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ASTM
E647-13e1 Standard Test Method for Measurement of Fatigue Crack Growth Rates
Edition: 2013
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Description of ASTM-E647 2013
ASTM E647 - 13e1
Standard Test Method for Measurement of Fatigue Crack Growth Rates
Active Standard ASTM E647 | Developed by Subcommittee: E08.06
Book of Standards Volume: 03.01
ASTM E647
Significance and Use
5.1 Fatigue crack growth rate expressed as a function of crack-tip stress-intensity factor range, d a /d N versus ? K , characterizes a material's resistance to stable crack extension
under cyclic loading. Background information on the ration-ale for employing linear elastic fracture mechanics to analyze fatigue crack growth rate data is given in Refs ( 1 ) and ( 2 ) .
5.1.1 In innocuous (inert) environments fatigue crack growth rates are primarily a function of ? K and force ratio, R , or K max and R ( Note 1 ). Temperature and aggressive environments can significantly affect d a/ d N versus ? K ,
and in many cases accentuate R -effects and introduce effects of other loading variables such as cycle frequency and waveform.
Attention needs to be given to the proper selection and control of these variables in research studies and in the generation of design data.
Note 1 ? K , K max , and R are not
independent of each other. Specification of any two of these variables is sufficient to define the loading condition. It is customary to specify one of the stress-intensity parameters (?
K or K max ) along
with the force ratio, R .
5.1.2 Expressing d a /d N as a function of ? K provides results that are independent of planar geometry, thus enabling exchange and comparison of data obtained from a variety of
specimen configurations and loading conditions. Moreover, this feature enables d a /d N versus ? K data to be utilized in the design and evaluation of engineering
structures. The concept of similitude is assumed, which implies that cracks of differing lengths subjected to the same nominal ? K will advance by equal increments of crack extension per cycle.
5.1.3 Fatigue crack growth rate data are not always geometry-independent in the strict sense since thickness effects sometimes occur. However, data on the influence of thickness on
fatigue crack growth rate are mixed. Fatigue crack growth rates over a wide range of ? K have been reported to either increase,
decrease, or remain unaffected as specimen thickness is increased. Thickness effects can also interact with other variables such as environment and heat treatment. For example, materials
may exhibit thickness effects over the terminal range of d a/ d N
versus ? K , which are associated with either nominal yielding ( Note 2 ) or as K max approaches the material fracture toughness. The potential influence of specimen
thickness should be considered when generating data for research or design.
Note 2 This condition should be avoided in tests that conform to the specimen size requirements listed in the
appropriate specimen annex.
5.1.4 Residual stresses can influence fatigue crack growth rates, the measurement of such growth rates and the predictability of fatigue crack growth performance. The effect can be
significant when test specimens are removed from materials that embody residual stress fields; for example weldments or complex shape forged, extruded, cast or machined thick sections,
where full stress relief is not possible, or worked parts having complex shape forged, extruded, cast or machined thick sections where full stress relief is not possible or worked parts
having intentionally-induced residual stresses. Specimens taken from such products that contain residual stresses will likewise themselves contain residual stress. While extraction of the
specimen and introduction of the crack starting slot in itself partially relieves and redistributes the pattern of residual stress, the remaining magnitude can still cause significant
error in the ensuing test result. Residual stress is superimposed on the applied cyclic stress and results in actual crack-tip maximum and minimum stress-intensities that are different
from those based solely on externally applied cyclic forces or displacements. For example, crack-clamping resulting from far-field 3D residual stresses may lead to partly compressive
stress cycles, and exacerbate the crack closure effect, even when the specimen nominal applied stress range is wholly tensile. Machining distortion during specimen preparation, specimen
location and configuration dependence, irregular crack growth during fatigue precracking (for example, unexpected slow or fast crack growth rate, excessive crack-front curvature or crack
path deviation), and dramatic relaxation in crack closing forces (associated with specimen stress relief as the crack extends) will often indicate influential residual stress impact on
the measured da/dN versus ? K result. ( 3 , 4 ) Noticeable crack-mouth-opening
displacement at zero applied force is indicative of residual stresses that can affect the subsequent fatigue crack growth property measurement.
5.1.5 The growth rate of small fatigue cracks can differ noticeably from that of long cracks at given ? K values. Use of long
crack data to analyze small crack growth often results in non-conservative life estimates. The small crack effect may be accentuated by environmental factors. Cracks are defined as being
small when 1) their length is small compared to relevant microstructural dimension (a continuum mechanics limitation), 2) their length is small compared to the scale of local plasticity
(a linear elastic fracture mechanics limitation), and 3) they are merely physically small (<1 mm). Near-threshold data established according to this method should be considered as
representing the materials' steady-state fatigue crack growth rate response emanating from a long crack, one that is of sufficient length such that transition from the initiation to
propagation stage of fatigue is complete. Steady-state near-threshold data, when applied to service loading histories, may result in non-conservative lifetime estimates, particularly for
small cracks ( 5- 7 ) .
5.1.6 Crack closure can have a dominant influence on fatigue crack growth rate behavior, particularly in the near-threshold regime at low stress ratios. This implies that the conditions
in the wake of the crack and prior loading history can have a bearing on the current propagation rates. The understanding of the role of the closure process is essential to such phenomena
as the behavior of small cracks and the transient crack growth rate behavior during variable amplitude loading. Closure provides a mechanism whereby the cyclic stress intensity near the
crack tip, ? K eff , differs from the nominally applied values, ?
K . This concept is of importance to the fracture mechanics interpretation of fatigue crack growth rate data since it implies a
non-unique growth rate dependence in terms of ? K , and R
( 8 ) .
Note 3 The characterization of small crack behavior may be more closely approximated in the near-threshold
regime by testing at a high stress ratio where the anomalies due to crack closure are minimized.
5.2 This test method can serve the following purposes:
5.2.1 To establish the influence of fatigue crack growth on the life of components subjected to cyclic loading, provided data are generated under representative conditions and combined
with appropriate fracture toughness data (for example, see Test Method E399 ), defect characterization data, and stress analysis information ( 9 , 10
) .
Note 4 Fatigue crack growth can be significantly influenced by load history. During variable amplitude
loading, crack growth rates can be either enhanced or retarded (relative to steady-state, constant-amplitude growth rates at a given ? K ) depending on the specific loading sequence. This complicating factor needs to be considered in using constant-amplitude growth rate data
to analyze variable amplitude fatigue problems ( 11 ) .
5.2.2 To establish material selection criteria and inspection requirements for damage tolerant applications.
5.2.3 To establish, in quantitative terms, the individual and combined effects of metallurgical, fabrication, environmental, and loading variables on fatigue crack growth.
1. Scope
1.1 This test method covers the determination of fatigue crack growth rates from near-threshold to K max controlled instability. Results are expressed in terms of the crack-tip
stress-intensity factor range (? K ), defined by the theory of linear elasticity.
1.2 Several different test procedures are provided, the optimum test procedure being primarily dependent on the magnitude of the fatigue crack growth rate to be measured.
1.3 Materials that can be tested by this test method are not limited by thickness or by strength so long as specimens are of sufficient thickness to preclude buckling and of sufficient
planar size to remain predominantly elastic during testing.
1.4 A range of specimen sizes with proportional planar dimensions is provided, but size is variable to be adjusted for yield strength and applied force. Specimen thickness may be varied
independent of planar size.
1.5 The details of the various specimens and test configurations are shown in Annex A1- Annex A3 . Specimen
configurations other than those contained in this method may be used provided that well-established stress-intensity factor calibrations are available and that specimens are of sufficient
planar size to remain predominantly elastic during testing.
1.6 Residual stress/crack closure may significantly influence the fatigue crack growth rate data, particularly at low stress-intensity factors and low stress ratios, although such variables
are not incorporated into the computation of ? K .
1.7 Values stated in SI units are to be regarded as the standard. Values given in parentheses are for information only.
1.8 This test method is divided into two main parts. The first part gives general information concerning the recommendations and requirements for fatigue crack growth rate testing. The
second part is composed of annexes that describe the special requirements for various specimen configurations, special requirements for testing in aqueous environments, and procedures for
non-visual crack size determination. In addition, there are appendices that cover techniques for calculating da/dN, determining fatigue crack opening force, and guidelines for measuring the
growth of small fatigue cracks. General information and requirements common to all specimen types are listed as follows:
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Section
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Referenced Documents
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2
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Terminology
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3
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Summary of Use
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4
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Significance and Use
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5
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Apparatus
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6
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Specimen Configuration, Size, and Preparation
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7
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Procedure
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8
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Calculations and Interpretation of Results
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9
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Report
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10
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Precision and Bias
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11
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Special Requirements for Testing in Aqueous Environments
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Annex A4
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Guidelines for Use of Compliance to Determine Crack Size
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Annex A5
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Guidelines for Electric Potential Difference Determination of Crack Size
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Annex A6
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Recommended Data Reduction Techniques
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Appendix X1
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Recommended Practice for Determination of Fatigue Crack Opening Force From Compliance
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Appendix X2
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Guidelines for Measuring the Growth Rates Of Small Fatigue Cracks
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Appendix X3
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Recommended Practice for Determination Of ACR-Based Stress-Intensity Factor Range
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Appendix X4
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1.9 Special requirements for the various specimen configurations appear in the following order:
The Compact Specimen
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Annex A1
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The Middle Tension Specimen
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Annex A2
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The Eccentrically-Loaded Single Edge Crack Tension Specimen
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Annex A3
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1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate
safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
ASTM Standards
E4 Practices for Force Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E8/E8M Test Methods for Tension Testing of Metallic Materials
E337 Test Method for Measuring Humidity with a Psychrometer (the Measurement of Wet- and Dry-Bulb Temperatures)
E338 Test Method of Sharp-Notch Tension Testing of High-Strength Sheet Materials
E399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness K Ic of Metallic Materials
E467 Practice for Verification of Constant Amplitude Dynamic Forces in an Axial Fatigue Testing System
E561 Test Method for K-R Curve Determination
E1012 Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
E1820 Test Method for Measurement of Fracture Toughness
E1823 Terminology Relating to Fatigue and Fracture Testing
Keywords
constant amplitude; crack size; fatigue crack growth rate; stress intensity factor range;
ICS Code
ICS Number Code 77.040.10 (Mechanical testing of metals)
DOI: 10.1520/E0647-13E01
ASTM International is a member of CrossRef.
ASTM E647
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