How Do Stress Change Tests Relate to Micromechanisms in Creep?

[darkelf]

Hello,

With creep, could anyone please give me any information with regards stress change tests especially in connection with mircomechanisms.

Thanks

 

[Astronuc]

What is meant by stress change tests?

Normally one loads up a specimen, either in compression or tension, then measures the deformation over time. Creep is a relatively slow process, but then one measures different creep rates at different loads and at different temperatures.

 

[darkelf]

Thanks Astronuc, Yes I was referring to creep. Could you by any chance refer me to a paper or book I can get information on experiments to perform stress change tests in creep? Or experiments that have been performed?

 

[Astronuc]

Please explain what one means by stress change test. The stress would change if the load is changed, or if the cross-sectional area changes, or perhaps if the temperature gradient changes.

Normally, load is constant. There are many papers on creep testing.

What is the material system of interest, and what is the stress and temperature ranges?

 

[darkelf]

I'm not too sure about the details. The procedure was just suggested to me by a professor. I found a paper that might have better information but unfortunately I seem to have no access to it.

 

[Astronuc]

Please elaborate on the professor's suggestion, and please provide the title of the paper. Perhaps I can find similar information.

 

[darkelf]

Well from what I have been able to gather what happens is that you peform a creep testat a set temperature under a set stress and determine the creep rate. Then you perform another test at another temperature and stress where the internal stresses of both tests are equal. then u suddenly change the stress to the original stress and measure the strain rate.

The paper is called 'instantaneous strains in stress change tests during steady state creep' by Longquan Shi

 

[Astronuc]

The paper is called 'instantaneous strains in stress change tests during steady state creep' by Longquan Shi

I'm having trouble locating that paper. Please provide the journal and year.


Perhaps the most common context of stress change test is 'stress-dip' test or 'stress dip', in which the specimen is unloaded by some 'small' incremental stress, then reloaded. It appears that temperature is not necessarily changed (increased).

Here is some background -

Deformation of Earth Materials

If dislocation glide is slower than climb, mechanical data from a laboratory study often show strain softening (controlled by dislocation multiplication). In contrast if dislocation climb is more difficult than glide, then work hardening will occur.

A stress-dip test is a useful technique in distinguishing glide-control from a recovery (climb)-control model (mechanism). The basic idea behind this test is the concept of "internal stress". When the creep rate is controlled by recovery, the during steady-state deformation, the applied stress is nearly balanced with the internal stress. Consequently, if one reduces the stress (stress dip) after steady-state creep, then the applied stress would be less than the internal stress and therefore deformation will completely stop until internal stress is reduced. In contrast, if internal stress is substantially lower than the applied stress, then strain rate after a stress reduction will be finite. This will be the case when dreep rate is controlled by dislocation glide.

Reference to Kurigarbagea et al (1989)

Here is a reference to Hiroaki Kurigarbagea paper:

Kurigarbagea H., Yoshinaga H., Nakashima H.
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B7598-48HRVT0-NS&_user=10&_coverDate=02%2F28%2F1989&_rdoc=17&_fmt=high&_orig=browse&_srch=doc-info(%23toc%2312948%231989%23999629997%23426323%23FLP%23display%23Volume)&_cdi=12948&_sort=d&_do
Acta Metallurgica
Volume 37, Issue 2, February 1989, Pages 499-505

In order to demonstrate that the high temperature deformation of f.c.c. and b.c.c. pure metals is controlled not by a glide process but by a recovery process, a thorough examination of distinctions between the two controlling mechanisms in issue is made, in particular in view of the effect of an experimental error involved in the strain rate change test. It is shown theoretically that the effect of the error behaves in a different manner corresponding to the two mechanisms and, in turn, the different behaviour provides an improved analysis to determine the high temperature deformation mechanism. It is presented that the result obtained by the application of the analysis to the high temperature deformation of pure Al and the other deformation characteristics of f.c.c. and b.c.c. pure metals reported so far are well explained by the recovery controlled process, in agreement with the authors' previous conclusion.

Some additional backgroud -

Starting on page 67 -
The physics of creep By Frank Reginald Nunes Nabarro, Heidi L. De Villiers
Stress-dip test discussed on page 69.

Austenitic stainless steels By P. Marshall, Springer; 1 edition (July 31, 1984)

FUNDAMENTALS OF CREEP IN METALS AND ALLOYS

Michael Kassner, Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, USA


Here is a good paper on the subject, but it involves an indentation test rather than a standard uniaxial tensile creep test.
http://www.mit.edu/~mingdao/papers/PM_2003_Indentation_Creep.pdf

Journals like JMEP, Metallurgical Transactions, Journal of Material Science, . . . should be available in the university library, or available through an online subscription (maybe).

 

[darkelf]

Whoa, that's a lot of information. Yes a stress dip test is a stress change test. The stress (load) is either increased or decreased at constant temperature to determine the back stress.

The information you required for the paper is 'Creep and fracture of engineering materials and structures: proceedings of the Fifth International Conference held at University College, Swansea, 28th March-2nd April, 1993 By B. Wilshire, R. W. Evans, Institute of Materials (Great Britain), Institute of Metals. Published by Institute of Materials, 1993'

Thanks ever so much

 

[Astronuc]

Thanks for the reference!

Here's a few more articles. One has to purchase them or find the journal issues in the univerity library. Most are many years old.

http://www3.interscience.wiley.com/journal/112432862/abstract
Initial and Steady-State Creep
Longquan Shi, D. O. Northwood
physica status solidi (a)
Volume 132, Issue 1 , Pages67 - 77
Copyright © 1992 WILEY-VCH Verlag GmbH & Co. KGaA


Instantaneous plastic strain associated with stress changes during the steady state creep of Al and Al-4.20 at% Mg alloy -
http://www.springerlink.com/content/m430170032w02831/
Journal of Materials Science, Volume 20, Number 3 / March, 1985
L. Moerner1, D. O. Northwood1 and I. O. Smith1
(1) Department of Mining and Metallurgical Engineering, University of Queensland, 4067 St. Lucia, Queensland, Australia


Work-hardening rates derived from instantaneous elongation upon sudden load-increments during creep of lead
http://www.springerlink.com/content/k41787kkg0338t4x/
Metallurgical and Materials Transactions A, Volume 12, Number 9 / September, 1981
Hiroshi Oikawa1 and Masanari Ohnuma2


Internal stresses, stress change tests and the formulation of the recovery creep equation
http://cat.inist.fr/?aModele=afficheN&cpsidt=19370094
Auteur(s) / Author(s)
BURTON B. ;
Philosophical magazine letters ISSN 0950-0839 CODEN PMLEEG
1990, vol. 62, no6, pp. 383-388
Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)
Berkeley Nuclear Laboratories, Berkeley Gloucs. GL13 9PB, ROYAUME-UNI



Dislocation network models for recovery creep deformation
http://www.springerlink.com/content/r627647133674741/fulltext.pdf?page=1
Longquan Shi1 and D. O. Northwood1
JOURNAL OF MATERIALS SCIENCE 28 (1993) 5963-5974
Volume 28, Number 22 / January, 1993


Recent progress in the modeling of high-temperature creep and its application to alloy development
http://www.springerlink.com/content/n4078q7882525782/
Volume 4, Number 2 / April, 1995
L. Shi1 and D. O. Northwood2
(1) Present address: Mechanical Engineering Dept., University of Waterloo, N213G1 Waterloo, Ontario, Canada - L. Shi, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22901, USA
(2) Engineering Materials Group, Mechanical Engineering Department, University of Windsor, N9B 3P4 Ontario, Canada

Northwood has been a prolific author, but he's now retired, and I can't find Shi anywhere, so perhaps he retired as well.

 

[darkelf]

Thanks. You've really been a great help. Just curious I know u are able to calculate the activation energy from the strain rate vs time graph but can you use the same method to calculate it from a stress vs time graph? Also how do you calculate the stress exponent from this graph?

 

[Astronuc]

Do you have a creep law in mind?

If $$\dot{\epsilon}\,\propto\,\sigma^n$$, then

$$log\,\dot{\epsilon}\,=\,const\,+\,n\,log(\sigma)$$

 

[darkelf]

Do you have a creep law in mind?

If $$\dot{\epsilon}\,\propto\,\sigma^n$$, then

$$log\,\dot{\epsilon}\,=\,const\,+\,n\,log(\sigma)$$

Steady state creep basically. So if stress is proportional to strain the the stress vs time graph holds for activation energy? I mean a log log graph? The stress exponent is still in question.

 

[Astronuc]

Steady state creep basically. So if stress is proportional to strain the the stress vs time graph holds for activation energy? I mean a log log graph? The stress exponent is still in question.

As far as I know, one has to plot strain rate vs stress (or log of each) to obtain the stress exponent, which implies varying the load/stress.

One has to plot strain rate vs 1/T, where T is temperature to obtain the activation energy, and this implies different specimens tested at different temperatures.

See these two papers.

The Temperature and Stress Dependences of the Steady-State Creep Rate of Ferritic Iron-Chromium Alloys
http://ir.library.tohoku.ac.jp/re/bitstream/10097/27714/1/KJ00004197935.pdf
Takeo Murata and Yunoshin Imai

Microstructure Changes and Steady State Creep
Characteristics in the Superplastic
Sn-5wt.% Bi alloy During Transition.
http://www.egmrs.org/EJS/PDF/vo291/101.pdf
B.A.Khalifa, M.R.Nagy*, G.S.Al-Ganainy and R.Afify.
Egypt. J. Solids, Vol. (29), No. (1), (2006)


For steady-state creep, or secondary creep, the strain rate is constant, with constant stress.

 

[Astronuc]

Here's a really interesting and useful poster.

A Steady-State Creep Model for the AISI 316 L(N) in the Technically Relevant Stress Range
http://www.extremat.org/ib/site/publication/downloads/Paper Rieth No. 1931.pdf

After an experimental period of about 10 years the creep tests have been either finished or aborted, and evaluated.

Also read pages 20-21 here
Heat-resistant materials By Joseph R. Davis

 

[darkelf]

Oh my word! I do apologise. Thanks ever so much. I just realized it might be a time to rupture graph not steady state. That would explain why its stress vs 1/time. I do ever so apologise.

Thanks so much Astronuc.

 

[Astronuc]

No apology necessary. Creep is a rather interesting and complex matter in materials science. There are a number of variables, and each material group or alloy system has its quirks.

Your questions are very good, and in looking for resources with which to respond, I've discovered some useful papers and texts of which I was unaware.

So, Thank you for the questions.

 

[darkelf]

Thanks really appreciate all your help. Is it safe to say that I can use the equation given in the heat resistant materials book calculate the stress exponent? given the time to rupture graph and that the slope is the activation energy?

 

[Astronuc]

Thanks really appreciate all your help. Is it safe to say that I can use the equation given in the heat resistant materials book calculate the stress exponent? given the time to rupture graph and that the slope is the activation energy?

That would give the 'stress exponent for rupture' which is not the same for 'stress exponent for creep'. Straining to rupture involves tertiary creep for short period at the end of the longer secondary creep period.

There is a chapter Design for Elevated-Temperature Applications (pp. 518-533) in the text Heat-Resistant Materials that gives a nice overview of high temperature behavior and creep. The text is one of the special publications available from ASM International, and if creep, particular at high temperature, is one's interest, then I highly recommend purchasing the text, or have one's professor or department purchase it for the library. I have a copy of Heat-Resistant Materials and the one on Stainless Steels, and there is one on Nickel, Cobalt and Ther Alloys.

The specialty hankbooks are currently $286, but ASM International members can buy them for $228. One can get an inexpensive student membership at ASM (and there is a joint ASM-TMS student membership). TMS publishes conference proceedings, Superalloys, which is an excellent reference on research in Ni-based Superalloys (Superalloys 718, 625, 706, and Derivatives).


Kassner's textbook on creep is also highly recommended.

http://ame-www.usc.edu/Books/kassner.shtml
http://www.elsevier.com/wps/find/bookdescription.cws_home/716677/description#description
FUNDAMENTALS OF CREEP IN METALS AND ALLOYS

Michael Kassner, Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, USA

1.0 Introduction

A. Description of Creep
B. Objectives

2.0 Five-Power-Law Creep

A. Macroscopic Relationships
1. Activation Energy and Stress Exponents
2. Influence of the Elastic Modulus
3. Stacking Fault Energy and Summary
4. Natural-Three-Power Law
5. Substitutional Solid Solutions

B. Microstructural Observations
1. Subgrain Size, Frank Network Dislocation Density, Subgrain Misorientation Angle, and the Dislocation Separation Within the Subgrain Walls in Steady-State Structures
2. Constant-Structure Equations
3. Primary Creep Microstructures
4. Creep Transient Experiments
5. Internal stress

C. Rate-Controlling Mechanisms
1. Introduction
2. Dislocation Microstructure and the Rate Controlling Mechanism
3. In-Situ and Microstructure-Manipulation Experiments
4. Additional Comments on Network Strengthening

D. Other Effects on Five-Power-Law Creep
1. Large Strain Creep Deformation and Texture Effects
2. Effect of Grain Size
3. Impurity and Small Quantities of Strengthening Solutes
4. Sigmoidal Creep

3.0 Diffusional Creep

4.0 Harper Dorn Creep
A. The Size Effect
B. The Effect of Impurities

5.0 Three-Power-Law Viscous Glide Creep, by M.-T. Perez-Prado and M.E. Kassner

6.0. Superplasticity, by M.-T. Perez-Prado and M.E. Kassner

A. Introduction

B. Characteristics of Fine Structure Superplasticity

C. Microstructure of Fine Structure Superplastic Materials
1. Grain Size and Shape
2. Presence of a Second Phase
3. Nature and Properties of Grain Boundaries

D. Texture Studies in Superplasticity

E. High Strain Rate Superplasticity (HSRS)
1. High Strain Rate Superplasticity in Metal-Matrix Composites
2. High Strain Rate Superplasticity in Mechanically Alloyed Materials

F. Superplasticity in Nano and Submicrocrystalline Materials

7.0 Recrystallization
A. Introduction
B. Discontinuous Dynamic Recrystallization (DRX)
C. Geometric Dynamic Recrystallization
D. Particle Stimulated Nucleation (PSN)
E. Continuous Reactions

8.0 Creep Behavior of Particle Strengthened Alloys
A. Introduction and Theory
B. Small Volume Fraction Particles that are Coherent and Incoherent with Small Aspect Ratios
1. Introduction and Theory
2. Local and General Climb
3. Detachment Model
4. Constitutive Relationships
5. Microstructural Effects
6. Coherent Particles

9.0 Creep of Intermetallics, by M.-T. Perez-Prado and M.E. Kassner
A. Introduction
B. Titanium Aluminides
1. Introduction
2. Rate Controlling Creep Mechanisms in FL TiAl Intermetallics During 'Secondary' Creep
3. Primary Creep in FL Microstructures
4. Tertiary Creep in FL Microstructures
C. Iron Aluminides
1. Introduction
2. Anomalous Yield Point Phenomenon
3. Creep Mechanisms
4. Strengthening Mechanisms
D. Nickel Aluminides
1. Ni₃Al
2. NiAl

10.0 Creep Fracture
A. Background
B. Cavity Nucleation
1. Vacancy Accumulation
2. Grain Boundary Sliding
3. Dislocation Pile-Ups
4. Location
C. Growth
1. Grain Boundary Diffusion Controlled Growth
2. Surface Diffusion Controlled Growth
3. Grain Boundary Sliding
4. Constrained Diffusional Cavity Growth
5. Plasticity
6. Coupled Diffusion and Plastic Growth
7. Creep Crack Growth
8. Other Considerations