Viscoplasticity Based on Total Strain. The Modelling of Creep With Special Considerations of Initial Strain and Aging

1979 ◽  
Vol 101 (4) ◽  
pp. 380-386 ◽  
Author(s):  
Erhard Krempl
Keyword(s):  
Strain Relaxation ◽  
Stress Test ◽  
Time Derivative ◽  
Constant Load ◽  
Primary Creep ◽  
True Stress ◽  
Initial Strain ◽  
304 Stainless Steel ◽  
Creep Tests ◽  
Creep Curves

A previously proposed theory is specialized for the uniaxial state of stress and its prediction for creep and relaxation is analyzed in detail. Constant true stress and constant load creep tests are simulated in the presence and absence of thermal aging together with the constant strain relaxation test. The signs of the creep rate and its time derivative as well as the relaxation rate and its time derivative are introduced as criteria. The constant load creep test can reproduce the normal creep curves and nonclassical creep curves (ε˙ > 0; ε¨ > 0 for all ε and for σ0 > 0). The capabilities of the constant true stress test are limited to primary creep if the work-hardening slope is positive. When aging is introduced almost any creep curve can be reproduced in both tests. The importance of initial strain is discussed and demonstrated by room temperature creep tests on Type 304 Stainless Steel. It is suggested that the initial strains together with the creep curves be reported in the future. Poisson’s ratio in creep needs to be measured and tests are proposed which will enable a quantitative assessment of aging.

Long-Term Creep Modeling of Modified 9Cr-1Mo Steel for a Sodium Cooled Fast Reactor

2010 ◽  
Author(s):  
Woo-Gon Kim ◽  
Jae-Young Park ◽  
Sung-Ho Kim ◽  
Chan-Bock Lee
Keyword(s):  
Constant Load ◽  
Primary Creep ◽  
Least Square ◽  
Creep Data ◽  
Creep Tests ◽  
Omega Method ◽  
Creep Curves ◽  
The Individual ◽  
Term Creep

This paper focused on long-term creep modeling for describing total creep curves of up to rupture for modified 9Cr-1Mo steel (G91). Creep data was obtained by a series of constant-load creep tests at 600°C. Three modified constitutive equations of modified power-law method (MPM), modified theta method (MTM) and modified omega method (MOM), described as a sum of a decaying primary creep and an accelerating tertiary creep, were proposed. A nonlinear least square fitting (NLSF) analysis was carried out on the basis of the creep data so that they provide the best fit to experimental data in optimizing parameter constants of the individual equation. Results of the NLSF analysis showed that in the lower stress regions of 160MPa (σ/σys<0.65), the MTM matched well with the experimental creep data compared with the MPM and MOM, but that in the higher stress regions of 160MPa (σ/σy > 0.65), the MPM revealed better agreement than the MTM and MOM. It was found that the MTM was superior in the modeling of long-term creep curves to the MPM and MOM. Long-term creep curves for the G91 steel were numerically modeled and its creep life was predicted by the MTM.

Creep Characterization of Ni-Based Superalloy IN-792 Using the 4- and 6-θ Projection Method

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
S. A. Sajjadi ◽  
M. Berahmand ◽  
A. Rezaee-Bazzaz
Keyword(s):  
Projection Method ◽  
High Strain ◽  
Operating Temperature ◽  
Constant Load ◽  
Least Square ◽  
Creep Tests ◽  
Wide Range ◽  
Creep Curves ◽  
Projection Techniques

This paper studies the accuracy of a technique which is capable of predicting and modeling a wide range of creep life in Ni-based superalloys. The θ-projection method was applied to characterize the creep behavior of the Ni-based superalloy IN-792 at 800 °C. Constant load creep tests have been carried out over a wide range of loads at the constant operating temperature. Creep curves were fitted using either 4-θ or 6-θ equation by the use of a nonlinear least-square technique. The results showed that both 4- and 6-θ projection parameters revealed a good linearity as a function of stress. Comparison of experimental creep curves with those predicted using both of the utilized θ-projection techniques showed that the techniques fit the experimental data at high strain values very well while the 6-θ approach describes much better the creep curves at low strain region.

scholarly journals Observations of the creep of polar firn

2021 ◽  
pp. 1-19
Author(s):  
Yuan Li ◽  
Ian Baker
Keyword(s):  
Total Porosity ◽  
Constant Load ◽  
Primary Creep ◽  
Secondary Creep ◽  
Micro Ct ◽  
Creep Tests ◽  
Thin Sections ◽  
Compressive Stresses ◽  
Optical Images ◽  
Before And After

Abstract Constant-load creep tests were performed at −10°C at various compressive stresses from 0.05 to 0.75 MPa on specimens taken every 10 m along a firn core extracted at Summit, Greenland in June 2017. The microstructures before and after creep testing were examined using both X-ray microtomography (micro-CT) and optical images from thin sections. An Andrade-like equation was used to describe the primary creep behavior and yielded the time exponent k of 0.17–0.76. The onset of secondary creep occurred at strains of ~0.5–3% but was sometimes not observed at all in shallow firn specimens and at stresses ⩽0.43 MPa even for strain up to 32%. For the 50–80 m firn crept at stresses ⩾0.55 MPa, secondary creep occurred at strains of 2.6 ± 0.28%, and the stress exponent, n, in Glen's law, was found to range from 4.1 to 4.6, similar to those observed for fully dense ice. Micro-CT observations of crept specimens showed that in most cases, the specific surface area, the total porosity and the structure model index decreased, while the structure thickness increased with increasing density. These microstructural characteristics are consistent with the densification of the firn. Optical images from thin sections showed that recrystallization occurred in some specimens that had undergone secondary creep.

Creep Behavior for Alloy 617 in Air at 950°C

2010 ◽  
Vol 654-656 ◽  
pp. 508-511 ◽  
Author(s):  
Woo Gon Kim ◽  
Song Nan Yin ◽  
Gyeong Geon Lee ◽  
Yong Wan Kim
Keyword(s):  
Creep Behavior ◽  
Primary Creep ◽  
Secondary Creep ◽  
Creep Tests ◽  
Alloy 617 ◽  
Very High Temperature Reactor ◽  
Creep Curves ◽  
High Temperature Reactor ◽  
Very High ◽  
Cr2o3 Layer

Creep behavior for Alloy 617, which is considered as one of the major structural materials of a very high temperature reactor, was investigated in air at 950oC. Creep experimental data was obtained by a series of creep tests with different stress levels at 950oC. Alloy 617 revealed little primary creep strains and unclear secondary creep stages. A tertiary creep stage was initiated from a low strain level and was dominant in full creep curves. The creep constants of A, n, m, and C in Norton’s power law and Monkman-Grant relationships were determined. In microstructure observations of crept specimens, it was found that a Cr2O3 oxidation layer was formed on the surface, and just beneath the Cr2O3 layer, an internal Al-oxide sub layer was formed with rod shapes. Also, below the internal sub layer, a thick carbide-depleted zone was developed due to reaction of the chromia and carbide precipitates. The thickness of the outer Cr-oxide layer increased with increasing creep rupture times. The increasing tendency showed a smooth slope like a parabolic curve.

Analysis of Creep Curves of Haynes 230 Superalloy

2010 ◽  
Vol 638-642 ◽  
pp. 2285-2290 ◽  
Author(s):  
Maurizio Maldini ◽  
Giuliano Angella ◽  
Valentino Lupinc
Keyword(s):  
Applied Stress ◽  
High Temperatures ◽  
Creep Curve ◽  
Creep Behaviour ◽  
Strong Dependence ◽  
Solution Treatment ◽  
Constant Load ◽  
Creep Tests ◽  
Haynes 230 ◽  
Creep Curves

The creep behaviour of the solid solution strengthened nickel-based superalloy Haynes 230 has been investigated under constant load and temperature conditions on as received, after conventional solution treatment, and on overaged conditions. The experimental results have shown a very strong dependence of the creep curve shape with the applied stress/temperature: in the tests performed at high stresses/low temperatures, the primary/decelerating stage takes an important portion of the creep curve. At these test conditions, the accelerating creep is mainly caused by the increase of the applied stress with the strain as it happens in constant load creep tests. In the tests performed at low stresses/high temperatures, the primary stage is very small and the following accelerating creep is characterized by different accelerating creep stages. The analysis of the creep curves on the as received and overaged alloys, has shown that a large portion of the accelerating creep at low stresses/high temperatures is caused by microstructural instability.

Primary Creep of Ni3(Al, Ta) Single Crystals at Room Temperature

1996 ◽  
Vol 460 ◽  
Author(s):  
M. D. Uchic ◽  
W. D. Nix
Keyword(s):  
Single Crystals ◽  
Applied Stress ◽  
Room Temperature ◽  
Primary Creep ◽  
Plastic Response ◽  
Stress Increment ◽  
Creep Tests ◽  
Creep Curves ◽  
Anti Phase Boundary ◽  
Test Stress

ABSTRACTThis study examines the time-dependent deformation of Ni3(Al, Ta) at room temperature. Tension creep experiments have been performed on single crystals with one {111}<101> slip system active at the start of the test, where the applied stress ranged from 66.4 MPa (the measured 0.01% flow stress) to 143 MPa (which produced approximately 9% plastic strain). All creep curves displayed primary creep leading to eventual exhaustion, where the measured creep strain declined at a rate faster than predicted for logarithmic creep. However, no correlation between the applied stress and the form of the declining creep rate can be made at this rime. Many creep curves can be obtained from one sample, as the creep curves from both virgin samples and samples with prior deformation history (at the same test stress) were indistinguishable. At the beginning of an incremental creep test, where the stress is increased by a small amount to reinitiate plastic flow in an exhausted sample, a significant retardation of the plastic response of the sample occurred when the stress increment was below 4 MPa. Preliminary TEM studies of a sample strained to 6% suggest that room temperature creep tests may not be ideal for examining the flow of Anti-Phase-Boundary (APB) dissociated dislocations.

Creep Behavior of Al-Si Die-Cast Alloys

2004 ◽  
Vol 126 (4) ◽  
pp. 378-383 ◽  
Author(s):  
Tim Jaglinski ◽  
Roderic Lakes
Keyword(s):  
Primary Silicon ◽  
Constant Load ◽  
Primary Creep ◽  
Silicon Phase ◽  
Creep Tests ◽  
Engine Operation ◽  
Casting Alloys ◽  
Die Cast ◽  
Cast Alloys ◽  
Creep Regime

Commercial, aluminum die-cast alloys are subject to long-term stresses leading to viscoelastic material responses resulting in inefficient engine operation and failure. Constant load creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al-Si and a 17% Si-Al alloys. Rupture occurred in the primary creep regime, with the eutectic alloy having the longest times to failure. Primary creep was modeled by Jt=A+Btn with A, B, and n dependent on stress. Poor creep performance is linked to the brittle fracture of the primary silicon phase as well as other casting defects.

An Experimental Study on the Effect of Prior Plastic Straining on Creep Behavior of 304 Stainless Steel

1993 ◽  
Vol 115 (2) ◽  
pp. 200-203 ◽  
Author(s):  
Z. Xia ◽  
F. Ellyin
Keyword(s):  
Stainless Steel ◽  
Plastic Strain ◽  
Strain Rate ◽  
Creep Behavior ◽  
Test Procedure ◽  
Constant Strain Rate ◽  
304 Stainless Steel ◽  
Creep Model ◽  
Plastic Strain Rate ◽  
Creep Tests

Constant strain-rate plastic straining followed by creep tests were conducted to investigate the effect of prior plastic straining on the subsequent creep behavior of 304 stainless steel at room temperature. The effects of plastic strain and plastic strain-rate were delineated by a specially designed test procedure, and it is found that both factors have a strong influence on the subsequent creep deformation. A creep model combining the two factors is then developed. The predictions of the model are in good agreement with the test results.

Field observations on the load–strain–time behaviour of geogrid reinforcement

1994 ◽  
Vol 31 (4) ◽  
pp. 564-569 ◽  
Author(s):  
R.J. Fannin
Keyword(s):  
Tensile Strength ◽  
Field Data ◽  
Constant Load ◽  
Reinforced Soil ◽  
Mean Annual Temperature ◽  
Creep Tests ◽  
Geogrid Reinforcement ◽  
Rapid Loading ◽  
Strip Test ◽  
Relationship Of

Field data are reported that describe the load–strain–time relationship of geogrid reinforcement in a reinforced soil structure. The data are for a period exceeding 5 years and reveal a continued strain in the reinforcement, which occurs at nearly constant load. The response to loading is attributed to creep of the polymeric material. A comparison of the field data with laboratory isochronous load–strain curves, from rapid loading creep tests performed at a temperature similar to the mean annual temperature in the backfill soil, shows the curves describe very well the magnitude of creep strains observed in the field. Implications of the load–strain–time performance data are assessed with reference to the use in design of a tensile strength established from the rapid-loading creep test and wide-width strip test. The need to clarify, in design of polymeric reinforced soil structures, between a safe and allowable tensile strength is emphasized. Key words : reinforced soil, geogrid, creep, tensile strength, strain.

Creep Behavior and Microstructural Stability of Lamellar γ-T1AI (Cr, Mo, Si, B) with Extremely Fine Lamellar Spacing

2000 ◽  
Vol 646 ◽  
Author(s):  
Wolfram Schillinger ◽  
Dezhi Zhang ◽  
Gerhard Dehm ◽  
Arno Bartels ◽  
Helmut Clemens
Keyword(s):  
Creep Rate ◽  
Creep Behavior ◽  
Lamellar Microstructure ◽  
Lamellar Spacing ◽  
Primary Creep ◽  
Vacuum Arc ◽  
Internal Stresses ◽  
Microstructural Stability ◽  
Thermodynamical Equilibrium ◽  
Creep Tests

ABSTRACTγ-T1AI (Cr, Mo, Si, B) specimens with two different fine lamellar microstructures were produced by vacuum arc melting followed by a two-stage heat treatment. The average lamellar spacing was determined to be 200 nm and 25–50 nm, respectively. Creep tests at 700°C showed a very strong primary creep for both samples. After annealing for 24 hours at 1000 °C the primary creep for both materials is significantly decreased. The steady-state creep for the specimens with the wider lamellar spacing appears to be similar to the creep behavior prior to annealing while the creep rate of the material with the previously smaller lamellar spacing is significantly higher. Optical microscopy and TEM-studies show that the microstructure of the specimens with the wider lamellar specing is nearly unchanged, whereas the previously finer material was completely recrystallized to a globular microstructure with a low creep resistance. The dissolution of the fine lamellar microstructure was also observed during creep tests at 800 °C as manifested in an acceleration of the creep rate. It is concluded that extremely fine lamellar microstructures come along with a very high dislocation density and internal stresses which causes the observed high primary creep. The microstructure has a composition far away from the thermodynamical equilibrium which leads to a dissolution of the structure even at relatively low temperatures close to the intended operating temperature of γ-T1AI structural parts. As a consequence this limits the benefit of fine lamellar microstructures on the creep behavior.

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