation temperature and pressure. Values as high as 6.7 × 10¹¹ dyne/cm² are obtained from samples extruded at 134°C and 2400 atm and tested at a strain rate of 3.3 × 10^?4 sec^?1. The effect of strain rate and frequency on modulus has also been evaluated by a combination of stress–strain data and dynamic tension plus sonic measurements over nine decades of time.     

The influence of low-temperature crystallization on the tensile elastic modulus of natural rubber

When natural rubber crystallizes at low temperatures, there is an increase in elastic modulus of up to two orders of magnitude. This phenomenon has been studied at various temperatures in the range 0 to ?55°C for samples held at tensile strains of up to 500%. There is an induction period associated with the nucleation of crystallites, before any increase in modulus is observed. The induction period increases with decreasing strain and passes through a minimum with increasing temperature at ?25°C. The growth rate subsequent to nucleation is successfully described in terms of Avrami-type rate relationships. The Avrami rate coefficient is independent of temperature and follows a simple exponential function of strain. The equilibrium extent of the modulus increase has also been studied by means of experiments of up to three months' duration. The equilibrium modulus increases with decreasing temperature—as predicted by Flory's thermodynamic theory.     

Generation and decay of peroxy radicals in deformed polyethylene

The rate of peroxy radical accumulation as a function of strain at various temperatures in AC1220 high molecular weight polyethylene has been determined by EPR spectroscopy. The results of isothermal radical decay experiments are used, where appropriate, to correct the apparent accumulation rate to the actual rate. An exponential dependence of radical concentration [R], on true strain is observed at all temperatures investigated in the range from 160 to 294°K. For constant effective strain, measured from the approximate strain at which radical accumulation initiates, it is found that d[R]/de exhibits two sharp transitions as a function of temperature. One of these, at low temperature, is believed to be associated with the glass transition of the amorphous phase of the material; the other, at higher temperature, is believed to occur as a result of a change in the rate-controlling mechanism of deformation.     

Viscoelasticity of Shear Thickening Fluid Based on Silica Nanoparticles Dispersing in 1-butyl-3-methylimidizolium Tetrafluoroborate

The viscoelasticity of shear thickening fluid (STF), a crucial property in the protective composite applications, with different silica nanoparticle concentrations in ionic liquid, 1-butyl-3-methylimidizolium tetrafluoroborate ([C₄min]BF₄), was studied at different temperatures and with shear frequencies through oscillatory shear, respectively. All STFs present strain thickening behavior. With increasing silica nanoparticle concentration, the critical shear strain for the onset of strain thickening decreased, while the complex viscosity, storage modulus, and loss modulus increased significantly. The critical shear strain increased with an increase of temperature, while the complex viscosity, storage modulus, and loss modulus decreased notably. The critical shear strain was constant with increasing the frequency of strain, while the complex viscosity decreases slightly. The storage modulus and loss modulus were independent with frequency in the strain thickening region. Nanoparticle clusters leading to strain thickening were demonstrated. The viscoelastic response of STFs to varying silica nanoparticle content, temperature, and frequency investigated here will help to design the specific application of STFs in soft protective composites and damping devices.     

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

We performed dynamic Monte Carlo simulations of stress relaxation in parallel-aligned and uniaxially stretched bulk amorphous polymers at low temperatures.We observed an extra-slowing down in the early stage of stress relaxation,which causes nonlinear viscoelasticity as deviated from Debye relaxation and Arrhenius-fluid behaviors observed previously at high temperatures.Meanwhile,fluctuation analysis of stress relaxation revealed a substantial increase in the stretch fractions of polymers at the transient periods of high-temperature Debye relaxation.Structural analysis of free volume further revealed the scenario that,at low temperatures,the modulus of polymer entropy elasticity decreases with temperature and eventually loses its competition to the imposed modulus (Deborah number becomes larger than one),and hence upon stress relaxation under constant strains,monomers are firstly accumulated nearby two stretching ends of polymers,resulting in tentative global jamming like physical cross-linking there,and thus retarding the coming transient state of stress relaxation.We concluded that intermolecular cooperation raises physical crosslinking for nonlinear viscoelasticity of polymer stress relaxation as well as the rubbery states unique to bulk amorphous polymers.The new microscopic mechanism of the fluid-rubbery transition of polymers may bring insights into the intermolecular cooperation mechanism of glass transition of small molecules,if the fluid-rubbery transition is regarded as an extrapolation of glass transition from low to high molecular weights.     

Blends of poly(methyl methacrylate) (PMMA) with PMMA ionomers: Mechanical properties

Rigid–rigid blends made of ionomer and ionomer precursor polymer, based on poly(methyl methacrylate) (PMMA), have been investigated. Two series of blends have been prepared for studying mechanical properties. In one series, dynamic mechanical properties were determined over a wide range of temperatures. As the weight fraction of the ionomer was increased, there was a modest increase of modulus at ambient temperature and a very large increase in the rubbery modulus at elevated temperatures above the glass transition temperature of PMMA. In a second series of tests, tensile stress–strain measurements, made at an ambient temperature, were carried out over a wide range of blend compositions. For all blends tested, the mechanical properties exhibited a synergistic enhancement, i.e., average values of modulus, strength and fracture energy were all higher than expected based on the rule of mixtures. Measurements of fracture toughness also exhibited synergy, with a maximum value, higher than the value of either blend component, being attained in blends containing about 30 wt % of the PMMA ionomer. These results are interpreted in terms of a higher resistance to fracture of the more chain-entangled ionomer phase and good interfacial adhesion between the two components of the blend. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1235–1245, 1998     

Gelatinization Mechanism of Rice Starch

ABSTRACT

The non-Newtonian behavior and dynamic viscoelasticity of rice starch (Nihonbare; amylose content, 15.8%) solutions were measured with a rheogoniometer. A gelatinization of Nihonbare starch occurred above 3.0% after heating at 100 °C for 30 min. The Nihonbare starch showed shear-thinning behavior at a concentration of 2.0%, but plastic behavior above 3.0% at 25 °C. The viscosity of Nihonbare starch at a concentration of 2.0% solution decreased gradually with increase in temperature from 10 to 55 °C, then it stayed at a constant value with further increase in the temperature. However, for 4.0% solution, rapid decrease in the viscosity was observed after the temperature reached 25 °C up to 50 °C, then it stayed at a constant value. The dynamic modulus of Nihonbare starch stayed at a constant value during increase in the temperature at 4%. The tan δ of the starch showed low values, 0.28, at low temperature range and stayed at a constant up to 30 °C, then it increased a little with increasing temperature. A little decrease of dynamic modulus of Nihonbare starch was observed at low temperature range upon addition of urea (4.0 M). The dynamic modulus, however, decreased rapidly when the temperature reached 50 °C, which was estimated to be a transition temperature. The dynamic modulus also decreased rapidly in 0.10 M NaOH solution above 50 °C. A possible mode of intermolecular hydrogen bonding between amylose and amylopectin molecules of Nihonbare starch is proposed. The short chains (A and B1) of the amylopectin molecules may take part in the intermolecular association in aqueous solution.     

Detecting Mechanochemical Degradation of Nitrocellulose by Combining Dynamic Mechanical Analysis with Mass Spectrometry

Summary: Dynamic mechanical analysis is combined with mass spectrometry to study nitrocellulose under oscillating strain. At a constant temperature (150–160 °C) and frequency (400–600 Hz) nitrocellulose fractures demonstrating a modulus drop and release of products with m/z: 30 and 44. At linear heating (2 °C · min⁻¹) and a frequency of 10–50 Hz similar products are released in two steps, the second of which demonstrates a modulus drop and a temperature increase indicating ignition.

Data for an isothermal dynamic mechanical analysis–mass spectrometry experiment performed at 160 °C.     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

Crystallinity in PEEK and PEK after mechanical testing and its dependence on strain rate and temperature

This work extends the results of previous workers on the influence of drawing on the crystallinity of PEEK and PEK to test temperatures well above T_g and to strain rates up to 10³ s⁻¹. The study thus includes measurement of crystallinity in samples tested in the strain rate regime where large increases in flow stress have previously been noted in these and other polymers. The results are in reasonable agreement with other workers on PEEK and are representative of the behavior of both PEEK and PEK at temperatures up to 200°C and strain rates up to 10² s⁻¹. However at a strain rate of 10³ s⁻¹ a dramatic increase in crystallinity and reduction in d-spacing is observed. It is speculated that a change in crystalline morphology induced by the high rate testing may account for the observed changes in spacing, crystallinity, and flow stress. © 1996 John Wiley & Sons, Inc.     

Thermal analysis of ring‐opening metathesis polymerized healing agents

Dicyclopentadiene (DCPD) and 5‐ethylidene‐2‐norbornene (ENB) and their mixtures were analyzed after ring‐opening metathesis polymerization (ROMP) in the presence of Grubbs catalyst as potential candidate healing agents for self‐healing composite materials using two complementary methods, rotational dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). Following isothermal DMA measurements at room temperature (RT = 25 °C) for 120 min, two consecutive dynamic temperature scan experiments were performed for each system. In the first dynamic temperature scans, there was an initial downward peak slightly above RT in the storage modulus versus temperature curve for samples with relatively slower reaction rates (i.e., DCPD and DCPD‐rich mixtures or low catalyst loadings) due to a combination of the glass transition followed by further residual reaction. However, no or negligible downward peaks were observed for the highly reactive ENB and ENB‐rich samples even at much lower catalyst loadings. Implications of the substantial decrease in storage modulus just above RT for the slowly reacting systems are discussed for healing of damage in composite materials at elevated temperatures. The maximum glass transition temperatures (T_g∞) from DMA of the fully cured samples were determined to be approximately 160 °C for DCPD and 120 °C for ENB, decreasing linearly with increased ENB in the blends. The glass transitions and further residual reactions above the glass transitions were confirmed by DSC. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1771–1780, 2007     

Photoviscoelastic behavior of amorphous polymers during transition from the glassy to rubbery state

Tensile stress‐relaxation experiments with simultaneous measurements of Young's relaxation modulus, E, and the strain‐optical coefficient, C_?, were performed on two amorphous polymers—polystyrene (PS) and polycarbonate (PC)—over a wide range of temperatures and times. Master curves of these material functions were obtained via the time‐temperature superposition principle. The value of C_? of PS is positive in the glassy state at low temperature and time; then it relaxes and becomes negative and passes through a minimum in the transition zone from the glassy to rubbery state at an intermediate temperature and time and then monotonically increases with time, approaching zero at a large time. The stress‐optical coefficient of PS is calculated from the value of C_?. It is positive at low temperature and time, decreases, passes through zero, becomes negative with increasing temperature and time in the transition zone from the glassy to rubbery state, and finally reaches a constant large negative value in the rubbery state. In contrast, the value of C_? of PC is always positive being a constant in the glassy state and continuously relaxes to zero at high temperature and time. The value of C_σ of PC is also positive being a constant in the glassy state and increases to a constant value in the rubbery state. The obtained information on the photoelastic behavior of PS and PC is useful for calculating the residual birefringence and stresses in plastic products. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2252–2262, 2001