ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation

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all of the fracture toughness methods discussed in the previous section except for the K-R curve method and the

crack arrest test method.

Transition Fracture Toughness Testing

The measurement of transition fracture toughness for ferritic steels has long been a problem. The fracture

behavior is usually brittle sometimes after an initial period of ductile crack extension. The toughness values

show extensive scatter and size dependency that cause difficulty in the characterization of toughness for the

evaluation of structures. The scatter and size dependency has been attributed to statistical influences and

constraint differences (Ref 14). Characterization of the toughness relies mainly on the statistical handling of the

data.

Test method ASTM E 1921 has been developed recently to handle the problems of transition fracture toughness

testing (Ref 15). The specimens, fixtures, instrumentation, test procedures, and calculation of toughness

parameters follow existing standards, for example, ASTM E 1820. The evaluation of the statistical aspects are

handled with a weakest-link Weibull statistical distribution (Ref 15). Six or more fracture toughness test results

are required at a given temperature. If the specimen size is not the unit size prescribed in the standard, a

statistical size adjustment is made to the fracture toughness values. From the toughness results and using a

statistical evaluation, a median value of toughness is identified. All median values of the distribution are

aligned on a master curve (Fig. 14). The assumption is made that in the standard, the master curve of median

toughness values is reproducible for the range of steel alloys with yield strengths between 275 and 825 MPa.

The master curve is positioned with a reference temperature, T

, which is the temperature where a median

toughness has a value of 100 MPa . All of the equations relating to the application of the Weibull statistics

and the determination of T

for the placement of the master curve are given in the ASTM E 1921 (Ref 15).

From the master curve and Weibull statistics, the toughness distribution at other temperatures can be

determined. Also, from the statistical distribution, a percentage lower bound confidence level of toughness can

be identified. For example, a 95% lower bound confidence level can be determined from the statistical

distribution as a function of temperature.

Fig. 14 Master curve of transition fracture toughness, K

, critical stress intensity based on the J integral

Fracture Testing of Weldments

Preparation of a test method for the fracture toughness testing of weldments is ongoing. Weldments do not

require a different set of parameters, specimens, or equipment for toughness testing; however, special problems

exist for the testing of weldments. Solutions to these problems are not covered in the other standards.

Weldments have a composite of materials containing base metal, heat-affected zone, and weld metal regions.

Such things as the placement of the notch for the sampling of the correct material, the precracking procedure to

get the crack to grow into the correct area, and the handling of such thing as distortion and residual stresses

present problems. These will be covered in the weldment standard. The parts that are common with the other

standards are not covered in this method, but the tester is referred to the other standards to complete the testing

and analysis after the special problems inherent to the testing of weldments have been addressed. In its first

version, this standard will be annexed to the existing ASTM E 1290 method on CTOD testing, and the test

result will be analyzed primarily with the δ parameter.

References cited in this section

11. “Standard Test Method for Measurement of Fracture Toughness,” E 1820, Annual Book of ASTM

Standards, Vol 3.01 ASTM

14. J.D. Landes and D.H. Shaffer, Statistical Characterization of Fracture in the Transition Region, Fracture

Mechanics: Twelfth Conference, STP 700, ASTM, 1980, p 368–382

15. “Standard Test Method for Determination of Reference Temperature, T

, for Ferritic Steels in the

Transition Range,” E 1921, Annual Book of ASTM Standards, Vol 3.01, ASTM

Fracture Toughness Testing

John D. Landes, University of Tennessee, Knoxville

Fracture Toughness Test for Nonmetals

The standardization of fracture toughness test methods for nonmetals is relatively new compared to

standardization for metallic materials. However, in the past ten years, several new standards have been written

for ceramic and polymer materials. These are usually patterned after similar standards for metallic materials.

The requirements to use the fracture mechanics approach for fracture toughness determination are that the

materials are homogeneous, isotropic, and have a macroscopic defect. Because no material meets this

requirement at all levels, it is required that it fits this criterion at some scale. Usually, this could be a scale of

the same approximate size as the defect length.

To develop the correct test procedure, the deformation behavior of the material must be considered to determine

the fracture parameter to be used to characterize the fracture toughness results. To determine whether the

fracture is characterized by a single point or by an R-curve, the fracture behavior of the material must be

considered. Brief discussion of fracture toughness testing of ceramics and polymer materials follows. A more

complete description is given in other articles in this Section specifically written for these materials. Some

fracture toughness standards have been developed for other nonmetallic materials including glass, rock, and

polymer matrix composites. These standards are not discussed in this article.

Fracture Toughness Testing for Ceramics

Discussion of the fracture toughness testing of ceramics considers two different groups, monolithic ceramics

and ceramic matrix composites.

Monolithic ceramics are brittle and fracture in a linear-elastic manner. The toughness, therefore, can be

characterized by the K parameter. A fracture toughness test procedure could be similar to the K

test procedure

given in ASTM E 399 or ASTM E1820 following the methods used for metallic materials. A major problem for

the fracture toughness testing of ceramics is the introduction of the defect. Because the toughness is so low,

failure can occur during a fatigue precracking procedure. One fracture toughness method that has been used for

brittle materials including ceramics is the chevron-notch fracture toughness test, ASTM E 1304 (Ref 16). This

is one of the test methods that do not require a fatigue precrack. The ASTM E 1304 method was developed for

metallic materials but can often be used for brittle ceramic materials. Because the fracture behavior of ceramics

is brittle, the toughness can be measured as a single point value. A new and provisional standard, PS 70 (Ref

17), has been developed for the fracture toughness testing of advanced ceramics at ambient temperatures. In this

standard, a defect may be introduced as a precrack, or a hardness indentation or a chevron notch can be used to

start the defect, as in ASTM E 1304. The precrack is popped in using a compression loading fixture; fatigue

loading is not used for precracking advanced ceramics.

Ceramic-matrix composites have a more ductile looking toughness character. These materials exhibit more of

an R-curve behavior. In some cases the deformation has a nonlinear characteristic. Although the nonlinear

behavior may not be the same as plasticity in metallic materials, the nonlinear fracture parameters may still

apply. Fracture toughness testing for these materials is largely in the experimental stages, and testing

information is available in a variety of articles on the subject. The article “Fracture Toughness of Ceramics and

Ceramic Matrix Composites” includes a good set of references relative to the fracture testing of ceramic matrix

materials.

Fracture Toughness Testing for Polymers

The fracture toughness behavior for polymers usually falls into two classes, below the glass transition

temperature T

, and above T

. Below T

, the deformation is nearly linear elastic, and fracture is unstable.

Therefore, a single-point toughness value, characterized by K, can be used. Above T

, the deformation is

nonlinear, and the fracture behavior is stable cracking. A J-based R-curve approach can be used. Two problems

that must be addressed in developing test standards for polymers that make them different from metals are the

viscoelastic nature of the polymer deformation behavior and the problem of introducing a defect by fatigue

loading. The viscoelastic deformation character makes the fracture toughness result dependent on the loading

rate, and that must always be specified in the test report. Comparison of toughness results for polymers should

always be made with awareness of the effect of loading rate. Also, due to the viscoelastic nature of polymers,

introduction of the defect is not easily accomplished with fatigue loading, and the crack is usually introduced

with a razor blade cut.

Fracture toughness testing of polymer materials has been standardized in the past few years. The ASTM D 5045

method, standardized in 1996 (Ref 18), is used for determining fracture toughness of plastic materials that fail

under essentially plane-strain and linear-elastic conditions. It has a basis in the ASTM E 399 method and

follows a lot of the same methods. For more ductile polymers, the ASTM D 6068 method, also standardized in

1996 (Ref 19), develops the J-R curve fracture toughness for plastic materials. It follows the ASTM E 813

method in that it is a multiple specimen technique used where each test generates a single point on the J-R

curve. It does not have a J

analysis as ASTM E 813 does, but it uses the entire R-curve as the fracture

toughness characterization. Both of these standard test methods for plastics require a reporting of the loading

rate during the test and an introduction of the defect with a razor blade cut. A single specimen method for

developing the J-R curve has not been standardized for polymers; however, the normalization method for

developing the J-R curve has been shown to work well as a single-specimen method for several of the more

ductile polymers (Ref 20).

More detailed information is provided in the article “Fracture Resistance Testing of Plastics” in this Volume.

References cited in this section

16. “Standard Test Method for Plane Strain (Chevron Notch) Fracture Toughness of Metallic Materials,” E

1304, Annual Book of ASTM Standards, Vol 3.01, ASTM

17. “Provisional Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient

Temperatures,” PS 70, Annual Book of ASTM Standards, Vol 15.01, ASTM

18. “Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic

Materials,” D 5045, Annual Book of ASTM Standards, Vol 8.03, ASTM

19. “Standard Test Method Determining J-R Curves of Plastic Mterials,” D 6068, Annual Book of ASTM

Standards, Vol 8.03, ASTM

20. Z. Zhou, J.D. Landes, and D.D. Huang, J-R Curve Calculation with the Normalization Method for

Toughened Polymers, Polym. Eng. Sci., Vol 34 (No. 2), Jan 1994, p 128–134

Fracture Toughness Testing

John D. Landes, University of Tennessee, Knoxville

References

1. “Standard Terminology Relating to Fracture Testing,” E 616, Annual Book of ASTM Standards, Vol

3.01, ASTM

2. J.D. Landes and R. Herrera, Micro-mechanisms of Elastic/Plastic Fracture Toughness, J

, Proc. 1987

ASM Materials Science Seminar, ASM International, 1989, p 111–130

3. G.R. Irwin, Analysis of Stresses and Strains near the End of a Crack Traversing a Plate, J. Appl. Mech.,

Vol 6, 1957, p 361–364

4. J.R. Rice, A Path Independent Integral and the Approximate Analysis of Strain Concentrations by

Notches and Cracks, J. Appl. Mech., Vol 35, 1968, p 379–386

5. A.A. Wells, Unstable Crack Propagation in Metals: Cleavage and Fast Fracture, Proc. Cranfield Crack

Propagation Symposium, Vol 1, Paper 84, 1961

6. “Standard Method for Plane-Strain Fracture Toughness of Metallic Materials,” E 399, Annual Book of

ASTM Standards, Vol 3.01, ASTM

7. “Standard Practice for R-Curve Determination,” E 561, Annual Book of ASTM Standards, Vol 3.01,

ASTM

8. “Standard Method for Determining Plane-Strain Crack Arrest Toughness, K

, of Ferritic Steels,” E

1221, Annual Book of ASTM Standards, Vol 3.01, ASTM

9. “Standard Method for J

, a Measure of Fracture Toughness,” E 813, Annual Book of ASTM Standards,

Vol 3.01, ASTM

10. “Standard Method for Determining J-R Curves,” E 1152, Annual Book of ASTM Standards, Vol 3.01,

ASTM

11. “Standard Test Method for Measurement of Fracture Toughness,” E 1820, Annual Book of ASTM

Standards, Vol 3.01 ASTM

12. “Methods for Crack Opening Displacement (COD) Testing,” BS5762: 1979, The British Standards

Institution, 1979

13. “Standard Method for Crack-Tip Opening Displacement (CTOD) Fracture Toughness Measurement,” E

1290, Annual Book of ASTM Standards, Vol 3.01, ASTM

14. J.D. Landes and D.H. Shaffer, Statistical Characterization of Fracture in the Transition Region, Fracture

Mechanics: Twelfth Conference, STP 700, ASTM, 1980, p 368–382

15. “Standard Test Method for Determination of Reference Temperature, T

, for Ferritic Steels in the

Transition Range,” E 1921, Annual Book of ASTM Standards, Vol 3.01, ASTM

16. “Standard Test Method for Plane Strain (Chevron Notch) Fracture Toughness of Metallic Materials,” E

1304, Annual Book of ASTM Standards, Vol 3.01, ASTM

17. “Provisional Test Methods for Determination of Fracture Toughness of Advanced Ceramics at Ambient

Temperatures,” PS 70, Annual Book of ASTM Standards, Vol 15.01, ASTM

18. “Standard Test Methods for Plane-Strain Fracture Toughness and Strain Energy Release Rate of Plastic

Materials,” D 5045, Annual Book of ASTM Standards, Vol 8.03, ASTM

19. “Standard Test Method Determining J-R Curves of Plastic Mterials,” D 6068, Annual Book of ASTM

Standards, Vol 8.03, ASTM

20. Z. Zhou, J.D. Landes, and D.D. Huang, J-R Curve Calculation with the Normalization Method for

Toughened Polymers, Polym. Eng. Sci., Vol 34 (No. 2), Jan 1994, p 128–134

Fracture Toughness Testing

John D. Landes, University of Tennessee, Knoxville

Selected References

• T.L. Anderson, Fracture Mechanics, Fundamentals and Applications, 2nd ed., CRC Press, 1995

• J.M. Barsom and S.T. Rolfe, Fracture and Fatigue Control in Structures, 2nd ed., Prentice-Hall, 1987

• J.A. Begley and J.D. Landes, The J Integral as a Fracture Criterion, Fracture Toughness, Proc. 1971

National Symposium on Fracture Mechanics, Part II, STP 514, ASTM, 1972, p 1–20

• D. Broek, Elementary Fracture Mechanics, 4th rev. ed., Martinus Nijhoff, 1986

• W.F. Brown, Jr. and J.E. Srawley, Plane-Strain Crack Toughness Testing of High Strength Metallic

Materials, STP 410, ASTM, 1966

• G.A. Clarke, W.R. Andrews, P.C. Paris, and D.W. Schmidt, Single Specimen Tests for J

Determination, Mechanics of Crack Growth, STP 590, ASTM, 1976, p 24–42

• G.A. Clarke, W.R. Andrews, J.A. Begley, J.K. Donald, G.T. Embley, J.D. Landes, D.E. McCabe, and

J.H. Underwood, A Procedure for the Determination of Ductile Fracture Toughness Values Using J

Integral Techniques, J. Test. Eval., Vol 7 (No. 1), Jan 1979, p 49–56

• M.G. Dawes, Elastic-Plastic Fracture Toughness Based on CTOD and J-Contour Integral Concepts,

Elastic-Plastic Fracture, STP 668, J.D. Landes, J.A. Begley, and G.A. Clarke, Ed., ASTM, 1979, p

307–333

• Fracture Toughness Testing and Its Applications, STP 381, ASTM, 1965

• A. Joyce and J.P. Gudas, Computer Interactive J

Testing of Navy Alloys, Elastic-Plastic Fracture,

STP 668, J.D. Landes, J.A. Begley, and G.A. Clarke, Ed., ASTM, 1979, p 451–468

• J.D. Landes and J.A. Begley, The Effect of Specimen Geometry on J

, Fracture Toughness, Proc. 1971

National Symposium on Fracture Mechanics, Part II, STP 514, ASTM, 1972, p 24–39

• J.D. Landes and J.A. Begley, Test Results from J

Studies—An Attempt to Establish a J

Testing

Procedure, Fracture Analysis, STP 560, ASTM, 1974, p 170–186

• J.D. Landes and J.A. Begley, Recent Developments in J

Testing, Developments in Fracture Mechanics

Test Methods Standardization, STP 632, ASTM, 1977, p 57–81

• P.C. Paris and G.C. Sih, Stress Analysis of Cracks, Fracture Toughness Testing and Its Applications,

STP 381, ASTM, 1965, p 30–81

• K.H. Schwalbe and D. Hellmann, Application of the Electrical Potential Method of Crack Length

Measurement Using Johnson's Formula, J. Test. Eval., Vol 9 (No. 3), 1981, p 218–221

• H. Tada, P.C. Paris, and G.R. Irwin, “The Stress Analysis of Cracks Handbook,” Paris Productions, St.

Louis, MO, 1985.

• K. Wallin, Statistical Modeling of Fracture in the Ductile to Brittle Transition Region, Defect

Assessment in Components—Fundamentals and Applications, J.G. Blauel and K.H. Schwalbe, Ed.,

ESIS/EGF, Mechanical Engineering Publications, 1991, p 1–31

• K. Wallin, Fracture Toughness Transition Curve Shape for Ferritic Structural Steels, Proc. Joint

FEFG/ICF International Conference on Fracture of Engineering Materials (Singapore), 6–8 Aug 1991,

p 83–88

• J.G.Williams, Fracture Mechanics of Polymers, John Wiley & Sons, New York, 1984

Creep Crack Growth Testing

B.E. Gore, Northwestern University, W. Ren, Air Force Materials Laboratory, P.K. Liaw, The University of

Tennessee

Introduction

HIGH-TEMPERATURE APPLICATIONS in the chemical processing, aerospace, nuclear and fossil power-

generation industries, and waste incineration industries are realizing a need for materials that can withstand

increasingly more strenuous working environments for longer periods of time. While many of the methods used

to predict the service life of a structural component involve tests based on the behavior of a material under pure

creep conditions, many applications require an understanding of how creep-fatigue conditions affect the life of

a component.

Using the fossil energy industry as an example, their systems are rarely operated under steady-state conditions.

Due to factors such as shutdowns for safety inspections and changing demands, there is often a thermal-

mechanical fatigue process introduced during service (Ref 1). These processes can affect the service life of a

material by inducing a redistribution of crack-tip stresses and by affecting the creep zone growth behavior (Ref

2) or by changing the creep crack propagation mechanisms (Ref 3). Therefore, the interaction of creep and

fatigue damage is an important concern in structural life assessments. There is also an increasing demand to

develop methods of increasing the service life of existing systems, as well as to develop more accurate

techniques of predicting the initial life span of a material (Ref 4). These demands are a result of not only

economic concerns but safety considerations as well (Ref 5).

Test methods for the evaluation of creep-fatigue interactions include fatigue-life testing with hold times and

creep crack growth testing with hold times. Fatigue-life testing involves stress-controlled (S-N) or strain-

controlled (ε-N) cyclic loading, where hold times and waveform patterns are used to evaluate the time-

dependent effects of creep conditions on fatigue life. These fatigue-life test methods are used to evaluate

materials for safe-life designs by either infinite life (S-N) or finite life (ε-N) criteria. Test methods for this

approach are discussed in more detail in the article “Fatigue, Creep-Fatigue, and Thermomechanical Fatigue-

Life Testing” in this Volume.

Creep crack growth testing is based on the concepts of fracture mechanics where subcritical crack growths are

evaluated from a preexisting flaw or crack. Although careful measures are taken to ensure that materials,

especially those designed for high-temperature applications, do not contain potentially damaging internal errors,

it is true that small defects, such as those due to machining or inclusions in the material, may elude inspections

(Ref 5, 6). Unintended defects typically serve as the point of origin of a crack, which can ultimately lead to the

failure of the material. For these reasons, it is important to understand not only the mechanisms by which a

crack propagates in a particular material, but also to be able to predict the rate of crack growth and to use

experimental data in order to develop a model for the behavior of a material prior to its application in real-

world situations. This article focuses on a description of the experimental method that should be followed in

conducting tests of creep-fatigue crack growth (CFCG) with various hold times and also provides an overview

of some suitable life-prediction models.

References cited in this section

1. W. Ren, “Time-Dependent Fracture Mechanics Characterization of Haynes HR160 Superalloy,” Ph.D.

dissertation, School of Material Science and Engineering, University of Tennessee, 1995

2. H. Riedel and J.R. Rice, Tensile Cracks in Creep Solids, Fracture Mechanics: Twelfth Conf., STP 700,

ASTM, 1980, p 112–130

3. M. Okazaki, I. Hattori, F. Shiraiwa, and T. Koizumi, Effect of Strain Wave Shape on Low-Cycle

Fatigue Crack Propagation of SUS 304 Stainless Steel at Elevated Temperatures, Metal. Trans. A, Vol

14, 1983, p 1649–1659

4. A. Saxena and P.K. Liaw, “Remaining Life Estimations of Boiler Pressure Parts—Crack Growth

Studies,” Final Report CS 4688 per EPRI Contract RP 2253-7, Electric Power Research Institute, 1986

5. R.H. Norris, P.S. Grover, B.C. Hamilton, and A. Saxena, Elevated Temperature Crack Growth, Fatigue

and Fracture, Vol 19, ASM Handbook, ASM International, 1996

6. A. Saxena, “Life Assessment Methods and Codes,” EPRI TR-103592, Electric Power Research

Institute, 1996

Creep Crack Growth Testing

B.E. Gore, Northwestern University, W. Ren, Air Force Materials Laboratory, P.K. Liaw, The University of Tennessee

Creep and Creep Fatigue

Static Loading (Creep Conditions). Testing involving a static load could also be characterized as a crack growth

test conducted with an infinite hold time. In such a case, there is little or no fatigue effect to be accounted for;

however, depending on the environment, there can be significant and sometimes fatal damage due to creep

(permanent deformation resulting from a steady load). A final failure would either occur due to widespread or

localized creep damage (Ref 5, 7). Creep damage is liable to be widespread if the material is in a uniform stress

and temperature environment. In this case, failure is likely to occur due to creep rupture. Failure of this kind is

most commonly observed in a component, such as a steam pipe or an inlet casing, where the material is thin

(Ref 8). Correspondingly, in a structural component, such as a turbine blade, one is apt to observe failure due to

creep crack propagation as opposed to creep rupture. In this case, the creep is localized as a result of

nonuniform stresses and temperatures (Ref 5).

Cyclic Loading (Creep-Fatigue Conditions). In a test run under cyclic loading conditions, the constant load is

periodically interrupted by unloading and reloading. In this case, the effects of creep-fatigue interactions during

transitory load periods play a major role in the initiation and growth of cracks along with the effects of creep

during the intervals of steady-state loads. This type of scenario might occur in a fossil energy system, for

example, when there are pressure and temperature fluctuations due to changes in output energy demands (Ref

1). Creep-fatigue damage might also be of foremost concern in turbine casings where it is often the primary

cause of crack initiation and propagation (Ref 8). It remains that creep contributes to crack growth in regions

where temperatures exceed 427 °C (800 °F), while thermal stresses are considered responsible for fatigue and

creep-fatigue crack growth in the lower temperature regions (Ref 5, 6).

References cited in this section

1. W. Ren, “Time-Dependent Fracture Mechanics Characterization of Haynes HR160 Superalloy,” Ph.D.

dissertation, School of Material Science and Engineering, University of Tennessee, 1995

5. R.H. Norris, P.S. Grover, B.C. Hamilton, and A. Saxena, Elevated Temperature Crack Growth, Fatigue

and Fracture, Vol 19, ASM Handbook, ASM International, 1996

6. A. Saxena, “Life Assessment Methods and Codes,” EPRI TR-103592, Electric Power Research

Institute, 1996

7. A. Saxena, “Recent Advances in Elevated Temperature Crack Growth and Models for Life Prediction,”

Advances in Fracture Research: Proc. Seventh Int. Conf. on Fracture, March 1989 (Houston, TX), K.

Salama, K. Ravi-Chander, D.M.R. Taplin, and P. Rama Rao, Ed., Pergamon Press, 1989, p 1675–1688

8. W.A. Logsdon, P.K. Liaw, A. Saxena, and V.E. Hulina, Residual Life Prediction and Retirement for

Cause Criteria for Ships Service Turbine Generator (SSTG) Upper Casings, Part I: Mechanical Fracture

Mechanics Material Properties Development, Eng. Fract. Mech., Vol 25, 1986, p 259–288

Creep Crack Growth Testing

B.E. Gore, Northwestern University, W. Ren, Air Force Materials Laboratory, P.K. Liaw, The University of Tennessee

Material Characterization

Creep-Ductile Materials. Materials that are classified as being creep-ductile have the ability to sustain

significant amounts of crack growth before failure. Examples of these materials would include Cr-Mo steels,

stainless steels, and Cr-Mo-V steels (Ref 5). Crack growth in this type of material is normally accompanied by

substantial creep deformation at the crack tip. As a result, in order to be able to make accurate predictions for

the lives of high-temperature components made from such materials, a complete understanding of the crack

growth mechanics and damage mechanisms is necessary. An example of the characteristic flow of this

methodology is shown in Fig. 1.

Fig. 1 The methodology for predicting crack propagation life using time-dependent fracture mechanics

concepts. Source: Ref 9

Typically, crack growth in this type of material is due to grain boundary cavitation. The cavitation is commonly

seen to initiate at second-phase particles or at defects along the grain boundaries (Ref 10). As the cavities

nucleate and grow larger, a coalescence of the cavities is observed that will eventually lead to crack propagation

and, ultimately, failure (Ref 11). This mechanism is considered to be characteristic of creep crack growth. Other

mechanisms for fatigue crack growth with hold times include an alternating slip mechanism (a crack-tip blunt

mechanism) and the influence of corrosive environment (Ref 5).

Creep-Brittle Materials. A second type of material is the creep-brittle material. These materials are typified by

the fact that creep crack growth is normally accompanied by small-scale creep deformation and by crack

growth rates that are comparable to the rates at which creep deformation spreads in the cracked body (Ref 5).

This can substantially influence the crack-tip parameters that characterize their crack propagation rates.

Examples of this type of material include nickel-base superalloys, titanium alloys, high-temperature aluminum

alloys, intermetallics, and ceramic materials.

References cited in this section

5. R.H. Norris, P.S. Grover, B.C. Hamilton, and A. Saxena, Elevated Temperature Crack Growth, Fatigue

and Fracture, Vol 19, ASM Handbook, ASM International, 1996

9. P.K. Liaw, A. Saxena, and J. Schaefer, Predicting the Life of High-Temperature Structural Components

in Power Plants, JOM, Feb 1992

10. J.T. Staley, Jr., “Mechanisms of Creep Crack Growth in a Cu-1 wt. % Sb Alloy,” MS thesis, Georgia

Institute of Technology, 1988

11. J.L. Bassani and V. Vitek, Proc. Ninth National Congress of Applied Mechanics—Symposium on Non-

Linear Fracture Mechanics, L.B. Freund and C.F. Shih, Ed., American Society of Mechanical

Engineers, 1982, p 127–133

Creep Crack Growth Testing

B.E. Gore, Northwestern University, W. Ren, Air Force Materials Laboratory, P.K. Liaw, The University of Tennessee

Time-Dependent Fracture Mechanics

Parameters for Static Loading

In time-dependent fracture mechanics, parameters have been defined for static loading and cyclic loading.

There are six crack-tip parameters considered to be applicable for cases of static loading. Each parameter is

specific to a particular set of testing conditions. These conditions are outlined below along with the definitions

and calculations that correspond to each parameter. All of the following information on parameters is found in

Ref 1.

In order to be able to characterize the creep crack behavior of a cracked body, the creep deformation and the

relationship among the creep strain, stress, and strain rate must first be considered. For cracks in creeping solids

under conditions of static loading, the following equation is most frequently used to describe this relationship

(Ref 12):

(Eq 1)