scholarly journals Stress relaxation by power-law creep during growth of a misfitting precipitate

2016 ◽  
Vol 96 ◽  
pp. 74-80
Author(s):  
F.D. Fischer ◽  
J. Svoboda ◽  
T. Antretter ◽  
E. Kozeschnik
Keyword(s):  
Stress Relaxation ◽  
Power Law ◽  
Power Law Creep

Creep Behaviour of Closed Cell Aluminium Foams from Stress Relaxation Tests

2009 ◽  
Vol 423 ◽  
pp. 131-136 ◽  
Author(s):  
B. Carcel ◽  
A.C. Carcel ◽  
P. Arrué
Keyword(s):  
Stress Relaxation ◽  
Power Law ◽  
Creep Behaviour ◽  
Solid Material ◽  
Constant Load ◽  
Creep Tests ◽  
Closed Cell ◽  
Power Law Exponent ◽  
Order Of Magnitude ◽  
Power Law Creep

Creep behaviour of closed cell aluminium foams and transitions from power law to power law breakdown (PLB) creep regimes are investigated from results of stress relaxation tests (SRT) carried out on Alporas foams with densities between 0.20 to 0.32 g/cm3. Tests were carried out at temperatures between 200°C and 300°C and stress relaxation was measured from the collapse stress under compression of the foams. Under similar foam density, temperature and stress conditions, the values of strain rate calculated from SRT tests were of the same order of magnitude than those previously reported in the literature from conventional constant load creep tests. Under stress values close to the collapse stress, the creep mechanism seems to follow a PLB regime, with values of the power law exponent n=10-17, much higher than those corresponding to the power law creep in the solid material (n=4.4-6.5) and with activation energy values close to Q = 150 KJ/mol. Having in mind the limitations of available creep models for closed cell foams and the need for additional experimental results, the use of SRT testing would offer advantages for the assessment of the high temperature behaviour of aluminium foams, due to its lower testing times and reduced experimental effort.

scholarly journals On the identification of power-law creep parameters from conical indentation

2021 ◽  
Vol 477 (2252) ◽  
pp. 20210233
Author(s):  
Yupeng Zhang ◽  
Alan Needleman
Keyword(s):  
Stress Relaxation ◽  
Stress Exponent ◽  
Power Law ◽  
Exponential Factor ◽  
Creep Stress ◽  
Uniaxial Creep ◽  
Creep Stress Exponent ◽  
Power Law Creep ◽  
Good Agreement ◽  
Conical Indentation

Load and hold conical indentation responses calculated for materials having creep stress exponents of 1.15, 3.59 and 6.60 are regarded as input ‘experimental’ responses. A Bayesian-type statistical approach (Zhang et al. 2019 J. Appl. Mech. 86 , 011002 ( doi:10.1115/1.4041352 )) is used to infer power-law creep parameters, the creep exponent and the associated pre-exponential factor, from noise-free as well as noise-contaminated indentation data. A database for the Bayesian-type analysis is created using finite-element calculations for a coarse set of parameter values with interpolation used to create the refined database used for parameter identification. Uniaxial creep and stress relaxation responses using the identified creep parameters provide a very good approximation to those of the ‘experimental’ materials with stress exponents of 1.15 and 3.59. The sensitivity to noise increases with increasing stress exponent. The uniaxial creep response is more sensitive to the accuracy of the predictions than the uniaxial stress relaxation response. Good agreement with the indentation response does not guarantee good agreement with the uniaxial response. If the noise level is sufficiently small, the model of Bower et al. (1993 Proc. R. Soc. Lond. A 441 , 97–124 ()) provides a good fit to the ‘experimental’ data for all values of creep stress exponent considered, while the model of Ginder et al. (2018 J. Mech. Phys. Solids 112 , 552–562 ()) provides a good fit for a creep stress exponent of 1.15.

Suppression of Stress Relaxation in MEMS Multilayer Film Microstructures by Use of ALD Nanocoatings

2002 ◽  
Author(s):  
Yanhang Zhang ◽  
Martin L. Dunn ◽  
Jeffrey W. Elam ◽  
Steven M. George
Keyword(s):  
Stress Relaxation ◽  
Power Law ◽  
Multilayer Film ◽  
Atomic Layer ◽  
Relaxation Mechanism ◽  
Isothermal Hold ◽  
Thermal Cycles ◽  
Al2o3 Layer ◽  
Full Field ◽  
Power Law Creep

We study the suppression of stress relaxation in MEMS multilayer film microstructures by the use of alumina nanocoatings realized by atomic layer deposition (ALD). Gold (0.5 μm thick) / polysilicon (1.5 or 3.5 μm thick) beam and plate microstructures were fabricated by the MUMPs surface micromachining process. The microstructures were then coated on both sides with a 40 nm thick amorphous Al2O3 layer by ALD. The beam and plate microstructures were initially thermal cycled between room temperature and 190°C tostabilize the gold microstructure. After the initial thermal cycles, the microstructures were cooled from 190°C to 120°C and held at 120°C for about 2000 hours (three months). We measured, using an interferometric microscope with a custom-built temperature chamber, full-field deformed shapes (and from these determined the average curvatures in x- and y- directions) of the microstructures during the initial thermal cycles, during the cooling process from 190 °C to 120 °C, and during the isothermal hold. Measurements were made on both coated and uncoated microstructures to assess the influence of the coating. We find that while the 40 nm thick coating has a small effect on the thermoelastic response of the microstructure, it significantly reduces the extent of stress relaxation during the isothermal hold. We modeled the curvature evolution with time assuming the stress relaxation mechanism is power-law creep in the gold, ε˙ = Aσn, and that the polysilicon and alumina deform elastically. The simple model describes the observed behavior reasonably well for the uncoated microstructures (when the power-law parameters are fit using the measured curvature), however, for the coated microstructures, the model predicts a decrease in the stress relaxation, but nowhere near the magnitude observed. This suggests that not only is the stress state in the gold film altered by the nanoscale coating, but also the fundamental deformation mechanisms are altered.

CREEP ANALYSIS FOR A WIDE STRESS RANGE BASED ON STRESS RELAXATION EXPERIMENTS

2008 ◽  
Vol 22 (31n32) ◽  
pp. 5413-5418 ◽  
Author(s):  
HOLM ALTENBACH ◽  
KONSTANTIN NAUMENKO ◽  
YEVGEN GORASH
Keyword(s):  
Experimental Data ◽  
Creep Rate ◽  
Stress Relaxation ◽  
Constitutive Model ◽  
Power Law ◽  
Creep Behavior ◽  
Minimum Creep Rate ◽  
High Stress ◽  
Stress Range ◽  
Power Law Creep

Many materials exhibit a stress range dependent creep behavior. The power-law creep observed for a certain stress range changes to the viscous type creep if the stress value decreases. Recently published experimental data for advanced heat resistant steels indicates that the high creep exponent (in the range 5-12 for the power-law behavior) may decrease to the low value of approximately 1 within the stress range relevant for engineering structures. The aim of this paper is to confirm the stress range dependence of creep behavior based on the experimental data of stress relaxation. An extended constitutive model for the minimum creep rate is introduced to consider both the linear and the power law creep ranges. To take into account the primary creep behavior a strain hardening function is introduced. The material constants are identified for published experimental data of creep and relaxation tests for a 12% Cr steel bolting material at 500°C. The data for the minimum creep rate are well-defined only for moderate and high stress levels. To reconstruct creep rates for the low stress range the data of the stress relaxation test are applied. The results show a gradual decrease of the creep exponent with the decreasing stress level. Furthermore, they illustrate that the proposed constitutive model well describes the creep rates for a wide stress range.

Calculation of Stress Relaxation Properties for 1CrMoV Bolt Material

2003 ◽  
Author(s):  
Fred V. Ellis ◽  
Sebastian Tordonato
Keyword(s):  
Stress Relaxation ◽  
Power Law ◽  
Uniaxial Stress ◽  
Flow Rule ◽  
Creep Equation ◽  
Relaxation Properties ◽  
Stress Levels ◽  
Long Time ◽  
Power Law Creep ◽  
Good Agreement

Analytical life prediction methods have been developed for high temperature turbine and valve bolts. For 1CrMoV steel bolt material, long time creep-rupture and stress relaxation tests were performed at 450°C, 500°C, and 550°C by the National Research Institute for Metals of Japan. Based on analysis of their data, the isothermal creep behavior can be described using a power law: ε=Kσn(t)m+1 where ε is the creep strain, t is the time, σ is the stress, K, n, and m are material constants. The time power is a primarily a function of temperature, but also depends slightly on stress. To obtain the value for the time power typical of low stress, the creep equation constants were found in two steps. The time power was found using the lower stress data and a heat-centered type regression approach with the stress levels taking the place of the heats in the analysis. The heat constants were then calculated at all stress levels and regression performed to obtain the stress dependence. For comparison with the measured uniaxial stress relaxation properties, the relaxed stress as a function of time was calculated using the power law creep equation and a strain hardening flow rule. The calculated stress versus time curves were in good agreement with the measured at initial strain levels of 0.10%, 0.15%, and 0.20% for all temperatures except 500°C. At 500°C, good agreement was found using the creep properties typical of a stronger (within heat variation) material.

Stress behavior of electroplated Sn films during thermal cycling

2009 ◽  
Vol 24 (4) ◽  
pp. 1522-1528 ◽  
Author(s):  
Jae Wook Shin ◽  
Eric Chason
Keyword(s):  
Thin Films ◽  
Thermal Expansion ◽  
Stress Relaxation ◽  
Thermal Cycling ◽  
Power Law ◽  
Bulk Material ◽  
Threshold Value ◽  
Considerable Effect ◽  
Whisker Formation ◽  
Power Law Creep

The mechanical behavior of electroplated Sn thin films was investigated using thermal-expansion induced strain. For stress above a threshold value, the stress relaxation observed during the thermal cycles is well-described by a power law creep mechanism with exponents similar to those of the bulk material. However, the stress relaxation showed significant thickness dependence so that the relaxation in thicker films is faster than thinner films. The surface oxide was also shown to have a considerable effect on retarding the relaxation by inhibiting diffusion to the surface. The relevance of the stress relaxation to whisker formation in Sn-based coatings is discussed.

scholarly journals Three Regions in the Power-Law Creep Regime of TiAl

1992 ◽  
Vol 33 (12) ◽  
pp. 1182-1184 ◽  
Author(s):  
Yukio Ishikawa ◽  
Kouichi Maruyama ◽  
Hiroshi Oikawa
Keyword(s):  
Power Law ◽  
Power Law Creep ◽  
Creep Regime

scholarly journals Unexpected power-law stress relaxation of entangled ring polymers

2008 ◽  
Vol 7 (12) ◽  
pp. 997-1002 ◽  
Author(s):  
M. Kapnistos ◽  
M. Lang ◽  
D. Vlassopoulos ◽  
W. Pyckhout-Hintzen ◽  
D. Richter ◽  
...  
Keyword(s):  
Stress Relaxation ◽  
Power Law ◽  
Ring Polymers
Sign in / Sign up

Export Citation Format

Share Document