Elida Hermida - Academia.edu (original) (raw)

Papers by Elida Hermida

Research paper thumbnail of Description of the Mechanical Properties of Viscoelastic Materials Using a Modified Anelastic Element

Physica Status Solidi B-basic Solid State Physics, 1993

Quasi-static and dynamical tests are employed to measure the modulus or the compliance of a mater... more Quasi-static and dynamical tests are employed to measure the modulus or the compliance of a material as a function of time or frequency, and to determine the respective distribution function in the case of a linear viscoelastic medium. The evolution of the mechanical properties, and consequently the distribution functions, are usually described by empirical expressions which cannot always be interpreted according to a physical model. Furthermore, as a rule these empirical distributions do not lead to closed expressions for all the viscoelastic functions. The mechanical properties of viscoelastic materials (amorphous polymers, metallic glasses, etc.) are described using a modified anelastic element (MAE). This modified element is analogous to the standard anelastic element, having a single characteristic time, τ, that is not a constant but a function of time or frequency. On considering a particular functional dependence of τ, it is demonstrated how several empirical viscoelastic functions are approximations of the mechanical properties of the MAE. Moreover, this functional dependence is related to the log-normal distribution the statistical and physical meanings of which are discussed in detail.

Research paper thumbnail of Gamma Irradiation Effects on Mechanical and Thermal Properties and Biodegradation of Poly(3-hydroxybutyrate) Based Films

Macromolecular Symposia, 2008

... Furthermore, they determined that chain scission occurred within amor-... Hence, g-irradiatio... more ... Furthermore, they determined that chain scission occurred within amor-... Hence, g-irradiation damages mainly the amor-phous phase by chain scission or, if the crystalline phase is dominant, it introduces defects in the ordered lamellae. ...

Research paper thumbnail of Transcrystallization kinetics at the poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/hemp fibre interface

Composites Part A-applied Science and Manufacturing, 2007

Transcrystallization of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) on hemp fibres was invest... more Transcrystallization of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) on hemp fibres was investigated using polarized optical microscopy. Nucleation and growth of crystals of the transcrystalline region (TCR) were measured during isothermal treatments of samples located in a hot stage. The nucleation rate, nucleation density at saturation and growth rate of the TCR, were determined at crystallization temperatures between 320 and 363K.The growth rate of crystals in the TCR is the same as that of spherulites in the bulk. Nucleation at the hemp fibres was properly described in terms of the heterogeneous nucleation theory and the transcrystalline growth agrees with the kinetics theory of polymer crystallization. From the empirical data we determined that the interfacial free energy difference function of PHBV on hemp fibres is Δσ=4.4erg/cm2, lower than the one in the bulk. This indicates that the substrate is not neutral but contributes to the formation of the TCR.From the intensity of the depolarization light, measured under different cooling rates, we determined the activation energies for crystallization in the bulk and transcrystallization.

Research paper thumbnail of Shear stress at the interface of coaxial composites determined by the resonance of low-amplitude vibrations

Philosophical Magazine, 2003

The aim of this work is to determine the viscoelastic behaviour of the interface in a coaxial com... more The aim of this work is to determine the viscoelastic behaviour of the interface in a coaxial composite material made of a tough shield and a ductile core. The elastic modulus and the amplitude-independent internal friction are measured using a longitudinal oscillating resonant system at 50 kHz. The contribution of the interface is modelled as a shear stress that modifies

Research paper thumbnail of Temperature dependence of the viscoelastic response of In, Sn and In–Sn alloys

Journal of Alloys and Compounds, 2000

The elastic modulus and the internal friction of Sn and the g-In-Sn phase are measured with a two... more The elastic modulus and the internal friction of Sn and the g-In-Sn phase are measured with a two-component piezoelectric oscillator 26 at 50 kHz in the temperature range from 296 to 390 K. For both materials the viscoelastic responses at strains below 10 are described in terms of a Debye peak characterized by an effective activation enthalpy of 54.8 and 20.4 kJ / mol, for Sn and the g-In-Sn phase, respectively. Due to their higher damping, the dynamic mechanical responses of In and the eutectic In-Sn were measured with a three-components piezoelectric resonator between 296 and 390 K at 50 kHz. The temperature dependence of the internal friction of the eutectic can be well described in terms of a single relaxation time with an effective activation enthalpy of 38 kJ / mol. Indium is not only amplitude dependent but also exhibits thermal hysteresis.

Research paper thumbnail of INFLUENCIA DE LA INTERFAZ EN EL COMPORTAMIENTO DINÁMICO DE COMPUESTOS BIFÁSICOS COAXIALES

Este trabajo presenta mediciones del módulo elástico y fricción interna de materiales compuestos ... more Este trabajo presenta mediciones del módulo elástico y fricción interna de materiales compuestos bifásicos dispuestos en paralelo, con geometría cilíndrica coaxial, mediante la resonancia de ondas acústicas longitudinales a 50 kHz generadas por un oscilador compuesto piezoeléctrico de tres componentes, a 5ºC y en vacío (10 -2 torr).

Research paper thumbnail of RELAXATION PHENOMENA AND THE TIME-TEMPERATURE SUPERPOSITIONPRINCIPLE

Le Journal De Physique Colloques, 1987

Research paper thumbnail of Nonlinear viscoelasticity determined from quasistatic and dynamic tests

Research paper thumbnail of Mechanical Relaxation of Linear Viscoelastic Materials Described by the Modified Anelastic Element

Physica Status Solidi B-basic Solid State Physics, 1995

The stress relaxation and creep in amorphous materials, the dielectric relaxation in conducting p... more The stress relaxation and creep in amorphous materials, the dielectric relaxation in conducting polymers, the spin relaxation in spin-glasses, are examples of processes described by the mathematical formalism of the theory of linear viscoelasticity. This description, given by a spectrum or distribution function, allows to express the temporal evolution of the relaxation through integral transformations. In many cases, however, this evolution is given directly using the empirical expression exp [—;(t/τ)γ], known as fractional exponential behaviour, where τ is a characteristic relaxation time and γ is a constant (0 < γ ≦ 1). It is shown that this empirical expression can be derived from the modified anelastic element (MAE) whose relaxation time depends on the time of the quasistatic test. From this dependence the spectrum for the MAE is calculated and correlated with the log—normal distribution. A novel procedure to calculate the parameters of the MAE is presented and applied to stress relaxation curves.

Research paper thumbnail of Scaling concept and the Williams-Landel-Ferry relationship

Journal of Materials Science, 1988

The superposition properties of the Williams-Landel-Ferry relationship, when obtained empirically... more The superposition properties of the Williams-Landel-Ferry relationship, when obtained empirically for different polymer systems and different temperatures, are formulated in terms of more general functions with a scaling property. Finally, an analytical description of these superposition properties is given and the influence of the selected reference temperature on the translations needed to obtain the universalα T(T - T s) curve is discussed.

Research paper thumbnail of Phenomenological description of strain rate and temperature-dependent yield stress of PMMA

Journal of Applied Polymer Science, 1995

A constitutive equation to describe the yield behavior of poly(methyl methacrylate (PMMA) is usef... more A constitutive equation to describe the yield behavior of poly(methyl methacrylate (PMMA) is useful not only from the technological point of view, but also for the comprehension of the nonlinear mechanisms acting in the material. In both compression and tension, the yield stress is usually represented as a function of the strain rate at different temperatures. In PMMA and other glassy polymers these curves are related by scaling, that is, they can be matched to form a master curve. Particularly in PMMA the temperature and strain rate dependence of the master curve has been characterized by two different models. The first involves two thermally activated rate processes, one acting only at high strain rates. The second model interprets the yield process as a cooperative movement of several independent structural units, all with the same activation energy. In this article it is demonstrated that only the second phenomenological model is correct because it provides a good fit to the master curve of PMMA both in compression and tension, and verifies the properties of a set of curves related by scaling. Moreover, it is pointed out that the first model leads to severe inconsistencies because it does not consider the nonlinear behavior of PMMA. Finally, the physical parameters obtained (internal stress, activation volume, and enthalpy) are compared with those given in the literature. © 1995 John Wiley & Sons, Inc.

Research paper thumbnail of Internal friction and loss tangent of nonlinear viscoelastic materials: different concepts, different results

Journal of Alloys and Compounds, 2000

This paper aims to give some insight into measurements of the nonlinear processes involved in the... more This paper aims to give some insight into measurements of the nonlinear processes involved in the dynamical response of viscoelastic materials. The main goal is to point out that these nonlinear effects can be observed with forced oscillating torsion pendulums, but by measuring the complete hysteretic loop, instead of just looking at the phase lag angle. The response of a nonlinear rheological model constructed with a standard anelastic element in series with a nonlinear dashpot is considered. The characteristic parameters of this model are according to previous empirical results reported in the literature. This system is dynamically driven as if it were in a forced torsion pendulum, which is almost the optimal device for the analysis of amplitude dependent internal friction at low frequencies. The intrinsic internal friction is calculated from the ratio between the dissipated energy per cycle and the maximum stored energy for the nonlinear rheological model. The simulation of the internal friction that could be measured with a torsion pendulum for the rheological model is then determined from the intrinsic internal friction after considering the non-homogeneous strain distribution. The intrinsic internal friction and the simulated internal friction data are compared with the tangent of the phase angle by which the strain lags behind the stress in order to show that the differences are important enough to be measured. In summary, the nonlinear response could potentially be measured with commercial equipment, but usually is not, because only the phase lag angle is considered. Automatic measurements of both the temporal evolution of stress and strain in a cycle will lead to the determination of the real value of the internal friction of nonlinear systems from the hysteretic s -´loop.

Research paper thumbnail of Stress relaxation in bending of AISI 316 at 773 K

Journal of Materials Science, 2006

In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hou... more In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hours is characterized. Samples were cut either parallel or transverse to the rolling direction and treated at different temperatures prior to the bending tests. The mechanical behavior shown by the longitudinal samples is quite different from that of the transverse samples and so we conclude there must be differences at the level of their microstructures. However, the presence of sigma phase precipitates in both cases is the consequence of a stress-assisted process. Besides, intragranular carbide density in either sample proved to be a function of the relaxation time.

Research paper thumbnail of Temperature and strain rate dependence of the tensile yield stress of PVC

Journal of Applied Polymer Science, 1996

Data on the tensile yield behavior of poly(viny1 chloride) (PVC), reported in the literature, are... more Data on the tensile yield behavior of poly(viny1 chloride) (PVC), reported in the literature, are interpreted in terms of a model involving a cooperative movement of several independent structural units, all with the same activation enthalpy. This analysis leads to physical parameters such as the internal stress, activation volume, and enthalpy, etc. These values are discussed and compared with those determined from thermodynamical considerations using stress relaxation tests and tensile curves at a constant strain rate. 0

Research paper thumbnail of Changes in the mechanical properties of compression moulded samples of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) degraded by Streptomyces omiyaensis SSM 5670

Polymer Degradation and Stability, 2009

Streptomyces omiyaensis SSM 5670 was characterized by its ability to use compression moulded samp... more Streptomyces omiyaensis SSM 5670 was characterized by its ability to use compression moulded samples of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as its sole carbon source. Biodegradation of PHBV in liquid mineral salts medium was investigated using scanning electron microscopy, gravimetric measurements, capillary viscometry, tensile testing and wide angle X-ray spectroscopy. The biodegradation of PHBV proceeds via surface erosion mechanism, resulting in the formation of pits by microbial attack. PHBV specimens lost about 45% of their original weight after 45 days of exposure. During the degradation process the elastic modulus reduces less than 10%. The formation of pores and microcracks initiated at the degraded pits determines the reduction of the elongation and stress at break. However, the true stress at break is practically independent of the degradation time. No significant changes of PHBV molecular weight or crystallinity were observed during biodegradation. The polymer chain cleavage occurred only at the specimen surface and does not discriminate between crystalline and amorphous states.

Research paper thumbnail of Glass transition temperature measured at high frequencies by piezoelectric excitation

Journal of Alloys and Compounds, 2000

As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition ... more As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition temperature, T , is reached where g polymeric materials undergo a marked change in properties associated with the virtual cessation of local molecular motion. In fact, as the thermal energy reduces, the distance between macromolecules, and consequently the specific volume of the sample, diminishes. However, this change is not abrupt, that is, the glass transition is not a rigorous phase transition from the thermodynamical point of view. Effectively, it depends not only on the polymeric structure but also on the heating rate, the measuring method, etc. Since there is a copious literature referring to traditional methods employed to measure T , this work will only present them as a frame to compare them with the g experimental procedure we used. The determination of T is based on the change of the real component of the dynamic modulus with g temperature. This modulus is measured at high frequencies on exciting the sample with a piezoelectric crystal attached to it. Glass transition temperatures of several glassy polymers are determined. These results are compared with values measured through other methods. Finally, the advantages of our procedure are discussed.

Research paper thumbnail of Stress relaxation of PVC below the yield point

Journal of Polymer Science Part B-polymer Physics, 1996

Stress relaxation of commercial poly(viny1 chloride) (PVC) is measured at strains below 3% and at... more Stress relaxation of commercial poly(viny1 chloride) (PVC) is measured at strains below 3% and at different temperatures below the glass transition temperature. First it is shown that below the yield point the material follows a linear viscoelastic behavior. Then the data a t a fixed deformation level (0.03) are fitted by considering a lognormal distribution function of relaxation times. Furthermore, from the measured stress-strain curves, the temperature dependence of the elastic tensile modulus is determined. The temperature dependence of the elastic modulus, the relaxation strength, and the parameters of the distribution: mean relaxation time, T,, and half-width, p, are given. Moreover, the distribution function and the temperature dependence of its characteristic parameters are discussed in terms of a cooperative model of the mechanisms involved in the mechanical relaxation of glassy polymers. Finally, the relationship proposed between the tensile modulus and the free volume helps explain the temperature dependence of the relaxation strength. 0 1996 John Wiley & Sons, Inc.

Research paper thumbnail of Stress relaxation in bending of AISI 316 at 773 K

Journal of Materials Science, 2006

In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hou... more In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hours is characterized. Samples were cut either parallel or transverse to the rolling direction and treated at different temperatures prior to the bending tests. The mechanical behavior shown by the longitudinal samples is quite different from that of the transverse samples and so we conclude there must be differences at the level of their microstructures. However, the presence of sigma phase precipitates in both cases is the consequence of a stress-assisted process. Besides, intragranular carbide density in either sample proved to be a function of the relaxation time. C 2006 Springer Science + Business Media, Inc.

Research paper thumbnail of Distribution Function Parameters Determined from Dynamic Mechanical Spectroscopy Data

Physica Status Solidi B-basic Solid State Physics, 1994

The dynamical properties of linear viscoelastic materials are related to the strength of relaxati... more The dynamical properties of linear viscoelastic materials are related to the strength of relaxation as well as to the statistical parameters of the corresponding distribution function. Nowick and Berry established numerically the relationships between the real and imaginary moduli or compliances, the strength of relaxation, and the parameters of a widely used distribution function: the log-normal one. These relationships are modified, applying a perturbation theory, in order to get the parameters from an internal friction peak measured as a function of temperature. Recently, a modified anelastic element (MAE) has been introduced for describing the mechanical properties, particularly the internal friction, of linear viscoelastic solids. It has been shown that the distribution function associated to the MAE is quite similar to a log-normal distribution. Therefore, in the present paper the perturbation theory developed by Nowick and Berry is applied to the internal friction peak for the MAE. It is found that the approximate values calculated for the MAE are in excellent agreement with the analytical parameters associated to this element and also with the approximate expressions derived for the log-normal distribution. However, this treatment is not enough to determine the temperature dependence of the parameters except if very accurate frequency data are measured. Consequently, an alternative procedure is proposed.

Research paper thumbnail of Analysis of Stress Relaxation and Creep Curves for a Log-Normal Distribution of Relaxation Times

Physica Status Solidi B-basic Solid State Physics, 1989

A procedure to determine the characteristic parameters of a log-normal distribution of relaxation... more A procedure to determine the characteristic parameters of a log-normal distribution of relaxation times, both for stress relaxation and creep tests, is presented. The method is based on the use of either the normalized stress relaxation or creep functions and their first derivatives. Furthermore, the limiting stress or strain can be obtained directly from the experimental data. Finally, the procedure developed is applied to actual stress relaxation curves reported for aluminum and polysulphide rubber and the results are compared with those obtained by approximate methods, based on the use of the error function.Es wird ein Verfahren zur Bestimmung der charakteristischen Parameter einer Log-Normalverteilung der Relaxationszeiten sowohl für Spannungsrelaxation als auch für Kriechtests angegeben. Die Methode beruht auf der Verwendung entweder der normierten Spannungsrelaxations- oder Kriechfunktionen und ihren ersten Ableitungen. Darüber hinaus kann die Grenzspannung oder -dehnung direkt aus den experimentellen Daten erhalten werden. Schließlich wird das entwickelte Verfahren auf aktuelle Spannungsrelaxationskurven für Aluminium und Polysulfid-Gummi angewendet und die Ergebnisse mit denen verglichen, die durch Näherungsmethoden auf der Grundlage von Fehlerfunktionen erhalten werden.

Research paper thumbnail of Description of the Mechanical Properties of Viscoelastic Materials Using a Modified Anelastic Element

Physica Status Solidi B-basic Solid State Physics, 1993

Quasi-static and dynamical tests are employed to measure the modulus or the compliance of a mater... more Quasi-static and dynamical tests are employed to measure the modulus or the compliance of a material as a function of time or frequency, and to determine the respective distribution function in the case of a linear viscoelastic medium. The evolution of the mechanical properties, and consequently the distribution functions, are usually described by empirical expressions which cannot always be interpreted according to a physical model. Furthermore, as a rule these empirical distributions do not lead to closed expressions for all the viscoelastic functions. The mechanical properties of viscoelastic materials (amorphous polymers, metallic glasses, etc.) are described using a modified anelastic element (MAE). This modified element is analogous to the standard anelastic element, having a single characteristic time, τ, that is not a constant but a function of time or frequency. On considering a particular functional dependence of τ, it is demonstrated how several empirical viscoelastic functions are approximations of the mechanical properties of the MAE. Moreover, this functional dependence is related to the log-normal distribution the statistical and physical meanings of which are discussed in detail.

Research paper thumbnail of Gamma Irradiation Effects on Mechanical and Thermal Properties and Biodegradation of Poly(3-hydroxybutyrate) Based Films

Macromolecular Symposia, 2008

... Furthermore, they determined that chain scission occurred within amor-... Hence, g-irradiatio... more ... Furthermore, they determined that chain scission occurred within amor-... Hence, g-irradiation damages mainly the amor-phous phase by chain scission or, if the crystalline phase is dominant, it introduces defects in the ordered lamellae. ...

Research paper thumbnail of Transcrystallization kinetics at the poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/hemp fibre interface

Composites Part A-applied Science and Manufacturing, 2007

Transcrystallization of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) on hemp fibres was invest... more Transcrystallization of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) on hemp fibres was investigated using polarized optical microscopy. Nucleation and growth of crystals of the transcrystalline region (TCR) were measured during isothermal treatments of samples located in a hot stage. The nucleation rate, nucleation density at saturation and growth rate of the TCR, were determined at crystallization temperatures between 320 and 363K.The growth rate of crystals in the TCR is the same as that of spherulites in the bulk. Nucleation at the hemp fibres was properly described in terms of the heterogeneous nucleation theory and the transcrystalline growth agrees with the kinetics theory of polymer crystallization. From the empirical data we determined that the interfacial free energy difference function of PHBV on hemp fibres is Δσ=4.4erg/cm2, lower than the one in the bulk. This indicates that the substrate is not neutral but contributes to the formation of the TCR.From the intensity of the depolarization light, measured under different cooling rates, we determined the activation energies for crystallization in the bulk and transcrystallization.

Research paper thumbnail of Shear stress at the interface of coaxial composites determined by the resonance of low-amplitude vibrations

Philosophical Magazine, 2003

The aim of this work is to determine the viscoelastic behaviour of the interface in a coaxial com... more The aim of this work is to determine the viscoelastic behaviour of the interface in a coaxial composite material made of a tough shield and a ductile core. The elastic modulus and the amplitude-independent internal friction are measured using a longitudinal oscillating resonant system at 50 kHz. The contribution of the interface is modelled as a shear stress that modifies

Research paper thumbnail of Temperature dependence of the viscoelastic response of In, Sn and In–Sn alloys

Journal of Alloys and Compounds, 2000

The elastic modulus and the internal friction of Sn and the g-In-Sn phase are measured with a two... more The elastic modulus and the internal friction of Sn and the g-In-Sn phase are measured with a two-component piezoelectric oscillator 26 at 50 kHz in the temperature range from 296 to 390 K. For both materials the viscoelastic responses at strains below 10 are described in terms of a Debye peak characterized by an effective activation enthalpy of 54.8 and 20.4 kJ / mol, for Sn and the g-In-Sn phase, respectively. Due to their higher damping, the dynamic mechanical responses of In and the eutectic In-Sn were measured with a three-components piezoelectric resonator between 296 and 390 K at 50 kHz. The temperature dependence of the internal friction of the eutectic can be well described in terms of a single relaxation time with an effective activation enthalpy of 38 kJ / mol. Indium is not only amplitude dependent but also exhibits thermal hysteresis.

Research paper thumbnail of INFLUENCIA DE LA INTERFAZ EN EL COMPORTAMIENTO DINÁMICO DE COMPUESTOS BIFÁSICOS COAXIALES

Este trabajo presenta mediciones del módulo elástico y fricción interna de materiales compuestos ... more Este trabajo presenta mediciones del módulo elástico y fricción interna de materiales compuestos bifásicos dispuestos en paralelo, con geometría cilíndrica coaxial, mediante la resonancia de ondas acústicas longitudinales a 50 kHz generadas por un oscilador compuesto piezoeléctrico de tres componentes, a 5ºC y en vacío (10 -2 torr).

Research paper thumbnail of RELAXATION PHENOMENA AND THE TIME-TEMPERATURE SUPERPOSITIONPRINCIPLE

Le Journal De Physique Colloques, 1987

Research paper thumbnail of Nonlinear viscoelasticity determined from quasistatic and dynamic tests

Research paper thumbnail of Mechanical Relaxation of Linear Viscoelastic Materials Described by the Modified Anelastic Element

Physica Status Solidi B-basic Solid State Physics, 1995

The stress relaxation and creep in amorphous materials, the dielectric relaxation in conducting p... more The stress relaxation and creep in amorphous materials, the dielectric relaxation in conducting polymers, the spin relaxation in spin-glasses, are examples of processes described by the mathematical formalism of the theory of linear viscoelasticity. This description, given by a spectrum or distribution function, allows to express the temporal evolution of the relaxation through integral transformations. In many cases, however, this evolution is given directly using the empirical expression exp [—;(t/τ)γ], known as fractional exponential behaviour, where τ is a characteristic relaxation time and γ is a constant (0 < γ ≦ 1). It is shown that this empirical expression can be derived from the modified anelastic element (MAE) whose relaxation time depends on the time of the quasistatic test. From this dependence the spectrum for the MAE is calculated and correlated with the log—normal distribution. A novel procedure to calculate the parameters of the MAE is presented and applied to stress relaxation curves.

Research paper thumbnail of Scaling concept and the Williams-Landel-Ferry relationship

Journal of Materials Science, 1988

The superposition properties of the Williams-Landel-Ferry relationship, when obtained empirically... more The superposition properties of the Williams-Landel-Ferry relationship, when obtained empirically for different polymer systems and different temperatures, are formulated in terms of more general functions with a scaling property. Finally, an analytical description of these superposition properties is given and the influence of the selected reference temperature on the translations needed to obtain the universalα T(T - T s) curve is discussed.

Research paper thumbnail of Phenomenological description of strain rate and temperature-dependent yield stress of PMMA

Journal of Applied Polymer Science, 1995

A constitutive equation to describe the yield behavior of poly(methyl methacrylate (PMMA) is usef... more A constitutive equation to describe the yield behavior of poly(methyl methacrylate (PMMA) is useful not only from the technological point of view, but also for the comprehension of the nonlinear mechanisms acting in the material. In both compression and tension, the yield stress is usually represented as a function of the strain rate at different temperatures. In PMMA and other glassy polymers these curves are related by scaling, that is, they can be matched to form a master curve. Particularly in PMMA the temperature and strain rate dependence of the master curve has been characterized by two different models. The first involves two thermally activated rate processes, one acting only at high strain rates. The second model interprets the yield process as a cooperative movement of several independent structural units, all with the same activation energy. In this article it is demonstrated that only the second phenomenological model is correct because it provides a good fit to the master curve of PMMA both in compression and tension, and verifies the properties of a set of curves related by scaling. Moreover, it is pointed out that the first model leads to severe inconsistencies because it does not consider the nonlinear behavior of PMMA. Finally, the physical parameters obtained (internal stress, activation volume, and enthalpy) are compared with those given in the literature. © 1995 John Wiley & Sons, Inc.

Research paper thumbnail of Internal friction and loss tangent of nonlinear viscoelastic materials: different concepts, different results

Journal of Alloys and Compounds, 2000

This paper aims to give some insight into measurements of the nonlinear processes involved in the... more This paper aims to give some insight into measurements of the nonlinear processes involved in the dynamical response of viscoelastic materials. The main goal is to point out that these nonlinear effects can be observed with forced oscillating torsion pendulums, but by measuring the complete hysteretic loop, instead of just looking at the phase lag angle. The response of a nonlinear rheological model constructed with a standard anelastic element in series with a nonlinear dashpot is considered. The characteristic parameters of this model are according to previous empirical results reported in the literature. This system is dynamically driven as if it were in a forced torsion pendulum, which is almost the optimal device for the analysis of amplitude dependent internal friction at low frequencies. The intrinsic internal friction is calculated from the ratio between the dissipated energy per cycle and the maximum stored energy for the nonlinear rheological model. The simulation of the internal friction that could be measured with a torsion pendulum for the rheological model is then determined from the intrinsic internal friction after considering the non-homogeneous strain distribution. The intrinsic internal friction and the simulated internal friction data are compared with the tangent of the phase angle by which the strain lags behind the stress in order to show that the differences are important enough to be measured. In summary, the nonlinear response could potentially be measured with commercial equipment, but usually is not, because only the phase lag angle is considered. Automatic measurements of both the temporal evolution of stress and strain in a cycle will lead to the determination of the real value of the internal friction of nonlinear systems from the hysteretic s -´loop.

Research paper thumbnail of Stress relaxation in bending of AISI 316 at 773 K

Journal of Materials Science, 2006

In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hou... more In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hours is characterized. Samples were cut either parallel or transverse to the rolling direction and treated at different temperatures prior to the bending tests. The mechanical behavior shown by the longitudinal samples is quite different from that of the transverse samples and so we conclude there must be differences at the level of their microstructures. However, the presence of sigma phase precipitates in both cases is the consequence of a stress-assisted process. Besides, intragranular carbide density in either sample proved to be a function of the relaxation time.

Research paper thumbnail of Temperature and strain rate dependence of the tensile yield stress of PVC

Journal of Applied Polymer Science, 1996

Data on the tensile yield behavior of poly(viny1 chloride) (PVC), reported in the literature, are... more Data on the tensile yield behavior of poly(viny1 chloride) (PVC), reported in the literature, are interpreted in terms of a model involving a cooperative movement of several independent structural units, all with the same activation enthalpy. This analysis leads to physical parameters such as the internal stress, activation volume, and enthalpy, etc. These values are discussed and compared with those determined from thermodynamical considerations using stress relaxation tests and tensile curves at a constant strain rate. 0

Research paper thumbnail of Changes in the mechanical properties of compression moulded samples of poly(3-hydroxybutyrate- co-3-hydroxyvalerate) degraded by Streptomyces omiyaensis SSM 5670

Polymer Degradation and Stability, 2009

Streptomyces omiyaensis SSM 5670 was characterized by its ability to use compression moulded samp... more Streptomyces omiyaensis SSM 5670 was characterized by its ability to use compression moulded samples of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as its sole carbon source. Biodegradation of PHBV in liquid mineral salts medium was investigated using scanning electron microscopy, gravimetric measurements, capillary viscometry, tensile testing and wide angle X-ray spectroscopy. The biodegradation of PHBV proceeds via surface erosion mechanism, resulting in the formation of pits by microbial attack. PHBV specimens lost about 45% of their original weight after 45 days of exposure. During the degradation process the elastic modulus reduces less than 10%. The formation of pores and microcracks initiated at the degraded pits determines the reduction of the elongation and stress at break. However, the true stress at break is practically independent of the degradation time. No significant changes of PHBV molecular weight or crystallinity were observed during biodegradation. The polymer chain cleavage occurred only at the specimen surface and does not discriminate between crystalline and amorphous states.

Research paper thumbnail of Glass transition temperature measured at high frequencies by piezoelectric excitation

Journal of Alloys and Compounds, 2000

As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition ... more As the temperature of a polymer melt or rubber is lowered, a point known as the glass transition temperature, T , is reached where g polymeric materials undergo a marked change in properties associated with the virtual cessation of local molecular motion. In fact, as the thermal energy reduces, the distance between macromolecules, and consequently the specific volume of the sample, diminishes. However, this change is not abrupt, that is, the glass transition is not a rigorous phase transition from the thermodynamical point of view. Effectively, it depends not only on the polymeric structure but also on the heating rate, the measuring method, etc. Since there is a copious literature referring to traditional methods employed to measure T , this work will only present them as a frame to compare them with the g experimental procedure we used. The determination of T is based on the change of the real component of the dynamic modulus with g temperature. This modulus is measured at high frequencies on exciting the sample with a piezoelectric crystal attached to it. Glass transition temperatures of several glassy polymers are determined. These results are compared with values measured through other methods. Finally, the advantages of our procedure are discussed.

Research paper thumbnail of Stress relaxation of PVC below the yield point

Journal of Polymer Science Part B-polymer Physics, 1996

Stress relaxation of commercial poly(viny1 chloride) (PVC) is measured at strains below 3% and at... more Stress relaxation of commercial poly(viny1 chloride) (PVC) is measured at strains below 3% and at different temperatures below the glass transition temperature. First it is shown that below the yield point the material follows a linear viscoelastic behavior. Then the data a t a fixed deformation level (0.03) are fitted by considering a lognormal distribution function of relaxation times. Furthermore, from the measured stress-strain curves, the temperature dependence of the elastic tensile modulus is determined. The temperature dependence of the elastic modulus, the relaxation strength, and the parameters of the distribution: mean relaxation time, T,, and half-width, p, are given. Moreover, the distribution function and the temperature dependence of its characteristic parameters are discussed in terms of a cooperative model of the mechanisms involved in the mechanical relaxation of glassy polymers. Finally, the relationship proposed between the tensile modulus and the free volume helps explain the temperature dependence of the relaxation strength. 0 1996 John Wiley & Sons, Inc.

Research paper thumbnail of Stress relaxation in bending of AISI 316 at 773 K

Journal of Materials Science, 2006

In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hou... more In this article, stress relaxation in bending of AISI 316 stainless steel at 773 K during 490 hours is characterized. Samples were cut either parallel or transverse to the rolling direction and treated at different temperatures prior to the bending tests. The mechanical behavior shown by the longitudinal samples is quite different from that of the transverse samples and so we conclude there must be differences at the level of their microstructures. However, the presence of sigma phase precipitates in both cases is the consequence of a stress-assisted process. Besides, intragranular carbide density in either sample proved to be a function of the relaxation time. C 2006 Springer Science + Business Media, Inc.

Research paper thumbnail of Distribution Function Parameters Determined from Dynamic Mechanical Spectroscopy Data

Physica Status Solidi B-basic Solid State Physics, 1994

The dynamical properties of linear viscoelastic materials are related to the strength of relaxati... more The dynamical properties of linear viscoelastic materials are related to the strength of relaxation as well as to the statistical parameters of the corresponding distribution function. Nowick and Berry established numerically the relationships between the real and imaginary moduli or compliances, the strength of relaxation, and the parameters of a widely used distribution function: the log-normal one. These relationships are modified, applying a perturbation theory, in order to get the parameters from an internal friction peak measured as a function of temperature. Recently, a modified anelastic element (MAE) has been introduced for describing the mechanical properties, particularly the internal friction, of linear viscoelastic solids. It has been shown that the distribution function associated to the MAE is quite similar to a log-normal distribution. Therefore, in the present paper the perturbation theory developed by Nowick and Berry is applied to the internal friction peak for the MAE. It is found that the approximate values calculated for the MAE are in excellent agreement with the analytical parameters associated to this element and also with the approximate expressions derived for the log-normal distribution. However, this treatment is not enough to determine the temperature dependence of the parameters except if very accurate frequency data are measured. Consequently, an alternative procedure is proposed.

Research paper thumbnail of Analysis of Stress Relaxation and Creep Curves for a Log-Normal Distribution of Relaxation Times

Physica Status Solidi B-basic Solid State Physics, 1989

A procedure to determine the characteristic parameters of a log-normal distribution of relaxation... more A procedure to determine the characteristic parameters of a log-normal distribution of relaxation times, both for stress relaxation and creep tests, is presented. The method is based on the use of either the normalized stress relaxation or creep functions and their first derivatives. Furthermore, the limiting stress or strain can be obtained directly from the experimental data. Finally, the procedure developed is applied to actual stress relaxation curves reported for aluminum and polysulphide rubber and the results are compared with those obtained by approximate methods, based on the use of the error function.Es wird ein Verfahren zur Bestimmung der charakteristischen Parameter einer Log-Normalverteilung der Relaxationszeiten sowohl für Spannungsrelaxation als auch für Kriechtests angegeben. Die Methode beruht auf der Verwendung entweder der normierten Spannungsrelaxations- oder Kriechfunktionen und ihren ersten Ableitungen. Darüber hinaus kann die Grenzspannung oder -dehnung direkt aus den experimentellen Daten erhalten werden. Schließlich wird das entwickelte Verfahren auf aktuelle Spannungsrelaxationskurven für Aluminium und Polysulfid-Gummi angewendet und die Ergebnisse mit denen verglichen, die durch Näherungsmethoden auf der Grundlage von Fehlerfunktionen erhalten werden.