Log Time Scale Research Articles

Quantitative Hydrogen/Deuterium Exchange Mass Spectrometry.

This Account describes considerations for the data generation, data analysis, and data interpretation of a hydrogen/deuterium exchange-mass spectrometry (HDX-MS) experiment to have a quantitative argument. Although HDX-MS has gained its popularity as a biophysical tool, the argument from its data often remains qualitative. To generate HDX-MS data that are more suitable for a quantitative argument, the sequence coverage and sequence resolution should be optimized during the feasibility stage, and the time window coverage and time window resolution should be improved during the HDX stage. To extract biophysically meaningful values for a certain perturbation from medium-resolution HDX-MS data, there are two major ways: (i) estimating the area between the two deuterium buildup curves using centroid values with and without the perturbation when plotted against log time scale and (ii) dissecting into multiple single-exponential curves using the isotope envelopes. To have more accurate arguments for an HDX-MS perturbation study, (i) false negatives due to sequence coverage, (ii) false negatives due to time window coverage, (iii) false positives due to sequence resolution, and (iv) false positives due to allosteric effects should be carefully examined.

Analysis of Isothermal Physical Aging of Polycarbonate from Tensile Creep Tests

Physical aging behavior of polycarbonate has been determined by performing a series of tensile creep tests at different temperatures. A series of short-term creep tests along with finite amount of relaxation time in between the creep tests were performed in a sequence on a thin film of polycarbonate at a temperature below its glass transition temperature (Tg). The sequence of tests was repeated at 3 different temperatures. Compliance during each creep test was measured and this data was fitted using Kohlraush functions. All the Kohlraush curves are then shifted on to a reference curve on a log-time scale using a program called Phyage. The amount of shift or shift factors of each Kohlraush curve onto the total reference curve are used to determine the shift rate at that particular temperature. The shift rate is used to predict of the amount of physical aging experienced by the material at that particular temperature. The shift rate of the polycarbonate film is determined at three different temperatures.

Stress limits for aramid fibres

Aramid fibres should have many structural applications: they should be used as tendons in prestressed concrete, as stay cables in bridges and as ropes in marine industry owing to their good tensile properties, low weight and lack of corrosion. However, uncertainty about their ability to carry significant loads for a long period of time has meant that engineers have been reluctant to adopt them. Two new techniques (stepped isothermal method and stepped isostress method) are now available that allow accelerated testing to be carried out at low stress levels, in such a way that the long-term creep and creep rupture properties can be determined without having to extrapolate more than one decade on a log-time scale. Such tests have now been carried out on two slightly different aramid fibres: Kevlar 49 and Technora. The paper shows how this information can be used to predict the safe operating stresses for these fibres when under sustained load, which is precisely the sort of application for which they are most economically suited. The effects of yarn variability are considered, as are the effects of different temperatures and varying loads. The aim is to be able to predict the behaviour of aramid fibres when subject to load durations of 100 years or more, without having to provide excess material because of a lack of applicable test data.

Geometrically Nonlinear Shell Element for Hygrothermorheologically Simple Linear Viscoelastic Composites

A triangular flat shell element for large deformation analysis of linear viscoelastic laminated composites is presented. Hygrothermorheologically simple materials are considered for which a change in the hygrothermal environment results in a horizontal shifting of the relaxation moduli curves on a log time scale, in addition to the usual hygrothermal loads. Recurrence relations are developed and implemented for the evaluation of the viscoelastic memory loads. The nonlinear deformation process is computed using an incremental/iterative approach with the Newton-Raphson Method used to find the incremental displacements in each step. The presented numerical examples consider the large deformation and stability of linear viscoelastic structures under deformation-independent mechanical loads, deformation-dependent pressure loads, and thermal loads. Unlike elastic structures that have a single critical load value associated with a given snapping of buckling instability phenomenon, viscoelastic structures will usually exhibit a particular instability for a range of applied loads over a range of critical times. Both creep buckling and snap-through examples are presented here. In some cases, viscoelastic results are also obtained using the quasielastic method in which load-history effects are ignored, and time-varying viscoelastic properties are simply used in a series of elastic problems. The presented numerical examples demonstrate the capability and accuracy of the formulation.

Recovery Behavior of Amorphous Poly(Ethylene Terephthalate) Film Uniaxially Drawn at Different Temperatures

Homogeneously deformed amorphous poly(ethylene terephthalate) (PET) film samples were obtained by uniaxial drawing at temperatures from near the glass transition temperature T g to far above T g. Isothermal dimensional recovery behavior was studied for the free stress state of the deformed sample. It was shown that the dimensional recovery behavior of samples drawn near T g was different from that of a sample drawn about 20°C above T g, indicating that the deformation mechanisms changed when the drawing temperature rose from near T g to far above T g. Recovery curves of samples drawn near T g show two distinct steps in log time scale; the second step follows a single exponential function. The infrared measurement of a drawn sample showed that the dichroic function decreased during recovery in the first step and was almost invariant at a very low value (<0.03) in the second step. However, the recovery curve of samples drawn about 20°C above T g demonstrated nearly a straight line in log time scale.

Thermoviscoelastic Analysis of Composite Structures Using a Triangular Flat Shell Element

To accurately model the structural response of e ber-reinforced polymer-matrix composites, it is necessary to account for viscoelastic behavior of the polymer matrix. Therefore, the geometrically linear dynamic analysis capability of a recently developed triangular e at shell element for elastic composite structures is extended here to include thermoviscoelasticity. The e at shell element is the combination of the discrete Kirchhoff theory plate bending element and a membrane element similar to the Allman triangle, but derived by transforming the linear strain triangle element. Linear viscoelastic composite materials are modeled, resulting in the relaxation moduli being expressed as Prony series. Hygrothermorheologically simple materials are considered for which a change in the hygrothermal environment results in a horizontal shifting of the relaxation moduli curves on a log timescale, in addition to the usual hygrothermal loads. The resulting hereditary integral terms are evaluated using a recursion relationship requiring only the previous two solutions. The Newmark method is used to incorporate the inertia terms. Numerical examples are presented to demonstrate the accuracy of the present formulation when compared with correspondence principle solutions and results available in the literature.

Thermoviscoelastic analysis of composite structures using a triangular flat shell element

To accurately model the structural response of fiber-reinforced polymer-matrix composites, it is necessary to account for viscoelastic behavior of the polymer matrix. Therefore, the geometrically linear dynamic analysis capability of a recently developed triangular flat shell element for elastic composite structures is extended here to include thermoviscoelasticity. The flat shell element is the combination of the discrete Kirchhoff theory plate bending element and a membrane element similar to the Allman triangle, but derived by transforming the linear strain triangle element. Linear viscoelastic composite materials are modeled, resulting in the relaxation moduli being expressed as Prony series. Hygrothermorheologically simple materials are considered for which a change in the hygrothermal environment results in a horizontal shifting of the relaxation moduli curves on a log timescale, in addition to the usual hygrothermal loads. The resulting hereditary integral terms are evaluated using a recursion relationship requiring only the previous two solutions. The Newmark method is used to incorporate the inertia terms. Numerical examples are presented to demonstrate the accuracy of the present formulation when compared with correspondence principle solutions and results available in the literature.

Stress Relaxation in Semi-Crystalline PEN Films: Physical Aging Effects

The effect of physical aging on the stress relaxation response of a semicrystalline PEN film was studied at temperatures ranging from the secondaryB* transition (-70'C) up to its "amorphous" glass transition temperature (-120'C). Within this temperature range, the response is thermorheologically simple, i.e., the effect of physical aging is to shift the stress relaxation function along the log timescale to longer times following a simple powerlaw superposition rule with a characteristic temperaturedependent exponents (= aging rate parameter). They (T) function peaks at -80'C, and it follows a general pattern commonly observed in other polymer systems. Within the temperature range studied, the relaxation function also obeys timetemperature superposition and the corresponding shift factor appears to follow Arrhenius activation. The apparent activation energy is, however, strongly dependent on aging timeit increases nearly threefold over the range of aging times coveredwhich can be rationalized on the basis of the effective time theory.

Development of an Accelerated Creep Testing Procedure for Geosynthetics, Part II: Analysis

Abstract A procedure for extrapolating creep strains to longer time intervals was developed. The procedure was based on applying time-shift factors to creep strain curves at elevated temperatures to establish a master strain curve for longer time intervals. The procedure was applied to creep test results on high density polyethylene (HDPE) geogrid tested at creep loads up to 40% of its maximum tensile strength Tmax and at temperatures up to 72°C (160°F). Temperature creep curves of 1000-h tests were shifted along the log-time scale to create 10 000-h master strain curves at each loading level. The established master curves were compared with experimental results of 10 000-h creep tests at room temperature and at the same loading levels. The shift factors were then applied to the creep curves to predict creep response for a duration of more than 100 000-h (about two cycles shift on the log-time scale from the initial 1000-h tests). The analysis demonstrated the applicability of applying shift factors to elevated-temperature creep test results to predict the response at longer times for this type of geogrid. The analysis was applicable to creep loads up to 40% Tmax. The procedure, however, did not accurately estimate creep response at higher loading levels and temperatures where accelerated creep failure occurred.

Broadband time-domain-reflectometry dielectric spectroscopy using variable-time-scale sampling

Methods for increasing the bandwidth of time-domain-reflectometry (TDR) dielectric spectroscopy using variable-time-scale sampling are presented. Consecutive segments of the TDR transient are sampled with increasing time increments and the entire transient transformed into the frequency domain using a running Laplace transform. Instrumentation artifacts are identified and controlled by examining reflected transients for stray artifacts prior to transformation, either on individual time scales or on a composite log time scale. Transform algorithms are verified using spice simulation in both time and frequency domains. Results are presented for a conducting salt solution, showing continuous dielectric spectra from 100 kHz to 5 GHz in the frequency domain, and requiring acquisition over six decades of time in the time domain to capture the entire response.

Two-level systems and low-temperature glass dynamics: Spectral diffusion and thermal reversibility of hole-burning linewidths

Intermediate time-scale time-dependent hole-burning measurements are reported on three glassy organic systems which undergo spectral diffusion: cresyl violet in ethanol at 1.30 and 2.13 K, resorufin in ethanol at 2.13 K, and resorufin in glycerol at 2.13 K. The hole width is observed to broaden on a log time scale from 0.1 to 5000 s for each ethanol system while no broadening is observed in the system of resorufin in glycerol. A detailed theoretical treatment is introduced which allows the raw data to be converted to the fluctuation rate distribution of the underlying modes responsible for dephasing. Using this theory, the broadening in ethanol is found to be the result of a distribution of glassy modes which is Gaussian on a log R scale with a center rate at ∼0.02 s−1. In addition, temperature cycling hole-burning results are reported on the system cresyl violet in ethanol. A hole is burned at 1.30 K and detected before, during, and after a temperature cycle to 2.13 K and back. The hole width is observed to broaden at the high temperature and then narrow again in a completely reversible manner when the temperature is again lowered. Theoretical calculations show this behavior to be entirely consistent with the tunneling two-level-system (TLS) model of glass dynamics but incompatible with other models such as particle or defect diffusion. The cycling data is shown to fall exactly on the theoretical curve calculated from the TLS model using no adjustable parameters.

Dependence of relaxation times of glassy polymers on their specific volume

This paper deals with the volume relaxation and simultaneous changes in mechanical properties that occur after complicated thermal histories. The histories were chosen such that, at the final measuring temperature, the specific volume v passes through a maximum with increasing time. It is shown that, for polystyrene, poly(vinyl chloride) and polycarbonate, the mechanical creep properties change as if the position on the log time scale of the creep curve is uniquely determined by the momentary value of v. Thus, a maximum in the volume relaxation curve implies that the creep curve first shifts to the left and later to the right.

The viscoelastic properties of wool and the influence of some specific plasticizers

The relaxation behaviour of Lincoln wool fibres in water and in three short chain aliphatic alcohols was measured. Applying a two phase model including a relaxation function based on a log-normal distribution of relaxation times a good systematic description of the data was found. The application of the model to the relaxation curves of wool fibers in the alcoholic media showed the invariance of the shape of the relaxation function, its shift on the log-time scale and the change of the elastical properties of the morphological components of the wool fibre depending on the aliphatic chain length of the alcohols. The importance of the apolar interactions for the elastic and viscoelastic properties of the morphological phases in the wool fibre is discussed.

Nonlinear viscoelastic behavior of wool fibers in a single step relaxation test

Experimental data and their analysis are presented on the nonlinear viscoelastic behavior of wool fibers in extensional stress–relaxation up to strains of 18.5%. The analysis, assuming a two-phase structure for the fiber, consists of superimposing the experimental curves onto the master curve for the nonaging material by multiplicative scaling and shifting on the logtime scale. The scaling and shift factors reflect the strain induced phase changes of the morphological components. Though different in their physical nature, these transitions show the same strain dependence.

NMR study of polyethylene crystallization kinetics under high pressure

AbstractCrystallization of polyethylene under hydrostatic pressures of 1–4.5 kbar is directly observed using pulsed proton NMR. The rate of growth of extended‐chain polyethylene crystals is measured over this pressure range and to a maximum temperature of 227°C. The observed crystallization isotherms are superimposable on a log time scale; this implies a consistent mechanism for extended‐chain growth over this pressure range. Avrami coefficients for high‐pressure extended‐chain crystallization are determined to be 1.3–1.7. A decrease of crystal nucleus surface free energies with increasing pressure is indicated. Findings are consistent with Wunderlich's model of initial folded‐chain crystallization followed immediately by chain extension. Future applications of this NMR technique are briefly considered.

Physical ageing of PMMA and polycarbonate in the region of secondary relaxation

Physical ageing of two amorphous polymers (PMMA and polycarbonate) was studied by dielectric measurements in the range 80Hz-10 5Hz and by thermally stimulated depolarization currents. It is shown that secondary relaxations do not age by simple shifting along the log time scale but rather through a reduction in relaxation magnitude.

Extension of Unoriented Nylon 66 Filaments

The orientation of nylon 66 filaments usually occurs by a cold-drawing process. This process has previously been described, in phenomenological terms, by the adiabatic theory of cold drawing of Muller and Jackel and Marshall and Thompson. To explain the process in structural terms, tensile creep experiments under low loads have been performed in an atmosphere of controlled temperature and humidity. The extension first increases uniformly with log time; then there is a rather sharp yield point where the extension quickly increases by a large amount. The location of the yield point on the log time scale is quite sensitive to environmental conditions. Phenomenologically, the behavior is similar to the propagation of Lüder's bands in metals. These results are not consonant with the adiabatic theory; therefore, the recent crystal defect model of nylon 66 proposed by Zaukelies has been used to suggest a qualitative explanation in terms of dislocations.

Erratum: “Theory of Viscoelasticity of Amorphous Polymers II. A Note on the Time-Temperature Reducibility of the Relaxation and Retardation Spectra”

Equivalence of temperature change and shift of log-time scale is usually assumed empirically in obtaining a master curve of relaxation and retardation spectra in polymers. In this paper the molecular theoretical basis of this assumption–which we call here the time-temperature reducibility –is elucidated by an examination of the generalized diffusion equation in the configuration space of a polymer chain. Several criterions for the reducibility are deduced as the results of the analysis which in fact give a gurantee to the reducibility in the case of simple intra-chain relaxation phenomena in amorphous polymers. Critical examinations of some other cases are also made.

Técnica para o estudo da estabilidade de agregados do solo

Os autores apresentam neste trabalho uma técnica para o estudo da estabilidade de agregados do solo. Vários tipos de solo sob diferentes condições de uso foram estudados. Amostras em triplicato, de 25 g de agregados entre 7 e 4 mm, foram agitados em agitador modêlo Wagner, com 40 r.p.m., durante 1, 2, 4, 8, 16, 32, 64 e 128 minutos e depois fracionadas pelo método da peneiragem lenta em água. Os agregados &gt; 2 mm diminuíram em pêso com o aumento do tempo de agitação, aumentando por outro lodo os agregados &lt; 0,2 mm. Os agregados médios (2-0,2 mm) variaram em peso de forma diferente nos vários solos. Estudou-se onaliticamente a relação funcional entre o tempo de agitação e o pêso dos agregados, pois os intervalos de tempos de agitação estudados formavam uma progressão geométrica, que proporciona intervalos iguais em escala logarítmica. A análise estatística revelou diferenças altamente significativas tanto para os teores médios das classes de agregados &gt; 2 e &lt; 0,2 como para a interação classes x intervalos de agitação. Um polinômio do 2.° grau quase sempre explica de forma satisfatória a relação funcional para essas duas classes de agregados. É apresentada, também, a marcho para o cálculo dos valores esperados resultantes da adaptação de um polinômio do 2.° grau. Os valores dos coeficientes A e B' dos diferentes polinômios permitiram estudar a influência, sôbre os agregados do solo, das diferentes condições de usa a que o solo estava sujeito. A técnica apresentada para o estudo do estabilidade dos agregados do solo foi bastante eficiente e apresentou pequena variabilidade experimental.

MECHANICAL BEHAVIOR OF WOOL FIBERS

Relaxation curves of wool fiber streched 10-25 per cent in aqueous media and phenol solution have been firstly determined. A conditioning temperature of 50°C was chosen, being as high as can be used without chemically damaging the wool fiber. Conditioning for half an hour at this temperature between measurements resulted in reproducible relaxation curves, as long as the period of extention of the fiber did not exceed 30 per cent.An apparatus used is constructed with the magnetostriction constructed for measuring the relaxation of stretched fiber. The output from the strain meter is applied to inkwriting oscillograph from the start for 10 seconds and after this time range value of valve voltmeter was measured.From the relaxation curves the reciprocal absolute temperatures corresponding to each test and the time required to attain the stress of 3.5×105g/cm2_??_4.75×105g/cm2 are obtained. The points for each specified stress are seen to lie in straight line. From the slope of these lines, the energies of activation for relaxation process in water are found from each deformation (15, 20 and 25 per cent) to be approximately 10Kcal, 20Kcal and 25_??_30 Kcal.The amount of breaking of hydrogen bonds depends on the concentration of phenol solution and temperature. With increasing temperature and concentration of phenol solution the rate of the stress relaxation becomes smaller than under the wet condition. Supplementary measurements of relaxation at various humidity were perfomed during 1/10sec_??_3000sec appling the former method. The each relaxation curve may be shifted log time scale and master relaxation curve may be constructed.