On the Nuances in the Power Law Description and Interpretation of High Homologous Temperature Creep and Superplasticity Data

2018 ◽  
Vol 385 ◽  
pp. 27-32
Author(s):  
K. Anantha Padmanabhan ◽  
S. Balasivanandha Prabu ◽  
A. Arsath Abbas Ali
Keyword(s):  
Grain Size ◽  
Steady State ◽  
Strain Rate ◽  
Linear Function ◽  
Power Law ◽  
Controlling Mechanism ◽  
Different Temperatures ◽  
Pi Theorem ◽  
High Homologous Temperature ◽  
Increasing Temperature

“Power law’’ representation is used to describe minimum creep rate and “steady state” superplastic deformation. In creep isothermal log stress – log strain rate relationship is linear for so long as the rate controlling mechanism remains unchanged. During optimal superplastic flow the slope of this curve changes even when there is no change in the rate controlling mechanism, i.e. the stress exponent, n, at a constant temperature and grain size is a function of strain rate. For a constant rate controlling mechanism, in both the phenomena, n decreases with increasing temperature. Grain size has no effect on creep, but its effect is significant in superplasticity. Therefore, analyzing creep and superplasticity data by treating n for any given mechanism as a constant independent of stress and temperature is questionable. In this analysis stress is normalized with respect to a reference stress, rather than the shear modulus. The microstructure dependence comes through the Buckingham Pi theorem. When contribution from microstructure terms to isothermal strain rate is constant, Laurent’s theorem helps generate a set of values for n. It is shown that the simplest solution, viz. n is independent of stress, but is a linear function of temperature, describes steady state creep. (The case n is independent of both stress and temperature follows as a special case.) The second simplest solution, viz. n is a linear function of both temperature and stress corresponds to steady state superplasticity. Using the equations, the values of n, activation energies for the rate controlling processes and strain rates at different temperatures and stresses could be estimated for both creep and superplasticity. The analysis is validated using experimental results concerning many systems. iiThis lecture is dedicated to the sacred memory of late Prof. Oleg D. Sherby.

scholarly journals Fluorescence investigation of epoxy resin LG 285 and mathematical description of the curing process

2019 ◽  
Vol 292 ◽  
pp. 01025
Author(s):  
Michaela Mikuličová ◽  
Vladimír Vašek ◽  
Vojtěch Křesálek
Keyword(s):  
Steady State ◽  
Fluorescence Spectroscopy ◽  
Epoxy Resin ◽  
Mathematical Description ◽  
Curing Process ◽  
Steady State Fluorescence ◽  
Two Component ◽  
Different Temperatures ◽  
Increasing Temperature ◽  
Steady State Fluorescence Spectroscopy

In this paper, steady-state fluorescence spectroscopy is used to investigate the curing of two-component epoxy resin LG 285. Moreover, the process of curing is mathematically described. The mixture of resin and hardener HG 287 is measured at five different temperatures (50 °C, 60 °C, 70 °C, 80 °C and 90 °C) for five and a half hours. The results indicate that the process of curing of epoxy resin decelerates with time and accelerates with increasing temperature. Furthermore, the energy of the barrier is calculated.

Superplastic Behaviour of AZ61-F Magnesium Composite Materials

2017 ◽  
Vol 36 (3) ◽  
pp. 279-283 ◽  
Author(s):  
Michal Besterci ◽  
Katarína Sülleiová ◽  
Oksana Velgosová ◽  
Beáta Balloková ◽  
S.-J. Huang
Keyword(s):  
Grain Size ◽  
Composite Materials ◽  
Strain Rate ◽  
Magnesium Alloys ◽  
Equal Channel Angular Pressing ◽  
Strain Rates ◽  
Maximum Strain ◽  
Plastic Deformations ◽  
Angular Pressing ◽  
Different Temperatures

AbstractDeformation of AZ61-F magnesium alloys with 1 wt % of Al2O3phase was tested at different temperatures and different strain rates. It was shown that at temperatures 473–523 K and the highest strain rate applied from 1×10–2s–1to 1×10–4s–1, a significant ductility growth was observed. The grain size of 0.6–0.8 μm was reached by severe plastic deformations by means of equal channel angular pressing (ECAP). Secondary Mg17Al12and Al2O3phases were identified. Maximum strain was gained at temperature of 473 K and strain rate of 1×10–4s–1.

Strain Rate Sensitivity of Mechanical Properties and Related Thermal Activation Process in a Two-Phase γ Titanium Aluminide

1996 ◽  
Vol 460 ◽  
Author(s):  
Dongliang Lin ◽  
T. L. Lin ◽  
Yu Wang ◽  
Yun Lin ◽  
Young-Won Kim
Keyword(s):  
Strain Rate ◽  
Strain Rate Sensitivity ◽  
Titanium Aluminide ◽  
Thermal Activation ◽  
Rate Sensitivity ◽  
Activation Process ◽  
Two Phase ◽  
Controlling Mechanism ◽  
Activation Theory ◽  
Different Temperatures

ABSTRACTTensile properties of a two-phase γ titanium aluminide with duplex microstructure are tested under different strain rates from 5×10-5 to 5×10-3S-1 at temperature from 1123K to 1273K. It is found that there exists approximate linear relationship between the flow stresses and the logarithm of the strain rate at different temperatures. The strain rate sensitivity can be explained by thermal activation theory, and dislocation climbing is identified as the rate controlling mechanism

Crack Initiation In Grain Boundaries Under Conditions of Steady-State and Cyclic Creep

1976 ◽  
Vol 98 (2) ◽  
pp. 132-139 ◽  
Author(s):  
R. Raj
Keyword(s):  
Steady State ◽  
Crack Initiation ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Power Law ◽  
Hold Time ◽  
Triple Line ◽  
Cyclic Creep ◽  
Grain Boundary Sliding ◽  
Power Law Creep

Nucleation of intergranular cracks during steady-state and cyclic creep is analyzed in detail. The basis of the analysis is that grain boundary sliding produces stress concentrations at inclusions, ledges and triple-line junctions which leads to nucleation of cracks. The analysis is built upon different facets of the mechanical behavior of grain boundaries such as: sliding with elastic accommodation, diffusional and power law creep accommodation, and the contribution of sliding to the strain rate under power law creep conditions. Inclusions play an important role in fracture since the energy of the inclusion-matrix interface is usually high and since sliding produces stress concentration at inclusions. Triple line fracture is shown to be important during transient creep conditions. Crack initiation is calculated as a function of the following engineering parameters: temperature, strain rate, grain size, inclusions, cyclic frequency and hold time. The results are in broad agreement with experimental observations.

scholarly journals Peculiarities of DRX in a Highly-Alloyed Austenitic Stainless Steel

Materials ◽  
2021 ◽  
Vol 14 (14) ◽  
pp. 4004
Author(s):  
Pavel Dolzhenko ◽  
Marina Tikhonova ◽  
Rustam Kaibyshev ◽  
Andrey Belyakov
Keyword(s):  
Grain Size ◽  
Stainless Steel ◽  
Flow Stress ◽  
Austenitic Stainless Steel ◽  
Dislocation Density ◽  
Strain Rate ◽  
Power Law ◽  
Peak Flow ◽  
Power Law Function ◽  
Grain Orientation Spread

The features of discontinuous dynamic recrystallization (DRX) in a highly-alloyed austenitic stainless steel were studied at temperatures of 800 °C to 1100 °C. Hot deformation accompanied by DRX was characterized by an activation energy of 415 kJ/mol. The frequency of the sequential DRX cycles depended on the deformation conditions; and the largest fraction of DRX grains with small grain orientation spread below 1° was observed at a temperature of around 1000 °C and a strain rate of about 10−3 s−1. The following power law relationships were obtained for DRX grain size (DDRX) and dislocation density (ρ) vs. temperature-compensated strain rate (Z) or peak flow stress (σP): DDRX ~ Z−0.25, ρ ~ Z0.1, σP ~ DDRX−0.9, σP ~ ρ1.4. The latter, i.e., σP ~ ρ1.4, was valid in the flow stress range below 300 MPa and changed to σP ~ ρ0.5 on increasing the stress. The obtained dependencies suggest a unique power law function between the dislocation density and the DRX grain size with an exponent of −0.5.

scholarly journals METHOD FOR MEASUREMENT OF GAS PERMEABILITY OF POLYMERIC FILMS

HortScience ◽  
1992 ◽  
Vol 27 (11) ◽  
pp. 1162d-1162
Author(s):  
Sannai Gong ◽  
Kenneth A. Corey
Keyword(s):  
Steady State ◽  
Oxygen Concentration ◽  
Flow Rate ◽  
Gas Permeability ◽  
Modified Atmosphere Packaging ◽  
Polymeric Films ◽  
Steady State Method ◽  
Different Temperatures ◽  
Time Required ◽  
Increasing Temperature

A rapid steady state method for measurement of gas permeability of polymeric films was developed. Films were sealed between two equal volume chambers with pure O2 and pure N2 flowing through opposite sides. Oxygen concentration in the N2 cell was measured over time until steady state was reached. The method was used to determine oxygen permeability of two different films. Results from four replications on each film indicated excellent repeatability with coefficients of variation less than 3%. The time required to reach steady state oxygen concentration was dependent upon film type, flow rate, and temperature. The higher the N2 flow rate the shorter the time to reach steady state O2 concentrations. The slowest measurement at the lowest flow rate of 27 ml/min took less than 3 hours to collect the data necessary to achieve steady state. Increasing temperature from 10°C to 20°C resulted in an approximately 40% increase in O2 permeability for both films tested. The technique will be a valuable tool for measuring permeabilities of new films and the same film at different temperatures, and for selecting the appropriate material for modified atmosphere packaging of fresh produce.

Deformation behavior and strain rate sensitivity of nanostructured materials at moderate temperatures

2005 ◽  
Vol 880 ◽  
Author(s):  
Cécilie Duhamel ◽  
Sandrine Guérin ◽  
Martin Hÿtch ◽  
Yannick Champion
Keyword(s):  
Grain Size ◽  
Strain Rate ◽  
Nanostructured Materials ◽  
Strain Rate Sensitivity ◽  
Deformation Behavior ◽  
Rate Sensitivity ◽  
Nanocrystalline Copper ◽  
Fcc Metals ◽  
Thermally Activated ◽  
Increasing Temperature

AbstractStrain-rate jump tests in compression are carried out on nanostructured copper (grain size = 90 nm) at moderate temperatures (353K - 393K). Strain-rate sensitivity m is measured as a function of temperature, T, and strain rate, έ. Increasing temperature or decreasing strain rate induces an increase in the strain-rate sensitivity. For (έ, T) = (1×10-5 s-1, 393K), m is equal to 0.17 which is the highest value reported for nanocrystalline copper. These results of enhanced m are encouraging in terms of gain in ductility. The measurements emphasize the existence of a thermally activated mechanism different from the normal rate-controlling process observed for microcrystalline fcc metals.

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