Plastic instability in amorphous selenium near its glass transition temperature

2010 ◽  
Vol 25 (6) ◽  
pp. 1015-1019 ◽  
Author(s):  
Caijun Su ◽  
James A. LaManna ◽  
Yanfei Gao ◽  
Warren C. Oliver ◽  
George M. Pharr
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Activation Parameters ◽  
Plastic Instability ◽  
Amorphous Selenium ◽  
Strain Rates ◽  
Creep Tests ◽  
Rate Dependent ◽  
Deformation Modes

The deformation behavior of amorphous selenium near its glass transition temperature (31 °C) has been investigated by uniaxial compression and nanoindentation creep tests. Cylindrical specimens compressed at high temperatures and low strain rates deform stably into barrel-like shapes, while tests at low temperatures and high strain rates lead to fragmentation. These results agree well with stress exponent and kinetic activation parameters extracted from nanoindentation creep tests using a similarity analysis. The dependence of the deformation modes on temperature and strain rate can be understood as a consequence of material instability and strain localization in rate-dependent solids.

Attenuation and Velocity of Ultrasonic Waves in Amorphous Selenium in the Vicinity of the Glass Transition

1977 ◽  
Vol 32 (9) ◽  
pp. 946-951
Author(s):  
Erwin Kittinger
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Ultrasonic Waves ◽  
Amorphous Selenium ◽  
Approximate Determination ◽  
Stabilization Process ◽  
Nonequilibrium States ◽  
Longitudinal Ultrasonic Waves

AbstractAttenuation and velocity of longitudinal ultrasonic waves have been measured in a-Se in the vicinity of the glass transition. Both quantities are shown to be affected significantly by the stabilization process below the glass transition temperature Tg. Equilibrium values of sound velocity are also reported for the range 25 °C to 45 °C. The decrease (increase) of attenuation (velocity) during stabilization is reversed at higher temperatures. The connection of both quantities is discussed in terms of an effective temperature which allows the approximate determination of equilibrium values of attenuation (and possibly of other structure related properties) from measurements performed in nonequilibrium states.

Glass transformation phenomena in bulk and film amorphous selenium via DSC heating and cooling scans

1990 ◽  
Vol 5 (4) ◽  
pp. 789-794 ◽  
Author(s):  
S. Yannacopoulos ◽  
S.O. Kasap
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Structural Relaxation ◽  
Amorphous Selenium ◽  
Retardation Time ◽  
Transformation Kinetics ◽  
Glass Transformation ◽  
Constant Rate ◽  
The Mean

Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature (Tg) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r, and cooling, at constant rate q, scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of a–Se, within experimental errors, log(r/T2gm) vs 1/Tgm plot where Tgm is the peak glass transformation temperature, and log(r) vs 1/Tgh plot, where Tgh is the glass transition onset temperature from DSC heating scans, are parallel to the log(q) vs 1/Tgc plot where Tgc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, τ, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, η(T), implying that the mean structural retardation time is proportional to the viscosity, τ ∼ η. The results also confirm that the earlier studies of glass transformation kinetics in a–Se utilizing only DSC heating scans remain meaningful.

Large Strain Mechanical Behavior of Poly(methyl methacrylate) (PMMA) Near the Glass Transition Temperature

2006 ◽  
Vol 128 (4) ◽  
pp. 559-563 ◽  
Author(s):  
G. Palm ◽  
R. B. Dupaix ◽  
J. Castro
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Methyl Methacrylate ◽  
Mechanical Behavior ◽  
Strain Hardening ◽  
Uniaxial Compression ◽  
Large Strain ◽  
Strain Rates ◽  
Poly Methyl Methacrylate

The mechanical behavior of amorphous thermoplastics, such as poly(methyl methacrylate) (PMMA), strongly depends on temperature and strain rate. Understanding these dependencies is critical for many polymer processing applications and, in particular, for those occurring near the glass transition temperature, such as hot embossing. In this study, the large strain mechanical behavior of PMMA is investigated using uniaxial compression tests at varying temperatures and strain rates. In this study we capture the temperature and rate of deformation dependence of PMMA, and results correlate well to previous experimental work found in the literature for similar temperatures and strain rates. A three-dimensional constitutive model previously used to describe the mechanical behavior of another amorphous polymer, poly(ethylene terephthalate)-glycol (PETG), is applied to model the observed behavior of PMMA. A comparison with the experimental results reveals that the model is able to successfully capture the observed stress-strain behavior of PMMA, including the initial elastic modulus, flow stress, initial strain hardening, and final dramatic strain hardening behavior in uniaxial compression near the glass transition temperature.

Looking at Rubber Adhesion

1979 ◽  
Vol 52 (1) ◽  
pp. 23-42 ◽  
Author(s):  
A. D. Roberts
Keyword(s):  
Surface Roughness ◽  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Surface Energy ◽  
Crosslink Density ◽  
Central Theme ◽  
Rubber Compound ◽  
Rate Dependent ◽  
Peel Adhesion

Abstract Rubber has been employed in contact with a variety of surfaces for decades, yet the adhesional mechanisms involved are not fully understood. This article describes fundamental investigations carried out over the last decade. Emphasis is placed upon the use of optical techniques for looking directly at the contact area rubber makes with another surface. The interpretation is based upon a rate-dependent surface energy approach. In this way it is possible to predict the level of peel adhesion between surfaces, for example, when a ball rolls on smooth rubber. Other examples treated in the same way are the time for detachment of a ball from a smooth rubber track under gravity, its resilience when bounced on the track, and its friction when slid over the track. The influence of surface roughness, electrostatic forces, surface bloom, and humidity are considered, together with rubber compound variables such as glass transition temperature, crosslink density, and fillers. The central theme is that the adhesional mechanisms all physically depend upon the product of surface properties and bulk viscoelasticity of the solids in contact.

High strain-rate shear response of polycarbonate and polymethyl methacrylate

1990 ◽  
Vol 429 (1877) ◽  
pp. 459-479 ◽  
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
High Strain Rate ◽  
Strain Rate ◽  
Polymethyl Methacrylate ◽  
Fracture Stress ◽  
High Strain ◽  
Rate Theory ◽  
Strain Rates

The high strain rate response of polycarbonate (PC) and polymethyl methacrylate (PMMA) are measured using a split Hopkinson torsion bar for shear strain rates Ẏ from 500 s -1 to 2200 s -1 , and temperatures in the range —100°C to 200°C. The yield and fracture behaviours are compared with previous data and existing theories for Ẏ < I s -1 . We find that PC yields in accordance with the Eyring theory of viscous flow, for temperatures between the beta transition temperature T β ≈ — 100°C and the glass transition temperature T g = 147°C. At lower temperatures, T < T β , backbone chain motion becomes frozen and the shear yield stress is greater than the Eyring prediction. Strain softening is an essential feature of yield of PC at all strain rates employed. Poly methyl methacrylate fractures before yield in the high strain rate tests for T < 80°C, which is close to the glass transition temperature T g 120°C. It is found that the fracture stress for both materials obeys a thermal activation rate theory of Eyring type. Fracture is thought to be nucleation controlled, and is due to the initiation and break down of a craze at the fracture stress τ f . Examination of the fracture surfaces reveals that failure is by the nucleation and propagation of inclined mode I microcracks which link to form a stepped fracture surface. This reveals that failure is by tensile cracking and not by a thermal instability in the material. The process of shear localization is fundamentally different from that shown by steel and titanium alloys.

Preparation and Mechanical Properties of Hafnium-based Bulk Metallic Glasses

2000 ◽  
Vol 644 ◽  
Author(s):  
Xiaofeng Gu ◽  
Li-qian Xing ◽  
T. C. Hufnagel
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Metallic Glasses ◽  
Bulk Metallic Glasses ◽  
Strain Rates ◽  
Casting Method ◽  
Glass Forming ◽  
Arc Melting ◽  
Potential Use

AbstractWe have prepared bulk metallic glasses of composition (HfxZr1-x)52.5Cu17.9Ni14.6Al10Ti5 (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content, which is advantageous for their potential use as kinetic energy armor-piercing projectile materials. The glass transition temperature and the melting temperature increase linearly with increasing Hf content. The reduced glass transition temperature (Tg/Tm) decreases from 0.64 (x=0) to 0.62 (x=1), indicating reduced glass-forming ability for the Hf- based alloy. The fracture strength in uniaxial compression at quasi-static strain rates also increases with increasing Hf content, reaching ∼ 2.2 GPa for Hf52.5Cu17.9Ni14.6Al10Ti5.

scholarly journals A Thermo-Mechanically Coupled Large-Deformation Theory for Amorphous Polymers Across the Glass Transition Temperature

2010 ◽  
Author(s):  
Shawn A. Chester ◽  
Vikas Srivastava ◽  
Lallit Anand
Keyword(s):  
Glass Transition ◽  
Transition Temperature ◽  
Glass Transition Temperature ◽  
Large Deformation ◽  
Deformation Theory ◽  
Amorphous Polymers ◽  
Polymer Processing ◽  
Constitutive Theories ◽  
Rate Dependent ◽  
The Relationship

Amorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. In this manuscript we briefly summarize a few results from our own recent research [1–4] which is intended to fill this need.

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